YEAST 43 



resulting in the formation of globules of fat, which accumu- 

 late in the vacuoles. 



Physiology. The yeast plant contains no chlorophyll; 

 therefore, it cannot manufacture its own food. Its food is 

 the substance found in fruit juices, sprouted seeds, bread 

 dough, etc. It obtains carbon dioxide and oxygen from 

 sugar, nitrogen and phosphorus from the soluble proteids or 

 from the soluble salts present in the solution. It cannot 

 take nitrogen from a compound as simple as a nitrate. It 

 can, however, extract the nitrogen from an ammonium 

 tartrate solution. 



Yeast grows best in a solution of malt. When barley is 

 sprouted, it forms malt, which contains diastase, a substance 

 converting the starch of the fruits into dextrose or grape 

 sugar. At the same time a ferment is formed that converts 

 the proteids of the grain into peptones. Malt when ground 

 and added to water forms a solution of soluble proteids, 

 carbohydrates and various inorganic salts and is known as 

 wort. In wort the yeast plant grows and multiplies very 

 rapidly when a temperature of 20 to 30 C. is maintained. 

 Under these conditions the yeast plant produces a ferment 

 called zymase, which converts the dextrose into alcohol and 

 carbon dioxide as follows: 



C 6 Hi 2 6 + yeast = 2C 2 H 5 OH + 2CO 2 



The above process, which is known as fermentation, is not 

 confined to the yeast cell. In fact, every living cell contains 

 one or more ferments which make it possible for it to break 

 up and recombine food to form protoplasm and to decompose 

 food, thus liberating heat and energy for carrying on the life 

 process. 



