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 for bacteria, 
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 pouter 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: 
