Physicochemical and structural insights into three Japanese rice varieties: Starches from the macromolecular to the nanocrystalline scale.

This study presents the physicochemical, structural, thermal, pasting and vibrational characterization of three Japanese rice cultivars: Hitomebore, Koshihikari, and Milky Queen. Proximate analysis showed moisture between 12.3 and 13.0%, protein 5.3-6.0%, fat 0.6-1.5%, and ash 0.14-0.42%. Amylose content ranges from 18% to 31%. ICP-OES revealed that P, K, Mg, and Ca are the predominant minerals, underscoring their nutritional value. SEM showed polyhedral starch granules with average sizes of 2-8 μm. X-ray diffraction confirmed that all starches are A-type, containing nanocrystals with an orthorhombic structure that undergo an order-disorder transition after gelatinization. Transmission electron microscopy of the nanocrystals revealed average dimensions of 18 nm in length, 14 nm in width, and a few nanometers in thickness. The presence of these nanocrystals could be associated with resistant starch type 2 content, ranging from 1.19% to 3.7%. Differential scanning calorimetry showed that gelatinization is an irreversible transition from ordered nanocrystals to an amorphous phase, with enthalpy change correlating with relative crystallinity. Rheological analysis indicated that pasting behavior involves multiple overlapping processes such as hydration, swelling, nanocrystal solvation, and polymer leaching. SEM analysis of the dry slurry at peak, setback and end viscosity showed the formation of microchannels, and FTIR confirmed that no chemical modifications occur during pasting, indicating that changes are primarily morphological. Overall, the results demonstrate that the functional properties of Japanese rice starch are governed by its hierarchical organization, particularly the arrangement of orthorhombic nanocrystals, providing insights relevant to food science and industrial applications.