GB2030038A - Method and apparatus for the thermal treatment of foodstuffs - Google Patents

Abstract

The distortion or bursting of air- tight packs containing foodstuffs including an evaporatable constituent such as water, during heating of the packs e.g. for sterilisation purposes, is reduced or eliminated by subjecting the pack to an electromagnetic high-frequency alternating field while the pack is surrounded on all sides by water which is pressurised to at least substantially compensate the internal pressure generated in the pack during the irradiation, the water pressure being subsequently lowered as the pressure within the pack falls as a result of the pack cooling.

Description

SPECIFICATION Method and apparatus for the thermal treatment of foodstuffs The invention relates to a method and apparatus for the thermal treatment of foodstuffs which contain water or other constituents which can evaporate during the thermal treatment and which are packed, preferably in a gastight manner, in an envelope consisting of non-conducting material, the packs of foodstuffs being immersed in water and exposed to at least one electromagnetic high-frequency alternating field.

A method for the thermal treatment of foodstuffs is already known (DE-AS 24 22 530) according to which packs of foodstuffs containing a gas-filled head space are to be made durable. For this purpose, these packs of foodstuffs, the envelope of which preferably consists of plastics material, are immersed in water up to the filling height of the foodstuff.

The packs of foodstuffs are exposed to an electromagnetic HF field by means of electrodes disposed laterally; the packs of foodstuffs are irradiated with a UHF field from above. In this known method of preserving, it is possible to manage with comparatively low temperatures. The known method is not suitable, however, for the sterilization of the foodstuffs packed in a gastight manner. During the sterilization, water contained in the foodstuffs is at least partially evaporated as a result of the great heating; in the course of this, the internal pressure in the pack of foodstuff rises considerably during the heating. At the same time, the plastics packs used for preference according to the known method would be exposed to temperatures which would greatly reduce their strength during the sterilization.The risk of a permanent severe deformation of the plastics packs and the possible damage to their contents, even their destruction, would be inevitable.

For these reasons, the known method is not suitable for the sterilization of foodstuffs.

According to another known sterilization method, foodstuffs packed in an airtight manner in the form of tinned food are introduced into the pressure compartment of an autoclave into which superheated steam is injected. This steam has a double purpose: it heats the goods packed in a gastight manner and coming into contact therewith and at the same time exerts a pressure on the surface of the envelope of the goods. This pressure is adjustable and it prevents a change of shape or expansion of the pack, in that it opposes the pressure rising inside the pack as a result of heating by a counter pressure acting from the outside. The effects of these two pressures on the pack thus cancel one another out.

The main disadvantage of this method of sterilization consists in that the heat can only be supplied to the goods from the outside. In order to reach a predetermined temperature in its middle, therefore, its surface and its outer regions must be heated for a prolonged time to a considerably higher temperature than the predetermined temperature in its middle. The temperature at the surface of the goods must naturally be selected the higher, the more quickly the predetermined temperature is to be reached in its middle. A uniform heating of the goods is therefore impossible with the sterilization times which can be used in practice. In addition, there is the risk that the surface and the outer layers of the foodstuff to be sterilized may be denatured because of their severe heating.

Further disadvantages of this known method are relatively long heating and keeping hot times, which exert unwanted effects on the albumen contained in the foodstuffs and last but not least the relatively high steam temperatures which make the use of plastics packings impossible because their strength would be too severely impaired as a result of the heating. Therefore the more expensive metal packings must necessarily be used. A minor difficulty, which must nevertheless not be ignored, consists in that it is not always possible, despite careful regulation of steam pressure and temperature, to really reach the predetermined temperature in the middle of the rack, and there may not only be a drop below this but it may also be exceeded.

The object of the invention is, inter alia, to develop a method of the kind referred to at the beginning so that on the one hand a gentle sterilization of the foodstuffs is possible and on the other hand, at the same time, deformation or destruction of the wrapping or of the enclosed foodstuffs caused by heat and rising internal pressure is avoided almost entirely.

According to the invention, this is achieved in that the packs of foodstuffs, which are substantially free of occlusions of gas, are at first surrounded by water on all sides, that then the water is put under pressure before the action of the alternating field or at least simultaneously therewith, the pressure being regulated so that it at least substantially compensates for the internal pressure of the pack of foodstuff occurring during the action of the alternating field, that the effect of the alternating field is carried out until the treatment temperature and possibly the sterilization temperature is reached, possibly with a following holding time for the foodstuff and is then terminated, and that the set pressure is maintained during the action of the alternating field and is then lowered at the earliest according to the internal pressure sinking as a result of the cooling of the pack of foodstuff.

By the method according to the invention, it is possible to heat the contents of the packs of foodstuffs extremely quickly through the ac tion of a high-frequency electromagnetic alternating field, the heating being effected substantially uniformly in all parts of the foodstuff so that at every moment all parts of the foodstuff have substantially the same temperature. There is no risk of overheating of marginal zones. The time from the beginning of the action of the alternating field until the predetermined sterilization temperature is reached (rise time) can be kept extremely short because the thermal conductivity of the foodstuff to be treated, which renders lengthy rise times necessary with the usual autoclave sterilization, can be ignored.

Although here water is always mentioned for the sake of brevity, all other liquids with a matching dielectric constant and a low tan 8 are included.

It is true that pure water is used as a preferred liquid. In all cases, however, the liquid should have as low a loss factor tan 8 as possible, so that energy losses of the high frequency electromagnetic alternating fields in the liquid, during the heating thereof. are avoided as far as possible. Furthermore, the liquid should have a relative dielectric constant which comes as close as possible to that of the foodstuff to be treated. The latter serves to avoid potential differences which could lead to unwanted disruptive discharges damaging or destroying the packs and the foodstuff to be treated.

When foodstuffs are referred to in this application, all substances containing water or other constituents which are volatile at sterilization temperatures to be supplied to the human or animal organism are included by this or expressions used in connection with other words.

As a result of the fact that the packs of foodstuffs are surrounded by water under pressure during the action of the alternating field, the internal pressure building up as a result of the heating of the foodstuff and the evaporation of the water contained therein can be compensated for practically completely.

This means that the risk of deformation or destruction of the pack and/or of the foodstuff contained therein can be eliminated.

Since, because of the uniform heating of all parts of the foodstuff, its marginal zones are not excessively heated, the pack is also largely protected because it is only heated to comparatively low temperatures. In addition, the pack is cooled by the water surrounding it. Therefore the strength of the pack is only slightly reduced during the thermal treatment of the foodstuff. Moreover, since the internal pressure in the pack is practically compensated by the water under pressure, the pack is only exposed to comparatively slight stresses during the thermal treatment so that light and inexpensive packings can be used.

The pressure of the water surrounding the packs of foodstuffs must be selected so that the pack is not overstressed by the internal pressures arising during the thermal treatment. Preferably the pressure of the water is selected so that it compensates the internal pressure in the pack of foodstuff practically completely all the time. Since an expensive regulation or control is necessary for this it is an advantage to proceed so that the pressure of the water before the beginning of the action of the alternating field is adjusted to the maximum internal pressure of the pack of foodstuff occurring during the action of the alternating field and this pressure is only lowered again at the end of the cooling of the pack of foodstuff.The pressure of the water may, however, be advantageously increased in a plurality of stages during the heating up of the pack of foodstuff and be correspondingly reduced again in a plurality of stages during its cooling down. The pressure of the water may be more or less lower than the internal pressure in the pack of foodstuff, depending on the strength of the packing material.

After the alternating field treatment, the pack of foodstuff is preferably rapidly cooled with cool water. The temperature of the cool water is selected according to the particular requirements. So long as the pack of foodstuff is still at a relatively high temperature level and a high internal pressure prevails therein, the cooling water must be kept at an appropriate pressure which compensates for the internal pressure of the pack to such an extent that this is not exposed to any inadmissible stresses even during the cooling with cool water.

Water which is free of lime and free of metal ions, particularly boiler water, may appropriately be used for the treatment according to the invention.

A high-frequency electromagnetic alternating field with a frequency in the range from 10 MHz to 1000 MHz may appropriately be used for the alternating field treatment. Which field is used in each individual case depends on various circumstances. On the one hand, of course, the freed industrial frequencies of 13.56 MHz, 27.12 MHz, 433 MHz and 915 MHz offer themselves. Higher frequencies appear inappropriate because of too low a depth of penetration. The selection of the frequency naturally also depends on the size of the packs of foodstuffs because with an inadequate depth of penetration of the alternating field (the depth of penetration is known to sink with increasing frequency) no adequately uniform heating of all parts of a pack of foodstuffs can be achieved.

Those foodstuffs which have a relative electricity constant e and a dielectric loss factor tan 8 of substantially the same size everywhere in their interior and in particular do not have any major occlusions of gas are particularly suitable for the treatment accord ing to the invention. If the foodstuff from the factory has relatively large discrete components with very different relative dielectricity constant e and/or very different dielectric loss factor tan 8, then these components must be finely comminuted and be thoroughly mixed together so that they are present in the product at least substantially statistically distributed.

The method according to the invention is therefore particularly suitable for mass products in which all the packs of foodstuffs have the same or at least very similar physical properties, that is to say in which, inter alia, the weight, initial temperature, dimensions, dielectric values and loss factor tan 8 of the product and its envelope are practically alike.

These units of mass goods are preferably enclosed in envelopes or containers of plastics or glass. Such envelopes disturb the treatment process only a little or not at all but protect the product from subsequent soiling or infection.

The water may advantageously be cooler than the foodstuff to be treated, during the action of the alternating field. If comparatively pure water is used, this has a comparatively low loss factor tan 8 and is accordingly scarcely heated during the alternating field treatment. Thus if the water has a temperature equal to that of the pack of foodstuff or below it at the beginning of the treatment, its temperature will always remain below that of the pack of foodstuff during the alternating field treatment. This is an advantage because the water can then be used to cool the pack and possibly marginal zones of the foodstuff to be treated during the heating of the pack of foodstuff. As a result of the cooling of the pack, its strength characteristics are largely retained so that it remains more stable to deformation.

At the beginning of the action of the alternating field the water preferably has substantially the temperature of the packs of foodstuffs to be treated before the action of the alternating field. This is a particular advantage if the packs of foodstuff are at environmental temperature because then the water for the treatment neither has to be cooled nor heated and no additional energy is necessary to adjust the temperature of the water.

For the use of the method on soft cheese, such as "Camembert" or "Brie" cheese, for example, it has proved particularly advantageous for the water to have a temperature between 2 and 1 roc, preferably about 5 to 7"C at the beginning of the action of the alternating field. As a result, the effect can be achieved that the white mould covering the surface which is traditionally present and valued with these sorts of cheese, is retained despite the thermal treatment because the marginal zones of the whole cheeses under the action of the surrounding cold water, do not come to temperatures at which the mould covering them would come to harm.

In particularly with packs of foodstuffs with pronounced edges and corners, it has been found in experiments that these can be damaged by overheating at the corners or edges during the alternating field treatment. Surprisingly it was found that if an electromagnetic alternating field with a frequency in the range from 10 MHz to 400 MHz is used these phenomena can be reliably prevented if, at the same time, a second electromagnetic alternating field with a frequency in the range from 400 to 1000 MHZ is radiated in at an angle of at least 20 to this first alternating field, this second alternating field having a density of energy in the range from 1:1 to 1:10 with respect to the first alternating field.

In an advantageous apparatus for carrying out the method according to the invention, a pressure treatment vessel is provided, furthermore electrodes are provided with which the interior of the pressure treatment vessel can be exposed to a high-frequency electromagnetic alternating field, the pressure treatment vessel comprises an opening for charging with packs of foodstuffs to be sterilized and water inlets and outlets and is provided at the inlet side and outlet side with pressure adjusting devices for the pressure of the water.

The pressure of the filling water for the pressure treatment vessel is preferably produced by appropriate pumps, the control or regulation of the pressure being effected by corresponding control or regulating devices.

Devices such as removable gratings, steps and the like, on which the packs of foodstuffs to be treated can be located in the pressure treatment vessel during the treatment may appropriately be provided in the pressure treatment vessel. In addition, the charging and unloading of the pressure treatment vessel can be carried out more quickly in this manner.

According to an advantageous further development of the invention, the pressure treatment vessel may only partially be filled with water, but to such an extent that all the packs of foodstuffs are completely surrounded by water, the pressure being produced by an appropriately compressed gas, preferably air, above the level of the water.

The pressure regulation can be effected in such a manner that the water is supplied to the pressure treatment vessel, already filled with gas, particularly air, at a suitable pressure, from the under side, in which case as the vessel is increasingly filled, the cushion of air above the water is compressed more and more. The pressure regulation can be effected, for example, by the amount of water introduced.

The pressure regulation may also be effected, however, in such a manner that the pressure treatment vessel is first partially filled with water, the air in the head space of the vessel at first being kept at atmospheric pressure and that then compressed air is blown in.

This procedure has the advantage that the water pressure can be regulated very well by appropriate adjustment of the pressure of the gas in the vessel.

The pressure treatment vessel preferably has a horizontally elongated shape. In the construction of the pressure treatment vessel, the supporting framework (consisting of nonconducting material, particularly plastics material) for the packs of foodstuffs in the pressure treatment vessel and the arrangement of the packs of foodstuffs on the supporting framework, and the formation of the packs of foodstuffs, care must be taken to ensure that he surface of the latter remains easily accessible to the cooling water and permits it to flow through rapidly.

The electrodes may be provided in the interior of the pressure treatment vessel or outside it, in which latter case, the pressure treatment vessel must naturally be made at least partially of a non-conducting material so as not to prevent the propagation of the highfrequency electromagnetic alternating fields.

Altogether, care must be taken to ensure that no parts of conducting material are provided between the electrodes because these would likewise hinder the propagation of the electromagnetic alternating fields.

The method according to the invention can be carried out particularly appropriately with an installation which is characterised in that, for the installation, a plurality of pressure treatment vessels is provided, that the pressure treatment vessels travel in succession through various stations on a conveyor path, the pressure treatment vessels being charges with the packs of foodstuffs in a first station, filled with water in a second station, the water being put under pressure at the same time and the packs are exposed to the electromagnetic, high-frequency alternating fileld, while retaining the water pressure, are taken to a further station in which cooling water is conveyed through the pressure treatment vessel under pressure and finally are conveyed to a discharge station in which the pressure treatment vessels are unloaded, after which they are again supplied to the first station in which they are again chrged with packs of foodstuffs.

The invention is explained below with reference to examples: Example 1 Sweet cream of ordinary commercial quality, that is to say with a fat content over 30% is to be made lasting.

The sweet cream is introduced, on ordinary filling installations which do not work aseptically, into preshaped enveloped of a relatively heat-resistant plastics material-in this case of polypropylene--and then closed so that only a very small amount of air-if possible none is enclosed in the resulting gastight pack.

At the beginning of the treatment the contents of the pack have a temperature of about 20 to 25"C.

Since with these goods it is important to adhere to short treatment and keeping-warm times, the rapid cooling immediately after the treatment plays a great part. Therefore the packs are accommodated individually in the holding grid and adequate space is left for the cooling water which is to run through between the packs. The packs were inserted with the holding grids in which they were inserted, in the pressure treatment vessel. The pressure treatment vessel was then filled with water at 20"C. The water is then brought to a pressure of 2.5 at.Then, through the treatment electrodes, an electromagnetic alternating field of 13.56 MHz is applied for the period of 35 seconds, the sweet cream being heated to 87 to 89"C. Immediately after the end of the action of the alternating field, the pressure treatment vessel is flowed through by cooling water at about 5"C inlet temperature to cool the packs of sweet cream, and is at first at a pressure of 2.5 at. which is lowered to 1 at. as the packs of sweet cream increasingly cool down. The cooling is terminated when the sweet cream reaches a temperature of 35"C.

In this manner, not only by the shock action of a sudden rise in temperature on the germs contained in the cream, but also by means of an extremely short keeping warm time, protection of the albumen structure can be contributed to to obtain the fresh quality of the product without the other advantages of this method being abandoned.

The product thus treated keeps for more than 20 days at a storage temperature of 18 to 22"C.

Example 2 By using the method according to the invention, the so-called ripening process in soft cheeses can be brought partially or completely to a standstill, without denaturing their albumen content by overheating or keeping warm too long, which would cause a smell or taste of cooked albumen foreign to the product.

Therefore, with these goods, the optimum for the maximum temperature, which lies between 52 and 62"C according to the sort, must be carefully adhered to.

A further difficulty arises from the fact that soft cheeses always contain very considerable amounts of free water which evaporates at the above-mentioned treatment temperatures.

Since this steam cannot emerge from the dense mass of cheese, it deforms the whole cheese during the thermal treatment until it becomes unwieldy. Such deformed portions no longer fit in the ordinary commercial prefabricated packs and therefore have no commercial value.

In order to counteract such loss or value, particular attention must be paid, during the treatment according to the invention, to the counter pressure of the water bath, which can be precisely precalculated and adjusted.

A third difficulty which must be taken into consideration during the treatment of soft cheese according to the invention is the necessity, with certain sorts of soft cheese, such as Camembert or Brie, of retaining the white mould covering the surface, which is traditioanlly present and is characteristic, despite the thermal treatment.

The measures to be taken will be given below.

In the treatment of soft cheese, in general the procedure is such that the standardized whole cheese is placed in a plastics envelpe which surrounds it closely, has an appropriate shape and is generally deep drawn, and which is then closed in a gastight manner. After the treatment is ended, this envelope can be made "accessible to air" by perforation.

The resulting packs are immersed in pure water in the pressure treatment vessel and exposed at a predetermined pressure of about 2 at. to a high-frequency electromagnetic alternating field which has a frequency of 13.56 MHz. The contents of the pack, which is warm at-between 16 and 18'Catthe beginning of the treatment is heated to 56 to 58"C by the alternating field in 1 20 seconds.

The keeping capacity obtained amounts to about 60 to 70 days without a change in quality at a storage temperature of about 20"C.

The water pressure is sufficient both during the treatment and during the following cooling, to prevent expansion of the soft cheese.

The small cavities (bubbles) present in the cheese do not cause any discharges adversely affecting the quality of the product or damaging the pack.

For soft cheeses with a moist surface, water at about 30 to 35"C is used at the beginning of the treatment. During the cooling, the water used has a temperature between 10 and 18"C.

During the preserving of sorts of soft cheese which traditionally have a surface covered with mould, without which they would lose considerable commercial value, such as "Camembert" or "Brie" for example, the procedure must be different from the above in order to retain this growth of mould.

Before the beginning of the treatment, relatively cold water at about + 4 to + 6"C is introduced into the pressure treatment vessel.

Since this pure water absorbs practically no field energy and since the treatment period, which in this case must be somewhat longer, at about 1 50 seconds, than otherwise, but is still relatively short, the water temperature remains far below the temperature of about 35 to 40"C which is harmful to the mould.

Because of the remarkably poor thermal conductivity of shoft cheese, this cooling of the surface only acts on the cheese to a depth of millimetres during the treatment period; it protects the mould from harmful overheating, however, while the cheese reaches the predetermined temperature practically in its whole mass. Although during the following cooling operation according to the invention, with the soft cheeses here in question, considerably colder water is used than otherwise, it takes considerably longer than the heating up.

In this manner, the growth of mould can be retained during the thermal preservation of soft cheeses.

The keeping period of soft cheeses thus treated naturally fluctuates according to the characteristics of the starting products treated.

With a storage temperature of about 20"C, however, it may generally be expected that no reduction in quality will appear during 8 weeks.

Example 3 Fish fillets enclosed in plastics envelopes are to be preserved.

First it is necessary to check whether the filleting has been carried out as carefully as is necessary for dielectric heating. Fish bones have a high dielectric loss factor and tend to overheating if they are exposed to a highfrequency electromagnetic alternating field.

This overheating is so severe that the fish meat in the vicinity of bones or dorsal vertebra may be damaged.

It is, however, perfectly possible to carry out the filleting with satisfactory accuracy.

Herring fillets were selected when carrying out the experiment because it was intended to show that the difference in electrical behaviour between a relatively fat material such as herring meat for example and one containing less fat, such as the tomato sauce with which the residual space was filled in in the packing after insertion of the fillets, has no influence on the practicability of the method.

The herring fillets are placed in a shallow bowl of relatively heat-resistant plastics material (polypropylene) and filled to the brim with a tomato sauce of ordinary commercial quality. The portion thus prepared is covered with a suitable plastics foil and then hot sealed.

Since, with this type of goods, less importance has to be attributed to rapid cooling than with milk products, the relatively flat packs can be accommodated in pairs on edge in the pressure treatment vessel according to the invention; even the spaces between the packs rendering possible the flow through of the cooling water can be so dimensioned that they merely suffice to keep the shape of thermoplastic material. With these goods the consumer expects cooked foodstuffs.

In view of what has been said above, the treatment according to the invention is effected in a high-frequency electromagnetic field with a frequency of 27.12 MHz. The treatment period amounts to 1 30 seconds, the end temperature 110 to 1 12'C, which is left at this height for 4 minutes. Only after the expiration of this period of time is the cooling process according to the invention begun, which takes about 1 5 minutes. Then the packs can be taken out of the pressure treatment vessel, because at the temperature of about 40"C which is then reached by cooling, there is no longer any risk of a change of shape or volume in the envelopes.

Claims (20)

1. A method for the thermal treatment of foodstuffs which contain water or other constituents which can evaporate during the thermal treatment and which are packed, preferably in a gastight manner, in an envelope consisting of a non-conducting material, wherein the packs of foodstuffs, immersed in water, are exposed to at least one electromagnetic high-frequency alternating field, characterised in that the packs of foodstuffs, which are substantially free of occlusions of gas, are at first surrounded by water on all sides, that then, before the action of the alternating field or at least simultaneously therewith, the water is put under pressure, the pressure being adjusted so that it at least substantially compensates the internal pressure of the pack of foodstuffs appearing during the action of the alternating field, at least during the action of the alternating field, that the action of the alternating field is effected until the predetermined treatment temperature and possibly sterilization temperature is reached, possibly with a following holding time for the foodstuff and is then terminated, and that the set pressure is maintained during the action of the alternating field and is then lowered according to the internal pressure therein falling as a result of the cooling of the pack of foodstuff.

2. A method as claimed in Claim 1, characterised in that, before the beginning of the action of the alternating field, the pressure of the water is adjusted to the maximum internal pressure of the pack of foodstuff appearing during the action of the alternating field.

3. A method as claimed in Claim 1 or 2, characterised in that the pressure adjusted during the action of the alternating field is maintained until the end of the cooling of the pack of foodstuff and is then lowered to the environmental temperature.

4. A method as claimed in one of the preceding Claims, characterised in that, after the alternating field treatment, the pack of foodstuff is cooled quickly with cool water.

5. A method as claimed in Claim 1, characterised in that water which is free of lime and free of metal ions (boiler water) is used.

6. A method as claimed in one of the preceding Claims, characterised in that a highfrequency electromagnetic alternating field with a frequency in the range from 10 MHz to 1000 MHz is used.

7. A method as claimed in one of the preceding Claims, characterised in that the water remains cooler than the foodstuff to be treated, during the action of the alternating field.

8. A method as claimed in one of the preceding Claims, characterised in that, at the beginning of the action of the alternating field, the water has substantially the temperature of the packs of foodstuffs to be treated, before the action of the alternating field.

9. A method as claimed in one of the Claims 1 to 7 for treating foodstuffs with a sensitive surface, such as soft cheese and particularly "Camembert" and "Brie" cheese, characterised in that, at the beginning of the action of the alternating field, the water has a temperature between 2 and 10"C, preferably about 5 to 7"C.

10. A method as claimed in one of the preceding Claims, characterised in that a first electromagnetic alternating field with a frequency in the range from 10 MHz to 400 MHz is used and that simultaneously a second electromagnetic alternating field with a frequency in the range from 400 to 1000 MHz is radiated in at an angle of at least 20 to this first alternating field.

11. A method as claimed in one of the preceding Claims, characterised in that the temperature of the water remains substantially unaltered during the action of the alternating field.

1 2. An apparatus for carrying out the method according to the invention, characterised in that a pressure treatment vessel is provided, that electrodes are provided with which the interior of the pressure treatment vessel can be exposed to a high-frequency electromagnetic alternating field, that the pressure treatment vessel comprises an opening for charging with packs of foodstuffs to be sterilized, that the pressure treatment vessel comprises water inlets and outlets and that pressure-setting devices are provided at the inlet side and outlet side to adjust the pressure of the filling water.

1 3. An apparatus as claimed in Claim 11, characterised in that the electrodes are disposed in the interior of the pressure treatment vessel.

14. An apparatus as claimed in Claim 11, characterised in that the electrodes are provided outside the pressure treatment vessel and that the pressure treatment vessel is made at least partially of a non-conducting material.

1 5. An apparatus as claimed in one of the Claims 11 to 13, characterised in that the pressure treatment vessel for the alternating field treatment is completely filled with the packs of foodstuffs and the water and does not contain any occlusions of air.

16. An apparatus as claimed in one of the Claims 11 to 14, characterised in that the pressure treatment vessel is only partially filled with water but to such an extent that all the packs of foodstuffs are completely surrounded by water, and that the pressure is produced by appropriately compressed gas, preferably air, above the water level.

17. An installation for carrying out the method as claimed in one of the Claims 1 to 10, using an apparatus as claimed in Claims 11 to 13, characterised in that, for the installation, a plurality of pressure treatment vessels is provided, that the pressure treatment vessels travel in succession through various stations on a conveyor path, and in a first station the pressure treatment vessels are charged with packs of foodstuffs, in a second station they are filled with water, the water being put under pressure at the same time and they are exposed to the electromagnetic high-frequency alternating field, while retaining the water pressure they are conveyed to a further station in which cooling water is supplied under pressure to the pressure treatment vessels and finally they are conveyed to a discharge station in which the pressure treatment vessels are unloaded, after which they are again supplied to the first station in which they are again charge with packs of foodstuffs.

18. A method as claimed in claim 1, substantially as hereinbefore described with particular reference to the Examples.

1 9. A method as claimed in claim 1, substantially as illustrated in any one of the Examples.

20. An apparatus as claimed in claim 11 or installation as claimed in claim 17, substantially as described herein.