BUTYRIC ACID FERMENTATIONS 285 



5. Butyric Acid Fermentations. When a mixture of 

 milk, old cheese, sugar, and chalk or calcium carbonate 

 is kept warm fermentation soon begins ; a great evolu- 

 tion of gas occurs, and butyric acid is formed, which, 

 when neutralized by the chalk, yields calcium butyrate. 

 In the process lactic acid or calcium lactate is first 

 produced, and this is fermented subsequently with the 

 formation of calcium butyrate. 



The nature and cause of this and similar reactions 

 where carbohydrates are broken down with the pro- 

 duction of butyric acid were studied by Pasteur, Van 

 Tieghem, and others. They showed that the fermenta- 

 tions were brought about by the action of a group of 

 anaerobic bacteria which are widely distributed in water, 

 milk, cheese, soil, dung, and other materials. 



A large number of butyric acid bacteria have been 

 described since Pasteur's time as if they were distinct, 

 but the recent investigations of Beijerinck, Schattenfroh, 

 and Grassberger have shown that most of those described 

 by earlier writers are varietal forms of a few distinct 

 species only. 



In the fermentation of calcium lactate this substance 

 is changed into calcium butyrate, but at the same time 

 certain bye-products are formed in variable amounts ; 

 the gases carbon dioxide and hydrogen are given off, 

 and small quantities of ethyl and butyl alcohol are 

 frequently formed along with propionic and other acids. 



The two stages in the change of milk-sugar into 

 butyric acid may be expressed thus : 



(i) CuH^Ou + H,0 = 4C.H.O, 



milk-sugar lactic acid 



(2) 2C 3 H 6 C>3 = C 4 H ? 0, + C0 2 + H 2 



lactic acid butyric acid carbon hydro- 

 dioxide gen 



