THE PRESERVATION OF FOOD BY COLD 473 



perature of zero, and many kinds multiply at a fraction of a degree 

 above that point. In order definitely to inhibit microbic activity, the 

 food must be frozen. When it is not frozen, bacteria continue to multi- 

 ply slowly at the lowest temperature of storage, and small variations 

 in the temperature and in the humidity of the atmosphere serve to 

 accelerate their activity. Such variations also accelerate diffusion 

 currents in the food substance and so tend to distribute the microorgan- 

 isms and their products. The extent of the resulting chemical changes 

 •in the food will depend upon these factors and upon the nature of the 

 food, the temperature and the length of the period of storage. , 



Changes after Storage. — This is a relatively short period, but in 

 many instances a very important one as regards change in the food. 

 If warmed too rapidly, vigorous currents may be set up in the food 

 mass by the great difference in temperature between the outer portion 

 and the interior, serving to distribute microorganisms and their 

 products. In the case of frozen foods rapid warming fails to restore 

 the original physical structure. , Dry cold foods are likely to condense 

 moisture from the warmer atmosphere unless it is particularly dry, 

 and this condensed water becomes another cause of diffusion currents. 

 In frozen foods the water, in melting, may fail to reenter the food struc- 

 ture, and exude and drip away,xarrying a portion of the soluble con- 

 stituents with it. At this time still more microbes are likely to be added 

 to the food, and, together with those already present, they multiply 

 with increasing rapidity as the temperature rises. As they may be 

 already pretty well distributed throughout the mass of the food, the 

 resulting chemical decomposition is the more rapid. It is well recog- 

 nized that, in keeping qualities, foods removed from cold storage are 

 much inferior to the corresponding fresh foods. 



Refrigeration of Certain Foods 



Meat, Fish and Poultry. — Meat, in this sense the flesh of 

 mammals, is preserved by cold in two ways, by storage above the 

 freezing-point (chilled meat) and by storage at —10° to —4" (frozen 

 meat). Fish and poultry are usually frozen for storage, often in the 

 undrawn condition. 



Mammals killed for chilled or for frozen meat are slaughtered and 

 carefully dressed. For chilled meat the temperature is reduced by 



