Competing effects of starch degradation and phenolic-starch complexation during extrusion: temperature-dependent...

The effect of extrusion on starch digestibility in starch-phenolic systems remains ambiguous, as heat and shear simultaneously gelatinize and degrade starch to accelerate digestion, or form starch-phenolic complexes inhibiting enzymatic hydrolysis. To identify which effect dominates, cold (40 °C) and hot (90 °C) extrusion were employed to examine how extrusion temperature and phenolic content regulate starch-phenolic complexation. Extruded noodles with buckwheat starch and phenolic extract were prepared at concentrations (0.50-2.00 %, w/w, based on starch). Molecular and crystalline structures of extrudates were analyzed and correlated with in vitro digestion. Compared with cold extrusion, hot extrusion enhanced starch gelatinization and chain fragmentation (average hydrodynamic radius, 41.74 nm) but simultaneously promoted the binding of phenolics with amylose (DP 100-5000) and short amylopectin branches (DP 28-100), facilitating the formation of V-type inclusion and non-inclusion complexes. These complexes increased V-type crystallinity and reduced hydrolysis rate and digestibility, despite greater molecular degradation. This study demonstrates that controlled molecular degradation under high-temperature extrusion can be strategically leveraged to facilitate the formation of enzyme-resistant starch-phenolic complexes, which provides a novel processing strategy for designing slow-digestion foods by tuning structural transitions towards nutritional benefits, rather than merely minimizing starch breakdown.