Starch is the most important of the carbohydrates from the standpoint of food. It is familiar to us all as the fine, white, glistening powder of "corn starch" and of laundry starch. We may easily, by washing it, obtain it also from grated potatoes and from flour. Starch is found only in the vegetable kingdom, and is manufactured by green plants and stored in different parts of the plant in the form of tiny grains lying within the plant cells.
(From Hygiene, by Parks).
The structure of these grains has been very hard to determine because of their minuteness. It was thought for a long time that they were composed of a cellulose envelope enclosing the true starch, and that by the action of water and heat these grains swelled and the cellulose envelope burst.
A later theory was that the starch grain was built up in alternate layers of starch cellulose and starch granulose.
The late work of a German botanist, Meyer, seems to show that the grains are in the form of sphero-crystals, each made up of many tiny particles. These radiate from a center, and at the same time are arranged in concentric layers. The particles are of two kinds called by Meyer alpha-amylose and beta-amylose. These may be compared to the starch cellulose and starch granulose of the older theory. Upon the application of heat and moisture the beta-amylose swells and becomes gelatinous, forming a solution. The alpha-amylose is affected only by a temperature much above the boiling point, or by long continued heating.
The starch grains in different plants differ much in form, size and general appearance, as shown in the illustrations. The relation of the difference in structure to digestibility is not well determined.