FERMENTATION 97 



96. Alcohol Formed. If a large quantity of fermenting 

 liquid as much as a pint or a quart is boiled, and the 

 vapor that first comes off is condensed to a liquid, this 

 liquid will have the characteristic odor of alcohol, and 

 will burn with a pale, almost colorless flame. It is, in 

 fact, alcohol, and it is on this account that this kind of 

 fermentation is called alcoholic fermentation, to distinguish 

 it from other kinds. Carbon dioxide and alcohol are, 

 therefore, two products of the fermentation of sugar by 

 yeast. If the proportions of yeast, sugar, and water, 

 and the temperature are properly adjusted, and if the 

 fermentation is allowed to continue long enough, it will 

 be found that nearly all the sugar has disappeared, having 

 been converted by the yeast into carbon dioxide and 

 alcohol. Not all of the sugar will be converted for, as 

 Pasteur, the great student of fermentation, demonstrated, 

 small amounts of other substances such as, for example, suc- 

 cinic acid and glycerine, are formed by fermentation, and 

 these finally begin to check the activity of the ferment. 



97. Heat Produced. If a delicate and accurate ther- 

 mometer is inserted into a fermenting yeast-mixture, and 

 the temperature recorded from time to time, it will be 

 found that the mixture grows gradually warmer, indicating 

 that fermentation produces heat. The experiment will 

 succeed best if the yeast mixture is placed in a Dewar flask, 

 or, what amounts to the same thing, in a " thermos" 

 bottle, which is double-walled, and thus retains more of 

 the heat produced than does an ordinary, single-walled 

 container. It is important to keep in mind the three 

 major results of alcoholic fermentation: (i) the formation 

 of alcohol; (2) the formation of carbon dioxide; (3) the in- 

 crease of temperature. We shall see, in the next chapter, 



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