Influences of pH on Gelling and Digestion–Fermentation Properties of Fish Gelatin–Polysaccharide Hydrogels

This study systematically evaluated the effects of pH (4-10) on the gelation properties, structural characteristics, and in vitro digestion-fermentation behavior of fish gelatin (FG, 6% (<i>w</i>/<i>v</i>)) hydrogels combined with either xanthan gum (XG, 0.07% (<i>w</i>/<i>v</i>)) or κ-carrageenan (κC, 0.07% (<i>w</i>/<i>v</i>)). The results revealed that the gel strength, hardness, and chewiness of the composite gels initially increased (pH 4-6) and subsequently decreased with rising pH levels. This trend correlated with the formation of a dense gel network structure. Furthermore, as pH increased, in vitro digestibility showed a similar pH-dependent trend, with FG-XG demonstrating superior enhancement compared to FG-κC. The addition of XG and κC resulted in increased gas production and a decreased pH during fermentation. Intestinal microbiota analysis revealed that both FG-XG and FG-κC improved the abundances of <i>Proteobacteria</i> and <i>Bacteroidete</i> while reducing <i>Firmicutes</i>. Compared to FG-XG and FG, FG-κC promoted higher levels of the genera <i>Lachnospiraceae</i> and <i>Bacteroides</i>, suggesting a more favorable impact on intestinal health. These findings provide valuable insights into the pH-responsive functional properties of FG-based hydrogels and their potential applications in designing novel food matrices with enhanced nutritional and probiotic attributes.