STRUCTURE OF THE STARCH GRANULE

Abstract

An hypothesis as to the anatomical structure of natural starch granules:[long dash]"The peculiar anatomical organization of the intact granule restrains its swelling beyond a certain point in cool water. This restraining anatomical structure is the parallel arrangement of the long-chain macromolecules of which the starch granules'' substance in large part consists. They are tied together in crystallites which are oriented radially in the granule. Moreover the starch-chain molecules possess an inherent tendency, probably due to intramolecular attraction of side groups for one another, to crumple or fold so as to reduce their own length. In crumpling, the macro-molecule increases its diameter laterally; it thickens. In the natural granule containing adsorbed moisture, the macromolecules are not crumpled or shortened to their ul- most because, being surrounded on all sides by other molecules which are also trying to thicken, room for complete contraction is lacking. The molecules at the same time endeavor to adsorb water, whereby they are also thickened. Thus, in swelling, a lateral pressure is exerted on the chains or bundles of chains, the micelles, which not merely prevents them from contracting but may even lengthen them so that the granule enlarges its diameter. In a sense, therefore, the macromolecules of a granule containing adsorbed moisture may be said to be stretched. They differ from the macromolecules in a mechanically stretched wool fiber in that the stretching force is a lateral pressure applied from all sides instead of a pull in the direction of the long axis of the fiber. The starch macro-molecule is lengthened by squeezing, as one flattens a sheet of corrugated-iron roofing by lateral pressure; wool macromolecules are lengthened by pulling, as one straightens a coil of soft wire. The micelles are prevented from swelling to capacity, because there is not room for them to thicken as much as would be necessary if they sorbed or associated water to the maximum possible for them in a mechanically free state. Therefore, the swelling of granules in cool water is limited. Conversely, as a granule is dried, the transverse diameters of the macromolecules crowd against one another with less force; space between them or between the micelles is made available to the extent that the water leaves. The granule shrinks until, in the completely anhydrous state, the chains approach a state of complete contraction."[long dash]This hypothesis[long dash]a combination of parts of older hypotheses, with the introduction of some concepts new for starch, though not new in themselves[long dash]is based on the following observations recorded in the literature or newly presented here: The starch granule may be made to swell and shrink by varying its water content. At ordinary temps., swelling in water is limited; at higher temps., it is great and may lead to ultimate dispersion of most of the granule substance (gelatinization). The granule, if chipped or cracked, swells greatly in cool water at the site of injury, and only there. It is insoluble in cold water, but if mechanically injured, is more or less soluble in cool water by partial dispersion of the swollen portion. The natural granule consists[long dash]at least in large measure[long dash]of material in orderly arrangement and exhibits a characteristic x-ray diagram, which may be changed by causing the granule to swell, and under some circumstances the process underlying the change in diagram is reversible; completely anhydrous starch, and thoroughly ground-up and mechanically disintegrated starch, exhibit no x-ray diagrams.