Device and method for chilling or deep-freezing food products by impacting jets   

The present invention relates to industrial devices and methods for chilling or deep-freezing food products.

The chilling of food products generally takes place by convective exchange between a cold gas and the products. The use of cryogenic fluids and fans is already known from manufacturers who wish to deep-freeze foodstuffs.

Deep-freezing or chilling technology is based on the fact that the products are chilled more quickly if they are subjected to impacting jets of a cryogenic fluid.

These impacting jets are created by pressurizing the area above a perforated plate, causing an increase in the speed of the fluid at the holes of the plate. On the industrial scale, these devices take the form of tunnels in which the pressurization above a perforated plate is created by a means for circulating the gas, for example a centrifugal fan, located above the belt on which the food products to be chilled are arranged.

However, the presence of humidity in the enclosed space in which the chilling takes place leads to the formation of frost and to snow deposits when the temperature of the cold gas is below 0° C. In cryogenic tunnels, the temperature can reach −130° C.

The humidity arises from the products which can become partially dehydrated by the presence of humid air mixing with the cold gas, in particular in the case of open-ended machinery such as tunnels.

When the method of convection is based on the speeding up of the cold gas when it passes through holes (which may be circular, oblong, rectangular, etc.), the deposits of snow or ice crystals tend to occur on the edges of these holes. Obstruction of the holes to a greater or lesser degree results therefrom, and they can even become completely blocked. Reducing the flow cross-section of the holes modifies the flow-rate characteristics of the cold gas, which adversely affects the chilling process and can even cause malfunctions.

Patent application EP-1 621 830 discloses a means for overcoming the frosting phenomenon by using mechanical vibrations with the aid of a vibrator mounted on the plate that has the holes. Other patent authors use heating systems to periodically defrost frosted surfaces.

However, these means consume a lot of energy and are difficult to implement.

The object of the present invention is a device and a method for chilling food products, equipped with means for preventing the clogging-up with frost of the holes forming the impacting jets, without the use of mechanical vibration or heating.

To this end, the invention proposes a device for chilling food products by impacting jets which consists of a tunnel comprising a moving belt, at least one plate located opposite (above or alternatively beneath) said moving belt and provided with through-holes, and according to one of the preferred embodiments at least two plates, an upper plate and a lower plate, located parallel to each other on either side of said moving belt, at least one plate, and preferably both plates, being provided with through-holes, and at least one means for circulating a cold fluid, said tunnel having a zone under pressure P1 above the upper plate or plates and/or beneath the lower plate or plates, and a zone under pressure P2 between the plate or plates and the moving belt, the pressure P1 being greater than the pressure P2, characterized in that it includes means that allow the pressure to be varied in the zone under pressure P1 and/or means for cooling the plate or plates down to a temperature below about −80° C., preferably below −90° C., and more preferably still below −100° C., and for maintaining that temperature.

The cold fluid can be cold air obtained by mechanical refrigeration or alternatively a cryogenic fluid.

The cold fluid is preferably a cryogenic fluid chosen from the group including, in particular, nitrogen, carbon dioxide, oxygen, or air, and mixtures thereof.

The chilling device according to the invention is preferably a device for deep-freezing food products. Deep-freezing is a means of freezing food very quickly. This is the technique of choice on the industrial scale when the foodstuffs are flat or small in size, such as, for example, beefburgers, pizzas or diced bacon. The heat-exchange surface area of these foodstuffs is large, and the thickness small, which favors rapid freezing.

In the device according to the invention, the plate or plates provided with through-holes are advantageously made from food-grade stainless steel. These plates can be inclined and disassembled to make it easier to clean them after operation.

Purely by way of example, the through-holes of the plate can have different shapes and, in particular, can take the form of cylinders, circles, elongated holes, trefoils or even cones with beveled or rounded edges. The plates can be flat, V-shaped or even corrugated.

In a conventional but non-restrictive manner, the means for circulating the fluid is a centrifugal fan driven by a motor.

The means for varying the pressure in the zone under pressure P1 and/or the means for cooling the plate or plates down to a temperature below −80° C. and for maintaining that temperature make it possible to prevent the clogging-up with frost of the holes in the plate without making the system more expensive in terms of energy.

According to a particular embodiment of the invention, this device is characterized in that the means for cooling the plate or plates down to a temperature below −80° C. and for maintaining that temperature are chosen from the group including a heat-exchange circuit attached to the plate, a plate that serves as a heat exchanger (also referred to as a plate/heat exchanger), a cold fluid bath on top of the plate or plates, and combinations thereof.

Surprisingly and paradoxically, the inventors have found that when the temperature of the plate is lowered to a temperature below −80° C., the ice crystals do not adhere to the edges of the holes, thus preventing the holes from becoming clogged up.

These cooling means are economical. Indeed, when the cooling of the plates is effected by a heat exchanger, and according to a preferred embodiment of the invention, the cryogenic fluid passing through the heat exchanger is taken from the circuit for supplying the chilling machinery. The cryogenic fluid contributes directly or indirectly to the cooling of the fluid that is in contact with the products.

These cooling means are easy to implement: the heat exchanger attached to the plate can be a tube fixed to one or both faces of the plate and within which a cold fluid circulates. The heat exchangers preferably have a form that allows them to pass as closely as possible to as many holes as possible, and thus to favor the cooling of the plate around as many holes as possible.

According to a particular embodiment of the invention, the heat exchanger attached to the plate has one or more passes required for cooling as many through-holes as possible. Another embodiment consists in using a material which is a good conductor of heat for the plate, or which would be in contact with the plate in order to cool the edge of the holes. This material would be cooled locally by a cooling circuit.

Furthermore, this heat exchanger attached to the plate has a form and a connection to the plate such that heat exchange is favored. By way of example, the use may be envisaged either of a weld of a form and made from materials that a person skilled in the art will be capable of defining and/or of a thermal compound.

It is preferable that the plate is cooled optimally at the through-holes and for as many holes as possible. The plate/heat exchanger can thus be formed by two plates that may or may not have baffles, for better distribution of the cold fluid. The use of baffles makes it possible to prevent the fluid following preferred channels, and thus promotes the cooling of as many holes as possible.

According to an embodiment of the device according to the invention where a bath of cold fluid is used as the means for cooling the plate down to a temperature below −80° C. and for maintaining that temperature, the plate is equipped with edges that are sufficiently high, with tubular holes and with a means for regulating the level of the bath. Such a device can make it possible to prevent the cold fluid from overflowing through the through-holes and over the edges of the plate.

According to a particular embodiment, and with the aim of further reducing the energy costs, the device can be designed in such a way that the fluid which is stirred within the zone under pressure P1 is cooled only in contact with the cold plate. In this embodiment, the fluid arrives in the zone under pressure P1 at an ambient temperature T1 and cools to a temperature T2 when it passes through the through-holes of the plate which is at a temperature T3 below −80° C., T2 lying between T1 and T3. The means for cooling the fluid can thus be the means for cooling the plate.

The means for cooling the plate down to a temperature below −80° C. and for maintaining that temperature have the advantage that they are easy to implement, in particular on existing and inexpensive cooling devices, as a person skilled in the art can simply swap conventional perforated plates with perforated plates provided with heat exchangers.

According to another aspect of the invention, the device is characterized in that the means for varying the pressure are chosen from the group including frequency inverters and pressurized storage tanks.

Varying the pressure in the zone under pressure P1 can be effected, in a particular embodiment of the invention, by varying the circulation rate of the centrifugal fan. If the circulation rate of the centrifugal fan increases, the pressure in the zone under pressure P1 increases. In this case, to obtain a variation in the pressure in the zone under pressure P1, the speed of the motor driving the centrifugal fan is varied according to controlled proportions.

Another way of generating a variation in the pressure in the zone under pressure P1 is to intermittently supply an additional quantity of pressurized fluid to the zone under pressure P1. This fluid can come from a pressurized tank connected to the zone under pressure P1 by means for releasing the pressure of the tank which are, for example, valves. The opening and closing of these releasing means are activated so as to generate the desired variation in pressure in the zone under pressure P1.

These two means for varying the pressure in the zone under pressure have the advantage of being able to be used on existing machinery for chilling foodstuffs.

The present invention also proposes a method for chilling food products by impacting jets, in which a fluid is cooled, a pressure P1 is applied to the fluid, the fluid is forced to pass through holes in a plate and to impact the food product to be chilled, characterized in that the temperature of the plate is maintained at a temperature below about −80° C., preferably below −90° C. and more preferably still below −100° C., and/or in that the pressure P1 of said fluid is varied over time.

According to a preferred embodiment of this method, the cold fluid is a cryogenic fluid chosen from the group including, in particular, nitrogen, carbon dioxide, oxygen, air, and mixtures thereof.

The chilling method according to the invention is advantageously a deep-freezing method.

The variation in the pressure P1 is preferably short and with a small amplitude, which has the advantage of being inexpensive in terms of energy and not requiring any significant modification to the chilling device.

The variation in pressure is preferably a notched sequence, the amplitude of which is a pressure difference ΔP between a normal operating value and a selected set value that is lower or greater than the normal value, said pressure difference ΔP lies between 10 and 1000 Pa, in particular between 200 and 800 Pa and preferably between 400 and 600 Pa, and the time for which the variation in pressure is applied Δt lies between 1 and 60 s, in particular between 2 and 30 s and preferably between 5 and 15 s.

The method of the invention is preferably implemented with the device described above.

This variation in pressure can be achieved by varying the rate at which the cold fluid is circulated in the zone P1 of the device.

This variation in pressure can thus be caused by the brief and sudden release of pressure coming from a pressurized tank connected to the zone under pressure P1. This release of pressure takes place when a release means such as, for example, a valve is opened. The opening and closing of this pressure release means can be automated.

The method according to the invention has the advantageous feature of being able to be applied to existing chilling or deep-freezing devices.

Other features and advantages of the invention will become apparent on reading the following description. This description is purely illustrative and must be read in conjunction with the attached drawings, in which:

FIG. 1 is a cross-sectional view of a chilling tunnel,

FIG. 2 is a photograph of the underside of a lower plate after the chilling device according to Example 1 has been operating,

FIG. 3 is a drawing of part of the chilling device, of the plate used in Example 2,

FIG. 4 is a photograph of the top side of the lower plate after the device according to Example 2 has been operating,

FIG. 5 is a diagrammatic drawing of the operating mode used in Example 3,

FIG. 6 is a photograph of the top side of the upper plate after the device according to Example 3 has been operating,

FIGS. 7a, 7b, 8, 9a and 9b are drawings of means for cooling the plate according to particular embodiments of the invention.

FIG. 1 is a cross-sectional view of a device for chilling by impacting jets according to the prior art. This device consists of a tunnel 1 within which is arranged a moving belt 2 which makes it possible to move the foodstuffs to be deep-frozen or to be chilled, which are arranged on said belt, from one end to the other of said tunnel 1. Plates 3 are present parallel to each other on either side of said moving belt. These plates 3 are provided with through-holes 4 that allow a fluid to pass through. Two zones can be distinguished within the tunnel 1. The first zone 5, termed the zone under pressure P1, is defined by the walls of the tunnel 1, the top side of the upper plate 3a, the underside of the lower plate 3b and pressure zone separating elements 6. The second zone 7 is termed the zone under pressure P2 and is defined by the walls of the tunnel 1, the underside of the upper plate 3a, the top side of the lower plate 3b and the separating elements 6. A centrifugal fan 8 is placed in the upper part of the tunnel 1 and within the zone 5 under pressure P1. Its role is to circulate the fluid within the zone 5 under pressure P1. The centrifugal fan is driven by a motor 9 preferably positioned outside the tunnel 1. Furthermore, a cold fluid feed 10 is provided in the zone 5 under pressure P1.

When the device in FIG. 1 is operating, the cold fluid that is supplied by the cold fluid feed 10 is stirred by the circulation means (centrifugal fan 8 and motor 9). This circulation generates a pressure P1 in the zone 5 under pressure P1 which subsequently forces the cold fluid to pass through the through-holes 4 of the plates 3 at a relatively high speed. At the outlet of the through-holes 4 the fluid takes the form of impacting jets that subsequently impact the foodstuffs arranged on the moving belt 2. Once it has impacted the foodstuffs present on the moving belt, the cold fluid then escapes via the zone 7 under pressure P2 which is open to the outside. Of course P1 is greater than P2 during the operation of the device, and P1 is equal to P2 when the device is not running. If the tunnel 1 is located in a room at atmospheric pressure, then P2 is also atmospheric pressure.

When the above-described device is operated in the presence of humidity supplied by a vapor generator such as that described in Example 1 below, it is observed, as can be seen in FIG. 2, that the through-holes 4 of the plates 3 are clogged up by the formation of frost. This phenomenon is representative of the actual phenomenon that occurs when food products are chilled or deep-frozen on an industrial scale. The residual humidity present in the tunnel and the humidity from the foodstuffs cause the holes to become clogged up, which means that the operators have to halt the device and clean the plates and holes.

The invention proposes to attach means for preventing such clogging-up to the known device for chilling foodstuffs.

In a first embodiment of the invention, such means can take the form of means for cooling the plate down to a temperature below −80° C. and for maintaining that temperature, as illustrated in FIG. 3 and in Example 2 below. FIG. 3 shows a drawing of a heat exchanger 11 attached to a plate 3 with the aim of maintaining the temperature at the through-holes 4 below −80° C. This heat exchanger 11 is provided with a cold fluid inlet 12 and a cold fluid outlet 13. The heat exchanger has several passes 14 in order to optimize heat exchange.

FIGS. 7a and 7b show other drawings of heat-exchange devices attached to a plate. The cold fluid is caused to circulate in proximity to the through-holes 4 of the plate 3 in a cold fluid circuit 15. In these figures it can be clearly seen that the pipe is fixed on the plate very closely to the edge of the holes and that its cross-section is such that it allows maximum heat exchange with the plate.

FIG. 8 shows another embodiment of the means for cooling the plate down to a temperature below −80° C. and for maintaining that temperature. In this figure, a plate 3 with high edges 16 and whose holes 4 are preferably tubular is arranged so as to create a cold fluid bath 17. The cold fluid is thus unable to pass through the through-holes 4.

A third embodiment of the means for cooling the plate down to a temperature below −80° C. and for maintaining that temperature is illustrated in FIGS. 9a and 9b. These figures are drawings of a heat exchanger incorporated into the plate. This heat exchanger can be formed by two plates joined together at the through-holes 4. A space between the two plates 15 permits the circulation of the cold fluid that enters through a feed end 12 and that leaves through an end 13 for extracting the cold fluid.

A device such as that described in FIG. 1 is used, which has: a moving belt, two perforated plates, an upper one and a lower one, that have circular through-holes with an 18 mm diameter and spaced 104 mm apart, a distance of 7 cm between the plate and the moving belt, a centrifugal fan driven by a motor, gaseous nitrogen at −70° C. as the cryogenic fluid, supplied by a ramp located in the zone under pressure P1, a water vapor generator equipped with a pipe, one end of which is fixed to the center of the belt and directed toward the gas recovery zone in order to artificially generate frost.

The vapor generator runs for about 4 hours from the beginning of the operating cycle.

The temperature of the tunnel is maintained at −80° C. During operation, the pressure in the zone under pressure P1 is measured by means of a manometer or pressure sensor placed in the zone under pressure P1 of the lower plate and variations in the pressure curve are observed, indicating partial blocking of the holes. After 4 hours of operating, the plates are photographed (lower plate seen from below in FIG. 2) and it is observed that the plates are completely blocked by snow and that there is ice in the center of the holes.

The device in Example 1 is used, modified by fixing a heat exchanger to the underside of the lower plate, which makes it possible to maintain the temperature of the latter in the region of −100° C., a few centimeters away from the heat-exchange tube during operation.

This heat exchanger, which is shown in FIG. 3, consists of two copper tubes with a 14 mm diameter which are mounted in hairpin fashion over two sets of 5 holes and welded to two copper plates that are themselves fixed to the underside of the lower perforated plate. Thermal compound ensures a better contact.

Liquid nitrogen at approximately −187° C. is passed through the heat exchanger, allowing the plate to be cooled down.

The holes cooled by the heat exchanger are the holes in the rows 4 and 5 and the lines 2 to 6 indicated in FIGS. 3 and 4.

The vapor generator is run throughout the operating cycle, i.e. for 4 hours.

The pressure P1 is maintained at 550 Pa and the temperature in the tunnel at −60° C.

After 4 hours of operating, the top side of the lower plate is photographed. The resulting photograph is shown in FIG. 4. In this photograph it can be observed that the holes of the lower plate closest to the heat exchanger are not blocked. These are the holes in the rows 4 and 5 and the lines 2 to 6, i.e. the holes whose edges are cooled by the heat exchanger.

The device in Example 1 is used and the pressure is varied over time in the sequence shown in FIG. 5. The variation applied is in the form of notches, the amplitude ΔP of which between a normal pressure value PN and a high pressure value PH is 500 Pa. The time for which the zone under pressure P1 is subjected to the pressure PH is Δt which corresponds to 10 s. The time between two notches is 5 minutes in FIG. 5.

PN is chosen to be equal to 550 Pa and PH to be equal to 1330 Pa.

The temperature of the tunnel is maintained at −60° C. The vapor generator is run throughout the trial, i.e. for 4 hours.

After 4 hours of operating, the top side of the upper plate is photographed (FIG. 6) and it is observed that a considerable amount of snow has accumulated on the plate but that all the holes have remained free.