100 Dairy Bacteriology. 



of two to three weeks, if the layer of the milk is not too 

 deep (an inch or less), the chemical reaction of the milk 

 becomes neutral or alkaline. 



(4) The putrefactive bacteria which found their way 

 into milk when it was first drawn, and which have re- 

 mained dormant in the sour milk, now find favorable 

 conditions for growth. As a result of their activity, the 

 milk soon undergoes a putrid decomposition, which is 

 marked by offensive odors. 



If the milk is placed under such conditions as will ex- 

 clude the growth of the mold, such as where the air is 

 excluded from the surface, the sour milk will remain in 

 that condition for an indefinite period, since the putre- 

 factive bacteria are inhibited in their development by 

 the acid, in a manner comparable to the preservation of 

 pickles in vinegar, or the keeping of silage because of 

 the acid that is produced as a result of the changes that 

 the plant tissue undergoes when excluded from the air. 

 The preservative effect of acids is of much importance 

 in the case of certain dairy products (see Chapter VIII). 



Fermented drinks from milk. Within the last few 

 years a great deal of attention has been directed toward 

 the preparation of various kinds of drinks from milk. 

 The use 'of such beverages has rapidly increased. But- 

 ter milk is one which meets with the greatest approval. 

 The true butter milk from cream that has been soured 

 by the desirable acid-forming bacteria has a mild agree- 

 able acid taste, wholly free from any sharpness that is 

 often noted in butter milk made from cream in which 

 considerable numbers of the undesirable acid-forming 

 bacteria have grown. Butter milk made from pasteur- 

 ized cream soured with pure cultures will have good 

 keeping qualities and is a most healthful drink for all 

 classes of people, even for young children. 



