Dendrobium officinale polysaccharide-spermidine composite hydrogels: Tunable rheological properties and improved in vitro colonic fermentation.

Dendrobium officinale polysaccharide (DOP) and spermidine (Spd) exhibit synergistic anti-aging effects in Caenorhabditis elegans, suggesting potential functional complementarity. Here, a DOP-Spd composite hydrogel (DS) was constructed to integrate structural tunability with fermentation-related functionality. Three weight ratios (18:2, 18:4, and 18:6, DOP:Spd) successfully formed hydrogels. Increasing Spd content enhanced gel strength and thermal stability, as reflected by higher hardness (149.62-353.75 g), chewiness (41.04-119.07), and DTG peak temperature (331.7-334.0 °C). All DS hydrogels exhibited shear-thinning and a clear linear viscoelastic region (0.1%-1.0% strain). Among them, the 18:2 formulation showed superior water holding capacity (76.41%) and was selected for subsequent digestion and fermentation studies due to its suitability for moderate dysphagia. The results showed that approximately 80% of the polysaccharide passed through the upper gastrointestinal tract and reached the colon in both DOP and the DS hydrogel. Both promoted the growth of beneficial taxa (e.g., Bifidobacterium). DS markedly promoted B. pseudocatenulatum, B. longum, B. ruminantium, Prevotella_2 and Bacteroides, while inhibiting Enterococcus and Flavonifractor. Functional prediction showed enhanced pathways related to carbohydrate and energy metabolism. Metabolomic analysis (OPLS-DA) revealed clear group separation. Compared with DOP, DS increased key metabolites, including propionic acid (1.40-fold), butyric acid (1.98-fold), malic acid (3.94-fold), NAD+ (2.87-fold), adenosine (4.16-fold), hypoxanthine (2.26-fold), and serotonin (2.43-fold), with pathway enrichment in pyruvate, galactose, glyoxylate/dicarboxylate, purine, and starch/sucrose metabolism, as well as the citrate cycle. These results demonstrate that the DOP-Spd (18:2) hydrogel integrates desirable rheological properties with improved microbiota-associated functionality, supporting its potential as a dual-functional food matrix.