Figure 13. Idealized freezing curve for fish muscle.

water is frozen rapidly, small crystals form and do not have time to increase in size due to the nucleation characteristics of a water molecule. Experience has shown that if seafood passes through the critical zone in ca 1.5 h, there will be little damage to cell structure as a result of large crystals formed during freezing. This is demonstrated by mi-crophotographs of cells frozen under different conditions. Figure 14 showns cells (magnified 640 X) of a rainbow trout that was rapidly frozen and passed through the critical zone in less than 1 h (15). Note the well-defined intact cell structure is similar to that of high-quality fresh flesh. Figure 15 is a similar microphotograph of cells from the other half of the same rainbow trout, which took approximately 5 h to pass through the critical zone. In this case extreme damage to the cell structure caused by large ice crystals can be noted. This is a dramatic demonstration of why fast freezing of seafood is essential to maintaining the quality of frozen products.

There is wide variation in commercial freezing facilities available to the seafood industry, and it is most important that commercial operators carefully study their options when purchasing and installing new equipment. The most efficient methods of freezing involve both conduction and convection freezing in an immersion system. Liquid refrigerants such as the freons have been used for this purpose

Figure 14. Cell structure of rainbow trout frozen rapidly ( X 640). Source: Ref. 15.
Figure 15. Cell structure of rainbow trout frozen slowly ( X 640). Source: Ref. 15.

but have proven too costly. Immersion freezing using refrigerated brine is fast and efficient but a considerable amount of salt is absorbed into the flesh during freezing and can be a problem to product marketing. Refrigerated seawater is used extensively, especially in Alaska for rapidly chilling and holding the fresh catch on shipboard.

Plate freezers rapidly freeze, by conduction, products that are in contact with the plates. Plate freezers are used to their best advantage when the plates can be brought together to contact both sides of a package. This is the case for rectangular packaged seafood portions or prepared formulations. A disadvantage is that irregular items, particularly large fish, can only contact the plate on one side and the exposed side must be frozen by convection. Another disadvantage is using plate freezers for whole fish, because the plate only contacts one side of the large irregular shaped item and causes one side of the finished product to be flat while the other side is the nice rounded shape of the live fish. However, unless the fish are suspended by hooks, this same condition prevails in any freezer where the fish are placed on shelves during freezing.

Convection blast freezing, where cold air is circulated over the products, is less efficient than conduction freezing but can be used for larger volumes. A disadvantage to blast freezing is that the rapidly moving dry air can dehydrate a product that is exposed for any length of time. This can be eliminated by proper packaging protection, but it also greatly decreases the heat transfer rate and lengthens the time of freezing and the subsequent time at which the product is in the critical range.

The problems with blast freezing can often be minimized or eliminated when a combined blast-plate-freezing system is used. This can be demonstrated by comparing Figures 16 and 17 in which chum salmon weighing 4 lb were frozen in two commercial units (16). Figure 16 is the freezing curve for a conventional blast freezer in which whole (headed and gutted) fish were placed on racks and frozen. Note that the time in the critical range was about 4 h and the total time to reduce the temperature to 0°F was about 8 h. The freezing curve shown in Figure 17 was that of a 4-lb chum salmon in a combined blast and plate freezer. The time in the critical range for this system was about 40 min, and the total freezing time to reach 0°F was slightly over 1.5 h. The fish frozen in the combined blast-plate freezer were far superior to those frozen in the blast freezer alone. Furthermore, the reduction in weight due to drip loss gave a significant economic advantage to the more rapidly frozen fish.

Holding Seafood in Cold Storage. Good freezing practices involving fast freezing and minimal dehydration of fresh seafood insures that high-quality products are delivered to the cold storage for holding. Equally important to the continuing maintenance of high quality is the environment under which the products are kept during the cold storage period. Unless frozen seafood is kept at extremely low temperatures, there is a certain amount of free water (not frozen) remaining in the product. This is a result of the antifreeze effect of soluble salts in the cell, which become more concentrated as water freezes. Depending on the specific food, the physical and chemical conditions of the food, and the composition (including water content), the point at which all of the water is frozen is in the range of — 45°F. Because few, if any commercial cold storages are held at that low temperature, there is a certain amount of free water remaining in all frozen food (9). This is depicted in Figure 18, indicating that the normal commercial cold storage temperatures are well above — 45°F. In fact, different commercial cold storage warehouses in the United States range in temperature from slightly above 0°F to a low that seldom is below - 20°F.

Not only is the average commercial cold storage facility maintained at a temperature at which several percent of the water remain unfrozen but there is normally a significant fluctuation in the base cold storage temperature. Thus when the temperature fluctuates above and below the average, some of the water in the product is continually frozen, thawed, and refrozen. The effect of this fluctuation is to greatly increase the effect of enzyme action that essentially digests the protein in the same manner that it is digested in the gut of an animal. This is emphasized by the fact that continual thawing and refreezing is not limited to the original free water and causes the enzyme action to spread throughout the flesh. This is why the meat in seafood held for long periods can be extremely soft when thawed. This effect is even more noticeable in many home freezer units where temperature fluctuations are greater than those found in commercial facilities. Hence, holding a product at a higher but constant cold storage temperature can result in a better product than when it is held at a fluctuating lower temperature. This concept is depicted in Figure 19.

It can be important to know how much fish a given size cold storage unit will hold. The true density of a seafood ranges from 70 to 80 lb/ft2. Because frozen fish blocks are essentially composed of solid fish flesh, they have about the same density as the natural flesh. At the other end of the spectrum, individually frozen and loosely packed fish range from 30 to 50 lb/ft2. A well-run cold storage, allowing for the average distribution of product forms and allowing for air spaces and movement within the room, usually holds about 20-30 lb/ft2.

Figure 16. Freezing curve for freezing of chum salmon (4 lb).


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