24 MARKET DAIRYING 
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 heated 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. 
