In vitro digestion of the amylosucrase treated waxy corn starches fitted to 1st-order kinetics

Abstract

Englyst et al. classified the starch into three fractions of rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS), according to the time of digestion. This classification has been argued because the in vitro digestion of starch can be described by a single rate constant k of a first order reaction. This study aimed to ascertain individual existence of RDS and SDS fractions in a single starch source. Waxy corn starch was modified by amylosucrase (AS) to obtain starch samples with increased SDS contents determined by Englyst method. Hydrolysis curves were obtained for native and AS-treated 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-treated starches revealed a discontinuity, demonstrating that digestion proceeded in two separated phases. It provided the evidence of a fraction that is digested more rapidly than the remainder, or RDS and SDS as a structural feature. Rate constants for digestion of AS-treated starches could be categorized into 2 groups and named kRDS and kSDS, respectively. The values of the kRDS and kSDS, and the contents of RDS and SDS estimated by LOS plot method of AS-treated starches were affected by the amount of AS employed. This intimated that highly extended branch chains would favor the formation of the more slowly digestible form of RDS and SDS. It seemed that the LOS plot approach would be a better investigative tool for classification of starch fractions by its digestibility. The structural characteristics AS-treated starches were examined before and after removal of RDS and SDS fractions. 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 removal of RDS and SDS fractions, chains of DP ≥ 37 and DP ≥ 25 decreased, respectively. The AS-treated 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. In conclusion, the branch chain length distributions of AP determined the primarily generated crystallite organization of AS-treated starches, causing the diversity of digestion property. The different values of kRDS and kSDS among starch samples reflected the different structures of RDS and SDS.