Page:The story of milk.djvu/48

 lactic acid produced by them would play havoc with the food and produce putrefaction or disease. Everybody who handles milk knows that pure sour milk or butter-*milk in which lactic acid bacteria abound keeps well for a long time, free from other fermentations which have no chance to develop in their presence. It is due to this purifying property that Metchnikoff recommended Bulgarian sour milk as a health food, asserting that it prevents harmful fermentations in the digestive channel.

The Control of Bacteria

The principal means at our disposal to prevent or control and regulate bacterial growth are:

Cleanliness, Heat, Cold and Disinfectants.

Cleanliness.—Only the most scrupulous cleanliness will prevent contamination. Hence the necessity of thorough scouring and sterilizing of all utensils, and the need for fresh air and pure water.

Heat.—The prevention of fermentation in milk by heating is called "sterilization" or "pasteurization" according to the intensity of the heat and the length of time the milk is subjected to it. Boiling destroys almost all bacterial life. Some germs require, however, exposure to a much higher temperature, up to 250° F. (boiling under pressure), to be entirely eradicated, but for all practical purposes a thorough boiling is considered sufficient to eliminate all danger of contamination. Such boiling is usually termed Sterilization.

Cooling checks bacterial growth and but few germs thrive at a temperature below 50° F. The following table and diagram show how quickly bacteria multiply