THE KINDS OF STORED FOOD 



177 



These characters are, in fact, so pronounced as to afford the 

 means of detecting adulterations in ground and powdered food 

 and drugs (see Fig. 97). 



The Hght and dark striations in the grains are layers of greater 

 and less density, and are possibly due to periods of greater and 

 less abundance of available sugar from which the starch is made. 

 Some starch grains when partially 

 digested, or swollen in a dilute solu- 

 tion of potassium hydrate, appear as 

 though made up of needle-shaped 

 crystals radiating from the organic 

 center of the grain, and it has been 

 suggested that the denser layers of the 

 grain are composed of denser groups 

 of these crystals. The crystalline 

 nature of the starch grain is also 

 demonstrated by its behaving in polar- 

 ized light as crystals do. 



Treatment of starch grains with 

 boiling water seems to show that 



they are composed of two kinds of starch, one insoluble and 

 the other soluble in boiling water, for the microscope reveals 

 that the paste made in this way is not a complete solution and 

 contains an abundance of undissolved remnants. The insoluble 

 part is called a-amylose and the soluble ^^-amylose. 



The percentage formula for starch is known (CgHjoOs), 

 but the exact number of atoms to the molecule has not been 

 definitely fixed. The number is tentatively expressed by mul- 

 tiplying the percentage formula by n, thus, w(CgHjpOg). 



Starch grains are almost always colored blue with an iodine 

 solution; but when undergoing digestion they may assume a 

 reddish color with iodine due to the dextrines that have ])een 

 formed from the amylose. In a few instances starch normallv 

 contains so much amylodextrin that it is colored red by iodine, 

 as in the seed-coats of Oryza and Chelidonium. 



At the close of the growing season the amount of stored starch 





Fig. 97. — Starch from different 

 sources. .4, curcuma starch; B, 

 com starch; C, tapioca starch; 

 D, rice starch, showing compound 

 grains. 



12 



