THE ELEMENTS OF PLANT FOOD 



43 



substance of the fruit or grain from which they are 

 made. 



Acetic fermentation often follows the alcoholic fer- 

 mentation. If it is not checked by bottling or cooling, 

 the alcohol is broken up into acetic acid and water. 



Alcohol 

 C 2 H 5 OH 



Oxygen from air 

 + 2 = 



Acetic acid 



Water 

 H 2 



Pure alcohol exposed to the air will not oxidize, but 

 when some nitrogenous substance is added to it the 

 acetic fermentation takes place. 



Vinegar consists of water with from about four 

 per cent of acetic acid. It may be made by allow- 

 ing weak spirits of wine or any other weak alcohol to 

 trickle slowly through a 

 cask filled with beech 

 shavings which have pre- 

 viously been soaked in a 

 strong vinegar. The pro- 

 duction of acetic acid is 

 caused by the presence of 

 an enzyme secreted by 

 cells which when collected 

 in masses is called mother 

 of vinegar. 



This mother covers the 

 shavings in the cask and 

 causes the oxidation of the 

 alcohol. This method is 

 called the quick method. If the spirits of wine is placed 

 in a cask with the bung removed to allow the entrance 

 of air, the same result will be produced, but it will 

 take some months to make the change complete. 



FIG. ii. Making Vinegar. 



