Structure―Digestibility Relationship of Recrystallized Starch by Using Amylosucrase-modified Starch as a Model System

Abstract

Starch can be classified into three fractions of rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS), according to the time of digestion as suggested by Englyst et al. SDS and RS are generally classified as low-digestible starch, and their production and health beneficial effects have been widely studied. However, the classification of RDS and SDS has been argued because the in vitro digestion of starch can be described by a single rate constant k of a first order reaction. With increasing interests on carbohydrate-controlling diets because of health concerns, food industry requires a unified strategy for the production of health-functional starch. To achieve this goal, actual existence of each starch fraction should be verified primarily.<br/> Waxy corn and waxy potato starches were modified by amylosucrase (AS) to obtain starch samples with different branch chain length distributions. Hydrolysis curves were obtained for native and AS-modified starches and then fitted to the logarithm of slope (LOS) plot, which can reflect the change of k during first order reaction. LOS plots for AS-modified starches revealed a discontinuity, demonstrating that digestion proceeded in two separate phases. It provided the evidence of a fraction that is digested more rapidly than the remainder, i.e., RDS and SDS, and their rate constants were defined as k_RDS and k_SDS, respectively. The values of the k_RDS and k_SDS, and the contents of RDS and SDS estimated by LOS plot method of AS-modified starches were affected by the amount of AS employed. The digestible properties related to RDS, SDS, and RS were investigated by both in vitro and in vivo study. This study could verify the existence of individual RDS and SDS fractions in a single starch source, and suggested the modified LOS plot approach as an alternative investigative tool for classification of starch fractions on the basis of its digestibility.<br/> The structural characteristics of RDS, SDS, and RS were investigated by tracing the changes according to serial removal of each fraction. The changes in branch chain length distribution implied the branch chains with certain DP contribute to the organization of each fraction. Undigested RS was composed of rather short chains with DP 13-24. After the removal of RDS and SDS fractions, chains of DP ≥ 37 and DP ≥ 25 decreased, respectively. Certain structures found in common among RDS and SDS fractions, respectively, strongly supported the existence of individual RDS and SDS possessing specific structural characteristics of each. The AS-modified starches displayed B type X-ray pattern, and the relative crystallinity increased with the amount of AS, and also according to the degree of hydrolysis of RDS and SDS fractions. The branch chain length distributions of amylopectin determined the primarily generated crystallite organization of AS-modified starches, causing the diversity of digestion properties. Highly extended branch chains would favor the formation of more slowly digestible form of RDS and SDS. The different values of k_RDS and k_SDS among starch samples reflected the different structures of RDS and SDS. <br/> The recrystallized starches prepared by using AS-modified waxy potato starch and amylose from potato starch by 3:1 ratio and the structures of their RDS, SDS, and RS were investigated. Co-crystallization of amylose and amylopectin occurred, where the branch chain length of amylopectin determined the length of amylose-amylopectin double helix and mobility of amylose. RDS was composed of singular amylose chains and amorphous double helices not long enough or not aligned ordered structure. Low digestible fractions, SDS and RS, were mainly composed of semi-crystalline or crystalline structure of double helices with DP 13-24. Unlike amylopectin-only environment, the amount of SDS did not show a linear increase according to the branch chain length. Certain length of amylopectin would prefer formation of SDS or RS by controlling the interaction with amylose. <br/> In short, this study provided the knowledge on the formation of low digestible starch under various environments and detail structure of each fraction within starch. It would be utilized in the production of customized starch for specific digestive properties with health functionality.<br/>