TERMENTATION 



95 



/f^l 



fundamental processes of plant physiology. Fermen- 

 tation is most commonly associated in our minds with 

 yeast. 



92. Yeast. — Practically everyone is acquainted with 

 yeast, which was the earliest recognized agent of fermen- 

 tation. We are now most familiar with it in the form 

 of small cakes, purchased at 

 the grocer's for use in making 

 bread and other "raised" 

 dough. Our grandparents 

 bought it in liquid form from 

 the local baker; and in brew- 

 eries and large bakeries it is 

 used in the liquid form in mak- 

 ing beer and bread. If a small 

 piece of a "compressed yeast" 

 cake, about the size of a pea 

 seed, is placed with a little 

 sugar and water in a fermenta- ^ ,, ^ 



. / . \ • ^'^^- ^^- — Fermentation-tube, 



tion-tube (Fig. 66), and set in /, level of fermenting liqu|d 



a warm place the mixture will 



soon begin to "work," and tiny 



bubbles of gas will be seen rising in increasing numbers 



to the top of the tube. The process which gives rise to 



these bubbles is alcoholic fermentation. 



93. Conditions Necessary for Alcoholic Fermentation. — 

 If two other tubes are prepared precisely like the first 

 one, except that ice-water is used in one and boiling water 

 in the other, and are set, the first in a cold place {e.g., the 

 refrigerator), and the second in a very warm place, fer- 

 mentation will occur either very tardily or not at all. 

 If a third fermentation-tube is set up with sugar but no 



space filled with gas (CO2) 

 given off by fermentation. 



