GB2207589A

GB2207589A - Microwave cooking

Info

Publication number: GB2207589A
Application number: GB08717842A
Authority: GB
Inventors: Philip John Fowler
Original assignee: Mardon Son & Hall Ltd
Current assignee: Mardon Son & Hall Ltd
Priority date: 1987-07-28
Filing date: 1987-07-28
Publication date: 1989-02-01
Also published as: GB8717842D0

Abstract

A package for food is made from a stock material (5) (such as cartonboard) to which is laminated a substate (3) having a deposit (4) of a semiconductive or ferromagnetic material. The deposit (4) heats resistively when subjected to microwave radiation, to brown or crisp food in the package. Deposit 4 can be ferric oxide, a ferrite, indium tin oxide, zinc oxide or tin oxide and the substrate can be a polyester or polyamide film. <IMAGE>

Description

MICRC)09AVB COOKING This invention relates to products (normally food) intended to be heated using a domestic or commercial microwave oven.

One disadvantage of microwave cooking is that, unless special measures are adopted, browning andlor crisping of the product does not occur, for example in pastry. This problem is discussed in detail in our UK patent application No. 8630889, to which reference is directed.

Attempts have been made to overcome this problem in disposable packages e.g. cartons, intended for microwaving. The proposal is to use a so-called "browning pad" which consists of, for example, a sheet of carton board to which is laminated a layer of metallised plastic film. Usually, this film carries a very thin, optically translucent coating of metal or metals such as aluminium or a metal alloy, such as stainless steel. Although in the prior art this is referred to as being of a "paint-like" nature, a preferred method of applying the metal or metal alloy is by vacuum deposition. Vacuum deposition is a well known technique whereby the metal or metal alloy is heated in a series of crucibles or "boats" and evaporates under vacuum onto the plastic web.

According to the present invention there is provided a package for food, the package comprising a stock material which is provided with a coating of semi-conductive or ferromagnetic material which coating is heated in the presence of microwaves in a microwave oven to a temperature sufficient to cause crisping and/or browning of food contained in the package adjacent the coating.

The coating may be achieved by the so-called "sputeering process. This is carried out, like the standard thermal vacuum deposition process, under vacuum, It utilises positive ions from a gas plasma to bombard the target material to be deposited, releasing surface atoms from the target in the form of an intense vapour stream. Whilst in transit from target to the substrate, the vapour can be deliberately reacted with small amounts of gas, such as oxygen or nitrogen. The material then deposited on the substrate is a reaction product of the target material and the introduced gas.

It is also possible to guide the deposited atoms at a controlled angle and to form a desired crystalline state e.g. colulnnar.

Although the full commercial exploitation of the sputtering process is in a comparatively early stage, it is known that, for example, coatings other than purely metallic types can be created. The process is very versatile and can deposit non-metallic coatings, for example semi-conductive coatings. More than one coating can be applied to a single substrate. Also, it is possible to deposit coatings of ferromagnetic compounds, such as ferric oxide or ferrites. Whilst these are mainly of interest in video and audio magnetic recording, they can also be used in the manufacture of a microwave package in accordance with the present invention.

Preferably, the coating is applied to a substrate in the form of a film of plastics material, such as a polyester, which is then laminated to the stock material, The stock material may, for example, be carton board.

The mechanism of heating in the microwave oven may not yet be fully understood, but in the case of the semi conductive compounds is believed to consist at the varying magnetic field in the oven generating electrical eddy currents, thus creating heat resistively in the semi-conductive coating. In the case of ferromagnetic compounds, the individual particles of the coating each respond to the varying magnetic field, generating heat as a result of the hysteresis loss experienced by each particle behaving as a tiny magnet or electromagnet.

The benefits of coatings of seml-conductive or ferromagnetic materials as used in the present invention, particularly when applied by the "sputtering" process include: 1) The coating can range in optical density from opaque to clear. This permits visibility of the stock material, for example white board, to which it is applied. The stock material may then carry a printed message or overall colour, if so desired, which is visible through the coating.

2) The coating, if insoluble and non-toxic in the sense accepted by authorities such as the American rood and Drug Authority, can be used in direct contact with the food product, thus making for a more effective heating than hitherto.

3) Whilst existing evaporated metal coatings exhibit some variation in their thickness due to unintentional "banding" of the deposit caused by the tow of individual heated boats used for their evaporation, a closer control exists in the sputtering process as the target is driven off from a continuous cathode of approximately full web width, typically heated by a DC magnetron or electron beam.

4) Improved adhesion of coating to plastics film.

For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be maze, by way of example, to the accompanying drawings, in which: Figure 1 shows a package including a browning pad; Figure 2 shows the browning pad of the package of Figure 1; Figure 3 shows the browning pad on an enlarged scale; Figure 4 shows an alternative form of browning pad; Figure 5 shows a third form of browning pad; Figure 6 shows a fourth form of browning pad; and Figures 7 and 8 show a packaging provided on its walls with heating coatings* Figure 1 shows a package 1 of carton board containing a "browning pad" 2. The browning pad 2 is illustrated as being on the floor of the package 1, so that food in the package rests directly on the browning pad 2.The construction of the pad 2 could be as shown in Figure 2, in which a polyester or polyamide film 3 (such as nylon), bearing a semi-conductive coating 4 (such as indium tin oxide, zinc oxide or tin oxide) is laminated to a carrier 5, such as carton board or another plastic film. The film 3 may be laminated to the carrier 5 using normal packaging techniques (e.g.

with polyurethane adhesive 8, as shown in Figure 2.

Alternatively, as shown in Figure 4, the semiconductive coating 10 may be uppermost.

The film 3 bearding the coating 4 need not be laminated to the carrier 5. It can instead be "window patched" or labelled, with a spaced discontinuous or fully continuous adhesive layer 16 into a carton 17 where required, as shown in Figure 5.

Further methods of applying the coating where required include a deliberate masking during the deposition process, to produce a striped effect 19 of the coating on the plastic film 3 which .can subsequently be registered into the final laminated, printed (if required) and die-cut carton.

In a further variation1 the coating could be applied by means of a hot-stamping foil. This involves the transfer of the required design of coating onto the carrier, for example the carton board from which a carton is made, from a hot die as described in our UK patent application No. 8630889. This process makes it possible to achieve patterning of the coating 4, as shown in Figures 7 and 8. The configuration of this patterning may be similar to the embodiments shown in our UX patent application No. 8630889.

A further variation again is that whilst browning/crisping requires close contact of the packaged product with the part of the carton 1 or the pad which is subjected to heating as a result of the coating 4, it can so be arranged that the semiconductive coating 4 is also spaced away from the food, above it, as shown at 27 or to the side of it, in such a manner that it intercepts microwave radiation and disperses the heat generated into the air (possibly through vents provided for that purpose) instead of radiating direct heat into the food surface.

The thickness of the coating 4 needs to be selected so as to achieve the required heating effect while avoiding arcing in the microwave oven. It is expected that the coating will perform satisfactorily if it is applied with a thickness which gives a resistance of 50 to 150 ohms per square.

Claims

1. A package for food, the package comprising a stock material which is provided with a coating of semi-conductive or ferromagnetic material which coating is heated in the presence of microwaves in a microwave oven to a temperature sufficient to cause crisping and/or browning of food contained in the package adjacent the coating.

2. A package as claimed in claim 1, in which the material is ferric oxide or a ferrite.

3. A package as claimed in claim 1, in which the material is indium tin oxide, zinc oxide or tin oxide.

4. A package as claimed in any one of the preceding claims, in which the coating is applied as.a continuous layer over the surface of the stock material.

5. A package as claimed in any one of claims 1 to 3, in which the coating is discontinuous.

6. A package as claimed in any one of the preceding claims, in which the semi-conductive or ferromagnetic material is desposited on a substrate which is laminated to the stock material.

7. A package as claimed in claim 6, in which the semi-conductive or ferromagnetic material is deposited on the substrate by a sputtering process.

8. A package as claimed in any one of the preceding claims, in which the semi-conductive or ferromagnetic coating has a resistance of 50 to 150 ohms per square.

9. A package substantially as described herein with reference to, and as shown in, the accompany drawings.

10. Stock material for use in the manufacture of a package for food, the stock material having a coating of semi-conductive or ferromagnetic material substantially as described herein.