ACID FERMENTATION OF MILK 191 



is sometimes difficult to secure with Bad. lactis acidi 

 alone. 



Buttermilk and cottage cheese are to be classed as types 

 of fermented milk. Both form excellent examples of a 

 material that has undergone fermentative changes, and yet 

 is a desirable and a healthful food. Milk in the fermented 

 form is widely used in the Southern States and in certain 

 tropical regions where it would be difficult to keep it in ihe 

 unfermented condition. 



Sweet curdling of milk. Sometimes milk becomes seeded 

 with other organisms than the usual lactic type, and oilier 

 fermentative changes arc produced. A common type of 

 change that is especially likely to occur in the absence of 

 the lactic fermentation is the sweet curdling change in 

 which the casein and albumen are acted upon, the former 

 being precipitated in a manner similar to that caused by 

 rennet. Since this change is occasioned by many spore- 

 forming organisms, heated milks (sterilized or boiled) are 

 peculiarly prone to undergo this change. In addition to 

 the curdling enzymes analogous to rennet, digestive en- 

 zymes are also produced that are similar in their action 

 to trypsin, which is found in the intestinal juices of all ani- 

 mals and which changes protein materials to soluble prod- 

 ucts. The bacterial trypsin gradually digests the curdled 

 casein, so that the change is frequently called the digestive 

 fermentation of milk. The digestive changes produced are 

 very similar to those that take place in the decomposition 

 of all proteins. 



Butyric fermentation. Those organisms that are able 

 to ferment sugars with the formation of the volatile butyric 

 acid are also found in milk, and at times, in the absence of 

 the lactic acid bacteria, produce their characteristic fer- 

 mentation, which is marked by the peculiar odor of the 

 acid. Gas is also produced, and in its initial stages the 



