THE MANUFACTURE OF VINEGAR. 453 



which is to expose the liquids prepared as above to the action of acetic 

 bacteria with full access of atmospheric oxygen at a suitable temperature. 

 The rapidity of the process depends on the number of active bacteria 

 present, the nutritive value of the liquid, the temperature, and especially 

 on the free access of oxygen. , 



STARTERS AND PURE CULTURES. The 10 per cent of vinegar added 

 to the liquid to be fermented usually contains sufficient bacteria to insure 

 a prompt start. Where this is not the case, a starter may be prepared 

 by exposing a suitable liquid in a shallow vessel to the air of a warm room 

 for several days. Any liquid containing about 4 per cent of alcohol, 

 2 per cent of acetic acid and a moderate amount of nitrogenous matter 

 is suitable. A decoction made by boiling 50 g. of fresh yeast in 1,000 c.c. 

 of water, filtering and adding the proper amount of vinegar and wine or 

 beer will serve. After thorough aeration, such a liquid in a few days 

 becomes covered with a film of acetic bacteria. This film may be used 

 as a starter by gently submerging the vessel in which it is formed in the 

 liquid to be acetified, or by removing with a clean sliver of wood which 

 is afterward floated in the liquid. 



In practice, such a starter gives a sufficiently pure fermentation of 

 acetic bacteria. The particular species of acetic bacteria, however, is 

 left to chance. Pure cultures of a particular selected form would in all 

 probability improve the certainty of the production of good vinegar, 

 but the method has not entered into general practice. 



APPARATUS. Most metals of all kinds should be avoided as much 

 as possible. The hoops of barrels and buckets may be protected by a 

 coating of paraffin. Pumps may be of wood or of the special alloys al- 

 ready mentioned, or they may be so constructed that they will not come 

 in contact with the liquids. 



DOMESTIC METHOD. A cask of convenient size (40 to 200 liters) is 

 fitted as illustrated in figure 89. 



The wine or cider to be acetified, after filtering, if necessary, is poured 

 into the cask until it is about one-half to two-thirds full, the object being 

 to have as large a surface as possible for the growth of the bacterial film. 

 Free circulation of air is insured by a 5 cm. hole in each head of the cask, 

 one near the surface of the liquid and one near the top of the cask. These 

 holes should be covered with varnished metal netting to prevent the en- 

 trance of flies. 



The top bung hole is then closed with a cork, through which a funnel 



