Extrusion-induced structural reorganization of starch-ca HMB complexes for modulating starch digestibility and controlling nutrient release.

Starch‑calcium β-hydroxy-β-methylbutyrate (starch-Ca HMB) complexes were prepared from cassava starch by twin-screw extrusion. The effects of extrusion temperature, screw speed, feed moisture content, and glycerol addition on the physicochemical properties, structural characteristics, starch digestibility, and Ca HMB release were systematically investigated. Compared with native cassava starch, extrusion markedly reduced peak viscosity and setback, indicating reduced paste stability and a lower tendency toward retrogradation. Dynamic rheological measurements showed that all complexes exhibited predominantly elastic behavior, with G' exceeding G″. Moderate extrusion conditions, particularly a temperature of 80-85 °C, a screw speed of approximately 198.0 rpm, a feed moisture content of 20%-25%, and 5% glycerol, promoted the formation of a stable viscoelastic network. Under these conditions, the water absorption index was relatively high, whereas the free Ca2+ content decreased to 69.23 mg/L. SEM revealed a transition from dispersed granules to a continuous, compact matrix with indistinct phase boundaries. FTIR and XRD analyses further showed that appropriate extrusion increased the I1500 value to 0.963 and reduced the relative crystallinity to 33.12%, indicating enhanced Ca HMB incorporation and structural reorganization of starch. In vitro digestion experiments showed that moderate processing effectively reduced starch digestibility and increased the resistant starch content to 55.68%. Simulated release experiments showed that gastric release was below 12% at 120 min, whereas the intestinal release from T80 and H20 reached approximately 68% and 69%, respectively. Pearson correlation analysis revealed significant associations among hydration properties, paste stability, short-range order, Ca2+ distribution, starch digestion fractions, and Ca HMB release. Overall, moderate extrusion produced a compact starch-Ca HMB network that enhanced starch resistance to enzymatic digestion and promoted the preferential release of Ca HMB during the intestinal phase.