384 AGRICULTURAL CHEMISTRY 



both chemically and mechanically to undergo readily in- 

 version changes. In the cooking and preparation of foods, 

 starch rarely undergoes more than the hydration 

 change. In bread-making, for example, only a small 

 portion of the original starch is converted into soluble 

 forms. 



The action of heat upon cellulose and cellular tissue 

 is mechanical rather than chemical. The mass is partially 

 disintegrated. and in the case of some of the cellulose, 

 hydration takes place to a limited extent. Human foods, 

 however, contain comparatively little of the cellulose 

 group of compounds. The sugars are partially caromel- 

 ized by heat, provided it is sufficiently intense, but in 

 ordinary cooking operations, they undergo little or no 

 chemical change unless associated with acids, alkalies 

 or ferment bodies, in which case they may be converted 

 into a number of chemical products. 



In the cooking of fruits, as the baking of apples, a por- 

 tion of the levulose of the fruit-sugar is partially carbon- 

 ized. In case the fruit is not fully matured, the pectose 

 substances or jellies are converted into a more soluble 

 condition by the action of heat. When heat is sufficiently 

 intense, the essential or volatile oils are expelled. 



Fats, as a class, undergo slight oxidation changes by 

 the action of heat. In the process of bread-making, for 

 example, the fat extracted from the bread is different in 

 character from that in the original flour. It is darker in 

 color, and chemical tests show that it is slightly oxidized. 

 Heat causes the proteids to undergo more complex 

 changes than any other class of nutrients. The soluble 



