High-level substitution slows fermentation and modulates the gut microbiota in modified starches esterified with short-chain fatty acids.

Many conventional resistant starches tend to undergo rapid fermentation in the proximal colon. This often causes gastrointestinal intolerance and limits their health benefits across the full length of the colon. In this work, we synthesized acetylated, propionylated and butyrylated corn starches with precisely controlled degrees of substitution (DS 1.0, 2.0 and 2.5), and systematically characterized their multi-scale structure, in vitro fermentation behaviors and regulatory effects on human gut microbiota. Esterification disrupted starch's native A-type crystalline structure, induced thermally stable V-type crystals with a characteristic high-temperature endothermic peak at 120-145 °C (DSC), and increased starch hydrophobicity in a DS- and acyl chain length-dependent manner. High-DS (2.5) starches showed desirable slow-fermentation properties, with steady pH maintained at 6.03-6.43 during fermentation, and enabled targeted short-chain fatty acid release matching their grafted acyl groups. Relative crystallinity, especially V-type crystallinity, is significantly negatively correlated with fermentation rate. They also significantly enriched beneficial bacteria including Bacteroides, Bifidobacterium and Faecalibacterium, reduced the relative abundance of the potential pathogenic genus Escherichia-Shigella, and enhanced gut microbial diversity. This in vitro work provides preliminary theoretical reference for developing slow-fermenting starch-based functional ingredients with potential intestinal regulatory capacity for subsequent in vivo intestinal health intervention research.