THE MILK IN ITS RELATION TO CHEESE 15 



Ordinary milk makes a very favorable medium for the 

 growth of bacteria, because it is an adequate and easily 

 available food supply. 



In milk, certain groups of bacteria are commonly 

 present, but many others which happen to get into it 

 live and multiply rapidly. A favorable temperature is 

 very necessary for such organisms to multiply. There is 

 a range of temperature, more or less wide, at which 

 each group of bacteria grows and multiplies with the 

 greatest rapidity. This range varies with the different 

 groups, but most of them find temperatures between 

 75 F. and 95 F. the most favorable for growth. Ex- 

 cessive heat kills the bacteria. Low temperatures stop 

 growth, but kill few if any bacteria. Temperatures of 

 50 F. and lower retard the growth of most forms of 

 bacteria found commonly in milk. Many forms will 

 slowly develop, however, below 50 and some growth 

 will occur down to the freezing point. Milk held at 50 

 F. or lower will remain in good condition long enough 

 to be handled without injury to quality until received in 

 the cheese factory. In the place of seeds, some groups 

 of bacteria form spores. The spores are exceedingly 

 resistant to unfavorable conditions of growth, such 

 as heat, cold, drying, food supply and even chemical 

 agents. This property makes it difficult to destroy such 

 bacteria. 



21. Groups of bacteria in milk. Milk when first 

 drawn usually shows an amphoteric reaction; that is, 

 it will give the acid and alkaline reactions with litmus 

 paper. Under normal conditions, milk soon begins to 

 undergo changes, due to the bacteria. Changes pro- 

 duced in this way are called " fermentations " ; the 

 agents causing them, "ferments." Normally the acid 



