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 (Ci2H22On -f EbO) breaks up 

 into four molecules of lactic acid (CsHeOs) 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 

 active 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, 



