Effect of Repeated Heat–Moisture Treatment Temperature on the Multi-Scale Structure, Physicochemical Properties, Rheological Behavior, and In Vitro Digestibility of Hard Proso Millet Starch

Repeated heat–moisture treatment (RHMT) is an efficient approach for modifying starch. However, the role of treatment temperature, a critical parameter, remains poorly understood. Therefore, this study investigated the effects of RHMT temperatures (80, 100, 120 °C) and cycles (1, 3, 5, 7) on the multi-scale structure and in vitro digestibility of hard proso millet starch, using native starch as a control. Compared with the severe 120 °C treatment, processing at 100 °C better preserved double-helical organization (supported by moderately retained enthalpy, ΔH) and short-range order, while maintaining granule integrity. These structural retentions restricted swelling, improved pasting stability, and reinforced the macroscopic gel network. Furthermore, multivariate analysis suggested that the rigidified internal granular architecture delayed initial enzymatic hydrolysis, maximizing slowly digestible starch (SDS) formation (47.44% in 100-RHMT-5). Conversely, 120 °C caused severe granular collapse and a drastic drop in ΔH, diminishing gel elasticity and triggering a surge in rapidly digestible starch (RDS, 59%). Overall, 100 °C RHMT yields an SDS-enriched starch, which may be a promising ingredient for the development of starch-based foods with slower in vitro digestibility.