BACTERIA IN THE INDUSTRIES. 113 



6. Cider-making. 



Acetic-acid fermentation in wine cider and other fermented alcoholic 

 substances is initiated by the Mycoderma aceti, collectively known as "mother 

 of vinegar." This is no doubt a mixed growth, representing several species 

 or varieties of acetic-acid forming organisms. While it is true that nature 

 invariably inoculates the substances named, resulting in the production of 

 vinegar, it is customary to use the top skin or pellicle (mother of vinegar) on 

 vinegar already formed, adding it to new wine or cider in order to hasten the 

 fermentation. As stated, this is not a pure culture representing a single 

 species. In fact, the tests with what were pure species have proven un- 

 satisfactory. The vinegar organisms require an abundance of oxygen. To 

 supply the necessary oxygen (of the air) it is customary to have the fermen- 

 tation barrels or casks only about two-thirds or three-fourths full and to 

 leave the bunghole open (generally with a plug of cotton) . In Germany a 

 quickened method is much in vogue. The wine or cider is allowed to trickle 

 slowly through a cask filled with wood shavings which are moistened with 

 old vinegar. The wood shavings offer a maximum surface exposure and 

 fermentation is as a result very much hastened. 



Occasionally the vinegar loses its acidity. This is due to the invasion of 

 a bacillus (B. xylenum) which, in the presence of oxygen, splits up the acetic 

 acid into other compounds. This change can be prevented by excluding 

 air from the containers. Vinegar should contain from 4 to 4 . 5 per cent, of 

 acetic acid (the legal standard) . 



