METHOD AND PLANT FOR IMPROVING KEEPING QUALITIES OF FOOD PRODUCTS

Abstract: For improving the keeping qualities of certain food products, especially sliced meat products, it is suggested that the food products immediately after they have been packed are subjected to a heat treatment by means of microwave energy, so that they are maintained at a temperature in the interval 50-80 DEG C, preferably 50-70 DEG C, during a time period of 5-60 seconds. The food products (12) are moved by means of an endless belt (9) in a path across an elongated wave guide (15, 21) which has at least one slot (19; 22, 23) extending across the whole food products movement path.

Description: Method and plant for improving keeping gualities of food products The present invention relates to a method and a plant for rapidly and simply improving the keeping qualities of separate food products. The background of the invention is that it has proved possible by means of a relatively moderate heat treat- ment substantially to improve the keeping qualities of the kind of food products which are not being deep frozen or sterilized but are maintained in a fresh state until prepared for consumption. This kind of food products are for instance sliced meat products, such as sausage, ham, etc. One reason for the fact that improved keeping qualities can be achieved particularly in connection with sliced food products is assumed to be that a small amount of bacteria often may be added in connection with the slicing operation and the subsequent packing, which puts a limit to the keeping qualities of the food products. It has proved that the effect of these bacteria may be reduced substantially if the food products are subjected to a relatively moderate heat treatment immediately after the slicing operation. The invention thus aims at enabling in a rapid and simple way improvement of the keeping qualities of for instance such food products. Plants are previously known for rapid heat treatment of packed food products by means of microwave energy. However, known plants of this kind, as a rule, have a relatively complicated construction and do often include sophisticated equipment for accomplishing an even heat treatment effect in the packed products. Thus, US-A-3.809.844, US-A-3.809.845 and US-A-3.814.889 describe plants in which food products are subjected to an accurately controlled heat treatment already before they are heated by means of microwave energy and, more- over, are cooled during the very microwave heating by means of a surrounding fluid in order not to get a too high surface temperature. EP-Al-0.287.760 describes a plant in which packed food products are placed at accurately determined intervals on an endless belt, on which they are moved inter¬ mittently and subjected to microwave heating in diffe¬ rent heat treatment zones. Microwave applicators arranged at the heat treatment zones are formed in a particular way with regard to the shape of the food products and are arranged to be moved towards and away from the food products during the microwave heating, when the endless belt keeps the food products stationary in the heat treatment zones. EP-Al-0.351.491 describes a plant in which packed food products are placed in several parallel rows on an endless belt and are moved by the belt under a number of microwave applicators, each one of which is arranged for heat treatment of the food products in only one of the rows. The food products have to be placed very accu¬ rately on the endless belt to be subjected to the intended heat treatment under the microwave applicators. At least one of the microwave applicators for each row of food products has the shape of an elongated wave guide extending along the movement path of the food products and having several slots distributed along said movement path, through which the microwaves may radiate and hit the passing food products. None of the above described known plants are, in practice, suitable for resolving the particular problem having formed a basis for the present invention. The known plants are either too complicated and expensive or imperfect from a functional point of view. An object of the present invention is, thus, to provide a method and a plant for rapidly performing a relatively moderate heat ' treatment of relatively thin separate food products for improvement of their keeping qualities. It is important that the heat treatment can be performed simply and cheaply, i.e. a plant for the performing of the heat treatment should be inexpensive and be very easy to use. For instance it should be simple to feed the food products into the plant. Furthermore, the plant should be usable without costly reconstruction for heat treatment of food products having somewhat different shape and size. For the fulfilment of these demands it is suggested according to the invention that food products, the keeping qualities of which should be improved, are first packed in a material transparent to microwaves and are then by means of an endless belt moved after each other through a heat treatment zone, microwaves being gene¬ rated in at least one elongated hollow wave guide and being caused to propagate in the longitudinal direction thereof. The food products are to be moved by means of the endless belt in a path across the longitudinal direction of the wave guide, and a microwave field is to be provided by means of the wave guide, extending across the whole of the movement path so that the food products in their entirety will pass through the microwave field. In a plant according to the invention said wave guide has at least one elongated slot extending across the whole movement path of the food products. Either, microwaves generated and caused to propagate within the wave guide may be allowed to leak out through such an elongated slot, the food products being moved by the endless belt past the wave guide outside the slot. It has proved possible in such an arrangement to maintain a very stable microwave field in the wave guide and, thereby, to accomplish a well controlled microwave heating of the food products passing outside thereof. Alternatively, the wave guide may have two slots which extend in parallel in opposite walls of the wave guide, the food products being moved by the endless belt in through one of the slots and out through the other. Preferably, a plant according to the invention comprises two or more wave guides, which are placed after each other along the movement path of the food products. Hereby, the temperature may be raised rapidly in the food products without any risk of overheating the surface layers of the food products. If the embodiment of the invention is chosen, in which microwave energy is allowed to leak out through a slot in a wave guide, two such wave guides may be arranged opposite to each other on the respective sides of the part of the endless belt that supports the food products. The endless belt then has to be transparent to microwaves. A plant according to the invention preferably is arranged for heat treatment of the food products such that these are maintained at a temperature in the interval 50-80°C, preferably 50-70°C, during a time period of 5-60 seconds. It has proved that such a heat treatment leads to a doubling of the keeping time possible for products such as sliced ham, sausage and liver paste. With regard to the gradual attenuation of the micro¬ waves, when they propagate into a food product, it is important that the food products are relatively thin. Therefore, their thickness should not exceed 10 mm and preferably be less than 8 mm. The extension of the food products in the plane of the endless belt, or the location of the food products on the endless belt, may be allowed to vary. It is important though that the extension of the food products across the endless belt is not larger than that of the generated microwave field. The invention may be used in a simple manner in connection with existing packing lines, which include for instance a vacuum packing machine. It can also be used in a simple manner in connection with manual packing of food products. The invention is described in the following with reference to the accompanying drawing, in which fig 1 is a block diagram of a plant in which the invention is used, fig 2 shows an apparatus for heat treatment of food products, fig 3 shows a longitudinal section through the apparatus in fig 2, fig 4 shows a cross-section through the apparatus in fig 2, fig 5 shows a combined wave guide/microwave applicator and a microwave generator connected thereto, fig 6 shows a cross-section through the wave guide in fig 5, fig 7 shows two wave guides/microwave applicators of the kind shown in fig 5 and arranged on the respective sides of a belt supporting food products, fig 8 shows an alternative embodiment of a wave guide/microwave applicator, and fig 9 shows a cross-sectional view of the wave guide/microwave applicator according to fig 8 and a food product being moved on a belt through the wave guide/microwave applicator. Fig 1 is a block diagram and illustrates a plant comprising a cutting machine 1 for slicing of sausage, ham or the like, a weighing machine 2 for weighing of the cut slices, a sorting device 3 for providing portions each containing a number of slices, a supplying device 4 and a packing machine 5, e.g. a vacuum packing machine. The supplying device 4 is arranged to receive portions from the sorting device and to place them on predetermined places in the packing machine 5. The plant in fig 1 further comprises an apparatus 6 for heat treatment of the packed portions, equipment 7 for weighing and labelling thereof after the heat treatment and a so called cartoning machine 8. Fig 2 shows a heat treatment apparatus 6. This has an endless belt 9 arranged to move packed food products as illustrated by an arrow 10 through a heat treatment zone in the apparatus. The reference numeral 11 concerns an operating unit for the heat treatment apparatus. Fig 3 and 4 show a longitudinal and cross section, respectively, of the heat treatment apparatus in fig 2. As can be seen, the endless belt 9 supporting a packed food product 12 is arranged to be driven around two rolls 13, 14. Resting on an upper horizontal part of the endless belt 9 the food product 12 is intended to be moved, preferably at a constant velocity, in a path past a number of microwave applicators 15. Fig 3 shows four pairs of microwave applicators (see also fig 7), which are all supported by a common supporting means 16. The supporting means 16 is adjustable vertically in relation to the endless belt 9 by means of separate adjusting means 17. Fig 5 and 6 show a microwave applicator 15 in closer detail. The microwave applicator comprises an elongated wave guide which at one end supports a magnetron 18, i.e. a microwave generator. The wave guide forms an elongated space rectangular in cross-section and confined by four thin surrounding walls. For the nor¬ mally used microwave frequency 2450 MHz the space of the wave guide should have a cross-section of 9 x 4,7 cm. A slot 19 extends along the wave guide through one of its about 9 cm wide surrounding walls. Opposite to this slot the other about 9 cm wide surrounding wall of the wave guide supports a partition 20 which extends up into the slot 19 and along the whole of the wave guide, so that the partition divides the space in the wave guide into two equally sized parts. Each elongated microwave applicator 15 is arranged in the heat treatment apparatus (fig 2-4) such that it extends across the path, in which the food products 12 are intended to be moved by the endless belt 9. The slot 19 is turned towards the movement path of the food products and has a length that is about as large as the width of the endless belt 9. Thereby, all parts of the food products being moved on the belt 9 will pass the microwave applicator opposite to the slot 19 and safely be radiated by microwave energy leaking out there¬ through. Within each wave guide, along which microwaves propagate from the magnetron 18, so called standing waves are generated, and microwave energy leaks out through the slot 19 in the form of a thin layer. When a food products passes through such a layer of microwave energy, i.e. a so called microwave field, successive portions of the food product will be rapidly heated but after that be subjected to cooling by the surrounding atmosphere as soon as they have left the layer. As can be seen from fig 7, two microwave applicators may be placed on respective sides of an endless belt, on which food products 12 are transported. The endless belt as well as the food product packaging material have to be transparent to microwaves. When two microwave applicators are placed in the manner shown in fig 7, it may be suitable to arrange them in a way such that tfie standing waves in one of them are 90° out of phase relative to the standing waves in the other. Hereby, the best conditions are obtained for an even heating effect in the passing food products. Fig 3 shows four pairs of microwave applicators, as illustrated in fig 7, arranged after each other along the endless belt 9. Each of these pairs of microwave applicators may be arranged in one layer of microwave energy to raise the average temperature within passing food products by 20°C. If in connection with such a temperature raise a certain temperature difference would come up between the surface layer and the interior of a food product, this temperature difference will rapidly disappear after the relevant parts of the food product have left the layer of microwave energy. The end temperature after the fourth temperature raise may be in the range of 65-70°C. Fig 8 and 9 show an alternative embodiment of a micro¬ wave applicator. Even this microwave applicator, designated 21, comprises an elongated wave guide. In two opposite walls the wave guide has slots 22 and 23 extending in parallel with each other in the longi- tudinal direction of the wave guide. In this case the endless belt 9 is intended to be arranged to move the food products across and through the wave guide 21, i.e. in through one of the slots and out through the other. Even in a wave guide/microwave applicator of this kind standing waves are generated, forming a microwave field through which the food products are moved. The standing waves are generated such that they do not to a substantial degree leak out through the slots 22 and 23. By means of a plant of the above described kind food products may very easily be subjected to a short duration heat treatment, so that their keeping qualities is substantially improved. If for instance the food products are cut to slices and then are packed manually, the packed food products may be placed without demands on accuracy on the endless belt 9 in a heat treatment apparatus according to fig 2-4. An important matter in connection with the invention is that the heat treatment is performed immediately in connection with the cutting and packaging of the food products.

Claims: Claims 1. A method of improving the keeping qualities of separate food products (12), the food products being first packed in a material that is transparent to microwaves and then moved by means of an endless belt (9) after each other through a heat treatment zone, and microwaves being generated in at least one elongated hollow wave guide (15, 21) and being caused to propagate in the longitudinal direction thereof, c h a r a c ¬ t e r i z e d i n that the food products (12) are moved in a path across the longitudinal direction of the wave guide (15, 21) and that a microwave field is provided by means of the wave guide, extending across the whole movement path, so that the food products in their entirety pass through the microwave field. 2. A method according to claim 1, c h a r a c t e ¬ r i z e d i n that the food products (12) are caused to pass through the microwave field immediately after they have been packed. 3. A method according to claim 1, c h a r a c t e ¬ r i z e d i n that the food products are cut, e.g. sliced, before they are packed. 4. A method according to claim 1, c h a r a c t e - r i z e d i n that the food products (12) are cut, e.g. sliced, immediately before they are packed, and are caused to pass through the microwave field immediately after they have been packed. 5. A method according to anyone of the preceding claims, c h a r a c t e r i z e d i n that the food products (12) are subjected to a heat treatment by means of microwaves such that they are maintained at a temperature in the interval 50-80°C, preferably 50-70°C, during a time period of 5-60 seconds. 6. A method according to anyone of the claims 1-4, c h a r a c t e r i z e d i n that the microwaves are caused to propagate out through a slot (19) in the wave guide (15), extending in the longitudinal direction of the wave guide, and that the food products (12) are moved past the wave guide (15) outside the slot (19). 7. A method according to claim 6, c h a r a c t e ¬ r z e d i n that microwaves are generated and caused to propagate within two wave guides (15), the microwaves being caused to propagate out through slots (19) in the respective wave guides (15) in a way such that they hit from opposite directions food products (12) being moved by means of the endless belt (9). 8. A method according to anyone of the claims 1-4, c h a r a c t e r i z e d i n that the food products (12) are moved through the hollow wave guide (21) across the longitudinal direction of the wave guide. 9. A plant for improving the keeping qualities of separate food products (12), comprising an apparatus (5) for packaging of the food products (12) in a material transparent to microwaves, and an apparatus (6) for heating of the packed food products (12) by means of microwave energy, the latter apparatus (6) comprising an endless belt (9) for moving the food products (12) along a path through a heating zone, at least one microwave generator (18) and at least one elongated hollow wave guide (15, 21) connected thereto, c h a r a c t e r i z e d i n that the elongated wave guide (15, 21) extends across said movement path, the wave guide has an elongated slot (19) extending across the whole of said movement path, the microwave generator (18) is arranged in a manner known per se to generate in the hollow wave guide (15, 21) microwaves having a propagation direction along the wave guide in order to provide a microwave field, and - the endless belt (9) is arranged to move the food products (12) through the microwave field. 10. A plant according to claim 9, c h a r a c t e ¬ r i z e d i n that the endless belt (9) is arranged for moving the food products (12) past the wave guide (15) outside said slot (19). 11. A plant according to claim 9, c h a r a c t e ¬ r i z e d i n that two wave guides (15) are arranged in parallel and opposite to each other on the respective sides of the endless belt (9), the slots (19) of the wave guides (15) facing the endless belt (9). 12. A plant according to claim 9, c h a r a c t e - r i z e d i n that the wave guide (21) has two slots (22, 23) extending in parallel in opposite walls of the wave guide (21), and that the endless belt (9) is arranged for movement of the food products (12) in through the one slot (22) and out through the other slot (23).

Inventors: NOREN TOMMY, NOREN, TOMMY
Applicants: ALFA LAVAL FOOD ENG AB, ALFA-LAVAL FOOD ENGINEERING AB
Application year: 1991
Main classification: H05B6
IPC codes: A21B2, A23L1, H05B6