GROWTH OF BACTERIA IN MILK. 159 



they have been studied, prove to be based upon the combined 

 action of yeasts and bacteria. Very likely the bacteria change 

 the milk-sugar into a fermentable form and at the same time sour the 

 product. The yeast is probably responsible in all cases for the 

 alcoholic fermentation proper, although in some the milk souring 

 by the bacteria is the primary feature, while the action of the yeasts 

 is secondary and is not regarded by some as at all essential to the 

 product. In several of these products the type of lactic acid 

 organism mentioned under the name of B. bulgaricus is present. 

 The beverages are generally regarded as more digestible than 

 ordinary milk. 



GROWTH OF BACTERIA IN MILK. 



The number of bacteria that may be in any sample of milk is, 

 in the first place, dependent upon the number and variety that get 

 into the milk during and aftei the milking. But the original 

 contamination is only a small factor in determining their number 

 at any subsequent time. Milk furnishes excellent food focJaacteria, 

 and when drawn from the cow it is warm. Hence a rapid multi- 

 plication of bacteria begins; but although the milk furnishes such 

 an excellent medium for them, they do not begin to multiply 

 at once. For a few hours their number remains the same or even 

 decreases. There seems to be something in fresh milk that injures 

 them. Whatever this may be, its influence ceases after a few hours. 

 This power of checking bacteria growth is sometimes called the 

 germicidal power of milk, and it lasts from three to twenty-four 

 hours, 'according to temperature, being less at higher temperatures. 

 After it has passed, the bacteria begin to increase rapidly and 

 the number present at any later period is more dependent upon the 

 extent of their multiplication than upon the original contamination. 

 The rate of multiplication of all bacteria depends upon temperature. 

 The majority of milk bacteria grow best at temperatures between 

 60 and 100 F., and, generally speaking, they grow more rapidly 

 at the higher temperatures. The effect of temperature is shown: 

 by the following example: 



