Industrial methods of freezing, thawing and subsequent chilled storage of whitefish скачать в хорошем качестве

Industrial methods of freezing, thawing and subsequent chilled storage of whitefish

Fish begins to spoil immediately after death. This is reflected in gradual developments of undesirable flavours, softening of the flesh and eventually substantial losses of fluid containing protein and fat. By lowering the temperature of the dead fish, spoilage can be retarded and, if the temperature is kept low enough, spoilage can be almost stopped. Rigor mortis, over a period of hours or days soon after death, can have a bearing on handling and processing. In some species the reaction can be strong, especially if the fish has not been chilled. The muscles under strain tend to contract, therefore, some of the tissue may break, especially if the fish is roughly handled, leaving the flesh broken and falling apart. If the muscles are cut before or during rigor, they will contract and in this way fillets from fish can shrink and acquire a somewhat rubbery texture. In many species, however, rigor mortis is not strong enough to be of much significance. The freezing process alone is not a method of preservation. It is merely the means of preparing the fish for storage at a suitably low temperature. In order to produce a good product, freezing must be accomplished quickly. A freezer requires to be specially designed for this purpose and thus freezing is a separate process from low temperature storage. 2.1 What happens during freezing Fish is largely water, normally 60-80 percent depending on the species, and the freezing process converts most of this water into ice. Freezing requires the removal of heat, and fish from which heat is removed falls in temperature in the manner shown in Figure 1. During the first stage of cooling, the temperature falls fairly rapidly to just below 0°C, the freezing point of water. As more heat requires to be extracted during the second stage, in order to turn the bulk of the water to ice, the temperature changes by a few degrees and this stage is known as the period of "thermal arrest". When about 55% of the water is turned to ice, the temperature again begins to fall rapidly and during this third stage most of the remaining water freezes. A comparatively small amount of heat has to be removed during this third stage. Figure 1 Temperature-time graph for fish during freezing As the water in fish freezes out as pure crystals of ice, the remaining unfrozen water contains an ever increasing concentration of salts and other compounds which are naturally present in fish flesh. The effect of this ever increasing concentration is to depress the freezing point of the unfrozen water. The result is that, unlike pure water, the complete change to ice is not accomplished at a fixed temperature of 0°C, but proceeds over a range of temperature. The variation of the proportion of water (which is converted to ice) in the muscle tissue of fish against temperature is shown in Figure 2. The figure shows that by the time the fish temperature is reduced to -5°C about 70% of the water is frozen. It also shows that even at temperatures as low at -30°C, a proportion of the water in the fish muscle still remains in the unfrozen state. Figure 2. Freezing of fish muscle. The percentage of water frozen at different temperatures Literature on the freezing of fish is confusing and often contradictory about what happens to fish as it freezes. This is particularly the case when reference is made to the difference between slow and quick freezing. One of the main reasons for this apparent confusion is that only in recent years has knowledge of the freezing process advanced sufficiently to explain these differences in freezing rates. The result is that much of the literature still in circulation is now outdated. At first it was thought that rapid freezing was unsatisfactory since sudden cooling could disrupt and tear the muscle tissue. It was also thought that, since water expands on freezing, it might be reasonable to expect the cell walls to burst under the induced pressure. There is some justification for both of these theories but they do not fully explain the differences between slow and quick freezing. For some time a widely held view was that slow freezing resulted in the formation of large ice crystals which damaged the walls of the cells. This would then result in a considerable loss of fluid when the fish was thawed. The smaller ice crystals formed, when fish is frozen quickly, were thought to do little damage to the cell walls and, as a result, little fluid was lost on thawing. Difference in size of ice crystal probably accounts for some of the differences between slow and quick freezing, but is has been shown that this still does not provide a full explanation. The walls of fish muscle cells are sufficiently elastic to accommodate the larger ice crystals without excessive damage. Also, most of the water in fish muscle is bound to the protein in the form of a gel, and little fluid would be lost even if damage of the above nature did occur original video    • Fishing and Processing on a Freezing Trawler