464 Bacteria in Relation to Country Life 



alcohol into acetic acid. Similarly, the spontaneous 

 souring of wine and beer was thought to be accomplished 

 by the condensation of oxygen in the pores of the mem- 

 brane formed on the surface of the liquid. The mem- 

 brane itself was regarded by the chemists as a lifeless 

 mass of albuminous matter. 



This purely chemical explanation of acetic fermenta- 

 tion fell to the ground, however, when Pasteur, in 1864, 

 upheld the contention of Kutzing in 1837. Pasteur 

 demonstrated that the " mother of vinegar," the mem- 

 brane on the surface of souring alcoholic liquids, was 

 composed of minute living cells, to whose activities 

 the formation of vinegar was due. He was not willing to 

 admit, however, that the bacteria were directly con- 

 cerned with the change of alcohol into acetic acid, but 

 thought that they helped merely to condense the oxy- 

 gen in the membrane and facilitated thus the oxidation 

 of the alcohol. 



Subsequent investigations showed that the micro- 

 organisms must play a direct part in the production of 

 acetic acid. It was found that the transformation was 

 accomplished best at certain temperatures. When the 

 latter was too low, acetic acid was formed very slowly 

 or not at all. When it exceeded 104 Fahr., the formation 

 of acetic acid stopped. Liquids containing more than 

 14 per cent of alcohol did not undergo acetic fermenta- 

 tion. On the other hand, the oxidation of alcohol by 

 platinum-black was favored by high temperatures, and 

 concentrated alcohol was changed as readily as was 

 dilute alcohol. A further step in advance was made 

 when the acetic ferments were isolated in pure culture 



