i54 THE POPULAR SCIENCE MONTHLY 



ing perishable products. As milk, for instance, perishes within a short 

 time, the nutritive constituents have been transformed for ages into 

 cheese, butter, fermented beverages, etc. During recent years milk and 

 eggs have been evaporated or desiccated. Their keeping qualities are 

 thus enhanced and by adding the requisite amount of water they -can 

 be restored to nearly their original condition. Frozen milk, which, 

 when thawed, represents milk almost equal to the fresh product, is 

 being exported from Denmark in large amounts. 



In a crude and empirical fashion many substances have been sub- 

 jected to preserving agencies for ages. Savages have desiccated meat, 

 or preserved it by immersion in sour milk. The farmer slaughters in 

 the fall and keeps the meat in his cellar, where low temperature and 

 the drying out of the outer layer protect the meat from serious decom- 

 position. Fresh fodder for cattle is fermented into silage and grapes 

 into wine. Acids for pickling vegetables, fruit and meat; sugar in con- 

 centrated solution for preserving berries and fruit; these means have 

 long been utilized. High and low temperatures also have been used ex- 

 tensively for preservation with success. These methods have been 

 reduced by science to certain fundamental principles, which may be 

 considered conveniently under five heads: (1) Harmless preservatives. 

 (2) Chemical preservatives. (3) Heat. (4) Desiccation. (5) Low 

 temperatures. 



These methods to prevent "spoiling" of foods were practised to 

 some extent before the causes of food decomposition were understood. 

 With the dawn of the science of bacteriology it was learned that decom- 

 position of organic matter was due almost exclusively to the vital activ- 

 ities of microorganisms. It became clear that the sole end was the 

 destruction or restraint of development of these minute organisms. 

 Microorganisms require food, but this food must be diluted with water, 

 consequently the abstraction of water by desiccation or freezing, or by 

 boiling in concentrated sugar solutions will restrain their multiplica- 

 tion. Some chemicals are poisonous to bacteria, but are used only to a 

 limited extent on account of possible injury to the consumer. High 

 temperature kills microorganisms rapidly, low temperature slowly, and 

 below the freezing point there is little, if any, multiplication. It must 

 be distinctly understood that in the absence of microorganisms many 

 years must elapse before organic matter can show any signs of decompo- 

 sition. If this fact is borne in mind it will readily be understood that, 

 if microorganisms are restrained from activity or destroyed, no matter 

 by what means, there can be but little change in food substances. Anti- 

 diluvian animals have been disinterred from a bed of ice, where they 

 were buried for ages, and the flesh has been found to be still in good 

 condition. 



The chief object of this article is the consideration of effects of low 



