SECTION II — YEASTS 



CHAPTER V 



YEASTS AND THEIR DISTRIBUTION 



Fermentation 



Yeasts are the natural agents which produce the phe- 

 nomenon called fermentation. This term has several mean- 

 ings to the scientists, but as the word is commonly used 

 it refers to a process by which alcoholic liquors are pro- 

 duced from sugary solutions. Fermentation is therefore 

 the basis of the various popular beverages known to 

 civilized as well as to uncivilized races. Fermentation is 

 also the foundation of another phenomenon apparently 

 quite different in its character ; for the raising of bread by 

 yeast is just as truly a fermentation as is the manufacture 

 of beer. 



The essential phenomena of fermentation are the 

 destruction of sugar and the production from it of two 

 other substances. The sugar is originally a solid, although 

 it is very easily dissolved in water. It is a somewhat com- 

 plex body, but by the action of yeasts it is easily broken 

 to pieces to form two simpler ones. One of these, alcohol, 

 is a liquid and remains in solution ; the other, carbon dioxide, 

 is a gas and usually passes off from the solution in the 

 form of bubbles (Fig. 31). It is this production of alcohol 



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