FOOD ACCESSORIES AND DRUGS 293 



unicellular plant, and when a small amount of it is added 

 to a solution of grape sugar or fruit sugar, it breaks up these 

 substances, chiefly into alcohol and carbon dioxide gas. The 

 latter passes off, while the alcohol remains behind in the 

 solution. In addition to these chief products of fermenta- 

 tion there are always formed other products in small quan- 

 tities, and to these, in part, the flavor of the fermented 

 mixture is due. Different varieties of yeast produce dif- 

 ferent kinds of fermentation. Thus one variety (domesti- 

 cated yeast) is used in making beer, and another (wild 

 yeast) in making wine. The amount of alcohol produced 

 differs with the yeast used, as do 

 also the character and quantity of 

 the secondary products. The growth 

 of yeast, like that of all living fer- 

 ments, is checked by the accumu- 

 lation of the products of its own 

 activity. Consequently when the al- 

 cohol produced reaches a certain per- 

 centage (usually less than ten per FIG. 117. Yeast cells 

 cent) the fermentation ceases. Alco- 

 holic drinks which contain higher percentages of alcohol are 

 prepared by special processes, which will be described later. 

 9. Malt liquors. Malt consists of sprouted grains (chiefly 

 barley). The grains contain a large amount of starch which 

 during the process of germination is converted into sugar 

 by diastase, an enzyme produced by the living cells of the 

 plant the action of diastase being essentially similar to 

 that of the ptyalin of the saliva. The germinating plant 

 thus comes to contain considerable quantities of sugar, to- 

 gether with salts, proteins, and other substances. The 

 watery extract of malt is known as wort, and it is this 

 which, after being boiled with hops, is acted upon by the 

 yeast. The liquid thus produced from wort by fermentation 

 is known as ale, beer, stout, porter, etc., according to the 



