19. SOME RELATED FACTS AND FIGURES


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 -1C
  55% frozen at -2.2C
  70% frozen at -5.0C

Table 30 Heat to be removed when freezing white fish (kcal/kg)

Initial temperature

Final temperature

(C)

-30C

-18C

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 -40C. The change in enthalpy between 10C and -30C 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 1C. Specific heat of the fish is a combination of sensible heat and latent heat at temperatures below 0C.

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Thermal conductivity of fish (kcal m/h m2 C)

Unfrozen white fish at 0C0.37 to 0.5
Frozen white fish at -1C1.12 to 1.49
Frozen white fish at -30C1.61

Density of white fish muscle (kg/m3)

at 0C1 054 at -20C 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