Structural Dynamics, Gut Microbiota Modulation, and Immunological Impacts of Shiitake Mushroom β-Glucan during In Vitro Intestinal Fermentation

This study investigated the structure–function relationship of shiitake β-glucan (SBG) through simulated digestion and colonic fermentation. SBG, composed of β-(1 → 3)-linked glucan backbones with β-(1 → 6)-linked side chains, exhibited notable resistance to enzymatic and acidic degradation in the upper gastrointestinal tract. During in vitro colonic fermentation, 59% of carbohydrates were consumed within 48 h, with a significant pH reduction and a 1.5-fold increase in short-chain fatty acid production. Microbiome analysis demonstrated that SBG enhanced Bacteroides and Lactobacillus populations, while suppressing Escherichia-Shigella. Within the first 12 h, SBG maintained a rigid triple-helix structure, with a slight decrease in branching from 48.02 to 44.26%. After 24 h, the triple helix unwound, and extensive depolymerization of the backbone occurred by 48 h. Immunomodulatory activity was preserved early in fermentation but decreased as the triple-helix structure broke down. These findings emphasize the critical role of molecular rigidity and conformational integrity in β-glucan’s functionality for food and therapeutic applications.