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TEXT-BOOK OF PHYSIOLOGY 



iooF. In the presence of heat and moisture the natural ferment of the 

 flour diastase converts a portion of the starch into sugar, which in turn 

 is split up into carbon dioxid and alcohol by the yeast plant. The sugar that 

 is added undergoes a similar change and hastens the process. The bubbles 

 of carbon dioxid, becoming entangled in the dough, cause it to swell or rise 

 and subsequently give the porous or spongy character to the bread. When 

 baked at a temperature of 4ooF., the alcohol is largely driven off; yeast cells 

 and other organisms are destroyed; the starch, particularly that on the 

 surface, is dextrinized. The principal salts contained in wheat flour are 

 potassium and magnesium phosphate. 



Composition of Vegetable Foods. The average composition of 

 some of the principal vegetables is shown in the following table: 



The vegetable foods, as a class, vary considerably in nutritive value and 

 digestibility, the latter depending on the amount of cellulose they contain. 

 A section of a vegetable shows not only the presence of an external cellulose 

 envelope, but also an inner framework which penetrates its substance in all 

 directions. The nutritive principles are contained in small cavities, the 

 walls of which are formed by the framework. Nearly all vegetables require 

 cooking before being eaten. When subjected to heat and moisture, not 

 only is the texture of the vegetable softened and disintegrated, but the 

 starch grains are hydrated and partially prepared for conversion into dextrin 

 and sugar. At the same time various savory substances are set free, which 

 make the food more palatable. 



Beans and peas contain large quantities of a protein, legumin, and 

 starch, and hence are especially valuable as nutritive foods. The presence 

 of the cellulose envelope, especially in ripe beans and peas, combined with 

 rather a dense texture, renders them somewhat difficult of digestion. Pota- 

 toes, though largely employed as food, are extremely poor in protein, 2 per 

 cent., and carbohydrates, 20 per cent. When sufficiently cooked they are 

 easily digested, owing to the small amount of cellulose they contain. 



1 With butter etc., added. 



