io6 



BACTERIA, YEASTS, AND MOLDS 



carbon dioxide gas are left as pores in the bread (Fig. 43). 

 This makes the bread light and porous, and gives it the 

 character that every one is familiar with in properly raised 

 bread. If it were not for these holes, the dough would 

 be a hard, tough mass, difficult to bake and more difficult 



to digest. 



The purposes of the 



raising of bread by yeast 



are three. 



1. It makes the ma- 

 terial lighter, i.e. more 

 porous, and hence easier 

 of mastication and more 

 palatable. 



2 . It renders it more di- 

 gestible, because the por- 

 ous material is more easily 

 acted upon by the digest- 

 ive juices than the more 

 solid unleavened bread. 



3. The yeast imparts a certain flavor to the bread 

 which enhances its value. This flavor, due to yeast, is 

 well shown by the difference in the flavor of bread raised 

 in the ordinary household and that sometimes raised by 

 bakers, where a different species of yeast is used. 



That the flavor produced by yeast is an important 

 factor may be realized also by comparing the flavor of 

 bread raised by yeast with that made light by chemical 

 or mechanical means. Any process which will fill the 

 dough with bubbles will make it light. In one type of 

 bread, known as aerated bread, the spaces or cavities in 



Fig. 43. The same material after bak- 

 ing, showing the cavities left after 

 the carbon dioxide is expelled. 



