¹H NMR studies of starch-water interactions during microwave heating.

The aim of the present study was to investigate the effect of microwave heating on water distribution and dynamics in starch granules during the gelatinization of starch. Starch samples treated with microwave heating, rapid conventional heating and conventional heating was measured by (1)H NMR to examine the water distribution and dynamics in rice starch granules at a water activity of 0.686. The system proton longitudinal and transverse relaxation times were determined using inversion recovery (IR) and Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences. The results showed that the T₁ of the water molecules in the samples treated with any of the three heating methods exhibited two distinct spectral peaks over the temperature range of 40-60 °C. With rising temperature, the long T₁ component and the short T₁ component approached each other, showing a trend of gradual convergence, while T₂ exhibited a single peak over the entire temperature range examined. In addition, significant differences were observed in the T₁ and T₂ of the water molecules in the samples heated by microwave, rapid conventional and conventional. The results show that the rapid heating effect of microwave inhibits the destruction of the hydrogen bonds between starch and water molecules. In contrast, the vibration motion of polar molecules caused by microwave heating accelerates the destruction of hydrogen bonds, producing a much stronger effect than the rapid heating effect of microwave.