Hydrothermal Treatment of Water Yam Starch in a Non-granular State: Slowly Digestible Starch Content and Structural Characteristics

Gelatinized water yam starch was subjected to hydrothermal treatment (25, 30, and 35% moisture content for 1, 8, 16, and 24 h at 100 °C) and characterized by X-ray diffractometry, solid-state ¹³C cross-polarization and magic-angle spinning nuclear magnetic resonance, differential scanning calorimetry, and digestibility analysis. The slowly digestible starch (SDS) content of the starch treated at 30% moisture content for 24 h reached 49.1%, 31.9% higher than that of the control starch. The B-type pattern of native starch was re-crystallized to the A-type by hydrothermal treatment. The SDS content showed negative correlations with T _o, T _p, T _c, and T _r, but showed a positive correlation with melting enthalpy. Furthermore, SDS was positively correlated with hydrothermal reaction time, moisture content, relative crystallinity, and the double-helix proportion. The structural changes in hydrothermally treated water yam starches resulted in the enhancement of SDS.