Freezing Technology

3.1.3 Thermal diffusivity

Thermal conductivity values alone will not provide accurate indication of rate of temperature change in any given substance as these values apply solely to the rate of heat (calories) conducted across at a constant temperature difference. Therefore, heat capacity of a substance must be considered, as this will influence change in temperature per unit change in heat energy. As thermal conductivity values expressed as volume basis and heat capacity on mass basis, it is necessary to introduce a third term that is density (Mass/volume) to reconcile the two terms. By combining all the 3 terms, we get a new term known as thermal diffusivity.  

The co-efficient of thermal diffusivity “” characterizes the rate of temperature equalization in an unevenly heated material. The C.T.D may be defined as increase in temp in unit time when temperature gradient per unit length is equal to 1 unit (^ºC) 

Thermal diffusivity is related to thermal conductivity, heat capacity and specific gravity of a material by the following expression.  

 = λ    

    -----    m²/hr

      cν

 Where    λ   = coefficient of thermal conductivity k cal/m.hr.^ºC

            C = Specific heat in kcal/kg^ºC             

                ν = density  kg/m³  

Thermal diffusivity of any substance is directly proportional to its thermal conductivity and inversely proportional to sp. heat and density of the substance. It increases as thermal conductivity increases and density and specific. heat decreases. Also thermal diffusivity of a substance indicates the rate at which the substance will undergo a temperature change. 

Thermal diffusivity “” of a substance characterizes the speed with which the temperature changes upon cooling or heating. The higher the coefficient, the quicker is the change in internal temperature of the substance.  

In unfrozen fish it remains almost a constant during the process of chilling (in 30^ºC to O^ºC). However it changes considerably during the process of freezing and the values are very high in completely frozen fish. This is important in frozen storage of fish because any change in the cold storage temperature brings about marked change in temp of frozen fish. Co-efficient of thermal diffusivity in unfrozen state is in the range of 0.000375 to 0.000475 m²/hr. The values for fish at O^oC to -3.8^OC is not considered as accurate due to drastic changes in the physical properties of fish meat in this temperature range and values are minimum in this temperature range. In frozen fish values range from 3.75X10^-4 to 15X10^-4 .