44 DAIRY CHEMISTRY 



milk, and as a result the milk sugar undergoes a 

 chemical change with the production of lactic acid. 

 The milk sugar is first split up into two other sugars, 

 dextrose and galactose, and these sugars are then 

 changed into lactic acid. One molecule of milk 

 sugar may produce either two or four molecules of 

 lactic acid, depending upon the nature of the fermen- 

 tation change which takes place. In addition to the 

 lactic acid ferments, there are a great many others 

 that may gain access to milk, develop there, and cause 

 various changes to take place. The action of these 

 ferments will be discussed in another chapter of this 

 work. 



51. Production of Lactic Acid in the Milk. Fresh 

 milk, even when first drawn from the cow, gives a 

 slightly acid reaction, due to the acid character of 

 the casein and to the composition of the mineral 

 matter or ash of the milk. In a few hours, if left 

 exposed to the air, milk readily increases its acid 

 content until a sour taste is developed. The milk 

 then contains from .3 to .4 of a per cent of lactic 

 acid. If the milk contains .4 of a per cent of acid, 

 it will curdle when boiled. The fermentation usu- 

 ally continues until .6 to .8 of a per cent of acid is 

 developed, and then the lactic acid ferments become 

 inactive. The products of the lactic acid ferments 

 are destructive to themselves. The amount of acid in 

 milk is directly proportional to its freshness, and for 

 many purposes milk with more than .1 of a per cent 

 of acid is not suitable. The rapidity with which the 



