430 SMITH'S INTERMEDIATE CHEMISTRY 



are nearly all carbohydrate (starch); "that lean beef when dry is 

 nearly all protein; that some seeds (wheat and beans) contain 

 almost no fat, some (oats) much more, and some (almonds and 

 nuts) a very large amount; and that lettuce and other leaves are 

 mainly water, with dissolved inorganic salts (valuable), contained 

 in a light framework of cellulose (non-digestible). 



Digestion of Starch. The carbohydrates, in most foods 

 which contain a large proportion of them, are mainly in the form 

 of starch. The exceptions are milk, sweet fruits, and sugar itself. 

 Starch is insoluble in water, and can not be directly absorbed. 

 But we have seen (p. 401) that, when boiled with a dilute acid, it is 

 hydrolyzed, giving glucose. When bread and potatoes are masti- 

 cated, an enzyme (p. 417), named ptyalin, contained in the saliva 

 (alkaline) turns a part of it, by hydrolysis, into a soluble sugar, 

 maltose. Later, in the small intestine, amylopsin completes this 

 process. Here also another enzyme, maltase, splits the mal- 

 tose into glucose. The glucose then passes through the intestinal 

 wall and so goes into the circulation, where most of it is oxi- 

 dized. 



The cooking of starch (baked bread, boiled potatoes, etc.) breaks 

 up the grains and makes the mixing with the enzyme more perfect 

 and the digestion more rapid and complete. 



Baking Powders. The purpose of the powder is to generate 

 carbon dioxide in the dough. The bubbles of the gas are re- 

 tained by the sticky gluten of the flour. They expand when the 

 dough is baked, and give to it the open texture which, when the 

 bread is eaten, facilitates access of the saliva to every particle. 



Baking soda NaHCO 3 , if used alone, will give off, when heated, 

 half the carbon dioxide it contains (p. 367). The sodium car- 

 bonate which remains in the bread, however, has an acrid taste. 

 By its action on the gluten in the flour, it gives also a yellow color 

 and an unpleasant odor. Finally, the carbonate of soda tends to 



