Influence of molecular structure on physicochemical properties and digestibility of normal rice starches

Normal rice starches were isolated from six different rice varieties grown in Korea and their molecular structure, crystalline structure, and in vitro digestibility were investigated. Apparent amylose content was the highest in starch from Junam cultivar (25.5%) and lowest in Hopum (22.4%). Starch from Hiami cultivar had the lowest molecular weights of amylose and amylopectin, average amylopectin chain length, proportion of short chains (DP 6–12), and proportion of long chains (DP ≥ 37) among the tested rice starches. The relative crystallinity and ratio of 1047/1022 ranged from 30.2 to 36.7% and from 0.638 to 0.652, respectively. Hiami had the lowest gelatinization temperatures and the highest gelatinization enthalpy. Hiami had the highest pasting temperature (92.1 °C), the lowest setback (515 cP) and final viscosity (876 cP), whereas Hanareum had the lowest pasting temperature (82.7 °C), the highest setback (1002 cP), and final viscosity (1580 cP). The rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) content ranged from 43.9–53.5%, 35.5–52.6%, and 0.5–15.6%, respectively. The Junam cultivar had the lowest RDS content, whereas Hiami had the highest RDS content. The RS content of Hanareum (16.2%) and Boramchan (14.5%) was significantly higher than that of other normal rice cultivars (3.0–6.5%).