ACID FERMENTATION OF MILK 189 



Only a portion of the sugar of the milk is fermented by 

 these acid-forming bacteria. The casein and the ash con- 

 stituents of the milk are able to unite with a certain amount 

 of acid, but as soon as free acid appears in the milk the 

 growth of these bacteria is checked. The amount of acid 

 normally formed ranges from 0.8 to 1 per cent. 



The acid-forming bacteria of milk are non-spore-bearing, 

 a fortunate provision, since it permits of the use of the 

 pasteurization process as an aid in the preservation of 

 milk. Each of the groups is able to grow both in the ab- 

 sence and the presence of air again a fortunate circum- 

 stance, otherwise the use of either group in dairy manufac- 

 turing would be impossible. 



Milk forms an excellent example of the preservative 

 action of acid against putrefaction. The increase of acid 

 due to the lactic bacteria quickly inhibits the development 

 of types capable of attacking the casein and albumen. As 

 long as the milk remains sour, it is not subject to the action 

 of putrefactive bacteria. But usually there appears on the 

 surface of the sour, raw milk in the course of time a white 

 mold, O'idium lactis, which uses the acid by-products as 

 food, gradually changing the reaction from acid to alka- 

 line. Under these conditions the putrefactive bacteria that 

 are always present are able to develop, and soon the milk 

 is changed to a vile smelling mass. Milk forms an excel- 

 lent example of the sequence of decomposition processes in 

 nature where one type of life is dependent upon the by- 

 products formed by another group of organisms. 



Representatives of another group of acid-forming bac- 

 teria are found constantly in milk. The type of organism 

 is usually called Bact. Bulgaricum. The members of this 

 group are long rods which often occur in threads of con- 

 siderable length. They do not form spores, are facultative, 

 and are often classed as thermophilic, since they grow best 



