Multiscale Structural Evolution and Digestion Kinetics of Starch in Commercial Swine Compound Feed as Affected by Steam Conditioning Temperature and Retention Time

Steam conditioning is a key hydrothermal process for ensuring the quality of pelleted commercial swine compound feed. However, the molecular evolution of starch during processing remains poorly understood, particularly under the industrial constraints of limited moisture and complex substrates. This study quantified the effects of conditioning temperature (60–90 °C) and retention time (2–4 min) on starch structural reorganization across multiple length scales and in vitro digestion kinetics in pig diets. Structural evolution exhibited a nonlinear, temperature-driven pattern, with the temperature region around 80 °C representing a critical transition in structural reorganization. Although high-temperature conditioning (90 °C) increased the initial digestion rate, a nonlinear structure–digestion response was observed: final digestibility decreased, and resistant starch content rebounded. Microstructural analyses revealed that severe heat treatment was associated with the possible formation of amylose–lipid complexes (V-type crystallinity) and a more continuous protein-associated matrix, which may have imposed additional physical constraints on enzymatic hydrolysis. Consequently, maximizing starch gelatinization does not necessarily guarantee optimal in vitro digestibility. These findings integrate multiscale structural analysis with digestion kinetics, providing a mechanistic basis for balancing energy consumption, pellet quality, and nutritional value in the feed industry.