296 PRESERVATION METHODS 



conditions where both the temperature and the relative humidity may 

 vary considerably. Temperature has a greater effect on product quality 

 than does relative humidity. However, the detrimental effects of a low 

 relative humidity should not be underestimated, particularly when stor- 

 ing unpackaged frozen seafoods. Slavin, et al}^, found that whole tuna 

 stored at 0°F and at a relative humidity of 70 to 80 per cent for 10)-^ 

 months lost 20 times more weight than tuna stored similarly except at 

 a higher relative humidity of 90 to 95 per cent. Results of these tests also 

 showed that packaged seafoods will lose weight at a significant rate when 

 stored at low relative humidities. 



It is therefore important that cold-storage plants used for long-term 

 storage of frozen seafoods be designed to maintain relative humidities of 

 90 per cent or higher. All evidence available clearly shows that the use 

 of low storage temperatures and high relative humidities will significantly 

 extend the quality of unpackaged and packaged frozen seafoods. 



Theoretical Considerations in Freezing Fishery Products 



The freezing of fish takes place in three steps. The sensible heat is first 

 removed, lowering the temperature of the product to the freezing point. 

 A change in state then takes place, and the latent heat of fusion is removed 

 from the food, changing a major portion of the water to ice. The third 

 step involves subcooling of the food from its freezing point to its terminal 

 temperature. The process of freezing is basically the transfer of heat from 

 one substance to another. The rate at which this heat flows depends on 

 many different factors, of which the most important are: (1) the heat 

 transfer method, (2) the temperature difference between the fish and the 

 cooling media, (3) the size, type, and thermal properties of the product, 

 and (4) the packaging materials and method of packing employed. 



The heat transfer method is a function of the particular freezer em- 

 ployed. In freezing, heat is removed from the product primarily by con- 

 duction and convection. The transfer of heat from the center of the prod- 

 uct to its outside surface is accomplished by conduction and is a function 

 of product conductivity, temperature difference, and product thickness. 

 The heat is generally transferred to the cooling medium, whether it be 

 air, brine, or refrigerant circulating in contact plates by forced convection 

 or radiation. The transfer of heat to the cooling medium varies with the 

 velocity of the gas or liquid, its thermal properties, and, of course, its 

 temperature. 



The time of freezing is a direct function of the average temperature 

 difference between the product and the cooling medium. Ordinarily the 

 freezing time decreases in direct proportion to the increase in temperature 

 difference between the product and the cooling medium. 



The time of freezing is also a function of product thickness. As a rule 



