54 CARBOHYDRATES 



blue color with iodine, whereas amylopectin gives a violet-blue color. 

 This color is attributed to the branched structure of amylopectin. 

 Glycogen, which is even more highly branched than amylopectin, gives 

 only a red-brown color in the iodine test. Neither component of starch 

 shows any reducing power unless very refined tests are employed. Thus 

 the usual Fehling's test is entirely negative with native starch. How- 

 ever, soluble starch, which is made by subjecting starch to a mild acid 

 and heat treatment, does give a positive Fehling's test. This effect indi- 

 cates that the process of making the starch soluble has also resulted in 

 some decomposition and liberation of aldehyde groups. 



Starch usually contains a few hundredths of a per cent of phosphorus, 

 probably as a result of the fact that it is formed in plants from glucose- 1- 

 phosphate. Fatty acids (for example, oleic, linoleic, and palmitic) have 

 been found in various cereal starches, but it is probable that they are 

 present as impurities rather than as actual constituents of the starch 

 molecules since they can be removed by extraction with boiling methyl 

 alcohol. 



Starch is found in almost all chlorophyll-bearing plants. It is es- 

 pecially abundant in the common cereals (wheat, rye, oats, and rice) ; 

 it makes up from 60 to 80 per cent of the seed. Also peas and beans 

 may contain 50 per cent of starch. In certain oily seeds {e.g., cottonseed, 

 flaxseed, and soybeans) fats, instead of starch, form the storage ma- 

 terial. As a general rule seeds grown in the tropical regions are oil 

 bearing, whereas those of the temperate regions are high in starch. Many 

 tubers, such as the potato, are made up largely of starch. When unripe, 

 the apple and banana contain considerable quantities of starch. While 

 these fruits ripen, the starch is converted into sugar. The changes in 

 cereals during the ripening period are just the opposite of those that 

 occur in the apple and banana. Sweet corn is a striking example of a 

 plant that contains an abundance of sugar when the kernals are young, 

 but only a little sugar and much starch when the seed is mature. 



In nature the starch molecules are built up to form a larger aggregate 

 called a granule. Every plant has its own characteristic starch granules, 

 with or without particular markings (see Figs. 3-2 to 3-5). For ex- 

 ample, the potato starch granule is large, oval, and marked by concentric 

 lines arranged around a point called the hilum, at one end of the granule. 

 That of wheat starch, on the other hand, is smaller and spherical in 

 shape, without any particular markings. Oat starch is made up of a 

 number of particles and forms what is known as a compound starch 

 granule. The different fragments fit together in the form of a mosaic. 

 Because of this distinctiveness in appearance, it is comparatively easy 

 to determine the kind of starch that is present in a food material. 

 Microscopic examination of spices and flours is of great help in determin- 

 ing whether or not these materials have been adulterated by the addition 



