22 THE STORY OF MILK 



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 recom- 

 mended 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 consid- 

 ered 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 

 * Farmers' Bulletin No. 602, U. S. Dept. of Agriculture. 



