116 Milk and Its Products 
tions almost immediately after the milk is drawn, and 
continue until the maximum amount of lactic acid has 
been produced. In the lactic acid fermentations one 
molecule ‘of milk-sugar (CwH20n + HO) breaks up 
into four molecules of lactic acid (C3H¢gO3) ‘without 
the formation of any secondary or by-product. The 
presence of the lactic acid serves to coagulate the 
casein, so that curdling of the milk is always an 
accompaniment of the lactic fermentation after it 
has reached a certain stage. The presence of lactic 
acid is unfavorable to the growth of the ordinary 
germs of lactic fermentation, and when a certain 
amount of lactic acid has been formed (about .8 of 1 
per cent of the whole milk), the further develop- 
ment of lactic acid ceases. In milk of ordinary 
quality, this occurs when about one-fourth of the 
milk-sugar has been changed to lactic acid. If the 
acid be neutralized with an alkali, the fermentation 
will then proceed until another portion of milk-sugar 
has been changed to lactic acid; showing that the 
lactic acid simply prevents the growth of the germs, 
and does not kill them. Lactic acid germs are most 
avtive at temperatures between 80° and 100° F.; at 
temperatures below 80° they gradually lose their ac- 
tivity, and below 50° little or no lactic acid will be 
formed. At these low temperatures they are simply 
inactive, not dead. At a temperature of 105° F., 
the lactic germs become inactive, and a large propor- 
tion of them are killed at a temperature from 135° 
to 140° F. In milk, lactic acid fermentation means 
simply souring, and it renders the milk unfit for use, 
