150 DAIRY FARMING 



Most of the curdling and digesting bacteria are spore 

 bearing and can thus withstand unfavorable conditions 

 better than the lactic acid bacteria. For this reason milk 

 that has been heated sufficiently to kill the lactic acid 

 bacteria, will often undergo the undesirable changes 

 attributable to the digesting and curdling organisms. 



3. BUTYRIC FERMENTATION. 



It was mentioned that many bacteria have the power 

 of producing lactic acid but that the true lactic acid fer- 

 mentation is probably caused by a single species. So it 

 is with the butyric acid bacteria. While a number of 

 different organisms are known to produce this acid, Conn 

 is of the opinion that the common butyric fermentation 

 of milk and cream is due to a single species belonging 

 to the anaerobic type. 



The butyric acid produced by these organisms is the 

 chief cause of rancid flavors in cream and butter. These 

 bacteria are widely distributed in nature, being particu- 

 larly abundant in filth. They are almost universally 

 present in milk, from which they are hard to eradicate 

 on account of their resistant spores. It is on account 

 of these spores and their ability to grow in the absence 

 of oxygen that the butyric fermentation is often found 

 in ordinary sterilized milk from which the air has been 

 excluded. 



The influence of the butyric acid bacteria is felt mainly 

 in butter and in overripened cream. The latter frequently 

 possesses a rancid odor which must be charged to these 

 bacteria, especially since it is known that overripened 

 cream possesses conditions favorable for their develop- 

 ment. Overripening should, therefore, be carefully 

 guarded against. 



