268 TEXTBOOK OF PLANT PHYSIOLOGY 



occurs only in the presence of air. In this respect it shows an 

 evident resemblance to respiration, but differs from the latter in 

 that the products of oxidation are not carbon dioxide and water, 

 but acetic acid. The quantity of energy liberated is consequently 

 smaller than in complete oxidation. Acetic-acid fermentation is 

 therefore characterized by the transformation of as large quanti- 

 ties of substances as are other types of fermentation. 



Acetic-acid fermentation is produced by several species of bac- 

 teria, and by certain yeasts and molds. It takes place according 

 to the following equation 



CH3CH2OH + 2 = CH3COOH + H 2 + 117 Cal. 



This rather simple chemical reaction may proceed quite 

 smoothly, even under the action of inorganic catalysts, like spongy 

 platinum. By a process similar to that used for obtaining zymase, 

 a special enzyme capable of oxidizing alcohol can be secured from 

 acetic bacteria, but its action is considerably weaker than that of 

 living cells. The bacteria causing this fermentation are consider- 

 ably less sensitive than other organisms, to acetic acid or vinegar, 

 the product of their vital activity. They can withstand as much 

 as 6 to 10 per cent of this acid, therefore, the presence of acetic- 

 acid bacteria in a liquid checks the development of all other 

 microorganisms. When the supply of alcohol in the liquid is 

 exhausted, they will proceed to a further oxidation of the pre- 

 viously accumulated acetic acid, ending in carbon dioxide and 

 water. Hence in technical production of vinegar care must be 

 taken to remove the product obtained from the place of its for- 

 mation. 



Acetic-acid fermentation is applied in industry for obtaining 

 table vinegar. According to a French method of preparing vine- 

 gar, wine is poured into broad shallow bowls and inoculated with 

 the so-called mother of vinegar, a film of vinegar bacteria. The 

 film spreads rapidly, soon covering the whole surface of the liquid 

 and within a few days the wine has become converted into vinegar. 

 According to a German method, the wine or beer is allowed to flow 

 slowly over beechwood shavings, which fill a high conical cask 

 provided with a ventilation funnel. The shavings serve as a firm 

 substratum on which the vinegar bacteria have settled. They oxi- 

 dize the alcohol in the liquid which runs slowly over them. 



