ACID FERMENTATION 157 



stances whose chemical constitution can be determined, that 

 a sure advance in our knowledge of the chemistry of vital 

 action is likely to be attained. 



The technical applications of the activity of acid-forming 

 bacteria are numerous and important. 



The production of vinegar is due to the activity of various 

 species of bacteria which bring about the oxidation of alcohol 

 to acetic acid. Different qualities of vinegar are obtained 

 according to the process used. In France wine is allowed to 

 become sour in vats which are first filled with vinegar, wine 

 being gradually added, with simultaneous withdrawal of a 

 portion of the vinegar. The wine becomes charged with 

 acetic acid bacteria and is rapidly converted into vinegar, 

 when the withdrawal of the vinegar formed and the addition 

 of more wine is repeated. 



A more rapid process is in use in Germany and also in 

 England, according to which dilute alcohol is slowly passed 

 over beech wood shavings contained in large vats, suitably 

 ventilated to allow free passage of air. The shavings are 

 previously sown with acetic acid bacteria. A rapid oxidation 

 of the alcohol takes place. 



In the tannery acid-forming bacteria also play their part. 

 In order to remove hair from hides, they are generally first 

 soaked in lime, which has to be thoroughly removed from 

 the skin before the tanning process. This removal takes 

 place partly in what is known as the ' puering ' or ' bating ' 

 process and partly in the subsequent ' drenching.' In the 

 ' puering ' process the skins are placed in a bath of dog's dung 

 or similar material, when, in addition to many other changes, 

 e.g., the action of proteolytic enzymes on the albumin con- 

 stituents of the skin, ammonium salts of butyric and other 

 acids are formed, which exercise a solvent action on the 

 lime. This is completed in the ' drenching ' process, where 

 the skins from the bate, after washing, are placed in an 

 infusion of bran. A mixture of organic acids, chiefly lactic, 



