Characterizing maize starch gelatinization by Low-Field NMR spectroscopy

Transverse relaxation time (T₂) of starch, measured by low-field nuclear magnetic resonance spectroscopy (LF-NMR), were analyzed to understand starch swelling and gelatinization. Three water proton populations were identified: T₂₁ (intra-granular water), T₂₂ (extra-granular water on the granular surface), and T₂₃ (free water or leached-starch interacting water). During heating at 40 °C or 60 °C, where granular swelling occurred, the relative proton concentration of T₂₂ (M₂₂) and T₂₁ decreased, while M₂₁ increased. At 80 °C, where gelatinization happened, M₂₁ and T₂₁ increased and then decreased to stable values, opposite to the trend of M₂₂. These observations suggest that starch swelling is facilitated by the migration of extra-granular water into intra-granular regions, while gelatinization is driven by the redistribution of intra-granular water back to extra-granular regions. Across all temperatures, T₂₃ and M₂₃ increased and then remained stable. Higher AC retarded and prolonged both processes, by restricting T₂₁ while enhancing T₂₂. Pearson correlation analysis further suggested that M₂₁ was positively correlated with relative content of amorphous region in raw starches, while after heating, M₂₁ had positive correlations with 1047/1022 ratio of the FTIR absorbances and gelatinization temperatures. These findings offer insights into applying LF- NMR in evaluating starch gelatinization dynamics.