94 GENERAL BIOLOGY 



If this formula be studied it will be discerned that the 

 chemical compounds of the food of yeast are intermediate in 

 kind between those of animals and those of green plants. 

 Some of the same mineral salts are used by both green and 

 colorless plants. The nitrogen is obtained from a somewhat 

 more complex compound in the latter. Only the sugar is 

 properly an animal food. Proteids such as animals require 

 are wholly lacking. It will be noted that there is carbon in 

 the formula aside from the sugar : the yeast will live, indeed, 

 in this solution if the sugar be omitted but its growth will 

 then be very slow. It will be noted also that the sugar is 

 present in very great excess of the need of the yeast for 

 carbon. The yeast plant contains a sugar ferment. It 

 utilizes only about one per cent of the sugar, and decomposes 

 the remainder into carbon dioxide and alcohol. The re- 

 action of the fermentative decomposition may be expressed 



as follows : 



Carbon 

 Sugar Alcohol dioxide 



C 6 H I2 6 - 2 C 2 H 6 + 2 C0 2 



It is the production of these two by-products that makes 

 yeast commercially important. Yeast produces the same 

 reaction in the sugars of cider and wines, and in the meta- 

 morphosed starches of the cereal grains, that are chiefly used 

 in commerce in the production of alcohol. The carbon 

 dioxide is also utilized in the making of bread. Yeast is 

 mixed with the dough, and, fermenting in it, evolves the 

 carbon dioxide gas, which "raises" it, making it porous, 

 and improving its digestibility and flavor. 



If a little fresh yeast be sown in a bottle of Pasteur's 

 solution (or even in a 15% sugar solution made with tap 

 water, which will be likely to contain enough of the mineral 

 salts for considerable growth), and kept in a moderately 

 warm place, within twenty-four hours abundant growth 

 will be evidenced by the increasing turbidity of the liquid, 



