Multiscale starch architecture of Korean-bred long-grain indica rice cultivars grown under temperate conditions: Relationship to cooked-rice starch digestibility.

Korean-bred long-grain indica rice cultivars have been developed for cultivation under temperate conditions, yet their multiscale starch architecture and cooked-rice starch digestibility have not been systematically investigated. In this study, three such cultivars (Slowmi1, Slowmi2, and IPS) were characterized for starch architecture and in vitro starch digestibility. They were compared with japonica cultivars, Korean Tongil-type cultivars, and internationally sourced long-grain indica cultivars. The Korean-bred indica cultivars had intermediate apparent amylose contents (22.0-24.9%), lower than those of the Tongil-type and internationally sourced long-grain indica cultivars. Relative to the japonica and Tongil-type cultivars, they exhibited a distinctive amylopectin architecture characterized by low proportions of fingerprint A chains (DP 6-8), high proportions of B1 chains (DP 13-24), and elevated B1/Acltr ratios (Acltr, A chains incorporated into amylopectin clusters, DP 9-12). Slowmi2 and IPS were distinguished from most comparison cultivars by relatively high gelatinization temperatures, narrow gelatinization ranges, and compact cooked-kernel cross-sections with low pore-area fractions. Slowmi2 also showed one of the higher XRD-derived relative crystallinity values. Slowmi2 and IPS showed lower rapidly digestible starch contents and lower estimated glycemic index values than most other cultivars analyzed, whereas Slowmi1 was more rapidly hydrolyzed despite having a broadly similar amylopectin branch-chain profile. Together, these results indicate that amylopectin branch-chain distribution alone did not account for cultivar-dependent digestibility. The lower rapid hydrolysis of Slowmi2 and IPS was accompanied by a broader multiscale structural profile that combined B1-enriched amylopectin architecture with higher FT-IR-based indices of short-range molecular order, higher XRD-derived relative crystallinity, and a compact cooked-kernel microstructure.