106 BACTERIOLOGY. 



these organisms induce is analogous to that produced by 

 the fat-splitting ferment of the pancreatic juice. 



Butter, as is well known, is apt to become rancid, 

 especially, if it has been imperfectly worked. In that 

 case, more or less casein and other milk constituents 

 remain in the mass of butter-fat and furnish the neces- 

 sary soil for the development of bacteria. Inasmuch as 

 butter contains, among other fats, a small amount of a 

 butyrate, the latter on decomposition will, of course, 

 yield butyric acid. The odor of strong butter is largely 

 due to this acid. It may be well in this connection to 

 mention that the aroma of a good butter is largely due to 

 volatile ethereal products made by bacteria. Certain 

 organisms, which possess the power of improving the 

 flavor, when planted in the cream will render an excellent 

 butter out of what otherwise might have been a poor grade 

 article. 



Fatty acids are not only liberated in the decomposition 

 of fats, but they may also form in the cleavage of proteins. 

 The fatty acids in the free state are not readily split up 

 into simpler products by bacteria. When they unite with 

 metals to form salts they are more prone to fermentation. 

 In that case they are oxidized and form carbonic acid and 

 other compounds. Thus, calcium formate yields calcium 

 carbonate, carbonic acid and hydrogen, whereas calcium 

 acetate will give rise to the same products except that the 

 hydrogen is replaced by marsh-gas. The calcium salts of 

 the higher fatty acids are split up into a variety of pro- 

 ducts. The glycerin, likewise, undergoes fermentation, 

 forming butyric, lactic and carbonic acids. 



Hydrogen sulphide fermentation. In many instances, 

 when certain sulphur compounds are present in a ferment- 

 ing liquid they are converted into hydrogen sulphide. In 

 minute quantity, this gas is formed by nearly all of the 

 pathogenic bacteria, and, inasmuch as it is a highly poison- 



