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Changes in Rice with Variable Temperature Parboiling: Thermal and Spectroscopic Assessment

¹United States Department of Agriculture, Agricultural Research Service, R. B. Russell Agricultural Research Center, Athens, GA 30604-5677. ²Corresponding author. E-mail: Dave.Himmelsbach@ars.usda.gov ³Food Research Institute, P.O. Box M. 20, Accra, Ghana. ⁴Natural Resources Institute, University of Greenwich at Medway, Chatham, ME4 4TB, Kent, UK.

Rapid visco analysis (RVA) and differential scannning calorimetry (DSC) provided overall assessments of the effects of variable temperature soaking at 30, 50, 70, and 90°C and steaming at 4, 8, and 12 min. Calculation of the relative parboiling index (RPI) and percent gelatinization provided good metrics for determining the overall effects of partial parboiling. FT-Raman and solid-state ¹³C CP-MAS NMR spectroscopies provided insight to conformational changes in protein and starch of paddy rice under various parboiling conditions. RVA showed lower pasting curves and DSC showed lower ΔH with increased temperature and steaming times. A large decrease in viscosity occurred with only the 30-4 treatment as opposed to raw rice. This observation was consistent with FT-Raman results that indicated substantial conversion of the protein from α-helix to other conformations. DSC indicated incomplete gelatinization of starch, even with 90°C soaking and 12 min of steaming. Solid-state ¹³C CP-MAS NMR spectroscopy confirmed this result. However, it indicated the percent of V_h/amorphous plus the remaining crystalline starch in the 90-12 treatment was equal to the amorphous and partially-ordered starch in commercially parboiled rice. These results suggest that partial parboiling, 90°C soaking, and more than 8 min of steaming (ideally ≈12 min) of paddy rice is sufficient to induce changes that inactivate enzymes and provide enough starch gelatinization to prevent kernel breakage.