4. The cooling rate of fish
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If we assume that ice is placed above and below a layer of fish, the fish at the centre of the layer will obviously take longest to cool, since they are furthest away from either layer of ice. It should also be appreciated that the cooling rate is not constant during the cooling period but slows down as the fish approaches the final temperature of 0°C.
These two conditions are illustrated by the following examples:
If the layer of fish is 10 cm thick, then the centre fish are 5 cm away from the nearest ice. If at the start of cooling the centre fish are at 10°C and the ice is at 0 C, there is a temperature difference of 1 0°C, and a temperature gradient of 2°C/cm. But, when the centre fish have cooled to 5°C, the temperature gradient is down to 1°C/cm, and the rate of cooling is consequently slower. As the temperature of the fish approaches that of the ice, the rate of cooling becomes extremely slow; it takes about 6 h for fish at the centre of a 10 cm layer to reach 0.5°C. When cooling times are given it is important to state the final temperature since, when this temperature approaches 0°C (the temperature of the ice), lowering the final temperature by even a small amount can make considerable differences to the cooling time.
This slowing down of the cooling rate at the end of the cooling period should be taken into account when any code of practice or legislation is introduced. Measurement of cooling times to the final equilibrium temperature will be subject to considerable variation since temperature differences will eventually to very small and thereby likely to be variable depending on the accuracy and sensitivity of the thermometer used. It is, therefore, more practical to define a completion temperature slightly above the final storage temperature, as in Table 4.
A typical curve for cooling fish in ice is shown in Fig. 8.
If the layer of fish is 20 cm thick instead of 10 cm, the middle fish are 10 cm away from the ice. The temperature gradient at the start is then 1°C/cm, that is only half the gradient at the start in the previous example; the less steep the gradient, the slower the heat will flow, and thus cooling takes longer. On the other hand, when the layer of fish is only 5 cm thick, cooling is rapid. The effect of the depth of fish in a box on the time taken to cool them is shown in Table 4, and presented in Fig. 9.
Fig.8. Cooling fish in ice
Fig. 9. Thick layers of fish take longer to cool than thin layers
Table 4. Time taken to cool fish in the centre of a box with ice top and bottom
layer of fish
10°C to 2°C at centre
A single fillet can be chilled very quickly in ice; a thick layer of fish or fillets will chill only very slowly. Therefore, to chill fish quickly it is essential to keep the distance between each fish and the nearest piece of ice as small possible. This means in practice that ice must be distributed uniformly throughout the fish. The correct procedure for icing a box of fish is discussed more fully in Chapters 8 and 9.
Size, shape and the arrangement of the fish also have an influence on cooling rate since these factors may affect packing density, contact areas and the flow of melt-water through the fish layer. Thermal conductivity and other physical properties also have an influence on the time taken to cool fish, since this will vary depending on species and their condition. The influence of all these factors however, will be small compared to that of the thickness of the fish layer.
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