Oleic acid treatment of rice grains reduces the starch digestibility: Formation, binding state and fine structure of starch-lipid complexes

Rice contains abundant starch and contributes to a rapid rise in postprandial blood glucose levels. Hence, it is crucial to directly modify rice grains for resistant starch (RS) content elevation while preserving their morphology. In this study, rice grains were treated with 6%-18% concentrations of oleic acid (OA) and 8-20 h of soaking time to promote the formation of starch-lipid complexes, thereby reducing rice digestibility. In OA-treated rice, the OA molecules exist in three binding states. OA-treated rice exhibited a significantly higher complexation index and OA content than natural rice. RS content increased from 20.50% to 32.46%. X-ray diffraction and NMR spectroscopy revealed the development of amylose-OA complexes within the rice grains and a V-crystalline structure of up to 3.62%. Raman spectroscopy and thermogravimetric analysis showed enhanced molecular ordering and structural stability of rice starch. Overall, OA treatment effectively promotes RS formation within rice grains, consequently reducing rice digestibility.