228 COMMON CHEMICAL PROCESSES 



( 242), breaking up the dextrose into carbon dioxide and 

 alcohol. Since dextrose occurs widely in fruits, this sort 

 of fermentation is very common. All 

 sorts of alcoholic stimulants wines, 

 whisky, etc. are made by allowing 

 something to become fermented. 



Yeast is a very low form of plant ; 

 in its nature it is somewhat like bac- 

 teria. Fig. 149 shows a few bits of 

 yeast, greatly magnified. It is found 

 frequently in the air, from which it easily gets into 

 many substances. 



Other sorts of fermentation are common. Apple juice 

 partly ferments and becomes cider; then another fer- 

 ment acts upon the alcohol in the cider, changing that to 

 an acid, so that the liquid becomes vinegar. The sour- 

 ing of milk is a process of fermentation. 



268. Bread Making. Yeast may be easily made to 

 grow till it forms a large mass. This is commonly done 

 by many cooks, who make what they call potato yeast ; 

 in this case the potato serves as a substance in which 

 the yeast plant may grow. Cakes of compressed yeast 

 may be bought of the grocer ; these are masses which 

 have been grown for the purpose in large quantities. 



Bread is made of flour mixed with milk or water 

 to form dough ; yeast is added to " raise " the dough. 

 Flour is largely starch. When the mass is put in a 

 warm place the yeast acts upon the starch, changing 

 it to dextrose; this is further acted upon, so that it 

 ferments, forming alcohol and CO 2 . The CO 2 cannot 



