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Correlation Between Starch Retrogradation and Water Mobility as Determined by Differential Scanning Calorimetry (DSC) and Nuclear Magnetic Resonance (NMR)

¹Graduate Institute of Food Science and Technology, National Taiwan University, Taipei 106, Taiwan. ²Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan. ³Corresponding author. Phone: 886-2-2789-8568. Fax: 886-2-2783-1237. E-mail: cylii@chem.sinica.edu.tw

Starches from nine varieties of rice, including four indica, three japonica, and two waxy cultivars, were used for the investigation of the correlation between retrogradation and water mobility. Retrogradation and water mobility were analyzed by differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) and expressed as enthalpy change (ΔH) and differential relaxation rate (ΔR₂) for water-¹⁷O. Water contents were measured by DSC and Karl-Fischer methods. The results indicated that three different profiles, based on amylose content, were observed for the ΔH changes of rice starch cooks during storage. They fit well to the nonlinear regression equations of exponential rise to maximum and exponential growth models. The water content, as measured with DSC, decreased during storage but increased as measured with the Karl-Fischer method. This discrepancy might be attributed to the different characteristics of water measured by the two methods. The ΔR₂ of rice starch cooks showed an increasing trend during storage but was more complicated than the ΔH trend. The nonlinear regression models were also applied to fit the changes of ΔR₂ for indica varieties in the initial six days and for waxy varieties up to 24 days. This resembled the ΔH changes.