Multiscale structural remodeling of highland barley starch by Monascus fermentation: Chain-length redistribution...

Monascus fermentation is a food-compatible strategy for regulating starch digestibility, but the multiscale structural basis of resistant starch (RS) formation remains insufficiently understood. In this study, highland barley starch (HBS) was fermented for 6, 8, 10, and 12 days to clarify the depolymerization-rearrangement-V-type ordering pathway. Fermentation markedly reduced molecular size, with Mw/Mn decreasing from approximately 76/17 × 10⁶ g/mol in HBS to 14/3 × 10⁶ g/mol at day 8 and 3/0.7 × 10⁶ g/mol at day 12. FBS-8 showed the greatest enrichment of DP 100-5000 chains and the strongest A-to-V structural transition, accompanied by the highest RS content (60.41%) and the lowest predicted glycemic index (pGI). FTIR and DSC results further indicated hydrogen-bond reorganization and the formation of thermally stable ordered domains, whereas prolonged fermentation partly disrupted the optimal precursor pool. These findings identify 8 days as the optimal fermentation period under the present conditions and provide a structural basis for developing high-RS cereal starch ingredients with potentially lower predicted glycemic impact.