Effects of removing endogenous non-starch components on the structure, physicochemical properties, and in vitro digestibility of Castanopsis sclerophylla flour.

Castanopsis sclerophylla is a natural slow-digesting starch resource, and the mechanism by which the endogenous non-starch components slow the digestibility of Castanopsis sclerophylla starch (CaS) remains unclear. In this study, protein, polyphenol, fiber, and pectin were removed from Castanopsis sclerophylla flour (CaF) through enzymatic digestion and water immersion. Their effects on the molecular structure, physicochemical properties, and in vitro digestibility of CaS were systematically investigated. The results showed that the removal of the non-starch components significantly increased the content of rapidly digestible starch but significantly decreased the content of resistant starch from 41.73%. The regulatory priority of each component was in the following order: pectin > fiber ≈ protein > polyphenol. Endogenous non-starch components synergistically inhibited starch digestion through a three-dimensional mechanism: (1) physical barrier-pectin, fiber, and proteins reduced enzyme accessibility by encapsulating starch granules or accelerating their agglomeration, with this effect more prominent in the gelatinized state and masked by the dense crystallinity of non-gelatinized starch; (2) structural stability-these components collectively maintained the short-range ordered structure of starch, reducing enzyme attack sites; (3) physicochemical regulation-pectin enhanced the viscosity of the system through gelation and restricted water mobility, while proteins inhibited starch swelling, which synergistically retarded the process of enzyme diffusion and starch gelatinization. This study reveals that the physical barrier effect of pectin in CaF is the dominant factor affecting starch digestion. Additionally, it provides a theoretical basis and practical guidance for developing low-glycemic index and slow-digesting foods.