Heat-shock and soaking modulate parboiled rice starch structure and digestibility via in situ amylase hydrolysis

This study examines the activation of endogenous amylase by heat-shock pretreatment during soaking and its effects on the structure and digestibility of parboiled rice starch. The results suggested that moderate heat-shock treatment (80 °C for 1-3 min or 100 °C for 20-60 s) increased amylase activity by 80%-270%, and this activation was associated with heat-induced release of enzyme proteins. During soaking, the enhanced in situ enzymatic hydrolysis preferentially attacked the amorphous regions of starch granules, leading to an initial reduction in crystallinity. However, the subsequent parboiling and cooling retrogradation processes significantly promoted amylose rearrangement and the reconstruction of ordered structures, ultimately resulting in a marked increase in the crystallinity of the final product compared with raw rice. Concurrently, the resistant starch content increased to 30.06%, and the estimated glycemic index (eGI) decreased from 75.66 to 64.84-72.06. Further structural analyses revealed that during parboiling, hydrogen-bond interactions among starch chains were weakened, while water migration channels were expanded, thereby synergistically modulating the gelatinization and retrogradation behavior of starch and improving the elasticity and hydration properties of the rice grains. These structural modifications-driven by the synergistic action of enzymatic hydrolysis and thermal treatment-at both the molecular ordering and aggregate levels, constitute the key structural basis for the reduced starch digestibility. In summary, this study provides the first evidence that precise regulation of endogenous amylase activity in combination with hydrothermal treatment enables targeted modulation of the structure and functionality of rice starch.