9. WEIGHT LOSS FROM FISH DURING FREEZING AND COLD STORAGE

9.1 Freezer weight loss
9.2 Cold store weight loss

9.1 Freezer weight loss

Weight may be lost by dehydration or due to physical damage of the fish during the freezing process.

Physical damage may be due to damage during freezing which results in small pieces being broken off; this is likely, for instance, in freezers where the product is fluidized by the cooling air.

The other form of physical damage encountered during the freezing process is due to fish adhering to trays or conveyor belts. If the weight loss on releasing fish from trays is excessive, the trays may be sprayed on the underside with water to assist release. Fish frozen in continuous freezers with stainless steel link or mesh belts may stiffer weight losses due to small particles being trapped in the belt. Losses due to physical damage in a freezer should be small and need not be more than about 1 percent if the freezer and freezing process is suitable for the product.

Weight loss due to dehydration in a freezer depends on a number of factors, and the weight losses in air blast freezers give rise to the greatest controversy.

Weight loss due to dehydration will depend on:

• Type of freezer

• Freezing time

• Type of product

• Air velocity

• Freezer operating conditions

Freezers such as plate freezers where the fish is frozen by contact and released by defrosting will have a negligible weight loss during freezing. Any measured change in weight will probably be due to loss of drip before the freezing started.

Dehydration losses occur mainly in air blast freezers and in other freezers which use a gas such as nitrogen or carbon dioxide in direct contact with the product.

The loss of weight in nitrogen, carbon dioxide and other cryogenic freezers will be low by virtue of the fact that freezing times are short. A direct contrast made between a carbon dioxide freezer and an air blast freezer showed that the weight lost from haddock fillets in carbon dioxide freezer was about half of the weight lost in the air blast freezer, 0.6 percent compared with 1.2 percent. Other cryogenic freezers are likely to give rise to weight losses which are about the same as that of the carbon dioxide freezer.

Time in a freezer, however, cannot be directly related to the weight loss since the rate of weight loss shown in Figure 43 is not directly proportional to time. More weight is lost at the start of a freeze than at the end.

Some weight losses are given in Table 18. The differences between different types of freezer are not great and not as high as some commercial literature would imply. It should also be remembered that some of the weight loss is due to the evaporation of surface water probably left from washing the fish, and this would have eventually been lost as a drip if the fish had remained unfrozen.

One fact that is seldom considered is that fish kept in ice for a number of days will generally lose more weight than is ever likely in a freezer.

Table 18 Weight lost from fish during freezing

In view of the weight losses quoted above, claims that fish may show signs of "freezer burn" or severe dehydration as the result of the freezing process would appear to be unfounded. The shape of the weight loss curve shown in Figure 43 would imply that freezing times would have to extend to many hours or even days for "freezer burn" to become apparent.

9.2 Cold store weight loss

Much has yet to be done to correlate the rate of weight loss with differences between storage conditions but the rate of weight loss has been shown to vary with the following:

• Temperature

• Temperature fluctuation

• Humidity

• Heat transfer

• Air flow over the product

• Radiation effects of lighting

• The product

• Shape and size of the product

• Type of wrapper

Most codes of practice only state the temperature for storage. Variations in the other factors that control the rate of dehydration can therefore result in cold stores having widely different storage conditions. The rate at which the product loses weight by dehydration can therefore vary considerably.

Table 19 shows rates of weight loss measured in a variety of stores. The losses are expressed as the weight changes per square metre of exposed fish surface. These results clearly show that there are great differences between the quality of cold stores which may be attributed both to their design and mode of operation as well as to the operating temperature. Apart from the physical loss in weight, excessive dehydration results in "freezer burn". The overall weight loss, however, cannot be used to define the point when "freezer burn" becomes apparent. Dehydration only occurs from exposed surfaces and the rate of dehydration is greater where the surface area to volume ratio is high. The edge of fish fillets and the corners of slabs of fish will therefore show signs of excessive dehydration or "freezer burn" long before the other exposed surfaces of the product. For this reason, "freezer burn" can even become apparent on glazed fish long before the overall weight loss is equal to the weight of glaze applied.

The rate of weight loss within a store can vary considerably with location. Fish stored near fan coolers, where they are subjected to high air velocities, will quickly show signs of dehydration. Fish stored against walls remote from the cooler may be subject to poor air distribution and heat gains from the store walls. This can cause temperature fluctuations in the product which inevitably results in high dehydration losses.

Table 19 Rate of weight loss from fish in cold storage

Note: The last store was a large store and the very high results were obtained at points where unfrozen fish were placed in the store.

Fish adjacent to roof coolers may also dehydrate more quickly since the path of moisture migration is considerably shorter. Fish near the roof or walls of stores which are affected by a high incidence of solar radiation may also be subjected to higher losses. Finally, fish in storage which have unfrozen or partially frozen fish frequently stacked alongside show the highest dehydration rates of all.