Technical Field

[0001] The present invention relates to freezing and thawing of seafood, especially fish. According to the invention, it comprises an arrangement where the seafood is frozen or thawed while submerged in brine.

Background Art

[0002] Brine freezing has been known at least since the 1930 ^th . This freezing principle is based on the fact that brine freezes at a substantially lower temperature than freshwater. In a saturated solution of water and salt, a freezing point as low as -21 °C can be obtained. Since fresh seafood contains far less salt, the seafood will have freezing point that lies remarkably higher than a high salt content brine. Consequently, submerging fresh seafood in in cold brine will freeze the water inside the seafood while the brine surrounding the seafood is still liquid. After being submerged for a sufficient time to freeze the water inside the seafood, it can be brought out of the brine and brought to a freezer for storage.

[0003] The above principle of freezing will bring the seafood down to the temperature were it is stored, which is typically around -18°C, much faster than if the seafood is placed directly into the storage where it is surrounded by air only. Moreover, the traditional freezing where the fish is placed in crates and transported through a freezer tunnel, a substantial share of the fish will be deformed to curved shape, so-called “banana fish”, due to the fish freezing unevenly. Such deformed fish is considered an inferior product. It is both more difficult to fillet after tawing and the fillets are misshapen.

[0004] It has been found that if the fish is frozen by submerging it in brine, the problem of “banana fish”, i.e. fish that gets a bent shape after freezing, is practically eliminated. It has also been found that brine freezing provides a slightly better quality of the fish after an extended storage in frozen state.

[0005] Some salt will be taken up by the fish when it is frozen in brine, but the amount is small and acceptable. [0006] Moreover, the energy consumption is more than halved compared to tunnel freezing.

[0007] There are several solutions known in the art for brine freezing. An early example is shown in GB342896, which shows a continuous freezing arrangement where fish is fed into a reservoir with a rotating drum. At the bottom half of the reservoir is a cold refrigerating liquid. The fish is brought slowly through the refrigerating liquid by the drum and lifted out to an opening from the reservoir. After exiting the opening, the fish is washed.

[0008] The rotating drum has to rotate very slowly for the fish to get a sufficient retention time in the refrigerating liquid bath. This means that it will take a substantial time to get the fish through the machine. The only possible way to increase the capacity, is to provide a very large reservoir and drum. In addition, the arrangement is complicated with many moveable parts. Movable parts with bearings and salty conditions is not a very good combination.

[0009] US 2909040 describes a freezer arrangement on board a fishing vessel. The arrangement comprises a tank into which brine is sprayed. Sea water is used to keep the tank cold and the fish is frozen to a few degrees below freezing. The fish stays at the bottom of the tank.

[0010] This system is not suitable for continuous freezing. It is only capable of achieving a temperature of the fish slightly below freezing and the fish is kept in the brine for a prolonged time, which increases the salt uptake of the fish.

[0011] US 5797271 describes a freezing arrangement mounted in a trailer. It has a tank filled with brine. The fish is fed into the tank and as it freezes it will float to the surface of the brine. A transport belt is submerged in the brine at the upper part of the tank. The frozen fish meets this transport belt and is transported out of the tank.

[0012] This solution is similar to a belt transport device that has been tested by the present applicant. It was found that such a transport belt had too little capacity. Moreover, moving parts with necessary bearings is not a good combination with salty environments. [0013] WO 2020202224 describes a freezer arrangement comprising a spiral pump pumping fish and cold brine therethrough. The brine is recirculated, purified and cooled through a separate process arrangement.

[0014] It seems that this arrangement is depending on a height difference between the inlet and the outlet of fish and brine. The spiral pump also has to be of a substantial size for the capacity to be sufficient.

[0015] W02020031215 describes an arrangement for freezing fish in brine. It comprises a tank with a horizontal rotating helix that forces the fish along its length.

In a horizonal arrangement, the fish will float upwards to the top of the tank as it freezes. Here it will be led along the upper wall of the tank. This means that the capacity of the tank will be poorly utilized, and the temperature will be higher near the top of the tank. This results in a prolonged time for the fish to reach the desired temperature.

[0016] WO20202223 also describes an arrangement having a tank with a horizontal rotating helix. It is stated that It can be used to thaw food. It will have the same disadvantages as other arrangements having a tank with a horizontal helix.

[0017] US20040020749 describes an arrangement with a tank containing two vertical transporters. One of these transporters transport the food downwards while the other transports the food upwards again. Although it is a little unclear, it seems that trays containing food items are transferred from the downward transporter to the upward transporter. For this system to work, the food items will have to be of a type that will lie still on the trays during the transport. Seafood in brine will not lie still but will float upwards in the brine as it freezes. Consequently, this arrangement is not suitable for freezing seafood.

[0018] It has been found that batch freezing is not a viable solution. When a fishing vessel makes a large catch, it is necessary for the fish to be frozen as quickly as possible to maintain the quality. Batch freezing forces a large portion of the fish to wait for the previous batches to finish. It also prevents the crew from working continuously. Summary of invention

[0019] It is an object of the present invention to be able to quickly freeze the fish that has been brought on board the vessel. A continuous operation of the freezing arrangement is therefore a condition. The arrangement also must have a sufficient capacity to feed the fish through the freezing process without the last fish having to be kept at ambient temperatures for any substantial length of time.

[0020] It is also an object of the invention to be able to control the time the seafood is kept in the brine, to ensure that it obtains the desired temperature.

[0021] The invention is taking advantage of the effect that the fish, as it freezes, will gain buoyancy in the brine. The invention aims to control the speed of ascend of the fish so that it can be ensured that the fish is completely frozen and has reached the desired temperature before it reaches the surface.

[0022] The above objective is achieved by an arrangement comprising a freezing or thawing tank containing brine, said tank having a helical baffle arranged therein, said baffle forcing said seafood to follow a helical path along the height of the tank.

[0023] The main objective of the present invention is to enable rapid freezing of the raw seafood. This is in particular important for pelagic trawlers, which receives a large quantity of fish during the high season.

[0024] An additional objective of the present invention is to enable both rapid freezing of the fish and subsequent thawing of the fish in small batches during the following time in order to create an even production of fillets in the time between the high seasons. The invention also provides a solution to this need.

[0025] Moreover, it is a desire to be able to perform grading directly when the fish is received and fillet some of the higher graded fish directly. Before filleting, the fish has to be cooled down to, e.g., about -2,7°C. The invention also provides a solution for this procedure.

Brief description of drawings

[0026] The invention will now be described in further detail, referring to the enclosed drawings, in which:

Figure 1 shows the process arrangement of the invention in freezer mode, Figure 2 shows the process arrangement of the invention in thawing mode, and

Figure 3 shows an extended process arrangement with the possibility to fillet some of the fish directly.

Detailed description of the invention

[0027] When in the previous and following description, the term fish is used purely as an example of seafood. It should be understood that the seafood may be any type of seafood that is suitable for freezing. When filleting is discussed, this indicates any treatment of the seafood where the is gutted, sliced, partitioned, skinned or otherwise treated by cutting tools.

[0028] Brine in the context of the present invention covers all mixtures of water and salt that are liquid at the temperature at which it is desired to freeze the seafood. Any type of salt or mixture of salts may be used that are considered safe to use in contact with food products.

[0029] All temperatures given are examples. These can be departed from as long as the quality of the seafood is substantially retained. It is within the ambit of the person of skill in the field to select suitable temperatures depending on the type of seafood.

[0030] Figure 1 shows a principle layout of the process arrangement of the invention in freezer mode. It comprises a buffer tank 1 with a fish inlet 2. The tank 1 is filled with brine and an outlet 3 of the tank is coupled to a pump 4. The pump is conveniently a coil pump, such as the one sold under the name RID Fish Pump by the present applicant. In this pump type a tube has been wound into a helix that is set to rotate about its longitudinal axis. The fish and brine are thus drawn along the hose from the pump inlet to the pump outlet while a suction is created at the inlet.

[0031 ] The outlet of the pump 4 is coupled to a further tank 5, which is also filled with brine. Preferably, the further tank 5 has a circular horizontal cross section with a diameter that is slightly larger than its height, such as a diameter of 4,4 m and a height of 4 m.

[0032] Within the tank 5 a helical baffle 6 is arranged. The helical baffle is preferably a continuous screw having a vertical axis. The helical baffle 6 forces the fish to move along the baffle, i.e. in a spiral movement. The baffle 6 may have perforations to let brine therethrough, but any openings are too small to let fish therethrough. [0033] The pitch of the baffle may be selected according to the desired travel time of the fish along the baffle. A steeper pitch will give a shorter travel time and a shallower pitch will give a longer travel time, provided all other conditions are the same.

[0034] The further tank 5 has an outlet 7 close to the bottom of the tank 5. A brine pump 8 is coupled to the outlet 7. The pump 8 outlet is coupled to a heat exchanger 9. The heat exchanger may be a part of a refrigerated sea water system (RSW) that works like a heat pump to exchange heat with seawater. Brine that has been cooled by the heat exchanger 9 is fed into the further tank 5 through a first lower inlet 10 and a second upper inlet 11. The upper inlet 11 may be at the very top of the tank 5.

[0035] The tank 5 has a fish outlet 12 above the helical baffle 6 close to the top of the tank 5. From here fish and some brine is fed into a draining chamber 13. The brine is drained from the fish in the chamber 13 and conducted to the buffer tank 1 , such as to the fish inlet 2. The now frozen fish is conveyed out of the draining chamber 13 and transported to a cold storage (not shown) where it is kept at freezing temperatures.

[0036] In the above arrangement the brine is cooled to a desired temperature in the heat exchanger 9. A convenient temperature is about -20°C. The cooled brine is fed into the further tank 5 where a downward flow is created. The downward flow is weak enough to allow the fish in the tank 5 to rise upward due to buoyancy.

[0037] As the brine meets the fish it is heated slightly, such as to a temperature just above -18°C when it leaves the tank at the outlet 7. As the temperature difference between the inlets 10, 11 and the outlet 7 is small, it takes little energy to cool the brine down again to -20°C.

[0038] Some of the brine will leave the tank 5 with the fish and enter the draining tank 13. This brine heats up somewhat and enters the buffer tank at about -18°C from where it is fed back to the further tank 5 through the fish and brine pump 4.

[0039] The amount of brine in the system is adjusted so that the buffer tank 1 , pump 4 and further tank 5 are substantially filled with brine during the whole process.

[0040] The fish, which at the time it comes on board the ship has a temperature similar to the sea temperature, is fed into the buffer tank where it meets the brine at - 18°C. The freezing process starts already at this point. [0041] The fish is then pumped through the fish and brine pump 4 and enters the further tank 5 below the helical baffle 6. Here it is allowed to rise upward due to buoyancy. The friction of the fish against the baffle 6 and the weak downward flow of brine act against the rising of the fish. Consequently, the fish will take some time to rise from the bottom to the top of the tank 5. This time can be adjusted by decreasing or increasing the downward flow of brine. A typical residence time in the tank 5 is 50 - 90 minutes. As the brine enters the tank 5 through two inlets, the flow split can also be used to adjust the flow against the fish in the lower part of the tank 5 relative to the upper part of the tank 5.

[0042] The increasing or decreasing the downward flow will also influence the temperature distribution in the tank 5.

[0043] It is preferred that the helical baffle is stationary, but it is also an alternative to rotate the baffle slowly, either against the rising of the fish or with the rising of the fish, to decrease or increase the time the fish uses to reach the top of the tank.

[0044] The arrangement ensures that the first fish into the system is also the first fish out of the system. The time that the fish uses to travel through the system may be detected in various ways. A simple way is to feed dummy fish having similar weight and size as a fish. These can be coloured objects, be equipped with an RFID tag or other identifications that enables recognition when they emerge from the draining chamber. The circulation time should be in the order of 50-90 minutes.

[0045] Figure 2 show the same process arrangement as in figure 1 but configured to thaw the fish.

[0046] In this configuration the heat exchanger is used to heat the brine flowing therethrough, such as from -5,4°C to -3°C. This can be done by reversing the RSW system.

[0047] Frozen fish is fed into the buffer tank 1 , which contains brine at a temperature of about -5°C. Here the thawing of the fish starts. The fish and brine are pumped by the fish and brine pump 4 to the further tank 5. Here brine, which has been cooled down by the frozen fish is pumped, by the pump 8, out of the tank 5 close to the bottom of the tank 5 and circulated through the heat exchanger 9 to be heater. The heated brine is returned to the tank 5 both at the top inlet 11 of the tank 5 and at the lower inlet 10.

[0048] As for the freezing process, the fish rises through the tank 5 and follows the helical baffle from the bottom to the top. As the fish rises, the temperature of the fish also rises. The travel time of the fish and the brine temperature is selected so that the fish reaches a temperature of a few degrees below freezing throughout the fish when it exits the tank 5. In the draining chamber 13, brine is drained from the fish and recirculated to the buffer tank 1. The fish is now ready for further processing, such as filleting 14.

[0049] Figure 3 shows a production line that incorporates the freezer arrangement of the present invention. Starting from the right-hand side of the layout, the fish is brought on board the fishing vessel at 20, holding a temperature of perhaps 1°C. It is immediately transferred to a receiving storage 21 where the fish is stored temporarily at about -1°C. From the receiving storage 21 the fish is taken out (either in batches or continuously) for grading 22.

[0050] In the grading step 22, the highest quality fish is sent to a cooling step 23 to be cooled down to an ideal temperature for filleting, which is around -2,7°C. The filleting takes place in a filleting/cutting step 24. Filleted fish is sent to packaging 25, such as vacuum packing for the consumer market, and the packaged product is sent to freezing 26. Flere a continuous brine freezing with a CaCl2 brine may be used in order to speed up the freezing. CaCl2 brine can be liquid down to slightly below -50°C. The fish is preferably frozen to about 18°C at this step.

[0051] After freezing, the consumer-packaged fish is packed in larger units for transport at 27 and brought to a cold storage, which can hold about -30°C at 28.

[0052] The offal is sent to a residual product line 29, where it, e.g., may be ensilaged, whereafter it is brought to a suitable storage 30.

[0053] The fish that is not graded for immediate filleting is frozen as whole fish (it may be gutted though) in a NaCte brine having a temperature of -21 °C, which brings the fish temperature down to about -18°C, at 31. The fish is brought out as single frozen fish at 32 and brought to a storage, which may conveniently be the same storage as for the packaged fillets. [0054] Single frozen fish may be taken out from the storage 28 and back to the cooling step 23, where it now will be tawed at a temperature of, e.g., -2,7°C and forwarded to further processing, such as filleting. This is illustrated by the line 33.

[0055] Both the freezing arrangement for the fillets and for the whole fish are made according to the present invention. A pump, preferably of the coil type mentioned above, is used to transport the fish and brine through the arrangement. Where fish or fillets are transported without brine, belt conveyers, chutes or similar may be used.

[0056] It is conceivable that the whole arrangement is on board the fishing vessel. However, some of the steps may also be done in a land-based facility. This is especially convenient for the thawing arrangement described in connection with figure 2.