72 BACTERIA, YEASTS, AND MOLDS 



wild yeasts. The juice must not, of course, be heated, 

 for this would kill the yeasts and prevent fermentation. 

 The fermentation is not very vigorous, and the amount 

 of alcohol developed not very great. After the fermenta- 

 tion has about stopped, the wine is placed in bottles or in 

 closed casks. The time required for fermentation may be 

 a few weeks (currant wine) or many months (grape wine). 

 In making cider nothing is necessary except to press the 

 juice from the apples and allow it to ferment sponta- 

 neously. Fermentation in any of these cases might be 

 hastened by the addition of yeast. This is occasionally 

 done, but is not a common practice. 



Whatever be the source of the yeast, the process of 

 wine making is simply an ordinary fermenting of the 

 sugar. The carbon dioxide that is produced is allowed to 

 pass off into the air undisturbed during the fermentation, 

 and the liquid gradually becomes filled with alcohol. The 

 final result is the wine, which always contains alcohol in 

 small percentage. After the yeasts stop growing, bacteria 

 may develop in the product and cause further changes, 

 so as to injure its taste, or even totally change its nature, 

 as in the formation of vinegar. (See Chapter IX.) 



The Use of Yeasts as a Source of Carbon 

 Dioxide 



The chief use of yeasts in the household is not to pro- 

 duce fermented drinks but to raise bread. The raising 

 of bread by means of yeast has been brought to a state of 

 great perfection, so that the method of producing a desir- 

 able fermentation in bread dough by means of this product 



