Page:Popular Science Monthly Volume 81.djvu/160

154 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 subjected 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 decomposition. Fresh fodder for cattle is fermented into silage and grapes into wine. Acids for pickling vegetables, fruit and meat; sugar in concentrated solution for preserving berries and fruit; these means have long been utilized. High and low temperatures also have been used extensively 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 decomposition of organic matter was due almost exclusively to the vital activities 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 multiplication. 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 decomposition. 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. Antidiluvian 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