Such a method is generally known in the prior art and is used for frying potato slices and to an increasing extent also for fruit and vegetable slices, in other words, food slices in general. Efforts are being made to obtain a product with an acceptable flavour, where in certain markets a low fat percentage is desired.

It is known in the prior art to carry out the frying process at reduced pressure in a vacuum vessel, which makes it possible to work with a lower fat temperature. On the one hand, this extends the service life of the fat and, on the other hand, the absorption of fat is limited. It is known in the prior art to subsequently subject crisps fried in such a way to a mechanical vibration treatment or the like outside the vacuum vessel used in the process, in order to remove the last of the fat from the slices. Steam is added in the process. One problem here is that the mechanical energy supplied to the slices must be limited. The fact is that if this energy supply is too great, the brittle, hardened, just baked crisps will break, which is not desirable. This means that in practice only the outside of the crisps is heated and fat removed from them there.

If, for example, potato crisps are being fried, it is possible to achieve a fat percentage of approximately 24% using such a method.

It is the object of the present invention to reduce such a fat percentage further, or to limit the fat consumed.

This object is achieved with a method as described above in that the removal of fat is carried out by a mechanical operation at reduced pressure. Such reduced pressure can comprise 100 mbar.

It has been found that the crisps still under reduced pressure in the vacuum vessel or device connecting thereto are still relatively soft. Hardening by means of the fat occurs only when they are removed from the vacuum vessel and the temperature of the crisps is reduced. In vacuum conditions it is hardly necessary, or not necessary at all, to supply additional energy in order to prevent a considerable fall in temperature from

occurring. In other words, during the mechanical treatment in the vacuum vessel the crisps are still at a relatively high temperature. It has been found that in those conditions the mechanical strength of crisps is considerably higher. This means that the mechanical treatment can be considerably more intensive, which results in a higher release of fat, with the result that the product finally obtained can acquire a lower fat percentage. A percentage of less than 17% is easily achievable, and percentages down to 10% have been found achievable in practice. This is also a result of the fact that it is not only the fat near the outside of the product that is removed, but likewise the fat inside the product. Owing to the process used, the pores of the crisps or other product are not yet closed, and the product is also still at the raised temperature over its full thickness, so that fat can also come out from the inside.

Such a more intensive fat removal treatment can comprise centrifuging. If crisps prepared according to the prior art were subjected to centrifuging treatment outside the vacuum vessel, they would be smashed to pieces. However, it has surprisingly been found that, by carrying out this centrifuging treatment at low pressure and preferably directly following the frying operation, negligible breakage occurs, but that, on the other hand, the fat percentage can fall considerably.

The selected reduced pressure depends, of course, on many different circumstances, including the type of product, the fat used, the desired residence time, the thickness of the crisps etc.

The process described above can be carried out continuously, but according to an advantageous embodiment of the invention it is carried out batchwise. This applies, of course, in particular to the centrifuging operation.

The degree of centrifuging and the time likewise depend on the mechanical load that the product will take, the desired final fat content and the available centrifuging time. As an example, the centrifuging is carried out with a drum which has a diameter of approximately 50 cm and a speed of revolution lying between 200 and 600 rpm. The centrifuging time is preferably approximately 30-50 sec at a fat percentage of 20- 16%.

It has been found that the frying of crisps can be improved in the optimum way if the products are not placed immediately in the fat bath in the vacuum vessel, but are first subjected exclusively to the vacuum conditions for some time. This time can be brief. If the crisps move over a height of less than half a metre in the vacuum vessel

before they come into contact with the fat, sufficient residence time is created. During this movement a sort of implosion occurs, in other words, the pores of the crisp open up. The external structure of the crisp changes radically, with the result that a different, more pleasant flavour and bite are obtained after frying. Here again, all this depends, of course, on the product to be fried, the fat used, and the thickness of the slices. It has been found that in the case of potatoes being subjected to such a prior vacuum treatment, contrary to what is known, the pores do not seal immediately when they come into contact with the fat bath.

It is assumed that the removal of fat from the product is also promoted as a result.

The invention also relates to a device for frying products such as slices in fat, comprising a vacuum vessel provided with a container for fat, supply means for said products, discharge means for said products, and removal means for water vapour, centrifuging means being present between said container means for fat and said discharge means. If the process with centrifuges is batchwise, it is preferable to use at least two centrifuges, with one centrifuge in operation and the other being in the process of being charged or unloaded. Furthermore, it is possible for the centrifuges to be rotating already during the charging operation. The centrifuges can be designed in such a way that they are self-unloading. In other words, by means of a low speed of rotation and a centrally provided opening, the crisps fall down by themselves. After the centrifuges, the crisps leave the vacuum vessel. Sluice areas can be present for this purpose. The sluice areas may or may not contain the centrifuges. The inlet is, of course, also provided with sluice means. If necessary, the vacuum released at the outlet can be used at the inlet.

Means for regulating the residence time of the crisps as uniformly and as accurately as possible can be present in the vacuum vessel. Combinations of conveyor belts and rotating wheels in the fat bath can be present for this purpose. Likewise, a separate belt, which forces the crisps into the fat, can be provided.

In the course of frying many foods, owing to the increased temperature of the fat, considerable quantities of water vapour are produced at the time of entry into a vacuum vessel. This increases the pressure in the vessel. In the prior art it is proposed that a large number of heavy vacuum pumps be used in order to remove this water vapour and maintain the vacuum.

It is proposed according to the invention that the water vapour be condensed in the vacuum vessel. This means that it is not necessary to use heavy vacuum pumps.

Such condensation can be produced by compressing the water vapour slightly using simple fans and guiding it along a cooled surface. It has been found that condensation occurs in the process. Of course, this measure can also be applied in the case of other vacuum installations known in the prior art.

The invention will be explained in greater detail below with reference to an exemplary embodiment illustrated in the drawing, in which: Fig. 1 shows the device according to the invention diagrammatically in longitudinal section; and Fig. 2 shows cross section II-II according to Fig. 1.

The device according to the present invention is indicated by 1 in the figures.

This device is composed of a vacuum vessel 2 of considerable length. The latter is provided with an inlet sluice construction 3 and an outlet sluice construction 4. Fat is present and is indicated by 5. This fat can be stored in a separate container. The fat level is kept constant by means of a fat storage container 8 and a fat supply line 9. A circulation system for fat (not shown in any further detail), in which the fat is filtered and heated up, is present. For a fat customarily used for frying potato crisps, an optimum temperature is lower than approximately 136°C.

Reference numeral 6 indicates a conveyor belt which moves through the liquid fat. Metering/control rollers, which accurately regulate the residence time of the product, are indicated by 7. A further conveyor belt is indicated by 16, and the arrow indicates that the latter is designed in such a way that it presses the products below the fat level. An elevator (extension of conveyor 6), which interacts with conveyor 18, is indicated by 17. This elevator is provided with a valve system (not shown in any further detail) for supplying the crisps present thereon either to centrifuge 19 or to centrifuge 20. In this case the centrifuges can be placed at an angle, in order to make as much use as possible of the space in the vacuum vessel. Sluice chambers 21 and 22, which can be shut off by means of valves 23, are present.

A vacuum pump is indicated by 13. A branch line for water vapour is indicated by 10. It contains a pump 11 in the form of a fan. The fans used can be so-called Rootes blowers. Reference numeral 12 indicates a condenser, which is connected to a further external heat exchanger 15. A collection tank for water is indicated by 14, which

collection tank can be isolated with respect to the vacuum in a manner not illustrated in any further detail, or which can be drained in some way.

The device described above functions as follows: The slices, such as potato slices, to be fried are introduced through the inlet sluice 3. During this process they fall downwards several tens of centimetres before reaching the fat level 5. During this fall the pores of the material of the slices are opened by the vacuum and the evaporating water. The pores do not seal immediately when the slices come into contact with the fat 5. The oil temperature is preferably 120°C. The product is then conveyed on belt 6, the residence time being accurately regulated by the rollers 7. A maximum residence time of approximately 360 seconds is mentioned as an example. All this depends, of course, on all kinds of characteristics, in particular the thickness of the crisps and the temperature of the fat. The crisps are pressed below the fat level by means of belt 16. The crisps are then subjected to a centrifuging treatment in centrifuges 19 and 20. The centrifuges have a diameter of 50 cm and preferably a speed of revolution of approximately 400 revolutions per minute. The centrifuging treatment is maintained for a maximum of 1 minute. The centrifuges are provided on the underside with a valve which can be remote-controlled (not shown in any further detail). This means that the product can be discharged towards the underside and subsequently falls down into sluice area 21,22 respectively. With such a construction it is possible to reduce the fat percentage of crisps without further special actions to approximately 12 per cent (by weight).

By means of the device described above, it is possible to subject the crisps to a full frying treatment and treatment for the removal of fat in the course of an extremely short residence time. It has been found that the temperature of the crisps hardly rises above 60° during the frying, in view of the brief residence time. Moreover, the temperature of the crisps hardly falls after frying during the conveyance along belts 17 and 18 and supplying to the centrifuges 19 and 20. Drops in temperature of approximately 5°C have been found. This means that the. crisps remain relatively soft and have great mechanical strength, in other words, they will not readily break during the centrifuging treatment. Furthermore, the vacuum hinders rapid ageing of the fat used in the system and prevents fat from solidifying in the crisps.

Although the invention is described above with reference to a preferred embodiment, it must be understood that numerous modifications can be made thereto.

Adaptations can be made, depending on the product to be prepared and the specifications set for the product. Of course, it is also possible to carry out the various steps in separate vacuum vessels connected to each other in some way. These and similar modifications immediately spring to mind for a person skilled in the art on reading the above description, and lie within the scope of the appended claims.