I4 2 PHYSIOLOGICAL CHEMISTRY. 



mentation there is usually a bacterial fermentation with pro- 

 duction of acid. In some kinds of bread this is extremely 

 important. In the preparation of sauerkraut and many pick- 

 les a lactic acid fermentation is the characteristic feature. 

 Several well-known beverages produced from milk are fer- 

 mented in such a manner that they contain lactic acid ; kephir 

 and kumyss are illustrations. Yeasts and the lactic acid bac- 

 teria work together in many instances and symbiotic products 

 are the rule, perhaps, rather than the exception in fermenta- 

 tions. In the milk industries these mixed fermentations are 

 apparently essential in the ripening processes, and in certain 

 distillery fermentations with yeast a lactic acid fermentation 

 is encouraged to prevent the growth and action of the. bacteria. 

 This fermentation lactic acid is found also in the stomach and 

 the intestine. In the stomach the formation of any large 

 amount is usually impossible because of the presence of hydro- 

 chloric acid. About o.i per cent of free hydrochloric acid is 

 sufficient to impede the lactic fermentation. Free mineral 

 acids are not present in the intestine ; the organic fermentation 

 acids may therefore be formed in appreciable quantities. 

 Fermentation lactic acid must not be confounded with the 

 isomeric sarcolactic acid found in the muscles. 



Butyric Acid Fermentations. Another very important 

 kind of acid fermentation is that which results in the forma- 

 tion of normal butyric acid : 



CeH 12 6 = 2H 2 + 2C0 2 + GHsOa. 



As in the case of lactic acid this butyric acid fermentation is 

 not the result of the action of one organism only, but it may 

 be produced by several, and furthermore several by-products 

 are always produced in quantity. The above reaction is then 

 merely a limit reaction, which is approached but never abso- 

 lutely realized. 



In the milk fermentation the lactic acid or calcium lactate 

 formed may be further changed to butyric acid, the necessary 

 ferment entering from the air. Most river waters contain 

 butyric acid bacteria, which bring about the characteristic fer- 



