322 BAOTERIOLOGICAL CHEMISTRY 



Acetic Acid Fermentation. Vinegar Fermentation.— 



The production of vinegar from plant sugars via alcohol 

 is one of the oldest fermentation industries. It involves 

 two stages — the conversion of the sugar to alcohol by the 

 action of yeasts, and secondly, the oxidation of the alcohol 

 to acetic acid by various bacteria of the genus Acetobacter. 

 The latter process is strictly aerobic and is accomplished 

 in practice by trickling the alcohol solution over wood 

 shavings impregnated with the bacteria. If too little 

 alcohol is present the acetic acid formed is further oxidised 

 to carbon dioxide and water and lost. The aeration has 

 to be adequate or the oxidation of alcohol stops at the 

 acetaldehyde stage. 



In the ordinary vinegar process the acetic acid is 

 probably formed by direct oxidation of the alcohol via 

 acetaldeh^^de : — 



/OH 



CH3CH2OH + > CH3CHO + H2O > CHgC^OH + — ^ CH3COOH + H2O. 



(acotaklehydo hydrate) - (acetic acid) 



The organisms A. ascendans, A. pasteuria7iimi and 

 A. xylinum have been shown capable of converting 

 acetaldehyde anaerobically to acetic acid by dismutation. 

 The reaction is catalysed by the enzyme aldehyde mutase 

 with the intervention of co -enzyme I as hydrogen carrier, 

 the aldehyde acting as both donator and acceptor of 

 hydrogen : — 



CH3CHO+ Co-enzyme I > CH3COOH+ reduced Co-enzyme I 



CH3CHO + reduced Co-enzyme I > CH3CH2OH + Co-enzyme I 



It is therefore possible that some of the acetic acid in 

 vinegar fermentation is formed by dismutation of the 

 acetaldehyde : — 



2CH3CHO — > CH3CH2OH + CH3COOH. 



The alcohol so formed is then oxidised to acetaldehyde, 



