Abstract: This study aimed to clarify the influence of natural fermentation on the molecular structure and retrogradation properties of sorghum starch. The surface morphology, functional groups, molecular mass and chain length distribution of starch prepared by alkaline extraction of naturally fermented sorghum were measured versus fermentation time. The results showed that after natural fermentation, the surface of sorghum starch granules exhibited some traces of erosion; no new chemical groups were formed during the fermentation process, and the peak position of each functional group remained unchanged. The weight average molecular mass of the starch was reduced after fermentation, and the percentage of low-molecular-mass starch was increased from 74.2% to 78.5%, while the percentage of high-molecular-mass starch decreased from 5.4% to 2.5%; the percentage of short-chain branch of amylopectin decreased by 1.8%–3.89%, whereas the percentage of long-chain branch increased by 2.01%. After fermentation, and the setback value increased by 466 mPa·s, and the peak viscosity also increased. Fermented sorghum starch was more likely to form gel. The findings of this study provide a theoretical basis and supporting data for the regulation of natural fermentation of sorghum and the processing of foods with retrogradated starch.

Key words: naturally fermented starch, molecular structure, retrogradation properties