The reversible swelling of starch granules investigated by solid-state nuclear magnetic resonance

This study explores the molecular mechanisms that underlie the reversible swelling of starch, using advanced solid-state NMR (ss-NMR) techniques. It focuses on four A-type starches—wheat, waxy wheat, rice, and tapioca—at two hydration levels (low and moderate). 13 C CPMAS and VCT-CPMAS NMR experiments were used to characterize structural transitions and water-starch interactions. Deconvolution of the C1 and C6 regions revealed distinct molecular organizations, including single and double helices and amorphous domains, allowing their evolution with water content to be tracked. Modeling of 1 H to 13 C polarization transfer kinetics enabled the quantification of spin diffusion environments, linking specific relaxation parameters to the starch components amylose and amylopectin. Correlations were established between swelling factors and these NMR-derived parameters, highlighting amylose as a key constituent that limits the granule swelling at low temperatures. This integrated approach demonstrates the sensitivity of ss-NMR to hydration-induced structural changes and provides novel insights into the quantification at the nanoscale of the starch swelling phenomenon induced at low or moderate hydration levels and based on the measurement of NMR spin diffusion times. • Solid-state NMR reveals starch hydration dynamics at low water levels. • VCT-CPMAS NMR differentiates amorphous and semi-crystalline starch rings. • Polarization transfer kinetics identify starch structural domains. • Amylopectin content strongly correlates with starch swelling factor. • Low-temp swelling is linked to amylose/amylopectin molecular balance.