308 MARINE PRODUCTS OF COMMERCE 



immediately surrounding the product, to prevent desiccation; (3) an atmosphere 

 free from oxidizing or volatile compounds which might induce "oflE" flavors in 

 the product. 



For economic reasons any facility, to be practicable, must be readily available 

 and inexpensive. Hence, water ice has long been thought of as an ideal sub- 

 stance for the holding of unfrozen fishery products; however, it does not entirely 

 fulfill the above requirements. In the first place its melting point is several degrees 

 above the initial freezing point of most fishery products; and secondly, the ice, in 

 melting, produces water which, if permitted to come in direct contact with the 

 product, has a leaching effect which may detract from the appearance and flavor 

 of the products. 



Most authorities agree that frozen fishery products should be stored at sub-zero 

 temperatures, the exact temperature depending upon the proposed storage life 

 of the product. Also, to prevent the product from disiccating and oxidizing, the 

 atmosphere surrounding it should be saturated with water vapor and free from 

 oxidizing agents. 



In order to make the transportation service economically sound the commercial 

 carriers must have the necessary flexibility in their facilities to accommodate all 

 commodities. The temperature requirements for general piirpose storage do not 

 impose a particularly difficult problem. Modern control equipment is readily 

 available, whereby temperatures within the range —20 to 55° F (—28.9 to 

 12.8° C) can be easily maintained within the necessary limits. 



The control of humidity would be equally simple if fish alone were to be 

 considered; but in the shipment of fruits and vegetables the relative humidity 

 should not vary beyond limits determined by the need to restrict evaporation on 

 the one hand and the growth of molds and bacteria on the other. If the refrigera- 

 tion system can maintain a relative humidity in the neighborhood of 85 per cent 

 it may be regarded as adequate in the light of present knowledge. 



The control of the atmosphere, as in the "gas storage" of fruit or chilled meat 

 in a certain concentration of carbon dioxide, presents a somewhat more complex 

 problem, particularly in regard to the shipment of "living" biological materials, 

 such as fruits, which respire and thus generate heat as well as carbon dioxide. 

 However, attempts are being made to control the atmosphere as well as tempera- 

 ture in cold storage rooms, and the following section will give a brief descrip- 

 tion of such a scheme. 



The Jacketed System 



In a refrigerated space (hold of a vessel, railway car, truck, etc.) filled with 

 frozen fish heat leakage through the walls, floor, and ceiling into the space con- 

 stitutes the principal refrigeration load since the load does not generate heat. On 

 the other hand if the same space is occupied by fruit, the heat generated by the 

 load may be of the same order as that coming through the insulation, or it may 

 be many times the heat leakage if the fruits have not been precooled. The two 

 heat loads vary in proportion with the temperature of the fruit. Since the two 

 thermal loads vary so much in their relative proportions and since they must be 

 removed, one from the outside, the other from the inside of the space, it is 

 reasonable to arrange the refrigerating system so that they are separately dealt 

 with as far as practically feasible. 



