Retrogradation in rice starch: Nanocrystals from orthorhombic to hexagonal crystal structure crystallization

This study examines the retrogradation process in gelatinized isolated rice starch. Scanning electron microscopy (SEM) shows that starch granules measure 12.32 ± 1.74 μm and are composed of nanometric internal rings containing free nanocrystals coated with amylose and amylopectin. Transmission electron microscopy (TEM) reveals that these intrinsic nanocrystals have an orthorhombic structure (17 ± 5 nm in length, 12 ± 1 nm in width, and 2-4 nm in thickness), while energy-dispersive scattering TEM detects the presence of Ca, P, Mg, Na, and S. X-ray diffraction (XRD) confirms that the diffraction pattern originates from these nanocrystals, which become amorphous after gelatinization due to solvation. Retrogradation monitored from 0 to 28 days reveals a long-term, nonlinear crystallization process that begins with the solvation of nanocrystals with orthorhombic structure and progresses to the formation of nanocrystals with hexagonal structure. Differential scanning calorimetry (DSC) shows that the solvation of these nanocrystals governs both gelatinization and subsequent crystallization in excess water, at low temperature, and over extended durations. These nanocrystals serve as the fundamental structural units governing retrogradation: although their lattice solvates during gelatinization, the underlying six-pyroglucan-unit base may remain partially intact and reorganize into two-dimensional hexagonal arrangements that evolve into three-dimensional nanocrystals with hexagonal structure. Retrogradation therefore arises from nanocrystal nucleation and ordering rather than from amylose or amylopectin chains. X-ray diffraction and DSC analyses indicate that the irreversible thermal behavior of starch is associated with these nanocrystals, underscoring their central role in gelatinization and crystallization processes.