WO1999044433A1

WO1999044433A1 - Method for freezing of food products, in particular fish

Info

Publication number: WO1999044433A1
Application number: PCT/IS1999/000006
Authority: WO
Inventors: Palsson Gudlaugur
Original assignee: Frostmark Ehf.
Priority date: 1998-03-06
Filing date: 1999-03-08
Publication date: 1999-09-10
Also published as: IS5609A; DK200001300A; IS2043B; IS7823A; AU2955499A

Abstract

The invention concerns a method for freezing food products, fish in particular, e.g. shellfish. The freezing is performed in two or more steps, where the first step is pre-freezing and in the latter steps post freezing, and when appropriate glazing of the surface of the products. The pre-freezing is performed by immersing the food products into a salt solution at an appropriate temperature in a sufficiently long time to obtain freezing of the whole body of the food product.

Description

METHOD FOR FREEZING OF FOOD PRODUCTS, IN PARTICULAR FISH

Technical field of the invention

The invention concerns a method for freezing food products, in particular fish, e.g. shellfish. The freezing is performed in two or more steps, the first step being pre- freezing and the following steps post-freezing and when appropriate, glazing of the surface of the food products.

Background of the invention Freezing is one of the most important techniques in modern food production, in fish production, in particular. The main function of cooling and freezing of the food products is to counteract the adverse effect of bacteria and enzymes. By freezing, the shelf life of the products may be increased without diminishing their freshness. The food products have unaltered taste, smell and colour after freezing and thawing if these are performed in the right way.

From the moment the fish is caught the deteriorating effect of bacteria and enzymes starts and can if no action is taken make the fish unfit for human consumption. Cooling and freezing of fish slows down this detoriating process.

It is possible to perform freezing in different ways and the method selected can have considerable influence on the quality and the shelf life of the product. The freezing rate tells how fast the freezing process is progressing and how long it takes to freeze a product at given conditions. The freezing rate is usually given in cm/hour and indicates how far the freezing has reached into the body of the product or how far the ice edge has reach into the product in one hour. In general it is said that there is quick freezing and slow freezing. The difference between the two is not only how long a time it takes to freeze the product. In slow freezing, large ice crystals are formed and they can often be seen by the naked eye. Very slow freezing results in the formation of ice in the space in-between the cells where the quantity of dissolved material is less than in the cell. Water diffuses from the cell into the space in-between the cells because of pressure difference and is frozen there. Increased quantity of dissolved material in the cell decreases the rate of freezing of water in the cell. Large crystals of irregular shape can be formed and may deform the cells.

By quick freezing water has no time to flow from the cells before it is frozen and ice formation has started earlier in the cells. Ice crystals do not grow to the same size in the space in-between the cells and the deformation of the cells is less. Because of the formation of ice crystals the concentration of dissolved material in the cells increases leading to more rapid detoriation of the cell. Dissolved material acts as catalyst for the detoriation until the material is frozen. Slow formation of ice crystals has effect on the water holding property of the fish flesh and it has been shown that drip is increased considerably from slow frozen fish flesh that has been frozen slowly. The yield is increased if the food product is quick-frozen.

Mainly two methods are known for freezing of fish. One is the so-called plate freezing method and the other liquid air freezing method. Plate freezing is mainly used when the purpose is to freeze the products into blocks, but with liquid air freezing, often called loose freezing, the intention is to freeze food pieces individually.

The following components influence the rate of freezing (freezing time). Size and form of the product, thermal conductivity (how well the product conducts heat from the inside to the surface), starting and final temperature, conduction of heat from the product to the cooling media and the temperature of the cooling media. By air blowing, the rate of freezing is faster than in still air. Heat is transported by conduction. Increased blowing of air on the product increases transport of warm air from the surface of the product.

Food products are considered frozen when approximately 75% of the freezeable water in them have been converted into ice.

Loose freezing of food products, in particular fish has until now mainly been performed by apparatus where liquid or cooled air is blown onto the products, e.g. on grids, transport bands or on shelves. Food pieces are kept apart, if possible. Different types of apparatus are known for this purpose, such as blow freezers where cooled air is blown through shelves loaded with the products that are to be frozen. So-called band-or flow freezers are also known.

One of the main disadvantages of the blow freezer is the high energy required for the freezing and the long freezing time. In blow freezing the freezing is generally performed in two steps where the first step is freezing of the surface and then the freezing of the product body.

Description of the invention The aim of the invention is to set forth a method for freezing of food pieces, shellfish in particular, in such a way that it is possible to shorten considerably conventional freezing time, lower energy consumption and simplify the apparatus required and thus lower the investment and operation cost.

This goal is achieved by the invention being unique in that the pre-freezing is performed by immersing the food products in a 8 - 12% salt dissolvent, at a temperature of -5 to - 9°C, and the food products are kept therein for 120 - 540 seconds, or until the products are fully frozen.

Food products are said to be frozen when approximately 75 % of the freezable water in the product have been converted into ice.

The invention is further unique in that the immersing solution contains 8 - 12% salt, more particularly 9 - 12%, most preferred 9 - 11%. The temperature of the immersing solution is -6,5 to -7,5°C. The dwelling time of the food products in the immersing solution is 240 - 360 seconds, more particularly 240 - 480 seconds. 0,1 - 6% of ice crystals are added to the immersing solution, more particularly 0,3 - 5%, most preferred 0,5 - 4%.

General description of the drawings

Figure 1 shows schematically an arrangement of the freezing process, and Figure 2 shows a more detailed arrangement of the pre-freezing.

More detailed description of the drawings and one preferred embodiment of the invention Figure 1 shows the arrangement and the whole freezing process. Food pieces are first immersed in a salt solution for a predetermined time. The temperature is controlled with cooling equipment, dissolving salt in the solution and if needed in mixing of ice crystals. After the pre-freezing, the fish is moved into one ore more steps of post freezing and followed by glazing where it is sprayed with water that freezes as soon as it comes into contact with the frozen fish. In the example herein described shrimp is being frozen. Shrimp or whatever material is being frozen is according to the method transported on a band (4) that is pulled through the pre-freezer (1) at a Ti. The temperature is preferably between -5 °C to -9 °C. The pre-freezer is a tank (2) filled with salt solution (3) at a given temperature T). The salt solution is circulated and constantly pumped with for instance spraying equipment (5), over the tank and the temperature of the salt solution is thus kept constant. Following the pre-freezing the product is dipped quickly into (ice)-water bath for a short period of time. The temperature of the water is about 0°C. By dipping into the water the salt on the surface of the product is removed. Thereafter the product is transported further on a band (4) into a (ice) water tank (31) where the food pieces are immersed into the water (32) for a short period of time (1 - 60 seconds) where the salt is removed from the surface of the product. Then the product is transported to the post-freezer (7) where cooled air is blown (8) onto the product in a predetermined time. After the post freezer (10), there is a spray unit (9) where water is sprayed over the product as such and a glazing is formed over the surface because the product has reached a temperature of -20 °C in the post freezer. Finally the product is transported into final post freezer for throughout freezing.

Figure 2 shows more detailed arrangement of the pre-freezing in the salt solution. The pre-freezer (1) is as shown equipped with a circulating system for water (salt solution) in such a way that it is possible to keep a constant temperature Ti in the tank (2). The salt solution is circulated with pumps (21, 24), through a sieve (22), condenser (25) and valves (23) and again into the tank with e.g. spraying equipment (5). A second tank (51) is shown with (ice)-water for salt removal of the surface of the product. A second water tank (31) is shown for cleaning or removing salt off the surface of the product. The product is immersed/dipped into water, preferably ice water at a temperature close to 0

Example 1

About 243 g of pre-cooked shrimp was used. The salt solution was set at 12 % and the temperature kept at -7 . The shrimp was kept for 6 minutes (360 seconds) in the salt solution. After dripping the shrimp weighed 282 g. The salt content in the shrimp meat was about 3%.

Example 2

About 245 g of pre-cooked shrimp was used. The salt solution was set at 12% and the temperature kept at -7 . The shrimp was kept for 6 minutes (360 seconds) in the salt solution. After dripping, the shrimp weighed 295 g. After having removed the shrimp from the salt solution it was dipped into ice water (about 0°C) for a short period of time (ca. 10 seconds) and then removed again for further testing. The salt content in the shrimp meat was about 1,9%.

Example 3

About 310 g of pre-cooked shrimp was used. The salt solution was set at 9% and the temperature kept at -5 . The shrimp was kept for 6 minutes (360 seconds) in the salt solution. After dripping, the shrimp weighed 366 g. The shrimp was not completely frozen but the salt content appeared to be satisfactory.

Example 4

About 223 g of pre-cooked shrimp was used. The salt solution was set at 12% and the temperature kept at -7 . The shrimp was kept for 6 minutes (360 seconds) in the salt solution. After dripping, the shrimp weighed 262 g. The salt content in the shrimp meat was about 4,1%.

By freezing the shrimp in about 11-12%) salt (NaCl) solution at a temperature around 6 - 7 for a period of 5 to 6 minutes and thereafter dipping the shrimp for a short period of time into water, preferably ice water at a temperature close to 0 it is possible to obtain complete freezing of the shrimp, increase the water content in the shrimp up to about 25% and keep the sat content to about 2%. Furthermore, the shrimps are frozen individually.

The invention described here above is not precisely limited to those details which have been specified, but can be elaborated upon in many ways without deviating from the central concept and spirit of the invention as defined in the patent claims below.

Claims

Patent claims

1. A method of freezing food products, in particular fish e.g. shellfish, where the freezing is performed in two or more steps; the first step includes pre-freezing and the following steps post-freezing and when appropriate, glazing of the surface of the food products characterised in that the pre-freezing is performed by immersing the food products in a 8 - 12 % salt solution, at a temperature of -5 to -9┬░C, for a period of 120 to 540 seconds.

2. A method as claimed in claim 1, characterised in that the immersing solution contains 8 - 12 % salt, more particularly 9 ΓÇö 12 %, preferably 9-11 %.

3. A method as claimed in claim 1 or 2, characterised in that the temperature of the immersing solution is -6,5 to -7,5┬░C.

4. A method as claimed in one or more of the claims 1-3, characterised in that the dwelling time of the food products in the immersing solution is 240 - 480 seconds, preferably 240 - 360 seconds.

5. A method as claimed in one or more of the claims 1-4, characterised in that prior to immersing the product into the salt solution, the product is immersed into a water bath, preferably ice water at a temperature close to 0┬░C.

6. A method as claimed in one or more of the claims 1-5, characterised in that 0,1 - 6% of ice crystals are added to the immersing solution, more particular 0,3 - 5%, most preferred 0,5 - 4 %.

7. A method as claimed in one or more of the claims 1-6, characterised in that spices are added into the immersing solution.

8. A method as claimed in one or more of the claims 1-7, characterised in that the food product being pre-cooked shrimps.

9. A method as claimed in one or more of the claims 1 -8, characterised in that the food product is kept in the second water bath for a period of 1 - 60 seconds, preferably 10-30 seconds.

10. A method as claimed in one or more of the claims 1 -9, characterised in that after the food product is immersed into the salt solution, the product a subject to immersing in a water bath, preferably ice water at temperature close to 0 - 6┬░C, for a short period of time, preferably 10-30 seconds for removing salt from the surface of the product.