THE PRESERVATION OF FOOD BY COLD. 397 



to inhibit microbic activity, the food must be frozen. When it is not 

 frozen, bacteria continue to multiply 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 microorganisms 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 stuff. 

 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 food stuffs are likely to condense moisture 

 from the warmer atmosphere unless it is particularly dry, and this con- 

 densed water becomes another cause of diffusion currents. In frozen 

 foods the water, in melting, may fail to reenter the food structure, and 

 exude and drip away, carrying a portion of the soluble constituents 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 decom- 

 position is the more rapid. It is well recognized that, in keeping quali- 

 ties, foods removed from cold storage are much inferior to the corre- 

 sponding 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, the former often in the undrawn 

 condition, the latter sometimes so. 



Mammals killed for chilled or for frozen meat are slaughtered and 

 carefully dressed. For chilled meat the temperature is reduced by storage 

 in a cold air chamber to about + 2 in 48 hours, and the meat is stored at a 



