Page:Popular Science Monthly Volume 81.djvu/161

Rh temperatures; the other methods will therefore he mentioned but briefly.

Sugar and acids are perhaps the most important of these. By adding large amounts of sugar to perishable foods, such as berries or their expressed juice, and dissolving the sugar by heat, the berries may be kept in good condition for unlimited periods. The same result is obtained by the use of acids in the preparation of pickled vegetables, pickled meat, the curing of hams, etc. Spices also act as preservatives and many delicacies are preserved in oil.

Of these benzoate of soda, formalin, boric acid and salicylic acid are used to some extent. Whether these are unwholesome in the small quantities used is still disputed. The relative amounts are certainly very small and harmful results can only be incurred by continued use. Alcohol is also a powerful preservative and is produced by allowing fruits, etc., to ferment. The perishable grape is thus transformed into stable wine.

The enormous canning industry uses heat as a preservative, followed by exclusion of air. First the microorganisms are destroyed by heat and then renewed invasion is prevented by sealing the containers hermetically. Temperatures, lower than those used for canning, are applied in pasteurizing milk and beer. Pasteurization is carried out by heating milk or beer from 140 to 145° F. for 20 to 30 minutes. This process does not kill all microorganisms, but the great majority, including disease germs. If pasteurized milk is cooled rapidly and kept cold it will remain sweet for days. Pasteurized beer will keep much longer, since the amount of alcohol contained in it aids in protecting against decomposition.

Desiccation—the abstraction of water—has been applied to the preservation of fruits, meat, eggs and milk. Fruits are dried in the open air exposed to the sun or by artificial heat. Desiccated products, of course, can be of no better quality than the original substance. Poor fruit, dirty milk, decomposed meat and aged eggs can not be expected to improve by desiccation. Milk should be clean before desiccation, but unfortunately this is not often the case, and the production of eggs is not as sanitary and well regulated as might be desired. Eggs are rarely, if ever, sterile when freshly laid, the term sterile applying to the total absence of microorganisms. It is certain that, after laying, bacteria