Freezing Technology

3.1.2. Thermal conductivity

 In an unevenly heat body, where there is always a difference in temperature, there is always a flow of heat from warmer to colder layers. This type of heat exchange depends on direct heat transfer between adjoining particles and this continues until the temperature of body is uniform. The rate of heat transfer depends on the co-efficient of thermal conductivity of the substance which is defined as thermal conductivity of medium in which a unit of heat (calorie) flows through a unit surface area of unit thickness at a temperature gradient of 1 unit.  Consequently expresses the amount of heat (calories) passing through a unit area of flat surface having unit thickness at a temp difference of 1 unit in unit time. The units of thermal conductivity are erg/cm. sec. ^ºC or cal/cm sec, ^ºC or kcal/m. Lr. ^ºC. 

When fish is heated or cooled, the rate of conduction of heat within the fish or rate of removal of heat depends on the co-efficient of thermal conductivity. It depends on chemical composition of fish, physical structure, specific gravity, specific heat and temperature of fish. The thermal conductivity in fish is important it determines the chilling, freezing and thawing process in it. Effect of temperature on the co-efficient of their mass conductivity can be illustrated by the following table. 

Temp ºC	Co-efficient of thermal conductivity
+30°C	0.4 kcal/m.hr. ºC
0ºC	0.401-405 kcal/m.hr. ºC
-1 ºC	0.522 kcal/m.hr. ºC
-2 ºC	0.638 kcal/m.hr. ºC
-4.5 ºC	0.94 kcal/m.hr. ºC
-30 ºC	1.5 kcal/m.hr. ºC

 

Co-efficient of thermal conductivity in fish can be theoretically calculated if its water and dry matter, content is known . lf=lo x XW +ln X N

 lf = coefficient of thermal conductivity of fish

lo = coefficient of thermal conductivity of water in fish

ln = coefficient of thermal conductivity of dry matter in fish ( 0.22 kcal/m.hr.^ºC)

 W and N are respective water and dry matter %age in fish.

 Co-efficient of thermal conductivity of ice is 2.07 kcal/m.hr.^ºC which is almost 4 times that of water (0.52 kcal/m.hr. ^ºC). Hence heat transfer in frozen fish is faster than unfrozen chilled fish.