GB2206769A - Container for dielectric - Google Patents

Abstract

A container is disclosed in which foodstuffs, of a kind which can conform with the inner surfaces of the container, can be cooked or heated by the application of radio frequency energy. The container is preferably made of a plastics material and bears at least one pair of mutually insulated electrodes e.g. aluminium foil or steel exposed to the foodstuff, or isolated therefrom only by an ultra-thin protective coating. The electrodes are connectable to a source of the radio-frequency energy and may be configured (Figs. 4, 6) to present surfaces exposed externally of the container in order to facilitate such energisation as the container is carried past spring loaded electrodes on a conveyor belt. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO TEIE COOKING OR HEATING OF FOODSTUFFS This invention relates to the cooking or heating of foodstuffs and it relates aore particularly to the packaging of food in such a manner that it can be cooked or heated by the application of radio frequency (RF) energy.

Cooking or heating of food cssn be effected very efficiently by means of BF energy of frequency in the range from a few tens of KHz to 50MHz, partly because there is direct and virtually complete, penetration of the food by the applied energy and, as a consequence, there is no need to rely upon the slow process of thermal conduction through the food to cook or heat the innermost regions thereof. A difficulty associated with the use of RF energy for these purposes, however, is that of physically applying the energy to the food, and this invention aims to alleviate that difficulty.

The simplest applicator construction for these purposes is the parallel plate capacitor and the potential efficiency of this kind of applicator is sufficient to render its use attractive. However, unless the electrodes are allowed to depart from the parallel plate relationship, they are unable to conform adequately to the shape of most kinds of food and thus their use has, despite the attractions, been avoided.

In accordance with this invention there is provided a container for food to be cooked or heated by the application of RF energy, the package incorporating electrodes for connection to a source of the RF energy.

Preferably, the container is made of a plastics material and contains two or more sutually insulated electrodes. The electrodes are preferably substantially planar and the food is preferably of such consistency that, when placed in the container, it is in intimate contact vith, or minimally spaced from, the electrodes.

In order that the invention may be clearly understood and readily carried into effect, some esbodiments thereof will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 shows, in schematic and perspective view, a container in accordance with one example of the invention, Figure 2 shows a cross-section through a container, Figure 3 shows a cross-section through an alternative construction of container, Figure 4 indicates, in sectional view, one way of securing an electrode to a container and of coupling the electrode to an RF source, Figure 5 shows a conveyor belt heating system, Figure 6 shows a rimmed container permitting contact to be made from above the container, Figure 7 shows a part of a container configured to accommodate an electrode, Figure 8a a shows an electrode arrangement for heating layeral food, and Figure 8b and 8c show, respectively, equivalent impedance circuits associated with the arrangement of Figure 8a.

The invention has special application for foods which can be either cooked or heated in specially constructed containers. Examples of such foods are stews, cached potatoes, custard, rice puddings etc; the general characteristic of such foods being that they can be caused to conform to the shape of the container. A suitable container is shown schematically and in perspective view in Figure 1, wherein a container of low-loss electrically insulating material e.g. polythene or other plastic material, is shown generally at 1.The container is generally rectangular and the two longer sides are both provided with electrically conductive material, as shown at 2 and 3, to form the electrodes of a parallel plate capacitor which are intended to be in direct contact with food placed in the container 1, or to be covered by an ultra-thin coating, "skin" of plastics material. The conductive material 2,3, is coupled to a source 4 of R.F. energy and the coupling can be such that one electrode is live and the other earthed or, alternatively, a push-pull arrangement can be used.

The kind of container just described has been used experimentally for the baking of sponge mixes and it was found that, due to the parallel plate geometry, and hence uniform field distribution of the RF energy, the sponge cake rose uniformly in the container.

In practice, the container 1 was made of plastic material and was 20cm long, 9.5 cm wide and 6cm in depth. Before the wet sponge cake mix was introduced into the container, both long walls were coated on their inner surfaces with aluminium baking foil, to provide parallel plate electrodes having a separation of 9.5cm. The foil was attached to the walls of the container 1 with double sided tape.

The sponge was a 'ready-mis' type of gross dry weight 312g.

Half was 'baked' in this particular container and required 500 W of BF power of 14 Maz for 3 minutes. Lecciiended time in a conventional oven is 15-20 minutes, plus a cooling tine of 10 minutes. Clearly, RF represents a significant time and energy saving, but no surface browning was observed in this configuration.

When the electrodes were positioned on the shorter walls, uniformity was not as good owing to the increased fringing fields outside the container.

The container should be constructed from a material having a low dielectric loss factor. This is to prevent the container from being heated directly with RF energy. Some plastic containers are designed to be suitable for use in a microwave oven. These equally suitable at RF, since the dielectric loss factor tends to increase, rather than decrease with applied frequency. In addition, the material should have suitable thermal properties, such that the container vill tolerant temperatures around 100 C for a prolonged period as a result of heat conduction from the food to the container.

Taking these points into account, suitable material of the container 1 are for example, polyethylene, PTFE (teflon) and some types of glass (e.g. Corning 707).

Container size is not crucial in terms of efficient injection of RF energy into food. Hence, a variety of lengths, breadths and heights can be used depending on the substance(s) to be heated. In all cases however the load must conform to the shape of the container.

In its simplest form the cross-section of a food container is shown in Figure 2. The metal is shown to be intimately contacting the load, since a thin plastic protective (isolating) skin could cause high electrode voltages to develop, resulting in a large electric field across the skin.

In the example shown in Figure 3, which is intended to illustrate the problem of non-contacting electrodes, the plastic skin is only 1 mm thick but the electric field is 40 times greater than inside the food load. Under such conditions undesirable corona discharges ray occur, and thus it is crucial to maintain intimate contact, or to have an extremely thin plastic insulating skin.

Figure 4 indicates how a metallic electrode can be attached to the wall of a plastic container. There is no complicated fixing mechanism and the metal is held tightly at points A and B. This arrangement prevents any capacitive coupling between electrode and plastic wall. Provision can be made for the metal electrodes to be inserted into either pair of walls, if the moulded plastic container has the required groove (A) around its base perimeter.

Instead of using a complicated RF connection to the electrode, a spring-loaded system can be used, whereby the container can be quickly inserted and removed from the RF heating enclosure by an unskilled operator. The arrangement also lends itself to continuous processes. For example, food containers can be heated whilst in motion on a slow conveyor belt system (Figure 5). Electrical contact is maintained with one container (thereby heating its contents) for the time required to pail the fixed spring contacts. Hence heating time c Ir be controlled by conveyor rate, or conversely, throughput can determine the necessary levels of RF power. This technique ii useful in sass catcring for re-heating chilled seals.The food is pre-packed inside the 'BF-container' by the supplier.

Alternatively, on a smaller scale (e.g. in pubs, cafes and restaurants), a more contact RF apparatus can be used to heat individual food containers.

Aluminium foil electrodes may be caused to adhere to the plastic wall and after use, the foil can be replaced and the container re-used. Alternatively the whole package could be completely disposable.

In order for the spring loaded contact mechanism to work, as described in Figure 4 a modification to the container is desirable, this modification being illustrated in Figure 6.

The container has a rim to support the thin electrode and electrical contact is made from above, rather than at the sides. Alteratively again folded stainless steel sheet may be used instead of aluminium foil, to render the container more rugged.

Another simple, but effective technique is to have runners down the vertical inside edges of the plastic container (Figure 7). The metal sheet electrodes can be readily slid into position. The top of the electrode may be designed to protrude from the top of the container in which case the 'sideways' spring-loaded contact arrangement can be employed.

So far, systems having only two electrodes have been discussed. For certain applications it is beneficial to use two or more pairs of electrodes to provide the desired heating pattern. For example, in the rectangular box described above, two pairs of plates could be used instead of one pair. Such a system can be used to cook foods having layers of alternate high and low electrical iipedance.

An arrangement is shown in Figure 8a. When electrodes 2 and 3 are energised, the equivalent circuit is shown in figure 8b. When these electrodes are retracted (or removed) and replaced by plates 5 and 6, the equivalent circuit is shown in figure 8c. In the former case the orientation of the layers is such that sost voltage drops across the high impedance material. This is heated preferentially to the material having a lower impedance. In the latter case, most of the electrical current passes through the material of least impedance and this is then heated preferentially. Clearly, in layered foods, such as lasagne, shepherd's pie and sponge and fruit puddings, one pair of electrodes may not be sufficient to produce uniform heating. In the catering trade where food is prepared and served from large rectangular trays, the technique just described would provide a better heating uniformity. More than two pairs of plates could be used if necessary, for example three pairs of plates in a hexagonal arrangement.

Claims (10)

1. A container for food to be cooked or heated by the application of radio-frequency energy, the container incorporating a plurality of electrodes for connection to a source of the said energy.

2. A container according to Claim 1 constructed of plastics material and containing two or more mutually insulated electrodes arranged so as to be capable of assuming intimate contact with the food.

3. A container according to either of Claims 1 and 2 wherein each of said electrodes comprises a metallic foil adherent or otherwise secured to a respective substantially planar region of said container.

4. A container according to Claim 3 wherein each said foil is an aluminium foil.

5. A container according to any of Claims 1 to 4 of box-like construction and with at least one pair of opposing, inwardly facing surfaces bearing respective electrodes.

6. A container according to any preceding claim wherein said electrodes are configured to present respective externally exposed surfaces.

7. A container according to Claim 6 wherein said externally exposed surfaces can be contacted by respective resilient contact members.

8. A container according to Claim 7 configured and disposed to be transportable past said contact members and wherein the arrangement is such that each of the said externally exposed surfaces remains in contact with its respective contact member for a predetermined time.

9. A container, as claimed in any preceding claim, containing food.

10. A container substantially as herein described.