MATERIAL GENERATING HEAT BY ABSORBING MICROWAVES

Abstract: This invention relates to a material which heats and cooks foods from outside by absorbing microwaves of an electronic range and by generating heat. A conductive substance or a nonconductive compound containing transition metals of atomic number 21-29 is mixed in a base material having an Fe group oxide as a principal component. Since the material has a rapid heat generating speed, the temperature in heat-generation is high, and further the material is cheap; it is effective as a material generating heat by absorbing microwaves.

Description: The present invention relates to a microwave-absorptive heat-generating material, which generates heat as a result of absorption of microwave energy and provides excellent heat-generating characteristics when used in an electric oven. An electronic oven is a device in which irradiating microwaves impart frictional vibrations to molecules of water and the like contained in an article to be cooked so that cooking is effected by temperature rise caused by the frictional heat. A merit of such an oven is that cooking can take place in a short time because food is heated by an internal heat source. However, since food cannot be cooked from the surface by conduction heat or radiation heat, as is the case with a gas cooker and a resistance heater, scorching (browning) and surface crispness (crispy nature) cannot be imparted to the food surface. Therefore, such ovens have not been suitable for cooking food which is to have a browned or crisp surface region, such as grilled fish, roasted meats and pizza pie. As a countermeasure to this shortcoming, a method of heating foods from the surface is known which uses a heat-generating material in an existing electronic oven, which material absorbs microwaves and generates heat, and thereby enables a browned appearance or crispness to be imparted to the food. However, the heat-generating speed with such heat-generating material is slow, and it is not generally sufficient for such material to be useful as a heat-generating material in an electronic oven in which rapid cooking is required. In this connection, US-A-4190757 discloses a food heating package for browning or crisping food wrapped therein when the package is heated in a microwave oven. The package includes a package body comprising microwave transparent non-lossy dielectric sheet material incorporating particles of microwave absorptive material of different particle size and a binder bonding the particles together. The particles can be of Fe₃O₄ and the binder sodium silicate. Other classes of materials which can be used include zinc oxide, chromium oxide, cobalt oxide, manganese oxide and nickel oxide. All these oxides are to be used singly. It is an object of this invention to provide a novel microwave-absorptive heat-generating material, which can heat up in a relatively short period of time and attain a higher temperature. According to the present invention, there is provided a microwave-absorptive heat-generating material in the form of an integral body and comprising Fe group oxide as base material, characterised in that an electrically non-conductive oxide selected from oxides of transition metals having an atomic number of 21-29, exclusive of iron, is mixed in with the base material in an amount not exceeding 0.7 mol per mol of Fe and which enables a surface temperature to be achieved on microwave heating which is higher than that achieved in the absence of a said transition metal oxide. This invention also provides a method of enhancing local heat generation at the surface of an item of food being heated in a microwave oven, which comprises operating the oven with said item of food and a microwave-absorptive heat-generating material according to the present invention disposed therein. The term "Fe group oxide" used herein means an oxide as such of Fe such as FeO, Fe₂O₃, Fe₃O₄ or a complex oxide in which a part of Fe is substituted by another metallic element. The microwave-absorptive heat-generating materials according to the present invention thus comprise an electrically non-conductive compound of a transition metal having an atomic number 21-29, exclusive of Fe, mixed in a base material principally consisting of Fe group oxide to form a solid product. The amount in which the electrically non-conductive oxide is present preferably does not exceed 0.5 mol per mol of iron (Fe) in the base material. In other words, according to the present invention, it has been found that oxides of transition metals having an atomic number 21-29 in general are effective in enhancing heat generation in Fe group oxide materials. Forming of the Fe group oxide of the base material mixed with electrically non-conductive oxides to form a solid or integral body may be achieved by sintering after shaping of the mixture obtained into a desired configuration. Alternatively, use may be made of a cold-setting adhesive. When solidifying by sintering, the sintering takes place sufficiently at about 800-1200°C. The following three mechanisms may be contemplated as taking place in a microwave-absorptive heat-generating mechanism according to the present invention, namely heat generations by magnetic loss, dielectric loss and ohmic loss. A ferrite which is a microwave-absorbing heat-generating material in the prior art, generates heat by magnetic loss inter alia , as a result of absorption of microwaves. However, in such method, the energy of microwaves cannot be converted efficiently into thermal energy, and hence neither a sufficient rate of generation nor a sufficiently high temperature can be obtained. The reason why the heat-generating speed of the microwave-absorbing heat-generating material used in this invention is extremely high, is thought to be as a result of the addition of the above-described electrically non-conductive substance, heat-generation caused by ohmic loss or heat-generation caused by dielectric loss being reduced in addition to that by magnetic loss. In addition, it can be considered that, on the basis of secondary and tertiary products produced through these reactions, even after release of the reaction heat, further microwave absorption occurs and it contributes to heat-generation. The following examples illustrate this invention: Example 1 Oxide powders of Ti, Cr, Mn, Co, Ni, Cu, Sr and Ba were respectively added to Fe₃O₄ powder at an element ration of 0-0.7 mol per mol of Fe. The mixtures were press-shaped and then sintered at 800-1200°C to prepare samples of 30mm diameter and 2mm thickness. Next, microwave-heating was effected for 30 seconds in an electronic oven operating at a frequency of 2,450 MHz and 1500 W in output power with samples present in the oven. The surface temperature of the samples was measured by means of a radiation thermometer. The results of measurement are shown in Fig. 1 of the accompanying drawings. As will be seen from Fig. 1, except with oxides of Sr and Ba, a tendency towards temperature increase as a result of addition of metal oxide, is marked, although if the element added in the form of an oxide exceeds 0.7 mol per mol of Fe, the temperature is, on the contrary, lowered. Thus it can be stated that the use of the oxides of a transition metal in the fourth period (atomic number 21-29) such as Ti, Cr, Mn, Co, Ni or Cu leads to a high temperature characteristic being obtained. Example 2 Oxide powders of Ti, Cr, Mn, Co, Cu, Sr and Ba were respectively added to Fe₃O₄ powder in a metal element ratio of 0.3 mol per mol of Fe. The mixtures were press-shaped and then sintered at 800-1200°C and the thus prepared samples of 30mm diameter and 1-8mm thickness were heated by microwaves for 30 seconds in the electronic oven of Example 1. The surface temperatures were measured by means of a radiation thermometer. The results of the measurements are shown in Fig. 2 of the accompanying drawings. When an oxide of Sr or Ba was employed, however, the surface was actually reduced when the thickness of the heat-generating body was thin. However, when a transition metal oxide was employed, a high efficiency was obtained even with a thin sample. Example 3 Metal powders of Ti, Mn and Sr were respectively added to Fe₃O₄ powder at an elemental ratio of 0.3 mol to 1 mol of Fe. Samples prepared by press-shaping the mixtures and thereafter sintering the shaped bodies produced were ground. Then the ground powder was coated onto one side surface of a crystallized glass sheet (Li₂O-Aℓ₂O₃-SiO₂ group) having a strong resistance to thermal impact and a diameter of 150mm in a thickness of 0.5mm by means of an inorganic binder to form a heat-generating film, and thus a heat-generating body was prepared. These coated sheets was heated by microwaves for 30 seconds in the electronic oven used in Example 1, and the surface temperature was measured by means of a radiation thermometer. The results of the measurements are as shown in Table 1. Added Elements Average Temperature (°C) Ti 511 Mn 402 Sr 314 In addition, a commercially available refrigerated pizza pie 150 mm in diameter was placed on the above-described heat-generating body and cooked in an electronic oven. The cooked state of the topping ingredients on the surface of the pizza pie as well as the pizza crust on the back surface were observed. The results of the observations are shown in Table 2. Added Elements Cooking Time 80sec. 60sec. 90sec. inside outside inside outside inside outside Ti ▵ ▵ ○ ○ X X Mn ▵ ▵ ○ ○ X X Sr ○ ▵ X ▵ X ○ ○ : well cooked ▵: insufficiently cooked X : excessively cooked The heat-generating body in whose production Ti or Mn was added had a high temperature. Hence, when cooking a pizza pie, crispness could be imparted to the pizza crust and the entire food could be cooked uniformly, and had good taste. As described in detail above, a heat-generating material according to the present invention is effective in defrosting frozen food and shortening a cooking time because it has a remarkably fast heat-generating speed as compared to the heat-generating material in the prior art. In addition, a high temperature suitable for cooking is obtainable with a smaller amount of the material than the heat-generating material in the prior art, and hence it is economical to use. In summary, thus, a material according to the present invention can be utilised effectively in an electronic oven as a material for heating and cooking foods from the outside by absorbing microwaves of an electric oven and generating surface heat at the time of cooking in an electronic oven.

Claims: A microwave-absorptive heat-generating material in the form of an integral body and comprising Fe group oxide as base material, characterised in that an electrically non-conductive oxide selected from oxides of transition metals having an atomic number of 21-29, exclusive of iron, is mixed in with the base material in an amount not exceeding 0.7 mol per mol of Fe and which enables a surface temperature to be achieved on microwave heating which is higher than that achieved in the absence of a said transition metal oxide. A microwave absorptive heat-generating material as claimed in claim 1, which contains not more than 0.5 mol of the transition metal oxide per mol of Fe. A microwave-absorptive heat-generating material as claimed in claim 1 or 2, which is in the form of a sintered product. A microwave-absorptive heat-generating material as claimed in claim 1 or 2, which is in the form of a solid body given integrity by incorporation of adhesive material therein. A method of enhancing local heat generation at the surface of an item of food being heated in a microwave oven, which comprises operating the oven with said item of food and a microwave-absorptive heat-generating material according to any preceding claim disposed therein.

Inventors: IKEDA MIKI KABUSHIKI KAISHA KO, KURITA SUMIHIKO, MATSUKI MASAHARU KABUSHIKI KAI, YOSHIHARA TOSHIAKI KABUSHIKI K, IKEDA, MIKI KABUSHIKI KAISHA KOURANSHA, KURITA, SUMIHIKO, MATSUKI, MASAHARU KABUSHIKI KAISHA KOURANSHA, YOSHIHARA, TOSHIAKI KABUSHIKI KAISHA KOURANSHA
Applicants: KORANSHA KK, KABUSHIKI KAISHA KOURANSHA
Application year: 1991
Main classification: B65D81
IPC codes: B65D81, H05B6