The following summary of facts and figures related to fish freezing is only presented as a guide. There will be differences between species, due to the effects of seasonal changes and due to processing methods. Even if the information were available, inexhaustible lists would have to be prepared to cover all eventualities. This obviously is not practical in a document such as this and when accurate information is required it should be obtained from the literature recommended in Selected Reading, (Appendix 1) or, in the case of freezing times, pilot scale laboratory tests
Freezing temperature of fish about -1°C 55% frozen at -2.2°C 70% frozen at -5.0°C
Table 30 Heat to be removed when freezing white fish (kcal/kg)
Initial temperature |
Final temperature |
|
(°C) |
-30°C |
-18°C |
40 |
107.7 |
100.9 |
30 |
98.8 |
92.0 |
20 |
90.1 |
8.3 |
15 |
85.7 |
78.9 |
10 |
81.3 |
74.5 |
5 |
76.9 |
70.1 |
Specific heat and heat content of a fish (See Table 31 and Figure 49)
Table 31 The enthalpy and specific heat
Temperature (°C) |
Enthalpy datum -40 °C (kcal/kg) |
Specific heat (kcal/kg °C) |
-40 |
0.00 |
0.44 |
-36 |
1.77 |
0.45 |
-32 |
3.60 |
0.47 |
-28 |
5.55 |
0.51 |
-24 |
7.67 |
0.55 |
-20 |
10.03 |
0.62 |
-16 |
12.69 |
0.72 |
-14 |
14.18 |
0.78 |
-12 |
15.84 |
0.87 |
-10 |
17.73 |
1.01 |
-8 |
19.99 |
1.27 |
-6 |
23.01 |
1.85 |
-4 |
28.05 |
3.61 |
-3 |
32.70 |
6.34 |
-2 |
42.16 |
15.68 |
-1 |
71.16 |
24.54 |
0 |
77.16 |
0.99 |
2 |
78.90 |
0.87 |
4 |
80.65 |
0.87 |
6 |
82.39 |
0.87 |
8 |
84.14 |
0.87 |
10 |
85.89 |
0.88 |
12 |
87.64 |
0.88 |
14 |
80.39 |
0.88 |
16 |
91.94 |
0.88 |
20 |
94.65 |
0.88 |
24 |
98.17 |
0.88 |
28 |
101.69 |
0.88 |
32 |
105.21 |
0.88 |
36 |
108.73 |
0.88 |
40 |
112.25 |
0.88 |
Note: Enthalpy is the heat content of the fish measured above an arbitrary datum of -40°C. The change in enthalpy between 10°C and -30°C will therefore indicate the amount of heat that has to be removed when freezing fish.
Specific heat is a measure of the heat that has to be added or subtracted to change the temperature of the fish by 1°C. Specific heat of the fish is a combination of sensible heat and latent heat at temperatures below 0°C.
Thermal conductivity of fish (kcal m/h m2 °C)
Unfrozen white fish at 0°C0.37 to 0.5
Frozen white fish at -1°C1.12 to 1.49
Frozen white fish at -30°C1.61
Density of white fish muscle (kg/m3)
at 0°C1 054 at -20°C 966
Specific heat of white fish (kcal/kg deg C)
Unfrozen 0.9
Frozen 0.4
Stowage rates for frozen fish
(m3/ t) Whole round fish 25 to 30 cm in length frozen in blocks 1.2 Whole round fish 30 to 100 cm in length frozen in blocks 1.02 to 1.12 Frozen whole fish 30 to 100 cm in length stored as single fish 2.08 to 2.5 Frozen whole fish 30 to 100 cm in length frozen in blocks with allowance for pallets, passageways etc. 2.0 Fillets.frozen in large blocks with allowance for pallets, passageways etc. 1.25 to 1.56 Frozen fillets in consumer packs in master carton with allowance for pallets passageways etc. 2.5
Yields from cod
Component Ungutted weight (%) Gutted weight (%) Head 21 25 Guts 7 (5-8) Liver 5 (2-7) Roe 4 (1-7) Backbone 14 1 Fins and belly flaps 10 12 Skin 3 4 Fillets, skinned 36 43 Total 100 100
