96 BACTERIOLOGY. 



this type of fermentation is that seen in the conversion of 

 alcohol into acetic acid. 



CoHeO + Oj = C,H.02 + H^O. 



Alcohol. Acetic acid. 



The oxidation of NH, to HNO2 and to HNO3 is another 

 illustration. 



Lastly, the fermentation process may be largely one of 

 reduction. Thus, nitrates may be reduced to nitrites and 

 even to ammonia. Glycerin may be changed in this way 

 into ethyl alcohol. Eeduction changes can be easily recog- 

 nized if the medium has been previously colored with, 

 litmus. The medium, in that case, becomes colorless*. 



Alcoholic fermentation.- — This fermentation is of the 

 utmost practical importance, inasmuch as all the alcohol of 

 commerce is thus obtained. The yeasts are especially 

 endowed with the power of producing alcohol. It should 

 be remembered that this group of plants differs from the 

 bacteria in many important respects (p. 16, also Chapter 

 XII). Moreover, there are a large number of species of 

 yeasts which differ not only in the amount of alcohol to 

 which they can give rise, but also in the amount and char- 

 acter of other by-products. It is the presence of these 

 secondary products which influences to a large extent, the 

 appearance, odor and taste of beer and other alcoholic 

 liquors. In this fermentation, as it has been carried on 

 since ancient times, it was always possible for diverse 

 yeasts and bacteria to develop, and thus, more or less, alter 

 the final product. This uncertainty in the process has been 

 overcome by the employment of a pure culture of a yeast 

 of known power. The work in large breweries has thus 

 been placed on a scientific basis*. The pure yeast which is 

 used must be free from other species of yeasts as well as 

 from bacteria. In this way a constant product is assured. 



