PLANT STRUCTURE AND COMPOSITION 



TABLE 8-2. Size and Proportion of Amylose and Amylopectin 



in Some Starches 



189 



economy of the plant is not apparent. The starch granules of the dif- 

 ferent plants also vary in size from 1 ja to 150 /j. in diameter. Potato 

 tubers produce large granules, whereas rice and buckwheat produce 

 very small granules. The size, shape, and markings on the granules are 

 sufficiently characteristic for the microscopic determination of the 

 origin of a starch in food products. There is some evidence that the 

 layering, or stratification, shown clearly in the apparently concentric 

 rings in potato starch, is due to the day and night alternation in rate 

 of formation of the grain. 



Amylose can be separated from amylopectin by treating the grains 

 with hot water. The linear amylose diffuses from the swollen granule 

 while the amylopectin fraction remains behind. On standing, a rather 

 pure amylose precipitates in low yield from the solution. A better 

 separation of the tw^o components of starch can be achieved by heating 

 starch with a butanol-water solution under pressure. Upon cooling, an 

 amylose-butanol complex precipitates, Amylopectin is recovered by 

 drying or by alcohol precipitation. 



None of these methods provides a commercially feasible method of 

 separation. For many commercial purposes it would be desirable to 

 have a preponderance of one type of polymer rather than a mixture of 

 the two components. The problem of commercial separation has been 

 partially solved by the plant geneticists who have bred strains of corn, 

 rice, and barley yielding only amylopectin. This work was accom- 

 plished before World War II and provided a source of starch with 

 good gelling properties at a time when importation of tapioca starch 



