56 HUMAN BIOLOGY 



a warm place, begins to rise. This is due to the fact that the yeast 

 cells change the sugar into alcohol and carbon dioxid. Bubbles of 

 gas are thus imprisoned in the sticky dough. While expanding and 

 seeking to escape, the gas makes the solid mass porous. After the 

 bread has risen sufficiently, it is kneaded in order to break up 

 the large bubbles and in order to distribute the gas throughout 

 the dough. When the bread is baked, the alcohol and carbon 

 dioxid pass off into the air, leaving the bread light and digestible. 



VI. Food Economy 



68. Importance of food economy. — It is said that in 

 a large proportion of American families more than half the 

 total income is spent for food, and that the remainder of 

 the income must serve for rent, fuel, clothing, doctor's bills, 

 and other expenses. Hence, any saving that can be made 

 in the annual food bill of a family should result in a surplus 

 which may well serve as a nucleus of a saving's bank account, 

 or may be used in improving the home surroundings or in se- 

 curing wider means of education and enj oyment. The average 

 American, however, is far from economical in the matter 

 of foods. In the first place there is often extravagance in 

 the purchase of food, and in the second place foods are fre- 

 quently wasted in the home. 



69. Comparative cost of foods. — (Home study.) The 

 chart shown in Figure 22 exhibits (1) the cost price of each of 

 the foods represented, (2) the weight of the food that may be 

 purchased for 25 cents, and (3) the weight of the soUd food 

 substances (except mineral matters) that may be purchased 

 in each food for 25 cents. Note at the top of the chart the 

 short vertical lines that indicate 1 pound, 2 pounds, .etc., of 

 solid nutrients; hence, if 25 cents is spent for wheat flour, 

 about f of a pound of protein can be secured, J of a pound 

 of fat, and about 6^ pounds of carbohydrates. 



