MILK FERMENTATIONS 81 



the fresh milk the various types of bacteria and molds con- 

 cerned in the changes which the milk will undergo. For 

 instance, it is frequently impossible to demonstrate by plate 

 cultures the presence of acid-forming bacteria or of acid- 

 destroying molds in fresh milk. If, however, the milk is 

 allowed to remain for some days at ordinary temperatures, 

 these forms nearly always appear. 



Exercise. In each of two liter flasks should be placed raw skim 

 milk to the depth of 1 inch, the flasks being closed with cotton 

 plugs. One of the flasks should then be heated to 75 C. for a few 

 moments. Incubate both flasks at 20 C. for at least a month, not- 

 ing at frequent intervals the changes occurring. 



From the white velvet-like growth appearing on the raw milk 

 (Oidium lactis) microscopic preparations, stained and unstained, should 

 be prepared. 



Explain the various changes noted in the two flasks. 



Butyric-acid fermentation. In the soil, in manure, and in 

 barn dust are constantly found bacteria that are able to fer- 

 ment various carbohydrates, such as dextrose and lactose, 

 with the production of butyric acid and other by-products, 

 such as lactic and acetic acids, carbon dioxide, and hydrogen. 



Milk usually contains representatives of this group of bac- 

 teria. Butter and cheese also contain them, and are some- 

 times injured by them. 



As a group the butyric-acid bacteria are characterized by 

 their relation to oxygen. They are anaerobic, and grow only 

 in the absence of free oxygen when in pure culture. In milk, 

 butter, and cheese the oxygen is consumed by the aerobic 

 forms present ; hence the conditions, as far as oxygen is con- 

 cerned, are favorable for the butyric-acid bacteria. Other con- 

 ditions are less favorable. The large amount of acid formed 

 in milk and cheese by the lactic-acid bacteria inhibits the 

 butyric-acid bacteria. 



