FOODS AND THEIR USES 55 



which causes the starch grains to swell. The cell walls are broken 

 open in this way, and when the grains burst, a larger surface is ex- 

 posed to the action of the digestive juices (Figure 21). This is 

 strikingly shown in popping corn. The crust of bread is more easily 

 digested than the softer parts, and toasting bread increases its di- 

 gestibility, because this browned starch (sometimes called soluble 

 starch) requires less change before it can be used by the body. 



66. Boiling vegetables. Experiments have shown that a good 

 deal of nutriment is lost by boiling vegetables in water. Much of 



FIG. 21. A, cells of raw potato with starch grains inclosed in the cellulose 

 walls. B, cells of a potato well steamed and mashed ; starch grains have 

 been burst by the heat. 



this waste may be avoided, however, if one heeds the following 

 directions: (1) vegetables should be cooked as far as possible in their 

 peels, for these outside coverings keep the sugar, proteins, and min- 

 eral matters from being drawn out by the water ; (2) if the vege- 

 tables must be peeled and cut up, the pieces should be relatively large, 

 as a smaller surface is thus exposed to the water ; (3) the amount of 

 water should be as small as possible, and the vegetables should be 

 cooked rapidly, in order to give less time for the solvent action to 

 take place. 



67. Bread making. When bread is made, water (or milk), 

 butter, salt, sugar, and yeast are added to flour. After the mixture 

 has been stirred together, a sticky mass of dough is formed, which, in 



