THE CHEMISTRY OF BACTERIA. 97 



In the production of wine, the yeast-plant is not added to 

 the grape- juice. It is present upon the surface of the 

 grapes, and hence, passes into the liquid when the grapes 

 are crushed. As yet, pure cultures of yeast have not been 

 employed in the manufacture of wine. 



The best known species of the yeasts is the saccharo- 

 myces cerevisioB. Some species will produce more alcohol 

 than others. Beer, as a rule, contains less than 5 per cent. , 

 but in other sugar solutions, as in wines, the amount of 

 alcohol may rise to 10 or 15 per cent. This may be consid- 

 ered as the maximum limit of alcohol formation, inasmuch 

 as this amount tends to inhibit the growth of the yeast. 

 The so-called "wild-yeasts" or torulae do not yield more 

 than about one per cent, of alcohol. 



The production of alcohol is not restricted to the yeast 

 family. There are many bacteria which can convert 

 glycerin into alcohol. Friedlander's bacillus of pneu- 

 monia when grown on sugar media produces alcohol and 

 acetic acid. The same is true of the typhoid bacillus. A 

 number of bacteria which are present in the mouth and in 

 the intestines of normal persons are capable of producing 

 alcohol. Many moulds, especially mucors, possess the 

 power of forming alcohol out of sugar, and at least one of 

 these organisms is utilized on a commercial scale for the 

 preparation of alcohol. 



In alcoholic fermentation alcohol is the chief, the 

 characteristic product. A large number of secondary or 

 by-products are always present. Among these may be 

 mentioned, carbonic acid, aldehyde, the higher alcohols 

 such as propyl, butyl and amyl, which are commonly known 

 as fusel oil. Glycerin, acetic and succinic acids are likewise 

 always present. 



The yeast-cell when destroyed by a process of grind- 

 ing yields a soluble ferment, an enzyme, which can convert 

 sugar solutions into alcohol and carbonic acid. This fer- 

 ment is made and stored up within the cell. It may possi- 



