Biologic Utilizations of Unmodified and Modified Food Starches

Abstract

Investigations of unmodified starches, including those from cereal grains (wheat, rice, maize and waxy maize), from aerial stems (sago palm) and from plant roots and subterranean stems (arrowroot, cassava, sweet potato and white potato) have shown these, under thoroughly comparable conditions of feeding, to possess wide differences in physiologic assimilability. Unmodified starches isolated from cereal grains and cassava roots evidenced very high degrees of assimilability coincident with roughly 98% digestibility; those from arrowroot, white potato and sago palm evidenced decidedly low degrees of digestibility which varied inversely with the intake or dietary level of these starches; and that from the sweet potato occupied something of an intermediate position. Viewed from the standpoint of physiologic processes, the data herein presented suggest that the relatively low digestibilities of unmodified sweet potato, potato, arrowroot and sago starches were occasioned by an insufficient amount or concentration of amylase, or of some agent in the digestive juices which is required to modify these starches prior to attack by amylase. Viewed from the standpoint of the digestion-resistant properties of unmodified sweet potato, potato, arrowroot and sago starches, it would appear that this property resides consistently and exclusively in the outermost layers of the organized granules and, therefore, according to modern concepts, is associated with the hydrogen-bonding of these structures or with a granule integument of non-starch substance. Conditions which increase the digestibility of these starches include various modifications which produce obvious hydration of the granules, distinct changes in chemical nature, or disruption of the organized granule structures. Despite the frequent association between large-sized starch granules and digestion-resistant properties, the evidence presented here suggests that this is merely coincidental and not exclusive. It has also been shown that lack of homogeneity as regards digestion-resistant properties among granules of such unmodified starches as those of the potato or the sago palm offers no basis for explaining the low digestibility coefficients of these starches. There is some suggestion that, teleologically speaking, the digestion-resistant properties of starch granules from certain sources exist as a protective factor during storage in situ under conditions of high moisture. According to the observations made here, unmodified wheat starch granules which show no signs of injury are quite susceptible to digestion in vitro by U.S.P. pancreatin preparations, and very likely the same is true of cereal starches in general.