96 BACTERIA, YEASTS, AND MOLDS 



the bottom. Most of the yeast is then removed, and 

 the liquid stored in casks or bottles. Here the fermen- 

 tation continues for a time, but rather slowly. Since the 

 vessels are closed the carbon dioxide gas cannot escape, 

 but, accumulating in the vessel, is partly dissolved in the 

 liquid itself. The gas exerts considerable pressure inside 

 the bottle or cask, and when it is opened the expansion 

 of the gas gives rise to the popping of the corks and the 

 bubbling and frothing of the beer ; in other words, to the 

 sparkle. Beers are usually drunken while they are toler- 

 ably fresh, and sometimes before fermentation has wholly 

 ceased, though some types of beer are kept for months. 

 In all such fermented products the carbon dioxide is 

 desired no less than the alcohol, since it contributes 

 materially to the flavor of the product. In the production 

 of sparkling wines a similar effect is produced by a sec- 

 ondary fermentation, forming carbon dioxide in the wine 

 after it is placed in the bottles (Fig. 45, b). 



For those interested simply in home problems the fer- 

 mentation of beers is of little importance. It is carried 

 on chiefly in the great breweries, where beer is made on a 

 large scale, but only to a very limited extent in the house- 

 hold itself. It has been, however, a somewhat common 

 procedure in certain households to make homemade beer 

 on a small scale. In previous years this was made from 

 certain roots and extracts of strongly flavored plants. 

 For example : home beers have been made from a mixture 

 of molasses and hops flavored with spruce extract ; or 

 sugar and ginger with lemon for flavor ; or a mixture of 

 sugar, crushed raisins, and lemon; but to-day highly 

 flavored extracts are purchasable at stores for a small 



