190 General Biology 



vital actions such as growing and dividing, and yet be able to produce 

 lactic acid. 



Yeast reproduces by budding, also called gemmation ( ). 



A valuable study by the great French bacteriologist, Louis Pasteur, 

 has shown that various inorganic substances could be made into a fluid, 

 and, if yeast cells were placed therein, they could utilize it for growth 

 and reproductive purposes. This ability to use and manufacture new 

 substances from wholly inorganic matter sets the yeasts apart as being 

 a sort of intermediate grouping between even the lowest plants and the 

 inorganic world. 



Yeasts must have oxygen, however, to carry on their work. The 

 anaerobic ( ) bacteria are the only exception among 



living organisms in not needing oxygen. 



It must be remembered that the yeast cell is an organism and is 

 already existent, only making use of these inorganic substances by con- 

 verting them into proteins and carbohydrates, by virtue of the chemical 

 enzymes within the yeast cell itself. 



Yeasts work at temperatures from 9° to 60° C. When fermentation 

 takes place, as in bread, the temperature is raised during the fermenta- 

 tion process by the release of energy. 



Yeast secretes an enzyme which is a sugar ferment. This enzyme 

 may, for example, convert starch into sugar, although yeast "utilizes 

 only about one per cent of the sugar, and decomposes the remainder into 

 carbon dioxide and alcohol. The reaction of the fermentative decompo- 

 sition may be expressed as follows : 



Sugar Alcohol Carbon dioxide 



C 6 H 12 0. 6 = 2C 2 H e O + 2C0 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 metamorphosed starches of the cereal 

 grains, which are chiefly used in industry 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. 



An interesting experiment may be performed by placing a little 

 fresh yeast in a bottle of Pasteur's solution (or even in a 15 per cent 

 sugar solution made with tap water which will be likely to contain 

 enough of the mineral salts for considerable growth). Keep this in a 

 moderately warm place. Within twenty-four hours abundant growth 

 will be evidenced by the increasing turbidity of the liquid, and by the 

 taste of the alcohol in it as well as by the odor of the escaping carbon 

 dioxide 1 arising from it. 



*A simple chemical test of the presence of C0 2 in the escaping gas may be made by thrusting a 

 glass rod with a drop of lime water suspended on it into the mouth of the culture bottle. The calcium 

 oxide (CaO), of which lime water is a solution, readily unites with free carbon dioxide to form a 

 white precipitate of calcium carbonate CaC0 3 (CaO+CO.,— CaC3 3 ) which may be seen to form 

 in the drop. 



