Molecular interactions between guar gum and coix seed endogenous constituents modulating starch digestion kinetics

The hydrophilic colloid–starch composite system has been widely employed to enhance the quality and processing performance of grain-based products. Extrusion, as a cost-effective and efficient processing technology, holds considerable promise for the production of starch-based foods. Using optimized processing parameters, this study investigated the effects of extrusion treatment and guar gum (GG) supplementation on the physicochemical properties, spatial structure, and related characteristics of Coix seed (B), Coix seed starch (BS), and defatted Coix seed starch (TB), both before and after extrusion. An in vitro digestion model was further established to clarify the interactions among GG, endogenous proteins, and lipids, as well as the mechanisms underlying these interactions. The results showed that: (I) the GG–BS group showed the strongest synergistic interaction and binding capacity; (II) among the extrusion-treated samples, extruded brown rice starch with GG (EBSG) displayed the lowest starch digestibility; and (III) extrusion treatment significantly decreased protein digestibility and free amino acid content in the B group. These results suggest that complexes formed between starch and GG hinder the binding of digestive enzymes, suppressing starch digestion and absorption. This study provides a valuable theoretical basis for the development of low glycemic index (GI) foods.