FOOD AND DIET. 377 



as much as possible of the water. Cheese is thus rich in protein and 

 fat, the protein being chiefly casein. 



BREAD. 



Flour contains 68 per cent, of starch, 12 per cent, of proteins, and 

 small quantities of cellulose, fats, and salts. When it is kneaded with 

 water, a change takes place in its proteins. These are two in number : 

 gliadin, which is soluble in alcohol, and glutelin, which is insoluble in 

 alcohol. When flour is mixed with water these two substances are 

 converted into the sticky material, gluten. Dough is thus formed 

 mainly of gluten and starch. It is made spongy by the liberation of 

 gases in its interior, usually by the action of yeast. In the process 

 of baking, the starch in that portion of the loaf which is most exposed 

 to the high temperature, namely, the crust, is partially converted into 

 dextrin and dextrose. 



BEEF. 



The composition of lean beef is given in the table on p. 378. Fat 

 beef contains nearly as much protein, more fat, and less water. Mutton, 

 poultry, and white fish contain about the same proportions of the 

 various constituents, but poultry contains rather more protein and a 

 smaller proportion of salts, whereas white fish contains less salt and 

 more water. 



EGGS. 



The white of egg contains three proteins, egg-albumin, egg-globulin, 

 and ovomucoid. The yolk contains a small amount of a phospho- 

 protein, vitellin, and a large proportion of fats, with smaller quantities 

 of cholesterol, lecithin, sugar, and salts. 



GREEN VEGETABLES. 



Green vegetables are of little value as a source of proteins, carbo- 

 hydrates, and fats. They have, however, three important functions in 

 a dietary: (1) as a source of iron, (2) as a source of vitamines, the 

 absence of which leads to the. onset of scurvy, and (3) as stimulants 

 to the peristaltic movements of the digestive tract, in virtue of the 

 indigestible cellulose which they contain. 



GELATIN. 



Gelatin is a sclero-protein, and is formed by boiling collagen, the 

 principal solid constituent of connective tissue. Gelatin cannot replace 

 other proteins in a dietary, because it is deficient in three essential amino- 

 acid groups, phenylalanine, tyrosine, and tryptophane. Tyrosine is oxy 

 phenylamino-propionic acid, tryptophane is indolamino-propionic acid ; 



