298 PRESERVATION METHODS 



The calculation of the rate of cooling solid bodies has attracted much 

 attention, and rather complete analytical solutions have been provided. 

 However, most of the work reported involves complex mathematical for- 

 mulas requiring much labor and knowledge to solve. In 1913 Plank devel- 

 oped his basic equation for freezing foods, and he later modified this'^^-'^-. 



There are many modifications of Plank's basic formula. Some make the 

 calculations unnecessarily difficult, whereas others make them perhaps 

 too simple and inaccurate. The formulas used by Levy^^ and Nagaoka, 

 et alr^, provide for calculation of the cooling and freezing time using a 

 straightforward equation that can be easily solved. 



Plate Freezing. Extensive studies on contact-plate freezing of fish have 

 been conducted by Watzinger, et al.,'^^ and Slavin^^ Application of Plank's 

 modified formula to the freezing of 2 inch thick packages of fish fillets 

 agree with experimental findings. 



Commercial Freezing Methods 



Refrigeration machines used for freezing fishery products in this coun- 

 try vary in design from rather simple batch-freezing units, which require 

 considerable labor in product handling, to automatic loading and unload- 

 ing units that utilize mechanical or electronic controls to regulate opera- 

 tion in accordance with the requirements of the product. These machines 

 usually are classified according to their general physical characteristics as 

 being of the sharp, air-blast, contact-plate, or immersion types. 



The Sharp Freezer. The sharp freezer consists of an insulated room, 

 usually maintained at —20 to — 50°F, containing a number of shelves 

 made from pipe coils through which cold brine, ammonia, or other re- 

 frigerant circulates. Fish frozen by this method are placed directly on the 

 shelves or on aluminum pans or plates covering the pipe coils. 



Heat is withdrawn from the fish by the combined effects of convection, 

 conduction, and radiation. The coils or plates covering the coils remove 

 heat by conduction; the shelf above, by radiation; and the surrounding 

 air, by convection. The heat transfer between the refrigerant in the pipe 

 and the fish at the point of contact is relatively high. However, the over- 

 all heat transfer rate is quite low because of the extensive surface area of 

 the product that is not in contact with the pipes and is cooled largely by 

 the natural circulation of the air within the freezer. The result is slow and 

 uneven product freezing. At evaporator temperatures of —50 to — 20°F, 

 14 and 16.5 hours are required to cool 2 inch thick and 2} 2 inch thick pack- 

 ages of fillets, respectively, from 50 to 0°F. Faster freezing can be accom- 

 plished by using lower temperatures or by forced circulation of air over 

 the product. 



The sharp freezer is largely outdated and its useis limited^toround or 

 dressed fish, such as halibut or salmon; panned fish, such as whiting, 



