62 



HOUSEHOLD BACTERIOLOGY 



These are the agents which make the vinegar. 

 Not all vinegar used in the household is made from 

 cider. The large manufactories usually use alcohol 

 or wine as the base of the process. Alcqhol and acetic 

 acid contain the same elements in different porportions, 

 the former having less oxygen. The bacteria of 

 mother-of-vinegar are able to take oxygen from the 

 air, cause it to unite with the alcohol, and thus make 

 acetic acid. In actual practice a weak alcoholic solu- 

 tion is allowed to trickle 

 slowly over beechwood shav- 

 ings. In this way a large sur- 

 face is exposed to the air. It 

 is found that if the shavings 

 are sterilized, that is, if all 

 micro-organisms are removed, 

 no acetic acid is formed, thus 

 proving that here again we 

 are indebted to our dust-plant 

 friends. 



Lactic acid, the acid of sour milk, and acetic acid, 

 the acid of vinegar, are two desirable acids due to 

 bacterial growth, while a third, butyric acid, not desir- 

 able to the housewife, results when such growth takes 

 place in fats. This is the chief cause of rancidity in 

 butter and other oily substances and the similar taste 

 or smell in old milk. To the housewife this means 

 loss of food supplies and therefore comes under the 

 unfriendly work of dust-plants. 



FIG. 38. Bacteria 

 "Mother-of- Vinegar. " 



