Correlation between starch retrogradation and water mobility as determinedby differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR)

Starches from nine varieties of rice, including four indica, three japonica , and two waxy cultivars, were used for the investigation of the correlatio n between retrogradation and water mobility. Retrogradation and water mobil ity were analyzed by differential scanning calorimetry (DSC) and nuclear ma gnetic resonance (NMR) and expressed as enthalpy change (DeltaH) and differ ential relaxation rate (DeltaR(2)) for water-O-17. Water contents were meas ured by DSC and Karl-Fischer methods. The results indicated that three diff erent profiles, based on amylose content, were observed for the DeltaH chan ges of rice starch cooks during storage. They fit well to the nonlinear reg ression equations of exponential rise to maximum and exponential growth mod els. 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 DeltaR(2) of rice starch cooks showed an increasing trend dur ing storage but was more complicated than the DeltaH trend. The nonlinear r egression models were also applied to fit the changes of DeltaR(2) for indi ca varieties in the initial six days and for waxy varieties up to 24 days. This resembled the DeltaH changes.