Production of an in Vitro Low-Digestible Starch via Hydrothermal Treatment of Amylosucrase-Modified Normal and Waxy Rice Starches and Its Structural Properties

We investigated dual modification of normal and waxy rice starch, focusing on digestibility. Amylosucrase (AS) was applied to maximize the slowly digestible and resistant starch fractions. AS-modified starches were adjusted to 25–40% moisture levels and heated at 100 °C for 40 min. AS-modified starches exhibited a B-type crystalline structure, and hydrothermal treatment (HTT) significantly (p < 0.05) increased the relative crystallinity with moisture level. The thermal transition properties of modified starches were also affected by the moisture level. The contents of rapidly digestible starch fraction in AS-modified normal and waxy starches (43.3 ± 3.9 and 18.1 ± 0.6%) decreased to 13.0 ± 1.0 and 0.3 ± 0.3% after HTT, accordingly increasing the low digestible fractions. Although the strengthened crystalline structures of AS-modified starches by HTT were not stable enough to maintain their rigidity under cooking, application of AS and HTT was more effective in waxy rice starch than normal rice starch when lowering digestibility.