Freezing Technology

3.1.1. Specific heat

Specific heat is the quantity of heat that must be supplied or removed from a gram of substance to bring about change in temp of 1^ºC. It is expressed in terms of kilojoules (KJ) or kcal in applied science. 

Heat capacity/specific heat of fish depends on its chemical composition; that is the content of water, fat and minerals. It is the total heat capacity of individual components which is given by the following formula. 

Cf = Cw X W+L+Cm X m 

Where,                                    

             Cf= heat capacity of fish (kcal/kg)

             Cw = specific heat of water (kcal/kg)

             CL = specific heat of fat/lipid (kg)

             Cm = specific heat of minerals (kg) 

W, L and M are contents of water, lipid and mineral in Kgs in a kg of fish.  

Example : Specific heat of fish also depends on temperature. There is no applicable change in specific heat. When fish is cooled from 30^ºC to 0^ºC (prior to freezing)and in this range of temperature, specific heat of major components of fish are given as follows. 

Water                             -       4.18 KJ/kg^ºC        or 1 kcal/kg ^ºC

Fat                                 -       2.075 KJ/kg^ºC     or 0.5 kcal/kg ^ºC

Dry matter other than fat -     1.505 KJ/ kg ^ºC or 0.36 Kcal/ kg ^ºC (i.e.,

                                              protein + minerals) 

Based on these values specific heat of various fishes were calculated and found to be between 0.74 to 0.91 Kcal / kg ^ºC between 30 to 0^ºC. As fish gets cooled further its specific heat goes on decreasing. Chilled fish has sp. heat of 0.845 kcal/ kg ^ºC which gets reduced to 0.440 kcal/ kg ^ºC, when the same fish is frozen. This indicates less quantity of heat is required to change the temperature by 1^ºC in frozen fish when compared to unfrozen fish. This difference is large especially when all the water in the fish is frozen (i.e., to below -8^ºC).