NOVEL MICROWAVE SUSCEPTOR COMPOSITION AND METHOD FOR MAKING SAME

BACKGROUND OF THE INVENTION

Brief Description of the Invention

This invention relates generally to the art of microwave heating by high frequency electromagnetic radiation or microwave energy. More particularly, the present invention relates to organic or inorganic fiber base, fired ceramic base or inorganic base compositions some of which may contain organic materials useful for fabrication in or into microwave heating susceptors, especially for disposable microwave packages for food products.

Description of the Prior Art

The heating of food articles with microwave energy by consumers has now become commonplace. Such microwave heating provides the advantages of speed and convenience. However, heating, for example, fried or crispy foods with microwaves often gives them a soggy texture and fails to impart the desirable browning, flavor and/or crispness of conventional oven heated products due in part to effects of oil and/or moisture. Unfortunately, if microwave heating is continued in an attempt to obtain a crisp exterior, the interior is generally overheated or overdone. Moreover, the

microwave fields in the ovens are uneven which can lead to uneven temperatures, that is, both hot and cold spots within the food item or packaged food item being heated.

The prior art includes many attempts to overcome such disadvantages while attempting to retain the advantages of microwave heating. That is, the prior art includes attempts at providing browning or searing means in addition to microwave heating. Several approaches currently exist in the art including employing permanent dishes or disposable packages to provide microwave heating elements which provide such browning or searing. These elements are referred to herein and sometimes in the art as microwave heating susceptors.

The industry standard or most common susceptor composition at the present time involves aluminum- sputtered susceptor boards. These electrically resistive films susceptors are generally cheap and easy to make and they are disposed of immediately after use. However, evidence has recently come to light which presents health concerns when the susceptor board reaches very high temperatures, sometimes above 350°C. In particular, it is believed that the degradation of the polymer coating material at those high temperatures allows entry of the polymer into the food product. The U.S. Food and Drug Administration is now considering guidelines requiring that susceptor boards have much lower limits on the maximum temperature which is achieved. Thus, the art is now in need of an alternative susceptor composition which maintains much lower temperatures, e.g., usually no greater than approximately 288°C (550°F) , during cooking and thus

does not present the health concerns associated with the aluminum sputtered material.

A number of suggestions have been offered to date for microwave suitable susceptor compositions. For instance. Winters et al, U.S. Patent No. 4,283,427 disclose the use of a chemical susceptor comprising an inorganic salt of Group IA and IIA, and a polar solvent.

Wolfe, Jr., U.S. Patent No. 4,518,651 discloses a porous dielectric substrate embodiment. An electrically conductive coating formed from a matrix of dielectric thermoplastic material and finely divided conductive particles are pressed onto one surface of a porous substrate such as paper or paperboard.

Spanordis, U.S. Patent No. 3,853,612 discloses a method for coating a glass-ceramic cooking receptacle with a semiσonductive tin oxide coating by heating predetermined areas and pyrolyzing tin oxide on those predetermined areas.

A number of patents by Seaborne including U.S. Patent Nos. 4,818,831, 4,808,780, 4,810,845, 4,825,024, 4,806,718, 4,820,533, 4,874,618, 4,950,857, 4,956,533, 4,965,423, 4,965,427 and 4,968,865 which are assigned to General Mills, Inc., deal with packages having a susceptor plate made from an unfired, or "green", ceramic material. Each of the Seaborne patents specifically notes that firing of the ceramic "profoundly" changes a large number of the ceramic properties. The Seaborne patents continuously emphasize that it is important that the susceptors be unvitrified, i.e., not subjected to a conventional firing operation, generally above 800°F to 1000°F (426 ^β C to 538 ^β C) . Seaborne believed that conventional

firing resulted in a fused ceramic composition substantially transparent to microwaves and was thus devoid of desirable microwave reactive properties; this is, of course, true in many cases at temperatures below 150°C if the ceramics are used by themselves.

In particular. Seaborne et al, U.S. Patent No. 4,825,024 discloses an unvitrified ceramic composition comprising a ceramic binder, a ceramic susceptor material and a metal salt temperature profile moderator. The ceramic susceptor material has a neutral lattice change and may be clay, talc, kaolin, silicate, aluminosilicate, sodium metasilicate or alumina. The ceramic susceptor material is mixed with the ceramic binder, the temperature moderator and water. The mixture is formed while soft and allowed to dry. The temperature moderator can be a "super accelerator" such as an alkali metal acetate or an alkali metal bicarbonate or an "accelerator" such as an alkali metal chloride.

Seaborne, U.S. Patent No. 4,808,780 discloses materials such as mixed layer clays which are made amphoteric by steeping in, for example, sodium bicarbonate. The amphoteric material is then mixed with a conventional ceramic binder, a temperature moderator and water.

Seaborne, U.S. Patent Nos. 4,810,845 and 4,818,831 do not call for a temperature moderator but are otherwise similar to the Seaborne '024 and '780 patents, respectively. Seaborne '845 concerns ceramic susceptor materials having a neutral lattice charge. Seaborne, U.S. Patent No. 4,818,831 deals with a material using a mineral susceptor material having a residual lattice charge.

Hart et al, U.S. Patent No. 4,369,346 is of more general interest for its teaching of a microwave baking utensil in which a porous ceramic material is patterned on one side with an ion implanted material having an n- type impurity and a p-type impurity.

Hsia, U.S. Patent No. 4,518,618 discloses a method for crisping and browning the surface of a food using a typical coating material such as flour, corn starch, corn meal, bread crumbs, bran flour or flakes along with a combination of three salts. Increased crisping is preferably achieved by adding a combination of potassium acetate, potassium chloride and potassium bicarbonate; or potassium acetate, potassium chloride and sodium bicarbonate.

Thus, the art discloses a variety of chemicals and material compositions as susceptors for microwave cooking. Unfortunately, each of those compositions has one or more disadvantages and thus commercialization may be difficult. Thus, there remains a long-felt need in the art for a susceptor composition suitable for microwave cooking that avoids the use of metal/polymer combinations as well as the associated health concerns.

SUMMARY OF THE INVENTION

Accordingly, in view of the above-noted problems with present microwave susceptors, an object of the present invention is to provide a device which will heat under the influence of microwave radiation up to an upper temperature limit where the composite reaches a steady state absorption of microwave energy and heating.

An object of the present invention is to provide a microwave susceptor system which can be nontoxic and

free of the potential disassociation problems encountered with current metal-polymer-adhesive- substrate based susceptor compositions.

Another object of the present invention is to provide a microwave susceptor composition comprising a material which can disperse a susceptive salt in such a way as to provide a very large effective internal surface area of said susceptive salt, said susceptive salt being constrained within a material to interact with moisture present.

Yet another object of the present invention is to provide a susceptor composition having a high heat content after heating wherein said heat is controlled by the thickness and surface of said susceptor composition.

A further object of the present invention is to produce a susceptive composition which is capable of rapidly achieving high temperatures without the problems encountered with current metal based systems.

An additional object of the present invention is a composition which is formulated to allow for variability in the amount of microwave energy passing through the susceptor surface thereby converting energy to heat in the susceptor as well as allowing actual microwave energy to contact the food item intended to be heated.

Another object of the present invention is to introduce options in the design of microwave product packaging by tailoring the amount of susceptor material per unit area of surface as well as tailoring the presentation of the heat generated to the food product by selecting specific susceptor surface size, shapes and roughness.

Yet another object of the present invention is to provide a microwave heating composition or susceptor which may be disposable and adapted for use with pre- prepared foods.

An additional object of the present invention is to provide a microwave heating susceptor which is suitable for longevity and re-use for heating food in a microwave oven.

Another object of the present invention is to provide a microwave heating composition or susceptor which is inexpensive to manufacture, safe to use and well adapted for its intended use.

Yet another objective of the present invention is to provide a susceptor and package of natural materials which are biodegradable and do not, ipso facto, require the use of common and layered plastics.

Surprisingly, the above objectives can be realized and new compositions provided which overcome the problems associated with previous materials which have been used for the fabrication of microwave heating susceptors. The present compositions do not exhibit uncontrollable runaway heating while generating relatively large and controllable amounts of heat. Indeed, the final heating temperature can be controlled quite closely. Also, the present compositions are comprised of materials which are commonly available and inexpensive, yet are safe and natural for contact with fpods.

The present invention provides novel compositions useful in the formulation and fabrication of microwave heating susceptors whose thickness and flexibility as well as heating rate provide surfaces for many food

compositions. The present compositions, for instance, comprise an organic and/or inorganic active microwave absorbing material. If the active microwave absorbing material is organic then a binder and/or surface sealant may also be required. For example, an edible shellac commonly used on food surfaces may be used to provide both a surface and an internal sealant to those susceptors. Other more conventional binders may be and are, of course, used herein.

Throughout the specification and claims, percentages are by weight and temperatures are in degrees Celsius, unless otherwise indicated.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a cross-section of one embodiment of a susceptor panel along with a food item contained in a cardboard tray sealed to fit an outer carton.

Fig. 2 is a cross-section of the embodiment of Fig. 1 when opened for heating in a microwave oven.

Fig. 3 is a cross-section of an embodiment of a susceptor panel, food item and cardboard tray combination to show the airflow if venting holes are used.

Fig. 4 is a cross-section of a cardboard food cup containing vertical susceptor panels designed for heating foods such as french fries where air venting through holes is incorporated.

Fig. 5 is a cross-section of a rigid susceptor panel which has a substrate for support and a surface sealant applied.

Fig. 6 is a cross-section of a flexible susceptor panel of similar design to show greater detail.

Fig. 7 through Fig. 13 depicts time/temperature response curves for the susceptor compositions exemplified in Examples 1-4, 6 and 8.

Fig. 14 shows a cross-section of a block 21 of microwave invert plastic which is cut into two pieces between which a susceptor material can be placed.

Fig. 15 depicts the microwave heating times for four different temperature probes placed in different locations in a susceptor composition.

Fig. 16 depicts another heating curve associated with the probe positions of Fig. 15.

Fig. 17 depicts microwave heating time for four probes placed at different places on a susceptor which are associated with the loss of water.

Fig. 18 through Fig. 21 depicts the temperature responses which were delivered to a susceptor by both gravity and natural hot air movement.

Fig. 22(a), (b) and (c) depicts a microwave food package which has been made into a miniature internal "oven".

DESCRIPTION OF THE PREFERRED EMBODIMENTS In its composition aspect, the present invention relates to compositions useful for the formulation and fabrication of heating susceptors for disposable packages for the microwave heating of food products. The compositions comprise defined microwave absorbing materials and generally a binder. In its article aspect, the present invention resides in a microwave heating susceptor for packaged food items, to packages for such items and to the packaged food items themselves.

The microwave absorbing materials useful herein surprisingly include a wide variety of materials, including ceramic materials.

By ceramic materials are meant materials comprising oxygen attached to non-carbonaceous elements, and primarily to magnesium, sodium, calcium, iron, aluminum, silicon and mixtures thereof. In some cases these materials can be of either organic or mineral origin, particularly if they are porous.

There are four basic types of microwave susceptor compositions claimed in the present invention. The first type can be described as a fiber base material. This composition requires a binder and a microwave susceptive material. A deliquescent salt or salts or an inorganic filler is optional. This composition is usually formed by methods well known to the paper or cardboard industry.

The second type of microwave susceptive material is a fired ceramic base material. For example, a terra cotta ceramic is fired as a coherent tile. The terra cotta acts as a porous matrix that is capable of achieving high temperatures without burning or charring. As an inert matrix material, the terra cotta acts as a support and/or host for the microwave susceptive material that imparts the microwave susceptive characteristics to the compositions. Thus, this embodiment also requires a microwave susceptive salt or salts. Optionally, sodium silicate is also included since it provides additional bound water and other desirable characteristics to the composition. Deliquescent salts which themselves may be susceptive when wet may also be included. In this embodiment, there is no need in many cases for a ceramic-like

binder because the firing performs that function. The microwave susceptive material may be introduced into the porous ceramic base by a variety of means including soaking, vacuum impregnation, or spraying. This particular composition typically consists of the microwave susceptor salt or salts in combination with a compound which will provide the water of hydration or hydroxy groups that will activate the microwave susceptive material.

The third embodiment can be described as a fiber/inorganic base formulation. This embodiment involves a fiber material as well as an inorganic material. The inorganic base may be a ceramic material. Terra cotta will be discussed as a typical inorganic material and the terra cotta component in this embodiment has been ground to a finely divided powder. The inorganic material is preferably fired; however, a non-fired inorganic material may be used depending on the desired characteristics of the final composition.

Finely divided powder of the inorganic material may be added to an aqueous based slurry. The slurry may contain a number of ingredients such as sodium silicate, other inorganic fillers, paper fibers and hydrated inorganic compounds that act as flame retardants, i.e., aluminum trihydrate. As is the case with the first embodiment these compositions are formed by methods familiar to the paper and cardboard industry. In this embodiment, the terra cotta powder acts as an inorganic filler and imparts additional high temperature stability and heat capacity to the composite composition. In addition, the inorganic filler, e.g., terra cotta powder, acts as a porous matrix that allows for the absorption of the active

ingredients that render the composition susceptive. If the inorganic material is fired, e.g., if pre-fired clays are used, then the pre-fired clays impart additional high temperature stability to the composition. In addition, these materials impart the color and texture of fired terra cotta to the dried composite susceptor composition that is formed by conventional paper slurry coating methods. The color and texture of those boards imparts particularly desirable visual and tactile aesthetic considerations to those formulations. This third embodiment also requires the presence of a binder such as sodium silicate, which is also a flame retardant and reduces tendencies to scorch at lower temperatures. However, the fiber also contributes significantly to the cohesive base to bind the other materials together. Further, with all porous substrates, e.g., terra cotta, specific degrees of surface roughness, channels and the like can be achieved for optimum moisture release and microwave interaction with a food substance.

The fourth embodiment involves inorganic compositions where the material is selected from a wide variety of materials disclosed herein other than "green" or unvitrified ceramics.

The fifth embodiment involves a variety of organic materials such as semi-porous foods.

Each of the above-identified compositions of the present invention are useful in the conversion of microwave energy to heat. The heat may be intended for both products and appliances including food and non¬ food items in a microwave oven. With careful design as part of this system, items can be heated in

predetermined combinations and ratios of microwave to radiant/convective heat. Package walls both vertically and horizontally placed with respect to a food can be produced with variable capability of limiting passage of microwaves, thereby shielding the contained product from all or some percentage of exposed microwaves. Microwaves are absorbed in the panels due to the susceptor salts which are activated by moisture and this causes heating of the panels. Moisture may then be pumped from the microwaved heat source and from the food product itself thereby reactivating other parts of the susceptor salt composition in passage. Porous and semi-porous surfaces are preferred. These systems can be produced in sufficient bulk so as to provide thermal storage and discharge of heat during and after microwaving. The heat which is produced at the susceptor also serves as a means of initiating a chemical change which allows the limitation and control of the maximum temperature achieved by the susceptive media, in particular, it is desired that the maximum temperature of the susceptive media remain in the range of approximately 120°C to 300°C (approximately 250°F to 575 ^β F) and it is important to be able to select a particular temperature or temperature range for different food products.

As noted above, the microwave susceptor compositions of the present invention contain a base substrate. The base substrate is the structural, generally porous, matrix that contains the microwave susceptive material. This structural matrix not only provides a support matrix for the microwave susceptive material but also is the thermal heat sink that heats to the high temperatures required to crisp the microwave convenience foods, generally 200 to 500

degrees Fahrenheit. The heat capacity of the base material of the present invention continues to maintain high temperatures for a controlled period of time. That temperature is a function of the thickness, mass, porosity, geometric design and concentration of the susceptive material in the matrix.

Suitable fiber components for use in the practice of the present invention include paper, natural or synthetic cellulose, synthetic organic fiber material, cotton fiber, wool fiber, wood based cellulose fiber, newspaper grade pulp, bleached white pulp, kraft pulp, and the like. A typical composition may contain about 5 to 75 weight percent fiber base material, preferably about 5 to 15 weight percent, about 5. to 85 weight percent sodium silicate which is a binder which adds heat capacity and flame retardancy to the composition, about 0.01 to 5 weight percent microwave susceptive salts, about 1 to 20 weight percent deliquescent salts which are also susceptive and about 1 to 20 weight percent additional inorganic fillers, such as micronized marble, talc or calcium carbonate chemical form and of mineral and ceramic lava (e.g., briquettes and barbecue type materials) . The foregoing percentage and those presented hereinafter are not intended to be limiting but merely offer guidance to one of ordinary skill in the art.

A typical fired ceramic base composition as described as the second embodiment of the present invention may contain a terra cotta or other ceramic base, a cardboard or paper base backing, about 0.01 to 5 weight percent microwave susceptive salt, about 1 to 20 weight percent deliquescent salt for hydrated water action and about 1 to 20 weight percent sodium silicate as a hydrated inorganic source of water.

Typical inorganic porous materials notable for use in the practice of the present invention include brick and terra cotta (brick tile) both of which may be in crushed form; micronized marble; natural calcium carbonate (e.g., sea shell or clam shell); and a natural or synthetic zeolite which preferably is in pelletized, roasted or other cintered form.

Any ivory would also be suitable in the practice of the present invention. Such ivory may be either natural or synthetic and have a non-linear porosity which decreases with depth into the material, that is, away from the active or newest surface. While not wishing to be bound by any theory, the present inventors believe that such a non-linear porosity is highly, desirable in order to obtain a water pump effect. That is, water is absorbed into the susceptor material, its phase changes to steam, that is, it is converted into heat energy thus helping to control the susceptor temperature and the steam is released through the porous surface and it is converted into heat energy.

Such an ivory material would further be advantageous because it probably falls within the generally recommended as safe (GRAS) category and it is inexpensive. With respect to GRAS, it would be advantageous to use a simple curing process and/or remove all traces of solvent by baking the material after curing.

Synthetic ivory is porous and can be characterized as one form of a family of imitation ivory materials sometimes known in the trade as "ivorines". Synthetic ivories can be prepared by a variety of techniques known in the art. However, for

purposes of preparing a susceptor composition, the synthetic ivory composition should preferably be made by the following technique:

Processing Steps Involved in the Extraction of Protein Base and Conversion to forms of synthetic ivory and coral

es

The formulation comprises susceptor salts. If, for example, the filler used after the milling stage is a course natural carbonate, a rough surface is obtained after the compression stages and the resulting material

closely resembles natural white coral. "Short cuts" are possible: the casein glue made in the above process is an ideal binder for any course carbonate or similar porous material wherein the combination becomes susceptive with the addition of a metal salt. However, an extruded material is preferred in the creation of susceptive food packages. One familiar with the art will now appreciate that a microwave food package can now be made into a miniature internal "oven" with these and other susceptive materials, one means being shown in Fig. 22.

Synthetic ivory may be made by crumbling fresh eggs and milk. The Enzyme lipass are added to break down fats. The material is coagulated and a filler is added. The filler is used to adjust the weight and texture for porosity. A metal salt or temperature profile moderator may then be added to the material. This egg shell and milk composition may also be referred to as synthetic coral.

A wide variety of fillers would be useful in the practice of the present invention. Titanuim dioxide or natural carbonates are preferred. Thus, micronized marble, finely ground natural shells, zeolites or magnesium hydroxide (which is also flame retardant) are suitable fillers for use in the practice of the present invention, especially in the ivory and coral embodiments.

The texture of the final product is similar to that of natural ivory in which moisture can move easily through the capillaries to and from the surface. A rough surface is generally preferable for a porous susceptor of this type and can be obtained by sanding the surface.

The coagulation and filling processes may both require an adjustment in pH. The filler will determine the density and hence the porosity of the material. The inventors believe that a pH adjustment, together with the coagulation time, will tailor the final temperature of the microwave susceptor after it has been impregnated with the metal salts or temperature profile moderators. Sodium chloride or potassium chloride alone may be sufficient to create a susceptor which can achieve temperatures in the useful range of about 130°C to about 200°C.

Certain semi-porous foods themselves also may be candidates for microwave susceptor substrates. Such semi-porous foods include a wide variety of cooked pastries, starch gels, rice, (e.g., rice paper, rice particle board and the like) mustard seed or any other food which may contain those materials. The use of semi-porous foods is particularly advantageous since it is possible for these compositions to effectively disappear after heating by perhaps leaving an edible brown residue.

An edible shellac which is particularly suitable in the practice of the present invention is confectionery glaze Food Grade SDA 35-A described as regular vacuum dry bleached shellac made by William Zissner and Co., Inc., of Somerset, New Jersey. Water-based latex glues and paints, with or without susceptor salts, can also be used.

A potato may be mashed and made into a polymer for microwave use. Such a biodegradable/recyclable composition is extremely advantageous. Other biodegradable/recyclable compositions also fall within

the scope of the present invention as long as the composition is useful as a susceptor.

The present inventors have now determined that a susceptor comprising a common starch and at least one metal susceptive salt would be an extremely advantageous susceptor composition, especially if the wet version of the susceptor were dried, cured or even fired. The principal advantage with such a composition is that it can achieve a high temperature at an extremely rapid rate.

In addition, ground or milled calcium carbonate or natural marble may be bound together by a starch in order to obtain a desirable susceptor composition.

It has been determined that a starch can be heated and a metal salt or other temperature profile moderator may be incorporated or rolled into the starch. The composition should then be heated once again. In any subsequent heat step, the starch/salt composition would then work as a better susceptor. The second heating step is preferred. While not wishing to be bound by any theory, the present inventors believe that the second heat step results in the formation of conducting paths beside and through simple polymer strands which create uniform flat susceptor tiles, rods or layers.

A common starch and a salt, i.e., a temperature profile moderator, may be combined and processed to make a particularly advantageous microwave susceptor which is able to heat rapidly to high temperatures.

Pure corn starch obtained from a registered supplier may be carefully cooked with ordinary water to create, from the smoothest and most uniform wet slurry, a low-density solid of uniform texture. The solid thus

created is allowed to cool. It is then mixed with a salt to create an electrolyte. For example, approximately 2g of potassium chloride may be added to approximately lOg of the cooked corn starch and mixed to a uniform consistency, creating a stiff paste. The paste may be then spread and cured to a porous dry solid by rapid, uniform heating, for example, in a microwave oven. The material so formed becomes microwave susceptive to temperatures in the range of at least 120°C to 200°C when water is added in small amounts. With care, the material will repeat the heating process after it has cooled.

A starch such as 100% pure corn starch by Best Foods Canada, Inc., Etibitoke, Ontario M9C 4V5 may be used. A ratio of approximately three to one, susceptor host to binder is satisfactory. However, other ratios are just as suitable as long as it allows the final composition to be suitable as a susceptor. The present inventors have found that corn starch when mixed carefully and heated goes from a smooth viscous liquid to a self-supporting "sponge". Approximately 30 grams of corn starch may be placed in 120 ml of cold water and exposed to medium heat with stirring. It may be stopped at any point in the transition from a highly viscous paste to a "sponge". The present inventors have taken the sponge and added a metal salt, i.e., a temperature profile moderator and found that the susceptor composition reached a temperature of approximately 208 ^β C.

While not wishing to be bound by any theory, applicants believe that the curing step polymerizes the starch thereby facilitating the creation of a porous electrolytic "stringbag" in the presence of water. Thus, the applicants have developed an electrolyte

starch formulation which has microwave susceptive properties.

Hydrogel polymers made from oil/water emulsions are also useful in the practice of the present invention. The hydrogel polymers are used as porous polymer scaffolds to hold and perhaps even to bind the metal salts, i.e., temperature profile moderators, into a solid structure. The hydrogel polymer also provides a maze of conducting paths through which water and steam move under constraint, i.e., pressure. Without wishing to be bound by any theory, the present inventors believe that the degree of constraint will determine the temperature that will be eventually reached under conditions of microwave heating. The preferred scaffold structure would be a rigid foam with holes and connecting capillaries. The present inventors believe that ligands can attach to metals in an aqueous solution and concentrate the metals. The ligand-type structures can "hold" or even concentrate many types of ions and then provide conducting "threads" within a host material when various salts are present along with steam.

The process for making suitable ligands involves linking monomers into polymeric chains in such a way as to achieve hydrophilic solids which are in effect constrained hydrogels. It is important that the polymer chemistry involve creating a suitable gel inside a porous polymer scaffold.

Mustard is a surfactant which aids in establishing and maintaining an emulsion in which the aqueous phase is required to be dominant. For example, oil-water emulsions can remain semi-rigid even though they are more than 70% water. That is the case with

mayonnaise or a "low fat spread" (butter substitute) . At that percentage all the water droplets touch each other and the oil is confined to the gaps between the water. Each water droplet thus encased in a "stringbag". The material is then polymerized through, for example, very rapid heating such as microwave heating. One means of partially replacing the water before heating in order to create subsequent electrolytic conduction paths is to mix the emulsion with, for example, 20% by weight of sodium chloride.

Synthetic or man-made fibers may also be used as an inorganic substrate. Such fibers may include orIon, polyester, rayon, Zytel, PBT or other commercially available synthetic fibers.

With respect to the ceramic base formulation of the second embodiment of this invention, since the composition is fired or vitrified, there is no need for a ceramic binder that performs the function of binding the ceramic materials into a solid mass because the firing process performs that function and, in general, creates a highly desirable porous medium.

In the ceramic industry, a distinction is made between "greenware," a ceramic composition before firing or vitrification, and the finished, fired or vitrified ceramic compositions prepared therefrom. The firing step or steps profoundly change a large number of the ceramic composition's properties as the individual constituents are fused into a homogeneous mass. Thus, broadly speaking, for purposes of the present invention, when reference is made to ceramics, it is primarily directed toward compositions which would be considered vitrified, not greenware, in the ceramic arts.

A wide range of fired inorganic materials are suitable in the practice of the present invention. A possible description of such materials are those materials having a neutral lattice charge and being comprised of non-carbonaceous, inorganic based oxides, e.g., oxides of Mg, Ca, Fe, Al, Si or mixtures thereof; however, the existence of a lattice charge may be desirable in some cases, nevertheless a neutral lattice charge is preferred but the present invention is not limited by that statement.

The general properties of the ceramic materials referred to are known and described generally, for example, in "An Introduction to the Rock Forming Materials," by Deer, Howlie, and Zussman, Longman Group Ltd., London, England, (1963), or in "The Potter's Dictionary of Materials and Techniques" by Frank and Janet Hamer, Watson-Guptill Publications (1986) and "The Potters' Complete Book of Clay and Glazes" by James Chappell, Watson-Guphill Publications (1977) , all of which are incorporated herein by reference. Materials as therein described are generally and conventionally classified as ortho and ring silicates, chain silicates, sheet silicates, framework silicates, non-silicates and the like.

Exemplary specific ceramic materials which may be fired include sodium metasilicate, talc, kaolin, alumina and activated alumina, clays (fine grained, natural, early argillaceous materials), aluminosilicates, non-siliceous ceramics, vermiculite including both native and exfoliated (i.e., having been subjected to roasting heat of about 1200°F whereby the vermiculite is expanded by the loss of bound water) , glauconite, bentonites montmorillonoids or smectites, phlogopite mica, biotite mica, zeolite, whether natural

or synthetic: general formula, M _x Dy[Al _x+2 ySi _n _ ₍ x+2y ₎ °2n3 ^H 2° ^wnere i. ⁿ M represents Na, Ka or other monovalent cations and D represents Mg, Ca, Sr, Ba and other divalent F cations, hectorites, chlorites, illites, attapulgites, saponite, sepiolite; ferruginous smectite, kaloinites, and halloysite.

Of particular interest is the use of natural and synthetic zeolites. Any zeolite material is considered suitable in the practice of the present invention as long as it has some susceptive characteristics. Such zeolites would be known to those of ordinary skill in the art. Zeolite microwave heating properties have been described by S. Komarneni and R. Roy "Anomolous Microwave Heating of Zeolites", Materials Letters, Vol. 4, No. 2, 1986, pp. 107-110, and their water or water vapor characteristics under microwave heating conditions have been described by Jean-Marie Thiebault and her colleagues in a paper entitled "Dehydration and Dielectric Permittivity Measurements of a Porous, Inorganic Material (13X Zeolite) Heated with Microwave Power" in IEEE Transactions or Instrumentation and Measurement, Vol. 37, No. 1, March 1988, pp. 114-120, the entire disclosure of which is incorporated by reference.

Suitable zeolites include, for example, molecular sieve materials Linde (Union Carbide) 3A, 4A, 5A, 13X and the like. U.S. Patent No. 3,884,687 discloses a variety of zeolites which are suitable in the practice of the present invention, the entire disclosure of which is incorporated herein by reference. For instance, a complete discussion of the naturally occurring crystalline zeolite materials such as chabazide, mardenite, erionite, analcite and faujasite is not believed to be necessary since there is an

adequate description of such zeolites in the literature.

Among the synthetic crystalline three-dimensional zeolites admirably suited for use in the present invention are crystalline zeolites A and X which are described respectively in U.S. Patent No. 2,882,243 and U.S. Patent No. 2,882,244 issued April 14, 1959, to R. M. Milton. Other suitable crystalline zeolites include those described as follows:

Zeolite F in U.S. Pat. No. 2,996,358 Zeolite Q in U.S. Pat. No. 2,991,151 Zeolite E in U.S. Pat. No. 2,962,355 Zeolite T in U.S. Pat. No. 2,958,952

The term "zeolite," in general, refers to a group of naturally occurring and synthetic hydrated metal alumino-silicates, many of which are crystalline in structure. There are, however, significant differences between the various synthetic and natural materials in chemical composition, physical properties and crystal structure, the latter as evidenced by X-ray powder diffraction patterns.

The structure of crystalline zeolite molecular sieves may be described as an open three-dimensional framework of Si0 ₄ and A10 ₄ tetrahedra. The tetrahedra are cross-linked by the sharing of oxygen atoms, so that the ratio of oxygen atoms to the total of the aluminum and silicon atoms is equal to two, that is, 0 ₇ (Al + Si)=2. The negative electrovalence of tetrahedra containing aluminum is balanced by the inclusion within the crystal of cations, for example, alkali metal and alkaline earth metal ions such as sodium, potassium, calcium and magnesium ions. One

cation may be exchanged for another by ion exchange techniques.

Of course, mixtures of these materials can also be used. Preferred materials include sodium aluminum silicate, clay, sodium metasilicate, kaolin and mixtures thereof due to the relatively flat or uniform final heating temperature.

Both zeolite and brick materials hold water; for example, brick and terra cotta materials can exceed 50% of air space, in the form of weakly-connected spherical, ellipsoidal and needle-like cavities. By the means described herein, for all porous materials, vacuum impregnation techniques advantageously allow for the replacement of the air with water and salts which then create very large internal active susceptor surface areas. In this way, either by soaking or solute transfer under vacuum, a controlled and desired amount of susceptor and other materials can be introduced into the substrate (host) materials, thereby tailoring the heating rate, maximum temperature and other parameters, including moisture movement, as desired, when these materials are made into suitable overall shapes and sizes.

The present compositions include an effective amount of the above described microwave absorbing materials. The precise amount will depend on a variety of factors including end use application, desired final temperature, and thickness of the susceptor to be fabricated from the present compositions. Good results are generally obtained when the microwave absorbing material comprises from about 0.1% to about 98% by weight of the present ceramic compositions. Preferred compounds include from about 20% to 98% by

weight of the microwave absorbing material. For best results, the ceramic compositions comprise about 40% to 98% by weight of the composite microwave absorbing materials. The particle size of the composite microwave absorption material or refractory is not critical. However, finely ground materials (e.g., practice diameter typically less than 600 μm) are preferred since the composite ceramic susceptors produced therefrom are typically smooth and uniform in texture. This is particularly the case when materials are finely ground, e.g., particle size less than typically 200 m.

Another essential component of the unfired ceramic composition embodiment is the use of a conventional ceramic binder. By the term "ceramic binder" is meant that the binder is capable of binding the present ceramic heating materials into a solid mass. The term is not meant to imply or require that the binder material itself is necessarily ceramic in composition although it may be. Such ceramic binders are well known in the ceramic art and the skilled artisan will have no problem selecting suitable binder materials for use herein. The function of the binder is to form the particulate microwave absorbing material into a solid form or mass. Exemplary materials include both ceramic and plastic binders, respectively, such as calcium sulphate, silica fiber, feldspar, pulverized Kevlar (a polyamide fiber), colloidal silicas, fumed silicas, fiberglass, pulp, cotton fibers and mixtures thereof. The binder can comprise from about 2% to 99.9% by weight of the present ceramic compounds, preferably from about 20% to 80%. Exemplary, conventional plastic based binders, both thermoplastic and thermosetting, are described in

U.S. Patent No. 4,003,840 which is incorporated herein by reference.

In one preferred embodiment, the present compositions include binders which are organic thermoplastic resins especially those approved as food packaging material such as polyvinyl chloride, polyethylene, polyamides, polyesters, e.g., polyethylene terephthalate (PET) , polycarbonates, polyimides, epoxies, and the like. In these embodiments, the thermoplastic resin binders can range from as little as 20% up to 60% of the composition and preferably about 30% to 50%. Such compositions are especially well suited for fabrication into shaped microwave susceptors, especially food trays, e.g., for TV dinners or entrees.

The present ceramic compositions can be fabricated into useful microwave heating susceptor articles by a simple admixture of the materials into a homogeneous blend, and by the addition of sufficient amounts of water if needed to hydrate the binder. When the materials are used as the binder, typically, water will be added in a weight ratio to binder ranging from about 0.4 to 0.7:1. While the wet mixture is still soft, the ceramic compositions can be fabricated into desirable shapes, sizes, and thicknesses and thereafter allowed to harden or dry to a moisture content ranging from about 2.5% to 10%.

The composite of the present invention may be contained in some type of package, e.g., a blister pack, and some water may be contained somewhere in the package in order to condition the surface of the composite. The consumer would be able to tear the

package open and discard the package before the susceptor is used.

One advantage of the present invention is that upon heating in a conventional microwave oven, e.g., 2450 MHz, the ceramic compositions will heat relatively quickly (e.g., within 30 to 300 seconds) to a final temperature ranging from about 120°C to 260°C (about 250°F to 500°F) which temperature range is very desirable in providing crisping and browning to foods adjacent thereto and consistent with safe operation of the microwave oven.

Another advantage of the composition of the present invention is that the substrate can be dried at temperatures above 180°F. Still another advantage of the present invention is that susceptors fabricated from the compositions of the present invention provide a microwave field modulating effect, i.e., they even out peaks and nodes (standing wave points) . This phenomenon is believed to.be independent of wattage. Thus, the compositions of the present invention have a thermal conductivity and heat capacity sufficient to do the levelling. This advantage is especially useful when sensitive foods such as cakes or cookie doughs are being microwave heated.

Another advantage of the present invention is that the compositions can absorb oil and/or moisture from food items, e.g., par-fried fish portions, without substantial adverse affect on heating performance.

A further advantage of the present invention is that the susceptor composition heats by virtue of external or internal moisture either in the air or on the surface of the susceptor composition, whereby said moisture causes said susceptor surface to heat rapidly

which in turn drives the moisture off causing a rapid evaporation rate.

The thickness of the substrate depends on a variety of factors. However, the typical thickness of the microwave susceptor composition is in the range of about 0.1 mm to 10.0 mm. The present compositions include an effective amount of the above described microwave absorbing materials.

In designing a suitable susceptor composition it is best to retain some flexibility in the product. Thus, straight pulp and brick may be too brittle and too fragile and unable to withstand normal transportation conditions. Likewise, the base material selected must be such that it does not leach off and transfer to the food _\ product itself.

Suitable temperature profile moderators for use in the practice of the present invention include metal salts, carbon black, ferromagnetic materials, metal salts of organic acids, such as sodium and potassium salts of acetic acid, and the like. The ferromagnetic materials are preferably that group of materials known as the ferrites. The dielectric microwave power absorption (loss mechanism) in ferrites is explained in Chapter 1 of "Ferrites at Microwave Frequencies," by D.J. Fuller, IEE Electromagnetic Wave Series 23 (1987) , published by Peter Peregrinus Ltd. of London UK, pages 1-16. It is possible to make ceramic-ferrite composites in, for example, the Ni-Zn and Ni-Co series whereby the overall electric and magnetic loss mechanism decreases sharply at the Curie temperature (T _c ) , above which temperature the magnetic loss is reduced theoretically to zero. The value of T _c can be chosen between 90°C and 600 ^β C and as, for example, in

the case of Ni-Zn, ferrites may be effective for these purposes at 300°C. Ceramic ferrites and susceptors may be made from materials in the garnet (Y, YA1, YGdAl) series, the nickel series and from magnesium ferrites (MgMn, MgMn Al) by inclusion of them, in desired proportions or in other ceramic materials.

Other metal salts of other organic acids such as propionic acid, isopropionic acid, lactic acid, succinic acid, maleic acid, malonic acid, glycolic, 2,2-dimethylpropanoic acid, butanoic acid (butyric acid) , pentanoic acid (valeric acid) , pivalic acid, glutaric acid, adipidic acid and the like will exhibit microwave susceptive characteristics in combination as with the other metal chloride salts. In the case of several of these materials, however, other properties such as odor (e.g., butyric acid, valeric acid), taste, or toxicity may obviate their utility. Thus, selection of the salts must be consistent with the final end use of the composite.

While not wishing to be bound by the following disclosure, useful metal salts for temperature profile moderators may include sodium chloride, potassium chloride, magnesium chloride, sodium sulfate, zinc sulfate, calcium chloride, calcium oxide, lithium hydroxide, ammonium chloride, sodium hydroxide, potassium hydroxide, sodium bicarbonate, potassium bicarbonate, potassium bromate, ferric chloride, sodium chromate, lithium hypochlorite, sodium hypochlorite, potassium hypochlorite, titanium dioxide (rutile and anatase) , sodium oxalate, ferrous ammonium sulfate, boron suboxide, sodium metaborate and the like.

The preferred microwave susceptive salts for use in the microwave susceptor compositions of the present

invention are potassium acetate, potassium chloride, sodium chloride, sodium silicate or any combination thereof. Potassium acetate is the preferred microwave susceptive salt. These salts may be used in various ratios depending on the desired performance characteristics.

In addition, in each formulation, performance is also based on the presence of water within the salt mixture. This water is typically bound and the performance can further be enhanced by the presence of humectants, e.g., glycols, glycerols, and the like, in particular, food-grade glycols. The microwave susceptor salts may be incorporated into the substrate by a variety of methods including th.e vacuum impregnation technique. The preferred compositions comprise from about 0.1% to about 6% by weight salt. While compositions can be formulated having higher amounts of salt generally no advantage is achieved.

Three subclasses of temperature profile moderators exist: (1) dampeners, (2) accelerators or enhancers, and (3) super accelerators. Accelerators, for example, may increase the temperature rate of increase with time when exposed to microwave heating. Accelerators may also increase the maximum obtainable temperature. Dampeners have the opposite affect while super accelerators exhibit a greater acceleration effect.

Exemplary dampeners may possibly be selected from the group consisting of MgO, CaO, B ₂ 0 ₃ , Group IA alkali metal (Li, Na, K, Cs, etc.) compounds of chlorates (LiC10 ₃ , etc.), metaborates (LiB0 ₂ , etc.), benzoates (LiC0 ₂ CgH5, etc.), dichromates (Li ₂ Cr ₂ 0 ₇ , etc.); also, all calcium salts, antimony chloride, ammonium

chloride, cupric chloride, copper (II) sulfate (blue vitriol) , magnesium chloride, zinc sulfate, tin (II) chloride, vanadyl sulfate, chromium chloride, cesium chloride, cobalt chloride, nickel ammonium chloride, titanium dioxide (rutile and anatase) , and mixtures thereof. Exemplary useful accelerators are selected from the group consisting of Group IA alkali metal (Li, Na, K, Ca, etc.) compounds of chlorides (LiCl, etc.), nitrites (LiN0 ₂ ) , etc., nitrates (LiN0 ₃ , etc.), iodides (Lil, etc.), bromates (LiBr0 ₃ , etc.), fluorides (LiF, etc.), carbonates (Li ₂ C0 ₃ , etc.), phosphates (Li ₃ P04, etc.), sulfites (Li ₂ S0 ₃ , etc.), sulfides (LiS, etc.), hypophosphites (LiH ₂ P0 ₂ , etc.), also barium chloride, ferric chloride, sodium borate, magnesium sulfate, strontium chloride, ammonium hydroxide. Tin (IV) , chloride, titanium (II) oxide, titanium (III) oxide, silver citrate and mixtures thereof. Exemplary useful super accelerators are desirably selected from the group consisting of B ₄ C (boron carbide) , Re0 ₃ (rhenium (IV) oxide) , cuprous chloride, ferrous ammonium sulfate, silver nitrate. Group IA alkali metal (Li, Na, K, Cs, etc.) compounds of hydroxides (LiOH, etc.), hypochlorites (LiOCl, etc.), hypophosphates (Li ₂ H ₂ P ₂ 0 ₆ Na ₄ P ₂ 0 ₆ , etc.), bicarbonates (LiHC0 , etc.), acetates (LiC ₂ H ₃ 0 ₂ , etc), oxalates (Li ₂ C ₂ 0 ₄ , etc.), citrates (Li ₃ C ₆ H ₅ 0 ₇ , etc.), chromate (Li ₂ Cr0 ₄ , etc.) and sulfates (Li ₂ S0 ₄ , etc.), and mixtures thereof.

Exemplary accelerators may include certain highly ionic metal salts of sodium, lithium, magnesium, silver, barium, potassium, copper, iron and titanium including, for example, sodium chloride, sodium sulfate, silver nitrate, silver citrate, magnesium sulfate, sodium citrate, potassium acetate, barium chloride, potassium iodide, potassium bromate, copper

(I) chloride, lithium chloride and ferric chloride. The most preferred accelerator useful herein is common salt due to its low cost and availability. The temperature profile accelerator can assist in reaching the final operating temperature of the ceramic composition more quickly. Also, the accelerator can modestly increase the final operation temperature of the ceramic composition.

The preferred composition comprises from about 0.01% to about 10% by weight of the microwave susceptive material, e.g., metal salt moderators. Preferably, the present compounds comprises from about 0.1% to 6% of the moderator. For best results, about 1% to 6% moderator is used.

The susceptive salt requires the presence of water molecules or the presence of an inorganic deliquescent salt such as, but not limited to potassium acetate or sodium chloride. A deliquescent salt will absorb moisture from the local packaging environment and provide the water required for effective absorption of the microwave energy and the subsequent conversion to thermal energy. Absorption of water from the local environment provides the additional benefit of acting as a dessicant and aiding in the prevention of the formation of ice crystals.

In addition to the use of a deliquescent salt as described above, the invention includes the presence of, but is not limited to a material such as sodium silicate. Sodium silicate acts as an inorganic binding matrix. The sodium silicate can provide not only the mechanical binding matrix to the microwave susceptor board, but it also retains a significant quantity of water within the matrix of the susceptor board. This

bound water aids in the activation of the susceptive salt or salts to temperatures above the normal boiling point of water. The sodium silicate may be used in a range of about 5 to 75 weight percent, preferably 20 to 40 weight percent.

Sodium silicate is useful as a binder and it limits the increase in temperature of the susceptor, that is, it is a flame retardant. Sodium silicate was determined by the present inventors to perform the same function as edible shellac or starch glue. The addition of sodium silicate also allows the susceptor to be heated for a longer period of time while allowing the maximum temperature that can be reached to be limited in some cases. This prevents scorching and discoloration of the susceptor surface.

Another inorganic compound, such as alumina trihydrate, will provide additional water to the susceptive salt mixture at specific temperatures and thereby enhance the susceptibility of the susceptive salt component. Alumina trihydrate releases its water of hydration rapidly at approximately 230°C (445°F) ; other similar materials such as magnesium hydroxide will release their water of hydration at other predetermined temperatures. These inorganic water complexed salts can be used to tailor the desired temperature rates and thermal profiles.

Aluminum hydroxide is a more effective moderator than sodium silicate. The effect of aluminum trihydroxide is to "drown" the susceptor at high temperatures (approximately 200°C) by decomposition into alumina and water vapor. The reaction is endothermic and decomposition begins at approximately

210°C when conventional heating times are less than three minutes.

Also, colorants, both organic and inorganic may be incorporated at appropriate levels into either the coating or body of the susceptor composition of the present invention to aid in aesthetics without adversely affecting the performance of the composition. In addition, non-toxic inorganic filler materials such as micronized marble, talc or calcium carbonate may be included in the base composition.

Essences and flavoring agents may be added to the susceptor composition of the present invention. Chemical and natural control of odors are presumed to be possible by the same impregnation means. Oil of clove would be a useful preservative for starch-based materials, in particular.

It is believed that each additive in the microwave susceptor composition can affect microwave performance. The present inventors have noted that when vertical moisture movement through and from a food and its package had been achieved, flavors can be carried from the susceptor to the food. Thus it may, for example, be desirable to add vinegar to a porous susceptor, for instance, with fried potatoes, or herb "essences" or spices with other foods. In this regard, when water-based glues and gums are used as susceptors or susceptor hosts, off-flavors appear to be overcome with the use of trace amounts of oil of clove extract or oil of wintergreen. It has been determined that 1 drop (about 0.25 mil/liter) is effective in the susceptor gums devised. Up to 2ml/liter have been used with pleasant odor release.

Samples of food retained in plastic with oil of clove after heating and susceptor packs incorporating oil of clove will ordinarily not become moldy after storage at room temperature for approximately 10 weeks. This same observation was made with respect to varieties of gum (starch-based) sealants, including the papers made therefrom.

With microwave heating there is some evidence that the rate of decomposition of this chemical is greater and begins at a lower (susceptor body) temperature. Thus sodium silicate and aluminum hydroxide are both valuable "moderators" with overlapping properties.

The compositions of the present invention also heat up relatively quickly. Generally, the compositions of the present invention heat up within about 15 seconds to 30 seconds. The compositions also remain at the final heating temperature, i.e., about 200°F to 500°F with a high degree of stability.

The susceptor composition of the present invention has several advantages over the prior art. The susceptor compositions of the present invention do not exhibit runaway heating, that is, upon further microwave heating their temperatures continue to increase only slowly, in some cases not at all. The susceptor compositions are also able to generate large amounts of heat. They are also relatively economical to prepare and may be easily disposed of after only one use.

Through proper control of the susceptive component in the composition, heat radiated and/or conducted to the product can be individually tailored.

This is one of several novel features of the present invention over the prior art.

Likewise, the amount of microwave energy allowed to pass through the susceptive surface can be filtered or limited. This allows for protection of the microwave sensitive products being heated. It also allows for a package which is capable of simultaneous microwave convection heating or a package capable of microwave, radiant and/or convection heating.

The microwave heating susceptor compositions of the present invention may be fabricated into trays, disks, pans, cups, cylindrical shapes, pocket shapes and the like. Thus, a wide variety of shapes can be envisioned for the microwave heating susceptor compositions of the present invention. The selected shapes may serve to both contain the food prior to purchase by a consumer as well as heat the food item.

The microwave susceptor compositions of the present invention also are useful in any number of microwave absorption applications. The present compositions are particularly useful for fabrication into microwave susceptors which in turn are useful as components in packages for foods to be heated with microwaves.

Par-fried frozen food items, especially fish, chicken and vegetables are popular food items. Upon conventional baking in an oven, the prepared food articles exhibit a relative evenness of temperature throughout and a crisp or crunchy coating. During oven heating both oil and water vapor are released and the exterior coating is toasted. The novel susceptive compositions of the present invention are able to

mimic the desired aesthetic and tactile properties associated with fried or baked foods.

A wide variety of food items may be packaged in and/or cooked in the microwave susceptor composition of the present invention. The art of microwave cooking has advanced to the point where the variety of microwave food products available for consumer purchase extends to almost all types of food products. Given a little imagination and perseverance almost any food product can be cooked (and not just reheated) using only a microwave oven.

The fiber based susceptor composition of the present invention may be prepared by mixing an aqueous slurry of discrete cellulose fibers and adding about 15 percent glycerin, or 5 to 15 percent of a food grade glycol, olive oil or the like, an inorganic filler and a sodium silicate binder solution. The mixture of the filtered solution is withdrawn using suction and oven dried to obtain a non-woven mat containing uniformly dispersed inorganic filler. The mass may then be infiltrated with a controlled amount of aqueous susceptive solution and hot pressed and dried at approximately 180°C to obtain a hard, semi-rigid board.

The reader will now appreciate that some microwave susceptors can be made very simply. For example, many powdered, or gritty materials which have an inherently rough or porous surface can be bound to a paper substrate by a glue and then made susceptive by the addition of an electrolytic solution such as a chloride in water. A simple electrolytic microwave susceptor comprises a metal salt, a course material or suitable surface properties, a binder and a substrate. The substrate may, of course, be, by itself, another

susceptor and one, for example, which is susceptible to microwave heating at and above the maximum attained by the first susceptor. In this way interactive microwave susceptor layers can be achieved and very high temperatures attained, in steps. If one layer burns at a particular temperature, and "disappears", it is useful if the temperature is sufficiently high to cause the substrate to start to absorb microwave energy and itself continue to heat.

As discussed above, the fired ceramic composition may be prepared by vacuum infiltrating a porous inorganic substrate such as terra cotta, lava, pumice or unglazed porcelain with an aqueous susceptor solution comprised of any or all of: potassium acetate, sodium chloride or other salts with desirable properties in the desired proportions and then oven drying the mixture at approximately 100°C (212 ^β F) to make a high heat storage susceptor article.

The susceptive salts may be introduced into a terra cotta substrate, for example, by soaking the porous tile in an aqueous solution containing approximately 150 grams of one or more salts in a liter of warm water and placing a crystallizing dish containing the solution and the tile in a vacuum oven. A vacuum would then be pulled in order to degas the sample and to pull the salt solution into the tiny crevices and pores of the terra cotta tile. It may also be desirable to package the susceptor in a hermetic package so as to ensure that the requisite moisture did not evaporate or subliminate from the porous tile matrix into the potentially desiccating environment of auto defrost, low moisture content freezers.

Alternatively, it may be possible to seal in the moisture by using additional coatings on the composition. The second coating material could be an inorganic coating such as a thin layer of sodium silicate that is coated onto the substrate after infiltration with the susceptive salt solution. This hermetic sealing coating could potentially be put on to the substrate by spray coating on a continuous web assembly, or alternatively, the coating could be applied by dip coating the tile in a solution of the sodium silicate. Either of these two approaches have the potential of drastically reducing the rate of evaporation or sublimination of the requisite water from the susceptive tile. Suggested coating include an edible glycol such as an edible shellac or an edible starch glue. Such coatings would, of course, be useful on all susceptor compositions of the present invention.

In another potential form of the susceptive tile, it would be possible to seal the requisite water into the tile by coating the salt infiltrated tile with a thin coating of an organic food sealant. There are many such materials that are available commercially that would fill this requirement. Alternatively, a high molecular weight polysaccharide, polyethylene glycol, polypropylene, or the like, have the potential to provide an organic alternative to the sealing of the tile. As in the above case with the inorganic sealant, these materials could be applied by spray coating or they might by applied by dip coating.

In another potential form of the susceptive composition, it would be possible to introduce additives to the infiltrated composition that would provide the requisite moisture to the susceptor file and by the nature of the material lock in the moisture

by forming a stable matrix with entrapped water, thereby greatly reducing the rate of evaporation or sublimation of the water from the matrix. These materials might be any of the inorganic or organic materials that have been noted thus far.

Typical ways to make paper include slurry papers, conventional molding, stress molding, rigid board, flexible paper, and the like.

A fiber base composition on a gram per liter of aqueous slurry basis may be prepared as follows: 6 grams newspaper grade (or better) pulp, 2 grams cotton fiber, 750 ml of a 36 wt % sodium silicate solution, 24 grams micronized marble or 24 grams ground terra cotta, and 24 grams aluminum trihydrate. The boards cast from this slurry may be dried and subsequently soaked in an aqueous solution of the susceptive salt mix. In a more preferred embodiment, the salt solution may consist of: 250 grams potassium acetate and 250 grams potassium chloride per liter water.

A binding matrix material may be used that in many ways acts as a glue to consolidate the fibrous component of the board. The matrix binding agent or glue that consolidates the board may be an inorganic or organic material. The inorganic based material is preferred. First, the inorganic binder alternative provides significantly higher thermal stability over the organic alternative. Secondly, the use of an organic binding agent may require the use of preservatives or additional additives that may or may not fare well in the high temperature environment of some susceptor boards. Thirdly, the introduction of an organic binder over an inorganic binder may introduce smoke or odor in the event of an unanticipated high

temperature scenario that brings about the burning, smoking, or smoldering of the organic binding matrix although this problem is reduced by the inclusion of sodium silicate. Some possible examples of useful materials are flours of wheat, rice, oats, potatoes, and the like. Any of these materials might function well in a controlled temperature scenario. In the event of a long-term microwave environment, i.e., product left in the microwave for a period significantly longer than the planned or prescribed period, the associated unpleasant breakdown products might be generated.

Cabosil fumed silica may be used as a thickening agent and flocculating agent for the slurry of the present invention, with the added benefit of introducing greater inorganic content to the final susceptor board and increasing the viscosity of the slurry.

Aluminum trihydrate may be included to release the water of hydration at temperatures from 200 ^β C to 250°C. This water of hydration is realized into the matrix of the susceptor board and can also act to reduce the likelihood/possibility of burning or smoldering of the material at high temperatures.

A particularly novel aspect of the present invention is that the susceptor composition acts as a form of heat pump. For instance, dry, hot potassium acetate does not heat significantly in a microwave oven; breakdown (carbonization) can occur, but this is not automatic even after extended periods of heating. The same is true if potassium acetate is included in a ceramic matrix which itself does not bind water. Both potassium acetate and the ceramic matrix are dry. As

potassium acetate and/or the matrix absorbs atmospheric water, or is made damp, microwave heating occurs. Both the heating rate and the final temperature depend upon the water content and the ceramic or fiber substrate. Thus, the susceptor composition acts as a form of heat pump due to the presence of either internal or external moisture in or through the food and the susceptor material. While not wishing to be bound by any theory, the present inventors believe that moist air is pumped at an elevated temperature through or within the temperature self-regulating susceptor combinations. Thus, in a moist airstream or on a wet surface, internal water is replaced or assisted by external replacement subject to the adjustment of the surface temperature and stability, and this process can be moderated by the inclusion of, for example, sodium or potassium chloride and/or sodium silicate.

Thus, a reasonable hypothesis for the surface chemistry of the susceptors involves the concept of a high temperature, moisture pumping susceptor as a flat or vertical porous or semi-porous surface adjacent to the food and/or a convective airstream created by the microwave heating process.

The present inventors have further observed that when potassium acetate, a hygroscopic chemical, is heated in a microwave oven it dries violently; in a glass container the material was seen to be mechanically turbulent. This thermo-dynamic instability is best observed if the material is heated by microwaves as a layer, or as a thick film on an already hot surface. One skilled in the art can demonstrate this by placing damp potassium acetate by itself in a layer of irregular depth on an aluminum film susceptor plate of the type used, for example, to

heat a pizza in a microwave oven. With the application of microwave power the acetate layer shows a thermodynamic instability, with a dry, fine dust of potassium acetate physically moving away from the rest of the material which still heats. In fact, the material shows signs of heating faster as moisture released below and around it moves by. The aluminum film reaches about 250°C (after about 1 minute at 650 W) . The drying potassium acetate's upper surface heats to about the same temperature, continuously activated, the present inventors believe, by internal and external moisture. This activation also increases, for any moisture level, apparently, with increasing temperature, counteracting the effects of radiant and convective heat loss, as well as that due to moisture expulsion into the surrounding hot air layer. The dry acetate powder, both expelled and remaining, is inert to the microwave power present until it cools and re- absorbs moisture. This is the principle of an air- moisture pumping system. It is presumed that the rate of heating exceeds the rate of moisture movement and that both rates increase with temperature for this to occur.

The present inventors postulated, and have since verified, that the preferred manner for creating a moisture pump, i.e., heats by virtue of external and internal moisture, is to use a hygroscopic material whose absorption of microwave energy increases rapidly with temperature over a practical moisture content range and at such a rate as to allow high temperatures to be generated. Potassium acetate is the preferred salt, although sodium chloride, for example, can be made to exhibit the property up to a lower but still useful temperature. Combinations of salts allow for

both the control of surface temperature and the moisture pumping rate. The moisture required can result from adjacent wet surfaces, wet oil and water bound in other chemicals and mixtures.

The absorption of microwave power in a volume of dielectric material is proportional to the microscopic dielectric loss factor (e") and the square of electric field strength (E ² ) of the microwave radiation within the volume considered. The latter, in a most general sense, wherein several field components can arise through different modes of propagation, as in a typical multimode domestic microwave oven, is determined by the dielectric constant, or relative (to air) permittivity of the material, e'. If e' is small, and is relatively independent of temperature (T) , the power absorbed per unit volume, for a given value of E, is determined by e", which is a function of temperature and moisture content. If free water is present de /dt is negative. Then water removal is stable even if it is rapid; as a paper (for example) dries, it absorbs less power in drier areas; further, as a wet area heats it also experiences a reduction in heating rate; thus, as both de^/άm (m: moisture content) and de"/dT are negative, heating, and thus drying, are both selective and self- limiting. Of course there are situations (including that of low moisture content, i.e., bound water) in paper and other materials where de"/dT is no longer negative and can give rise to thermal-runaway (burning) .

To achieve a microwave susceptor which reacts with and is activated by water, either present in the material, or captured from the surrounding air, an adjacent surface or even from that semi-bound to an oil or gel, or by a reaction due to another polar (lossy)

material, a porous surface (a substrate host) is required for the heating reaction to occur. This host is preferably one which constrains the salt within capillaries (e.g., in a brick or a lava) or cages, as in a zeolite (natural or synthetic) . While not wishing to be bound by any theory, by constraining the active salt in pores or capillaries the present inventors postulate that the overall polarizability of the wet material («••: charge separation, capacitance) is reduced without negating charge mobility (e": loss, power absorption) on these susceptor surfaces activated by water, both in bound and free form. By these means the present inventors teach a method for maximizing the macroscopic ratio, e"/e', which is well-known to electromagnetic and power engineers as the tan δ or the loss tangent of a material, or a measure of the material's ability to act as a resistance as well as a capacitance. If brick, tile, lava or pelletized zeolite (for example) are placed in a solution of a salt or salts and the whole system is placed under vacuum, air is withdrawn, and the solution fills, in whole or part, the voids in the host. The host material is thus impregnated with the salt solution wherein salts are deposited in ultra-thin layers on what can be a very large internal surface area. For example, the inventors can achieve an active susceptor grit by placing 30 g of crushed and sieved red brick (particle size range <590μm and >425μm) in a solution of 100 ml double-distilled water to which both 20 g of potassium chloride and 20 g potassium acetate have been added, first warmed to encourage dissolution and then cooled to 20°C; this system is then placed under a vacuum of 25 mm Hg for 1 hour. By allowing 1 hour at this pressure, all air bubbles have ceased; care is

required, by controlling the vacuum pressure, to prevent the mixture "boiling", although if this happens it does not apparently prevent the exchange mechanism from occurring. Equally, it is advantageous to start with dry brick materials. Treatments of the above type may also be repeated to ensure maximum penetration of the salts solution and this may be detected when all air bubbles from the host have ceased.

By virtue of the vacuum process, after draining and filtering the container, it is observed that smaller brick particles will create also, as a supernatant, an active susceptor, both in solution and on filter paper substrates; thus, in that case, the impregnated solid grit, the filter paper and the supernatant are all active susceptor materials.

In an attempt to demonstrate why the susceptive material combinations of the present invention work so well, the present inventors measured dielectric data to demonstrate the advantage of the new materials of the present invention. Very simple experiments have been used to demonstrate the advantage of the new materials, an advantage referred to as a "gain" G in the following equations. G is a ratio for comparison purposes of the tan δ of the active substance to that of pure water, the dominant constituent of most foods as far as microwave heating is concerned: tan δ of chemical mixture or dispersion

G = tan δ of pure water over the initial heating range, approximately 20 to 70°C. The method uses a microwave network analyzer whose transmitting coaxial waveguide is terminated as an open-circuit placed within the material, which, as a

dielectric termination to the transmitter, is electrically large enough so that the far boundary of the sample and its glass container causes a reflection which results in only a small interference with the original signal transmitted from the coaxial line ^• s abrupt, open-circuit, end. Anyone familiar with the art will appreciate that, for lossy materials, including pure water, the method is practical, repeatable and offers an acceptable accuracy which is often in the range of ±5%. However, if the samples are less than 5 cm in diameter (and depth from the probe) when, for example, e'is high (>50) and e" <5, the error can reach ± 10%.

For water it is known from the literature that good estimates of values can be stated as 23 50 73°C

Pure water e'=dielectric constant values e"=dielectric loss factor literature tan $=ratio of above average

at 2.45 GHz. The present inventors designate this tan 6 ^* as the denominator in the ratio G above, or alternatively, G for water is the row:

which can be added to the above table. In establishing the calibration of the network analyzer used at 2.45 GHz the present inventors recorded values for water samples which averaged as:

23 50±2 73°C

Pure water 82±2 65±4 e•=dielectric constant samples 12±1 5±2 e"=dielectric loss factor

0.15+ 0.08+

0.02 0.03 tan <S=ratio of above.

which is a reasonable and expected difference between different laboratory measurements.

Potassium acetate/potassium chloride saturated solutions were also measured and were found to be typically:

2 50 7 °

where it can be seen that the gain ratio, G, is the potential heating improvement compared with ordinary water and is about 40 at 23°C and then about 20 times this value at 73°C. In this and other measurements, the probe was recording the values in the solution and not those in the undissolved residue. The present inventors expect this gain to increase at higher temperatures, as long as sufficient water (or atmospheric moisture) is available to react with the salts, when the material becomes a constrained fluid or even a paste within the pores or capillaries of a host material such as brick. Sodium silicate also changes the loss factor of water and has itself, as a solution, a significant gain, G; for example:

23 50 73°C

Sodium silicate 58

(20g) in 40 ml 92 of water double distilled 1.58

11.3

whereas aluminum hydroxide as a stirred dispersion in water has the effect of reducing slightly both e ' and e" without a significant change in G, for example: 3 50 73°C

A starch gel, or glue, designated G3, and described in Table 3 is used to create thin substrates and which itself contains 10-20% by weight of sodium silicate and has, when wet, i.e., freshly made, a small gain by itself, with de"/dT negative:

23 60°C

and which becomes potentially active with the inclusions of salts, for example: 23 60°C

(freshly made) 103(107) 117(108) e"=dielectric loss factor

+10%w/w 2.19(2.16) 2.85(2.77)tan <S=ratio of above

NaCl(or KC1)* 16 48 G

♦values in brackets are for 10% w/w KC1.

which illustrates that when the host is uncured starch gel, as G3, the value of G increases rapidly with the salt concentration and temperature. This causes one to except that a gel-like salt paste, with an increasing relative salt concentration occurring as it dries, creates a hyperactive susceptor [de"/dT and dG/dT positive] with a G value probably far exceeding 1000 above 120 ^β C when the volume decreases rapidly and which drops to very low values for all the parameters as the material finally cures (dries) to a fine flaky brown (sodium silicate/residue) film. This happens with microwave heating; if the material is dried in air, or slowly by any means, it sets in a larger (porous) volume which remains reasonably permeable to water even after, for example, 24 hours as a thin (1-2 mm) film in dry (10% RH) air. This semi-permeable dry starch film

has low values of e' and e" and a gain G which is less than 1 when the original starch gel was made and had included 10% W/W NaCl. Measurement in this dry case requires a different method from which only estimated values can be obtained.

While not wishing to be bound by any one particular set of observations, or theory, the present inventors have noticed that similar films of the organic material have become plastic during microwave heating. Thus, when films made from potato and salts, for example, were cured rapidly (>100°C/minute) , but incompletely with microwave heat, they had a flexible, plastic-like surface in localized areas. It is postulated that in this rapid heating. case, the starch melts just before decomposing; and that the water present, and in combination with the oil, acts as a plasticizer or lubricant above the normal boiling point of water. The water retained in bound form within the starch composite, is thus by itself and in association with salt exhibiting an ability to absorb microwave energy at surprisingly high temperatures above 100 ^β C. In the process of such microwave heating and incomplete "curing," a material of this type might be molded into a variety of shapes since it is rather pliable. It has also been observed that when a partially cured susceptor involving a starch of the G3 type is heated again with microwave energy, it attains a high temperature more rapidly the second time. Further, partially cured films and composite materials using these starches have been found to be more permeable to water the higher the original salt concentration. These films were found to dissolve slowly if left in water for several hours, thereby demonstrating at least a degree of biodegradability.

The starch gel can be used, uncured, as a susceptor with added salts and fire-retarding chemicals; it can be painted onto papers and other materials. By means of a "blister pack" water can be added to a surface protected from the air, or unsealed prior to heating by other means as illustrated in Fig. 1 and Fig. 2. In cured form, with a fiber, it provides a host material of controllable porosity and other properties for salts and/or salt-impregnated porous particles which are then "thermal seeds" to the combined susceptor. The same principle has been found to work for other gel-type hosts; for example, the starch gel described may be replaced by a water-based white glue, e.g. Elmer's school glue ^' products. Susceptor constituents, then, have "gains" over a wet (free water) surface in the range 10-1000 below 100°C and can be expected to exceed this by at least one order of magnitude in the next 100°C if the susceptor is still active. By a similar argument the theoretical gain for a typical aluminum susceptor film is 10 ⁵ but with a heat capacity that is at least 10 ³ lower than, for example, a 2mm thick susceptor composition of the present invention.

The measurement of the properties of thin film resistive attenuators in a waveguide is well-known. In this case, a rectangular S-band waveguide was used and an attenuation "substitution technique" adopted. With the film placed transverse to and centrally across a (.1.32" x 2.84") waveguide section, the attenuation introduced by the susceptor film to the propagating electromagnetic wave with the wave having its electric field strength vector (E) in the plane of the film, is compared to that recorded on a calibrated waveguide

attenuator instrument with the film absent. Accuracy is improved if a detector placed beyond the two means of attenuation is adjusted to read the same value, i.e., the attenuation introduced by the film is removed from the instrument by reducing it attenuations by the necessary amount, as read from the instrument's scale. Accuracy is further improved if the ends of the film to be measured are tapered into the incident wave and also into the waveguide space beyond the film sample. This reduces wave reflections. Both precision and accuracy require that the film be thin and of low dielectric constant (e'), i.e., a small perturbation only is then introduced into the waveguide system, although it is known that the logarithmic attenuation per unit length, e.g., that given in db.m" ¹ or Neper.m" ¹ , which is proportional to both the loss factor e" and the film thickness is quite accurately recorded even for high values of e' .

Aluminum susceptor films used commercially have attenuation values between 1 and 2 db/cm at 2.45 GHz when measured by this technique. A preferred and typical average value for one of these aluminum films is 1.5 db/cm at 20°C but it should be noted that a wide variation over any one film, and between different films, is acceptable. By plating, for example, 6.4 g of fresh glue, to which 1 part in 20 of either potassium acetate or potassium chloride had been added, onto thin cardboard, an attenuation of between 0.9 and 1 db/cm was recorded initially; the attenuation increases linearly with the thickness of the film (as will the heating rate and the heat capacity upon use in a microwave oven) . Attenuation increases also with the concentration of a salt and decreases, as the film sets, with time, or heating (drying) , to a very low

value, typically 0.1 db/cm. 60% of this final value is found to be caused by the substrate. The temperature at which the material loses its high attenuation is determined by the salt used, its concentration and the available surrounding moisture. Even after exposure in dry air (15 hours, about 35% RH) a film which contains at least 5% potassium acetate will re-absorb useful amounts of water, or (for example, vinegar) , with the attenuation returning (it was found in separate experiments) to more than 50% of the initial value. Further, the material maintains its attenuation if stored at -6°C. However, methods have not yet been developed to stabilize the material, if used by itself, for long-time storage. As susceptors, white glues and starch gels are generally inferior products to the impregnated porous ceramic host materials, but they, or similar materials, may be required in the fabrication of some of the susceptor compositions described herein.

The low-power measurement techniques teach methods of formulation for these susceptors and offer at least partial explanations of the electrochemical phenomena involved. In all practical cases, physically small amounts of a susceptor are used in a microwave package; between 5 and 50 g, (typically 10 g) of a material are exposed to 400-700 W, together with a small food item generally in the range 25-250 g. Heating rates are consequently very high if the combination performs correctly. In fact, it is preferable to make them as high as possible while maintaining food quality and appearance. Whereas it is possible to judge the result of cooking with any susceptor, it is difficult, for the above fast-rate reasons, to record temperature distributions in the

short time interval involved, particularly in a manner which allows analysis of the interactions.

Figs. 1-6 offer an aid in conceptualizing several embodiments of the present invention.

In Fig. l, for instance, outer carton or package 3 surrounds the cardboard tray 1 and contains the susceptor panel 2 as well as the food item (e.g, pizza) 6. The food 6 is independently surrounded by a plastic seal or bag 5. The susceptor panel is in turn surrounded by plastic seal 4. The air holes or gaps in the tray 7 are typically 0.5 cm in diameter. The air holes or gaps 8 in the susceptor panel are likewise approximately 0.5 cm in diameter. Air currents 9 flow through the circular holes and cutaway segments. Fig. 2 and Fig. 3 are variations of Fig. 1.

Fig. 2 is a cross-section of Fig. 1 when opened for heating in a microwave oven. Fig. 3 is a cross- section of this embodiment of a susceptor panel 2, food item 6 and cardboard tray 1 combination to show the airflow if venting holes in Fig. 1, 2 or 3 are used.

Fig. 4 depicts a cardboard food cup 10 for french fried shoe string potatoes having a tapered circular or oval section. The stand section or base 11 allows holes 12 for air circulation. Such holes are an optional aspect of the present invention. Vertical susceptor panel 13 divides the cup into 2 or 4 segments.

Fig. 5 represents a rigid panel, whereas, Fig. 6 represents a flexible panel. Edible shellac spray 14 is placed in the semi-porous (sealed) surface. Brick or other porous material in particle form 15 is impregnated with salts and binders. Substrate 16 is a two-ply dried starch or fiber. Underseal (shellac) 17

can be placed on substrate 16. In the flexible panel of Fig. 6, rice or starch paper (1 ply, 0.25 mm) 18 contains sodium silicate (10%) . A fine brick or other host material 19 impregnated with salts in starch gel may be used as a thin layer on the rice or starch paper 18.

While the invention has now been described with reference to several preferred embodiments, those skilled in the art will appreciate that various substitutions, omissions, modifications and changes may be made without departing from the scope or spirit thereof. Accordingly, it is intended that the foregoing description be considered merely exemplary of the invention and its implementation .and not a limitation thereof.

EXAMPLE 1

General Process

A general process for preparation of the susceptor composition of the present invention is set forth below. Typical ingredients used included the following: ground fired terra cotta powder aluminum trihydrate micronized marble sodium silicate (added in the form of an aqueous basic solution) papyrus paper fiber (a medium grade cellulose based wood paper fiber) cotton fiber potassium acetate potassium chloride

The cellulose fiber (papyrus fiber) was first prepared by cutting and grinding unbleached paper board stock used in paper plates. This material was obtained from Star Paper in Salem, Massachusetts. The paper board stock was first cut in small pieces on the order of 1-inch square and placed in a food processor (either Cuisinart, Panasonic or the like) . Tap water (250 to 500 ml) was added to the paper fiber and then the mixture was processed into a paper pulp. The fiber would generally process from the paper plate stock material to the fully suspended fiber in approximately 2 to 3 minutes. Next, an aqueous solution of sodium silicate obtained from Fisher Scientific or the PQ Corporation (36 weight % sodium silicate) was added to the suspended paper fiber. In some cases prior to the addition of the sodium silicate a portion of cotton fiber board stock was added to the food processor. This material was also cut into small 1-inch square pieces and was reduced to suspended fiber in 2 to 3 minutes.

Next, the additional materials were added to the suspension. The ground terra cotta powder was obtained by pulverizing pieces of broken terra cotta pottery, i.e., from flower pots obtained from a local hardware store. This terra cotta powder was of a large distribution of particle sizes, but the average particle size was on the order of 0.01 to 0.25 mm in diameter. This material was quickly reduced to this particle size through the use of an industrial grinding/pulverizing device of the type that is available through many manufacturers. Prior to the use of this industrial grinding/pulverizing apparatus, the terra cotta powder was prepared by hand or by the use of a ball mill, neither of these two approaches proved

as effective as the use of the industrial grinding apparatus.

Upon the addition of the terra cotta powder to the previously described mixture, the mixture was blended in the food processing equipment for approximately 1 to 2 minutes. This insured good mixing of the constituent components. Next, the aluminum trihydrate (Fisher Chemical) was added to the above described mixture and was also blended to an additional 1 to 2 minutes in the food processor.

In some of the formulations of this type of susceptor board, additional materials such as talc, fumed silica, as a thickening agent (Cabosil Corp.) and defoaming agents were added to the mixture.

A slurry was then obtained containing all of the components in the final susceptor board composition (prior to the infiltration of the susceptive salts) . One of the significant variables in the product development of the susceptor board was the final concentration of the slurry. Therefore, the final concentration of each of the components was finally adjusted by the addition of some portion of tap water or sodium silicate to bring the final concentration of the slurry to one that was amenable to further processing. It was important that the slurry not be too viscous or it would not be possible to rapidly produce the susceptor board via the static filtration process or slurry casting process that is described in the following paragraph.

All of the samples of the susceptor boards of the above type were prepared by a modification of the classic slurry casting method that is employed in the paper industry. In the specific approach used in this

example, the aqueous slurry described above was poured into a static filtration apparatus that was adopted for this purpose. A 10-inch and a 24-inch diameter Buchner funnel were used as filtration/casting apparatus. The advantage of the Buchner funnel for this application is the "porous" or open support on the bottom of the equipment that allows for the vacuum filtration of whatever material is poured into the funnel. In order to adopt this equipment for application to the slurry casting of the slurry used in the preparation of the above described susceptor boards, the bottom of the filtration apparatus was lined with a paper machine clothing type porous cloth. This type of cloth is commonly used in the paper making industry and the type used in these samples was particularly effective as it provided a porous base onto which the slurry was cast. On top of the porous paper machine type clothing fabric was placed a commonly found commercially available flexible nylon screen, of the type used as insect screens in home windows.

The slurry was mixed by hand under constant agitation with a large paddle to ensure even distribution to obtain a homogenous composition in the resulting susceptor board that is cast from the slurry. The paper fiber based aqueous slurry was then cast evenly over the bottom of the Buchner funnel and a vacuum was then applied. Upon application of the vacuum pump the excess water in the slurry was pulled down into a collection chamber placed between the vacuum pump and the Buchner funnel.

The actual casting process, once the equipment was in place required approximately 5 to 10 minutes depending on the size of the batch, i.e., 10 or 24-inch diameter Buchner funnel. Next, the now consolidated

susceptor board was removed from the Buchner funnel and placed in a drying rack or in a low temperature (approximately 40°C) oven. Upon the partial drying of the sample, they were then cut into many smaller 4x5- inch rectangular boards.

After the 4x5-inch boards were, prepared as described above, they were further dried, labeled and then subsequently treated by soaking in an aqueous solution of the susceptive salts (potassium acetate and potassium chloride) . The concentration of the susceptive salt solution used for the infiltration of the boards varied throughout the developmental process of the boards, but in the more preferred composition of the boards the concentration of the solution was approximately 150 grams per liter of each of the two salts, for a total solids content of approximately 300 grams per liter. It should be noted that more rapid and complete infiltration of the susceptor boards was obtained by raising the temperature of the salt solution to approximately 60°C and/or subsequently placing the container and the boards soaking in the solution under vacuum. The use of a vacuum when combined with a 60 ^β C solution of the susceptive salt mix produced the most rapid and effective infiltration of the susceptive salts.

Test results on a susceptor board of the above preferred composition and treatment are shown in Fig. 7 where a 4" x 5" sample was used to heat 75 gram of frozen, pre-cooked french fried shoe string potatoes. The food product was placed in a single layer on the board, which was raised 2 cm above the base of a 625 ± 25W domestic microwave oven. One fiberoptic thermometric detector was placed inside a randomly selected fry and three other such detector probes

(Luxtron, Mountain View CA Fluoroptic™ Multichannel Thermometer, Model 755) were placed at different positions just below the surface of the susceptor. For the time/temperature response shown in Fig. 7, line 1 is for the french fry, and lines 2, 3, 4 are the values recorded in the susceptor board, the average value of these being shown by line 5. The product heating was complete at six minutes; in this case it was allowed to continue beyond that time to show the stability of the board.

EXAMPLE 2

Preparation and Evaluation of Fiber Base Susceptor Board

A 10 inch diameter susceptor board was fabricated by preparing a paper based slurry. The slurry was formulated as follows:

200 ml of H ₂ 0

24.5 grams chinet cellulose fiber

168 grams aluminum hydroxide

200 ml (36 wt % Fisher brand) sodium silicate

All of these components were blended together in a mechanical agitator for complete mixing and were continuously agitated prior to casting the slurry into a Buchner funnel.

The paper based slurry was then poured into a static filtration apparatus (Buchner funnel) composed of a large 10 inch diameter container with a nylon screen and a porous bleeder cloth with a vacuum line below the screen. Next, the slurry was allowed to briefly settle by gravity and vacuum was applied. The paper board was briefly dewatered by suction and then

removed from the funnel and placed in a low temperature oven at approximately 40°C overnight.

The 10 inch board was then cut into two 4x5- inch rectangles. The rectangles were labelled and tested in a 4x5 inch microwave food box. The test samples performed well with an acceptable thermal profile after cooking the susceptive salt solution of the acetate/hcc. The test results are depicted in Fig. 8.

EXAMPLE 3

Preparation and Evaluation of Fiber Base Susceptor Board

A fiber base susceptor board was prepared by the method described in Example 2. The fiber slurry in Example 3 had the following composition:

250 ml H ₂ 0

7 grams papyrus paper fiber 64 grams micronized marble 32 grams aluminum hydroxide 150 ml sodium silicate.

The board was partially vacuum dried and cut into two 4x5 inch boards. These were dried in an oven at 60°C overr^ght and soaked in an aqueous solution containing 250 grams potassium acetate and 250 grams potassium chloride in 1 liter of H ₂ 0.

The fiber susceptor board was tested and a time versus temperature graph is depicted in Fig. 9.

Fig. 9 uses the same Luxtron unit described in the previous example. Probe 1 was placed within the fry. Probes 2, 3 and 4 were inserted into the board.

A separate plot in Fig. 9 sets forth the average temperature of probes 2, 3 and 4.

EXAMPLE 4

Preparation and Evaluation of

Fiber Base/Terra Cotta Powder Susceptor Board

A fiber base/terra cotta susceptor board was prepared by the method described in Example 2. The fiber board had the following composition:

250 ml H ₂ 0

7 grams ground papyrus fiber

32 grams aluminum trihydrate

32 grams ground terra cotta powder

750 ml sodium silicate solution (36%)

The board was prepared as described in Example 3; however, the board was soaked with 150 grams potassium acetate and 150 grams potassium chloride in 1 liter of H ₂ 0.

The fiber susceptor board was tested and a time versus temperature graph is depicted in Fig. 10.

EXAMPLE 5

Preparation of Fiber Starch Gel Microwave Susceptor Composition

A flame retarded paper was made out of a starch and sodium silicate solution. The solution described, when cured, took the form of a thin "rice-like" paper, almost transparent, or a thicker "cardboard", and was able to be moulded. Pliability was achieved by adding an oil (e.g., olive) and preservative against mold (including "off odors") by the addition of trace amounts of oil of clove or oil of wintergreen. The

paper samples were used as substrates for microwave susceptor materials up to 800°F (427°C) without burning, although charring (darkening) did occur above about 400°C.

The formulation prepared is set forth below: lOOg wheat flour lOg Na ₂ Si0 ₃

200 ml cold water

2 teaspoons olive oil, about 5 ml

1 drop oil of clove, about 0.25 ml

600 ml hot water, between 60-80°C sufficiently hot to form a starch "gel".

The above formulation can be varied for pliability, viscosity and temperature stability. The flour was mixed with Na ₂ Si0 ₃ in cold water and blended, then the oils were added. To the smooth, uniform batter, hot water was added slowly and the mixture was encouraged, by heat or more water, to achieve a desirable viscous gel format.

The gel was then rolled, for test purposes, onto a commercial, rough paper towel, made by recycling newsprint. Used recycled paper which had been bleached, and which has a rough, grainy surface was also used in tests. Each "ply" of raw paper (thickness about 0.25 mm) in the present samples were 5mg/cm ² although 10mg/cm ² appeared to be satisfactory.

Each ply was soaked and scraped and then allowed to air-dry, preferably between smooth, rigid plastics grids in the form of gratings and made into layers of the required thickness. One-ply is generally sufficient for a susceptor made from vacuum impregnating salts in brick powder painted onto the surface. 4-ply has been used for heavy brick and terra

cotta powder layers suitable for pizzas. The paper so formed can be cured, i.e., dried or set, in a microwave oven.

The susceptor surfaces were made as follows: A commercial red grade brick was crushed and sieved. Particles which passed a 590 μm pore sieve and were trapped in a 425 μm pore sieve were used. This is a suitable particle size for pizza, french fries and the like. These particles were placed in a solution consisting of 40 g potassium acetate and 40 g potassium chloride in 100 ml of distilled water. The solution was warmed and placed in a bell jar which was then evacuated to a pressure of 25mm Hg absolute and maintained for 30 minutes. The time was judged by the end of air bubbling from the particles. Initially, the degree of vacuum was increased slowly to prevent surface violence and adjusted accordingly.

The particles were drained. The supernatant and the material on the filters were retained. The filtration impregnation process released fine brick particles. These are typically less than 425 μm and probably all are less than 200 μm. A "brick paint" was made of the remaining impregnated material. The impregnated 425 μm particles were then spread into the dried paper substrate and pressed with a clamp. The surface thickness of the particle layer was judged to be from 1 to 2 mm above the 4-ply (approximately 1 mm thick) substrate. After air-drying, the surface had the appearance of rough brick. The oil used was sufficient to eliminate a tendency for the susceptor to shatter. An edible shellac or a starch gel can be used to seal the surface, partial sealing is preferred. In another formulation the impregnated brick material was mixed with a starch gel, rolled into sheets and dried.

It is desirable with some compositions to wet the surface before use.

The supernatant material was then painted on 1- ply substrates as uniformly as possible to ensure a dark red-brown surface. When dried, the susceptor material could be roughed up. This is generally a desirable property.

It was noted, in making these thick into thin susceptor surfaces, that

(a) the absorption of microwave energy can be tailored very simply by changing the thickness of the coating, the concentrations of the potassium solutions and, possibly, the impregnation time of the brick particles under vacuum;

(b) the moisture content (bound and "free" water) is relatively insensitive to temperature (below 30°C) , i.e., samples have been stored at 10°C, 0°C and -6 °C without affecting their subsequent performance;

(c) the susceptor has been proven to be stable over at least a -week period;

(d) adhesion of the brick to the substrate was excellent even under cooling stress except apparently for thick coatings (>2.0 μm) of particles <425 μm in size; and

(e) the substrate gel, or a pure sodium silicate solution, can be used over the brick particle surface, for greater adhesion or greater fire retardancy; however, this may reduce sensitivity by changing the moisture content.

A four-ply 7" diameter sample suitable for pizza was also made and tested. 25 cm square 1-ply sheets were prepared and compared to aluminum film susceptors of the same size.

Based on comparative tests on commercial frozen microwave pizzas using prior art aluminum film pizza trays from Pillsbury, the substrates of the present invention were found to have as good or better surface browning (with a time reduction between 10 and 20%) , uniform body heating and an acceptable "mouth feel" as the prior art susceptor composition.

Freshly-made uniform 5 inch diameter thick pizzas purchased from Safeway were placed on 7 inch brick susceptors. Excellent products were obtained after four minutes, at 650W microwave. Edge browning was improved in comparison with other conventional aluminum film susceptors and it was found desirable to place six 0.25 inch diameter holes in the susceptor and its raised support to increase air and thus moisture movement.

Using 1-ply susceptor "wrap" with two 50 g pre¬ cooked, frozen fried fish fillets in comparison with top and bottom aluminum susceptor pads (commercial microwave fish product) , it was determined that there was:

(a) less "oiliness" on the product;

(b) crisper surfaces; and

(c) a harder, over-heated center but was still an acceptable "fast-food", in three minutes versus four to four and a half minutes for an aluminum susceptor.

For the susceptors tested, and for other samples tested separately, there was no evidence of burning or scorching when the food items which were covered on their surface.

EXAMPLE 6

Preparation and Evaluation of a Fiber Starch

Microwave Susceptor Composition Using Potato and/or Potato Peel

The fiber starch composites of Example 5 were made from potato and susceptor chemicals. The objective was to show that different susceptor compositions and concentrations will give different maximum temperatures. Further it is shown that the susceptor's surface temperature is increased when an edible shellac (which partially seals one surface from internal water loss) is used.

The basic formulation prepared is set forth below:

350g of raw potato or 200g of potato peel

1 liter of water x g of potassium acetate and/or y g of potassium/sodium chloride and/or z g of sodium silicate lOg of paper fiber where the amounts x, y, z of susceptor chemical materials are selected to achieve a particular temperature. Of course, any of or all of the x, y and z can be zero, although it is preferred always to have some sodium silicate present with compositions of this type. Equally other chemicals can be used, including different salts, preservatives, essences and oils, the last for additional pliability.

Formulations of the above type were boiled until the potato material was soft and then blended with the paper fiber. The formulation is designed to make a thick paste which will pour after boiling; additional water may be added if necessary during either the

cooking or the blending stage. Samples (1, 1.5 and 2 mm final average thickness) were allowed to air dry for two to three days at 23°C and a relative humidity of 10-20%. Pairs of samples of the same composition and thickness, all typically 9cm in diameter, were selected; one sample in each pair was lightly painted on one surface with the Zinsser edible shellac described previously. The thickness of the shellac was intended to add about 2g to a typical 9cm diameter susceptor which then weighed between 12 and 15 grams, in total, when the average thickness (B) was 1.5 mm.

Susceptor samples were placed on a 1cm thick piece of dry Ceraform fiber insulation (Manville) . Another piece of the same Ceraform material was used to cover the upper susceptor surface which was shellaced in alternate cases. After the susceptor had been coated with several small piles of different Omega temperature-indicating wax crystals, the susceptor was heated in a 650W (non-rotating table) microwave oven for one, two and three minutes between the Ceraform pieces as described and all in a Ceraform insulating box (25cm wide, 25cm long, 17cm deep, 2.5cm thick). At the end of each minute, the susceptor and boxes were removed from the microwave oven to check the surface temperature and then returned to the microwave oven for subsequent heating. An infra-red temperature sensor was also used to confirm the surface temperatures indicated by the melted wax crystal on some samples which were not then reheated. In this manner, surface temperatures could be estimated and replicated to within approximately ±12°C at 240°C, ±15 ^β C at 300°C over a three minute heating period. One of ordinary skill in the art will appreciate that the tests are for surface temperature comparisons only as the load on the

susceptor upper surfaces was not a food but rather an insulating, porous cover which aids in estimating the maximum susceptor surface temperature likely to be achieved in a practical situation.

In Table I, the temperature values in °C are those for 2 and 3 minutes and are shown in Fig. 3 and Fig. 4. The samples were 1.5mm and were 9cm in diameter. All samples weighed between 12 and 15 grams. The shellac when used accounted for approximately 2 grams. The microwave oven power was 625 + 25W at 2.45GHz. The surface load on the susceptor samples was a 1-cm thick layer of Ceraform to absorb moisture and insulate the susceptor from heat loss.

TABLE I

TEST X Y Z B Shellaced Unβhβllacβd Fig. /Line Temp. σrams) ^{fwm Muwh} r

Table I specifies 15 samples which were tested for the effect of different salt concentrations (x, y, z) and thickness (B) with and without the use of a surface sealant. The temperatures attained by the samples after the first one minute heating period are also given in Table I and are shown for three minutes in Fig. 11 and Fig. 12. The values are not exact but lie typically within the range ±12°C at 200°C, ±15°C at 300°C of those given in the Table and illustrated in the Figures.

In all the tests shown the effect of the dry shellac surface sealant is seen to increase the temperature, typically by 50%, in some cases slightly more. It will also be noticed that there is little difference in any respect between the use of potato and potato peel. sealing the surface with the dry shellac increases the temperature in the zero-added salts example (Test 0) from 107 to 150°C approximately, and this value is constant over the next three minutes. Fig. 11, lines 1 and 2. The use of sodium silicate increases the susceptor's temperature, the effect being more pronounced when a shellac has not been used. Fig. 11, lines 7, 8.

When too much potassium acetate is used with a sealed surface, samples start to darken and burn; this occurred when surface temperatures were observed to be above about 275°C. Tests B and G illustrate this limit. Fig. 11 and Fig. 12, lines 5, 6 and 14, 15, respectively. Tests D, E, F (shown in Fig. 11, lines 9, 10 and Fig. 12, lines 11, 12 and 13) illustrate how a range of surface temperatures from approximately 100 to 300 ^β C can be achieved. The preferred temperature

range is 150 to 275°C and the actual susceptor composition needs to be adjusted according to a particular food's surface characteristic and temperature/heating time requirements.

EXAMPLE 7

Preparation of Microwave Rice Board Susceptor

Approximately 50 grams long grain rice, 100 to 120 ml of water and 5 grams of a mixture of potassium chloride and sodium chloride or 5 grams potassium acetate or 2.5 grams sodium silicate were used in the Examples.

All ingredients were mixed in a pot, with the lid on, and cooked under high heat until it boiled. The mixture was stirred to prevent the rice from sticking to the pot. The lid was secured and the heat was turned to medium. The mixture was allowed to cook for a few minutes until the rice was almost dry. The rice was then about 80% cooked. The rice was simmered under the lowest heat for approximately five minutes for final cooking of the rice.

The rice was mashed and pasted onto 2-ply starch gel (G3) cured paper. A roller was used to obtain an even and thin surface. The roller was then wet with water to prevent the rice from sticking onto the roller. Typical values were: Area = 10 cm ² ; thickness = 1.0 - 1.5mm; weight = lOg dry.

The rice coated paper was placed in an Ceraform™ container of 1 cm thick material and heated in a 600W, non-rotating table microwave oven for 3 minutes. One half of each sample was covered with a 1 cm thick piece of Ceraform. OMEGA temperature crystals were used as temperature indicators of surfaces and interfaces. The

composition of the rice paper sample as well as temperatures for the uncovered composition and the composition covered with the Ceraform strip are set forth in Table II with, for comparison, an aluminum film susceptor sample of the same surface area. Temperatures are within ±12°C.

TABLE II Susceptor Surface Temperature °C

2.5g KAcetate

Rice + 5g KC1 + 204 (a) 163 5g KAcetate

Rice + 5g KC1 218 (a) 149 + 5g KAcetate + 2.5 Na ₂ Si0 ₃

Aluminum film 246 218 (approx. 1.40 ohms/sq.) (for comparison)

(a) Starch substrate browning, maximum internal temperature about 300°C.

EXAMPLE 8

Zeolite Susceptors

A number of zeolite materials were tested for characteristics desirable for susceptor compositions. The following legend is provided.

Fig. 10 presents a comparison between conventional aluminum susceptor film (140 ohms/sq.) and Linde 13X zeolite with and without G3 substrates (total thickness 2mm) .

Pellets of Linde 13X zeolite were impregnated by the vacuum technique previously described from a 40% w/v solution of NaCl in distilled water. The impregnation time was 1 hour, with the degree of the vacuum increased down to a pressure of 25 mm Hg. All bubbling had ceased before this time. The pellets were then removed and allowed to drain. One hour later, they were crushed and ground by hand, until the size appeared to be similar to that achieved for terra cotta in Test 7 of Table 4. Together with the following procedure this was a judgmental attempt to prepare susceptor boards which would have time/temperature responses similar to those estimated and recorded previously and up to the performance achieved with aluminum films. Approximately 2mm thick samples were prepared using a 2-ply G3 starch paper (Table 3) of area typically 4"x5" (130 ± 15 cm ² ) . These were prepared a) by laying a film of the zeolite preparation onto the wet G3 and allowing it to dry. These samples are called "13X unshellaced." b) With the same procedure and then, after air drying, lightly shellacing the upper surface with approximately 2 g of the shellac previously described. These samples are called "13X shellaced." c) Mixing the prepared zeolite with the G3 in the proportion 1:2 and then preparing the 2-ply susceptor. These samples are called "13X + G3" and one was shellaced after it had dried for 24 hours. One more sample was made with the "13X + G3" procedure and heated in a domestic 600 W microwave oven for three minutes after it had air-dried for 24 hours and before it was used in comparative heating tests with the other samples. The time/temperature responses of all the samples described above are shown in Fig. 10.

The samples were tested using the procedures described previously and for comparison with aluminum film susceptor material -line 5 in Fig. 10. The sample which had been microwave cured before use reached the highest temperature and yet did not burn. It will be noticed that binding the zeolite into the G3 material (G3 + 13X) raises the temperature response above that of a shellac surface sealing. Lines 1 to 4 in Fig. 10 shows the comparative values.

EXAMPLE 9

Evaluation of Susceptor Board

To record and compare the performance of a susceptor (by itself and when interacting with a food) , a thermocouple probe, temperature reactive (microwave- inert) wax temperature markers and an infra-red (non- contacting) surface temperature detector were used. Where desirable, samples were placed in an insulated container made of a material which withstands high temperatures (>600°C) without deterioration or microwave absorption provided it is kept dry, moisture content less than 10% dry weight basis.

This one additive on the microwave susceptor material of this example is a wax which is intended to melt at a specific temperature. The wax is essentially microwave inert and is relatively insensitive to water. The wax is not intended to be a part of the final product but rather is intended for testing purposes only. The wax is used to determine whether the susceptor composition has attained a certain temperature.

The wax used to measure temperature was 20 types of Omega Engineering Inc., Stamford CT; Melting Point

Standards. Each wax had a different melting temperature and color and was uniformly distributed in the range of 52 to 315°C. The waxes were used in both a powder and a liquid form. NBS was traceable for each wax's melting point. Also, the Omega Marker Test Kit which contained 20 similar temperature-sensitive waxes in the form of pencils and covered the range of 52 to 427°C in approximately equal steps was used. Of these waxes, three were not used. One wax was dissolved by salt solutions. The other two waxes were suspected to react with microwaves near to but below their melting points.

In the following Tables, which summarize other tests discussed, the abbreviations us.ed are those listed below:

NOTES: a. vi - vacuum impregnated. b. P - sample, in pellet form, was first impregnated and then crushed. c. CP - sample, in pellet form, was first crushed before impregnation. d. [cj - unless otherwise stated, no surface load; placed in the insulated rectangular Ceraform container. e. β - total thickness of sample plus treated paper. f. IP - single ply, substrate thickness approximately 0.2 mm. g. 2P - two ply, substrate thickness approximately

0.4 mm.

The IR Thermometer used in the present example was Wahl Inc. (Los Angeles, CA) IR Camera HSQ6E P/H Heat Spy Automatic, 100-1000 ^e C. The digital

thermocouple was Fluke Instruments, Model 2190A Digital Thermometer, 8 channel, K-type.

The heat insulator was by Manville Inc. and was called Ceraform. One container had dimensions of (13.5 x 13 x 14)cm ³ with a removable top and a wall thickness (laminated) of 2 cm. A second larger container was also used which had dimensions of (20 x 20 x 13)cm ³ .

In heating tests 1, 2 and 3 in Table 3 below, the starch glue used was formulated for flexibility when cured on thin paper towel substrates. G3 is the latest development of the basic (papier mache) idea Gl as discussed previously; olive oil could be replaced by a food-grade glycol. Oil of clove is included to prevent (or retard) mold growth and off-smells in a material which is flexible and does not burn at 300°C. The final column represents tests where the cured material was subjected to a range of food temperatures of interest. G2 and G3 differ only in flexibility and storage characteristics.

TABLE III

Starch / Glue No. Recipe Comments

1 . G l all purpose flour no sign of burning sodium silicate not very flexible (no oil added) cold or iuke warm water 15 / 11 / 1989 boiling water

(no specific proportion used)

2. G2 100 g of all purpose flour no sign of burning unless directly heated

10 g of sodium silicate more flexible than Glue 1

1 drop of clove oil 20/ 11 / 1989

10 ml of olive oil

200 ml of cold or luke warm water

600 ml of boiling water

3. G3 100 g of all purpose flour no sign of burning (subsequent) 10 g of sodium silicate much more flexible

2-3 drops of clove oil 23 / 11 / 1989

30 ml of olive oil

300 ml of cold or luke warm water

500 ml of boiling water

Crushed commercial red brick, selected as a clean sample from an old building, was washed, air-dried for

two days and crushed. The material was then sieved; all material that would pass a 425μm sieve was used in test 4 and was impregnated as indicated under Chemical Treatment. A basic starch Gl was plated onto two pieces of two ply (substrate thickness 0.4mm) soft paper towel and allowed to dry in air (21°C, 40%RH) between plastic grids under sufficient pressure to create a flat rigid, dry, substrate 24 hours later. The treated brick susceptor was drained and the spread uniformly on the substrate to a total average thickness of 1.1 mm.

Omega Temperature Waxes, as described, were then placed on the susceptor surface, in groups of 5 different temperature colors, in 3 places. A part of the surface area of 7 x 11 cm ² susceptor was covered with 21 g of wet donut-type bread to create an irregular, heating and drying surface in order to determine the highest temperature and to estimate where the highest temperature might occur. The susceptor and irregular food sample were placed in a Ceraform container and heated for approximately 2 minutes at 650 W in a domestic microwave oven.

By inspection, a temperature in excess of 427 ^β C was achieved after 2 minutes heating. The highest temperature occurred at the edge of the susceptor surface nearest to, but not touching, the bread; elsewhere temperatures exceeded 270°C and in general, as best as could be estimated, the surface, (on which it is known that power illumination varies in practice) had a temperature greater than 230°C and a maximum of less than 427°C. The bread surface adjacent to and touching the susceptor browned severely. The susceptor showed only minor areas of darkening without smoke or burning. Test 4, in context with Tests 5-9 summarized

in the Table below, were used to develop a 7" diameter susceptor "plate" for pizza, and smaller susceptor areas for pre-fried, frozen fish.

In the tabulated report of these Tests, particle size and layer thicknesses were adjusted from a "best guesstimate" from the immediately previous data. In Tests 5 and 7 the material used passed a 590μm sieve and was trapped by a 425μm sieve, i.e., 425μm. Under the Table heading "Test", "non-wave" means a commercial product not specifically prepared for domestic microwave heating. One susceptor used (control Test 6) was a commercial (Ore-Ida) aluminum susceptor tray, α= 1.6 ± 0.2 db/cm. The addition of water to the new susceptor of the present invention before use is desirable under low relative humidity storage

conditions or for re-use; additional amounts of oil (G3 v G2) may diminish the need for additional water. In addition, such remedies as edge-sealing (moisture locking) or the inclusion of the susceptor in an air¬ tight "blister-pack" are possible.

Tests 10-17 were subsequent "control tests" and an extension to micronized natural sea shell. The symbol (c) designates that no food load was used, with heating at 650 W microwave, in a Ceraform container. Temperature waxes and the IR camera were used.

Brick samples in Test 12 achieved 370°C very rapidly. With crushed sea shell (Tests 13, 14, 15) the temperatures were far lower. However, in a separate test a 55 g half clam shell was washed and dried and then treated whole, in the same fashion as the crushed brick in Test 12. The shell was allowed to air-dry for one hour and then microwave heated in the same fashion as the samples in, for example. Tests 13, 14 and 15. A

surface temperature in excess of 250°C was recorded after one minute. Repeating these tests with other shells indicated a wide variation in surface temperatures achieved. Natural carbonates may well be irregular in structure; volcanic lava samples demonstrated the same variable heating effect and results demonstrating only that temperatures in excess of 120°C can be easily achieved. Crushed brick remains the preferred material at this time, possibly because greater effort has been devoted to its development. The present inventors believe that the sieved size of a particular porous, or semi-porous material, related to its water and salt mobility, determines the critical parameters. The only conclusion is that the porous materials used can achieve and in some cases exceed the temperature performance of metal-film susceptors. Comparative Tests 16 and 17 in the above Table illustrates this general result.

Test 16-26 illustrate the susceptor action possible with synthetic zeolites. The tests reported were used to develop a zeolite susceptor coating whose performance is shown by Test 25. Coating thickness, zeolite type, and moisture content after impregnation determine the maximum temperature achieved. Tests 26 and 27 compare a salt-doped zeolite susceptor to an aluminum film susceptor. Thus, it appears that both brick and zeolite substrates could replace metal films in microwave food susceptor applications. Method and comment in the Table illustrate just how critical are the particle size and the method of preparation.

In a series of tests performed prior to making the above zeolite susceptors, the present inventors attempted to establish some guidelines for three types of synthetic zeolite in terms of their time/temperature microwave susceptor response at small (but not untypical) amounts when heated. Vacuum impregnation was used for Test series 26-30. These tests showed that surface temperatures in the range 120 < T < 427 ^e C can be achieved for 5 to 10 g samples in less than 4 minutes without destructive consequences.

EXAMPLE 10

Preparation of a Polymer Converted

Hydrogel - Partially or Fully Polymerized Starch

Approximately 30g of 100% pure corn starch flour (Best Foods Canada Inc., Box 500, Etibitoke, Ontario, M9C 4V5) was mixed with 12Og of water over a medium heat with continuous stirring and allowed to thicken into a smooth stiff paste and then, slowly thickened into thicker, self-supporting cakes of the material. Approximately 2g, of KC1 were added to lOg of the corn starch cake and mixed thoroughly by hand in a porcelain crucible.

The porcelain crucible with the 12g mixture was heated by itself in a 650 W microwave oven to about 105°C and allowed to dry. The temperature was determined by using a Luxtron fiber-optic temperature probe. The resulting material resembled a fired clay, having porous, irregular layers. Approximately lg of water was added after the crucible and its dry, powdering material had cooled to below 40°C. The water was added at the Luxtron probe's position; -it was noted that the water dissolved the material rapidly. The sample was reheated, in the same position in the same microwave oven. The temperature exceeded 200°C in 90 seconds and then dropped rapidly. After cooling and with additional small amounts of water added, microwave heating again raised the temperature of the sample. The maximum temperature attained declined with each test after the second one.

After the initial microwave drying, the first step in the above, the inventors observed long, thin

needle-like threads of semi-transparent, yellow- brownish material attached to the surface of the crucible were observed. It was further observed that when one of these areas of the crucible was made wet again after cooling, it transferred heat rapidly when exposed to microwaves. This was observed by using, in a replication of the above experiment, three temperature probes: probe one in the re etted 'cured ¹ material, probe two under the crucible below an area where the needle formation occurred and a small amount of water had been added, and probe three firmly attached to a clean part of the rim of the crucible. The temperatures after 1 minute of microwave heating at 650 W were 145°C, 120°C and 58°C, respectively.

EXAMPLE 11

Preparation of Corn Starch Susceptor Composition

The present invention describes a microwave susceptor formulation which comprises two susceptive materials; that is, a metal salt and a cooked starch, together with a mineral filler (of large surface area per unit weight) which also has a useful heat capacity. The temperature obtained by microwave heating and the heat capacity can both be controlled by the thickness and the ingredients of the susceptor. The embodiment allows the susceptor composition to heat rapidly by virtue of its retained moisture and to reheat (and to continue to reheat) to useful temperatures above 120 ^β C from moisture present in the food; moisture which is delivered to the susceptor by both gravity and natural hot air movement. This is illustrated by the responses shown in Fig. 18 through Fig. 21; the measurements were made in a 600 +/- 25 W, 2,450 MHz domestic microwave

oven using a microwave - inert plastic holder which is illustrated in Fig. 14 and is described below. This plastic holder allows for fiber optic temperature measurements to be made during microwave heating under different simulated food conditions and for additional water to be added to the susceptor when desired during a microwave heating test simulating a food.

Fig. 14 shows a cross-section of a block 21, of microwave invert plastic of dimensions 15 x 15 x 4 cm ³ cut into two identical pieces between which a susceptor material 22, can be placed. The block is designed to be used in a domestic microwave oven. The block 21 is grooved, 23, to allow free air movements across a part of the susceptor. Pressure is applied to combination by an additional weight 26, in the range 100-500 g. Luxtron fiber optic temperature probes 25, (marked L on Fig. 14) are positioned as required on or within the susceptor. To simulate air restraint between a susceptor and (for example) a moist food surface, a small piece of fiber insulating material 24, is laid on the surface of the susceptor. In this example the fiber material "slip" 24 was made from a Ceraform (Manville) fibrous insulating material which had been previously dried and cured at 300°C. Water can be added to the susceptor 22, via the air space 23, so as to reach selected areas of the material. This can be done by pipette or spray, rapidly, between microwave heating cycles in a domestic microwave oven.

A high purity mineral limestone, (Vicron 15/15; Minerals, Pigments and Metals Division Pfizer Inc., 235 East 42nd Street, New York, N.Y. 10017; 96% CaCo ₃ , 1.5% MgC0 ₃ , 1.2% Si0 ₂ , 0.3% A1 ₂ 0 ₃ , <0.1% Fe ₂ 0 ₃ ; of specific gravity 2.7 and surface area 3.1 m ₂ /g, and of average particle size 3.5 microns) was first impregnated with

susceptor salts. Approximately 50g of the material was placed in a solution in a Pyrex glass beaker consisting of 100 ml of distilled water, at room temperature, to which had been added, while stirring, 20g of potassium chloride and 20g of potassium acetate. The solution was placed under vacuum for two hours at a pressure of 20 mm Hg. (It should be noted that soaking glues would be sufficient.) Several such samples were made and tested; these were sealed in plastic containers and stored for periods up to 35 days at 20°C. Before use each sample was drained but not dried.

Samples were first used to determine an optimum binding, i.e., an ideal putty - like composition with a starch. For this, approximately 30g .of corn starch was placed in 120 ml of water and partly heated until a thick smooth paste was obtained. By using the salts, impregnated Vicron 15/15 limestone described above with the corn starch paste in the ratio 3:1 an ideal putty was obtained, which can be spread onto or laid between, for example, filter papers. A wide variation in the ratio host to binder is acceptable; ratios of 3:1 and 4:1 have been used successfully with the Vicron limestone described and with a courser material known as Wyoming White Marble (Basins Inc., Wheatland, Wyoming 82201, average crystal size l to 1.5 mm). With these materials bound by a cooked starch tiles and flexible panels of significant heat capacity can be formed in the thickness range at least 0.5 to 2.5 mm. By using the combined susceptive characteristics of the salts with corn starch and, when necessary, with salts embedded into marble grit, susceptors of different microwave energy absorption and heat capacity can be made, with maximum useful temperatures above 100°C, and in the preferred range of 125-200°C.

In order to determine the susceptive potential of the impregnated Vicron 15/15 samples referred to above by themselves, 20g of drained material were placed in a hemispherical porcelain crucible of 150 ml capacity and heated in a 650 W microwave oven. A Luxtron temperature probe generated the following data:

and temperature then falls.

The reader will now appreciate that water activates, and reactivates the susceptor at temperatures above 100 ^β C.

Fig. 18 to Fig. 21 show the results of measurements made on 10 x 10 cm ₂ tiles of 2 mm thickness. These tiles were made with (a) the above salts-impregnated Vicron 15/15 and the cooked corn starch also in the ratio 4:1 (Figs. 20 and 21).

Each tile was mounted in the microwave inert plastic holder (Fig. 14) in such a way that parts of the surface were held firmly between plastic, whereas other parts were exposed to the air and, in one place, to a thin layer of water absorbent fiber insulating material. The insulating material, when dry, was

known not to absorb microwave energy. The assembly was placed in a 650 W microwave oven. Luxtron temperature sensing probes were placed (LI, L3 in Figs. 18 and 19) between the plastic holder and the susceptor, where it was not possible for moisture to escape readily (no free air) ; L2 was placed inside the susceptor tile below a free-air region; L4 between the susceptor's surface and the insulation material. The temperature recorded by L4 simulates the situation of a food which is initially cold and relatively dry. It will be seen from the results that the surface temperature of the susceptor is, in part, controlled by the ability of the water to escape from the surface. The tile was then allowed to cool and air-dry after the 120 seconds heating time (Fig. 18) . It was then reheated in the same fashion, with the result shown by Fig. 19.

The material between the plastic was drier and heated more rapidly, shown by LI, L3 in Fig. 19. The portion of the susceptor originally heated in free air heats far less, L2 in Fig. 19. After 120 seconds of reheating the microwave was turned off, (1 in Fig. 19) , about 2 g of water were quickly added, (2 in Fig. 19) , and the microwave heating repeated for the third time (3 in Fig. 19). (The numbers 1, 2, 3 refer to Fig. 19, which is a dual heating cycle after the heating recorded in Fig. 18.) It will be seen that the water causes the susceptor to remain active and repeatedly reach useful temperatures; that the susceptor pumps water out of its active regions and attains a working temperature in the range of about 120 to 180°C. Further, the composition described comprises sufficient materials to create a susceptor which is inherently stable and self-limiting in the temperature which it can attain.

Fig. 20 and Fig. 21 show the results of case (b) above, using the coarser Wyoming marble in susceptor tiles. These tiles were also approximately 1 mm in thickness. The testing methods used were the same for case (b) and case (a) . It will be seen that the results are very similar and that the surface area and particle size of active natural filler material (which is, in these cases, types of natural marble grit) provide a small but useful control on heating and heating rates of these susceptors, both of which have a large water permeability and reactivity.

EXAMPLE 12

Susceptor Comprising a Cooked

Corn Starch and Metal Salts on Paper and/or Unwoven Polyester Fabric

A pure corn starch (Best Foods Canada Inc. , Box 500, Etibitoke, ON, Canada, M9C 4V5) was cooked by the method previously described and allowed to partially dry with additional heat. 25 g of potassium chloride and 20 g of potassium acetate were added to 100 g batches of the cooked and partially dried corn starch; a uniform mixture of these constituents was made by hand-stirring and rolling the materials together. A few ml of cold water were then carefully added to create a thick paste which could be rolled out, as a type of putty, onto paper.

Tiles were made by rolling the susceptor putty onto filter paper and then gently laying on the upper surface a non-woven polyester terephthalate (PET) fabric (of approximately 0.5 mm in thickness) obtained from Veratec Corporation, an affiliate of International Paper Inc., 100 Elm Street, Walpole, MA 02081. A

susceptor of any uniform thickness between 0.5 and 2.5 mm can be made in this way. For this example tiles of approximately 1 mm thickness were made using PET and the tiles were heated very rapidly, in pairs, each of size 5 x 10 x 0.1 cm ² , in a microwave oven for 10 minutes on medium power, that is, 650 +/- 50 W for 25% of the time. The tiles were then left after this curing and partial drying treatment in 21°C still air for 1 hour. The curing process was judged so as to heat the tiles rapidly for approximately 2 seconds after every 5 seconds in such a way that they cured but were not left completely without water. The tiles were flexible when cold and presumed still to contain water. One tile treated in this way was placed in a plastic block illustrated by Fig. 14 and then heated in a 650 W domestic microwave oven. The plastic block with the Luxtron fiber optic temperature probes is described in Example 11.

The results are shown in Fig. 15; Curve 1 shows the temperature where the air and moisture movement are restricted on the surface by the fiber material (24 in Fig. 14) ; Curves 2 and 3 are records of the temperature where the plastic block restricts the air and moisture movement and Curve 4 records the temperature in an area of unrestricted air and moisture moment, zone 23 in Fig. 14.

The weight of one of these susceptor tiles before microwave heating was 4.55 g (size 5 x 10 x 0.1 cm ³ ); after the 210 seconds microwave heating cycle shown in Fig. 15, the weight was 3.50 g. The susceptor was slightly scorched. Approximately 4 ml of cold water was then added to the susceptor after it had cooled to about 60°C. The water was added drop by drop as uniformly as possible onto the upper surface of the

tile. The susceptor was then heated again, in the same manner as before. The new heating curves are shown in Fig. 16, where the numbers 1, 2, 3, 4 refer to the probe positions defined for Fig. 15. The weight of susceptor after wetting and before this reheating was 7.50 g; after the reheating process shown in Fig. 16 it was 3.45 g.

Tiles were also tested to determine the amount of water which could be transferred (or pumped) by a susceptor of this type in a short period of time. A tile of the same material and construction as the previous two was cured in the same fashion. The maximum amount of water this tile (3.65 g; 5 x 10 x 0.1 cm ³ ) would accept over a 30 minute period was 3. 0 g. After 80 seconds of microwave heating, under the same conditions as the previous tiles experienced, it weighed 3.00 g. The temperature responses associated with this loss of water are shown in Fig. 17. The inventors believe, therefore, that the susceptive properties of the tile were not destroyed by water saturation even when the susceptor comprises only a metal salt, a cooked starch and fibrous binder such as filter paper or PET.

EXAMPLE 13 Preparation of Re-usable Susceptor From Eggshells

Crushed chicken egg shells having a size distribution centered on 600 μm (that is a range approximately from 800 μm down to a fine dust at about 50 μm) were impregnated from a mixture of potassium acetate and potassium chloride over a two hour period at pressures down to about 20 mm Hg. Some of the treated material was kept in the salt solution for

several days, drained, but not dried, and placed in a microwave-inert alumina dish. After rapid microwave heating, drops of water were added to just dampen the material which had caked. Microwave reheating was observed six times as 20°C to 150°C; 110°C to 161°C; 95°C to 141°C, 88 to 135°C; 125°C to 152°C, and 125°C to 138°C, when small quantities of water were added near the area of the material in which a Luxtron temperature probe was embedded. Thus, a re-usable susceptor material was formed, which also acts, during the reheating of foods, as a high-temperature drying means activated by water or steam.

EXAMPLE 14

Macaroni Susceptor Rack

Uncooked macaroni sticks ("Macaroni Long", Cantelli Inc., P.O. Box 1200, Montreal, PQ, H3Z 3B7) were cut into lengths suitable to make the susceptor oven rack shown in Fig. 22. The individual sections 31, 32 were glued together with a non-toxic white glue 35, etc., after the material was made susceptive by forcing a solution consisting of 2.5 g of potassium chloride in 25 ml of tap water at 14 ^β C through the hollow, semi-permeable center of each macaroni stick. In a preferred embodiment of this microwave package the macaroni sticks 31, or similar material, would be line cut, as shown 33, in Fig. 22, to create a large, semi- confined internal surface area on which water (and fats containing water) activate, and continue to activate the susceptor during either microwave reheating, or microwave cooking of a food product. The rack dimensions shown in Fig. 22, detail 24, are suitable for 3.5 oz. portions of pre-cooked french fried potatoes but other dimensions, and many other foods,

can be cooked, heated or reheated on racks comprising the materials described herein.

Faster susceptor heating rates have been observed when the macaroni is impregnated with a metal salt and then first rapidly dried in, for example, a microwave oven. Although this method of preparing the susceptive macaroni is preferred, it is not essential and it has been found more difficult to make the racks with structural integrity after curing. However, the material can be ground or "chipped" and reconstituted into almost any shape, or surface.

When the salt-impregnated macaroni described was heated in a microwave oven starting at 20°C it was found that the temperature on the surface of a 20 g bundle of the sticks went rapidly to temperatures in the 120 to 130°C range and then, with additional water, to temperatures above 170°C, rapidly changing color to a light brown. However, the material did not burn as air-spaces were present in. the bundle. The reader will appreciate the need for package designs using materials of this type to allow air flow and moisture movement; preferred designs of the type described comprises such venting and moisture movement means.

While the present invention is described above in connection with preferred or illustrative embodiments, these embodiments are not intended to be exhaustive or limiting of the invention. Rather, the invention is intended to cover all alternatives, modifications and equivalents included within its spirit and scope, as defined by the appended claims.