Rice with lower amylose content could have reduced starch digestibility due to crystallized resistant starch synthesized by linearized amylopectin.

Background: High resistant starch (RS) rice with a low glycemic index (GI) is digested more slowly, resulting in a slow increase in the postprandial blood glucose level. People with milled rice as a staple diet, and with sedentary lifestyles, are more prone to acquire type 2 diabetes in the long term. With an increasing population of diabetics worldwide, one possible solution is the development of rice-based, low-GI foods with high RS content rice. In addition to amylose content (AC), linear chains of amylopectin also affect the rate of starch digestibility.

Results: The values of GI (52.49-63.0), RS (0.64% to 2.28%), and AC (3.82% to 24.52%) were found to vary widely in 110 rice genotypes. Genotypes IG 23 and IG 40, with contrasting AC (15.65% and 24.52%, respectively), revealed that amylose alone did not affect digestion rate. Starch morphology, bioaccessibility, and pasting properties differed noticeably among genotypes. A starch debranching enzyme pullulanase assay indicated the role of the linear amylopectin chain in crystallized RS formation within the grains of lower AC genotype IG 23.

Conclusion: Enhanced activity of the starch-debranching enzyme pullulanase produced linearized amylopectin resulted into reduced starch digestibility in low-amylose rice. This is probably the first report on the natural presence of high RS (crystallized) in rice with lower AC (IG 23). Development of rice-based food with low GI and high RS could be a promising strategy for lowering the prevalence of type 2 diabetes. © 2024 Society of Chemical Industry.