NMR relaxometry as a tool to understand the effect of microwave heating on starch-water interactions and gelatinization behavior

- Microwave heating on starch gelatinization investigated by LF NMR Relaxometry.
- Lowest T2 change in 1:1 starch to water ratio of corn, wheat and rice starch.
- No difference in T2 of wheat starch after 1:1.5 starch to water ratio.
- Disruption of starch granules after 20 s of heating for 1:1 wheat starch.
- Rice starch granules had better hydration and more homogenous gelatinization compared to corn and wheat starches.

Starch-water interactions during gelatinization by microwave heating was investigated by Low-field 1H Nuclear Magnetic Resonance (LF NMR) relaxometry experiments. Effect of microwave heating time on longitudinal (T1) and transverse relaxation times (T2), gel matrix structures, changes in proton populations and firmness values were evaluated. One to one corn starch (CS) to water ratio samples had the lowest T2 change during gelatinization indicating lower granule expansion which was attributed to the high amylose content of CS. Non-Negative-Least-Square (NNLS) analysis of the transverse relaxation curves provided detailed information on starch-water interactions during gelatinization. Rice starch (RS) samples had relatively lower changes in peak areas which was related to the better hydration of RS granules. While microscope images revealed the changes in the granule structures after distinct gelatinization intervals, firmness measurements supported the structural changes. With increasing microwave heating time, both T1 and T2 showed a decreasing trend. Two distinct proton populations detected on relaxation spectrum indicated the exchanging protons during gelatinization. A noticeable reduction in firmness at the later moments of microwave heating was detected. The results showed that starch gelatinization by microwave heating had some distinct features in terms of mechanism of the gelatinization and could be investigated by NMR relaxometry.