Solid-state NMR spectroscopy reveals polishing-induced supramolecular disorder in sake rice starch

While the brewing suitability of sake rice is influenced by its structural features, the molecular basis of its starchy core (shinpaku) remains unresolved. Here, Yamadanishiki, a widely used sake rice cultivar, polished to 40% was characterized using solid-state nuclear magnetic resonance (NMR) spectroscopy, scanning electron microscopy (SEM), gas-chromatography-mass spectrometry (GC-MS), and amylopectin chain-length analysis. Solid-state ¹³C NMR revealed progressive disruption of A-type crystalline order with polishing, particularly in shinpaku, whereas the amylopectin backbone remains unchanged. Complementary solid-state ¹H NMR experiments showed that polishing removes lipid-derived signals, consistent with quantitative GC-MS data. SEM observations showed that shinpaku contains loosely packed starch granules with abundant voids, unlike the dense outer endosperm. These results establish solid-state NMR as a powerful tool for probing starch disorder and demonstrate how polishing enhances water penetration, koji invasion, and brewing suitability. This study presents an integrated solid-state NMR and SEM analysis.