| Starch, as a widely used food material, has complex structure and composition, a lot of studies of starch and starch material food under microwave focused on the gelatinization progress and the application level. Otherwise, there is lack of study focused on the changes of the starch structure under microwave, which may due to a security problem. As a result, the lack of research in the field of starch structure changes under microwave, will hindering the security for microwave heating progress and also limits the application of microwave technology in starchy food industry.This article selected potato starch, a stable and widely used starch, as the research object, set up a rapid heating method of oil bath and a traditional slow heating method as a control to study on the effect of microwave on the chemical group, skeleton vibration, the component of disordered and double helices, crystallinity and semi-crystallization layer of potato starch. The methods above were based on the characteristic of microwave heating progress which leads to a rapid heating rate and some special effects departure of microwave. This article also tried to determine the primary mechanism of microwave heating on starch.Based on the principle of molecular vibrational spectroscopy, we used Fourier transform infrared spectroscopy (FTIR) and laser confocal micro-Raman spectroscopy (Raman) to find the changes of vibration intensities of chemical groups and skeleton of potato starch before and after microwaved. The results indicated the absorbance number of vibrational peaks in FTIR and Raman spectra of microwace sample did not changed, while the intensities of the absorbance peak increased sharply. The FTIR results showed the thermal effects of microwave was the main factor which influenced the changes of vibrational intensities of chemical groups in starch. The microwaved sample had similar trend with rapid heating samples. The Raman results showed after gelatinization point, the special effects departure of microwave had the main efficts on the changes of vibrational intensities of skeleton related modes, such as glycosidic bond and pyran ring.13C CP/MAS NMR and XRD technique were used to learn the component changes of double helices and disordered structure and the changes of crystallinity. The results indicated the thermal effect and the special effects departure of microwave both influenced the starch granule. Compared to the native starch, the content of disordered structure of microwaved potato starch increased by29%, the content of double helices of microwaved sample decreased by22%. Compared to the native starch, the crystallization degree of microwaved potato starch dreased by30.8%which was, which was higer than30.6%of rapid heating sample and lower than31%of the slow heating sample. The thermal effect made more contribution to the changes of the content of disordered structure, double helix structure and crystallinity degree.The semi-crystalline area of starch has significant scattering information on the small-angle X-ray scattering (SAXS) Figure, so SAXS was used to learn the semi-crystalline growth rings and other submicroscopic structure of starch.The results showed before the gelatinization point, the starch samples had a scattering peak between q=0.6-0.7. As the temperature rose, the peak disappeared. The thickness of microwaved sample and traditional heating samples both showed a S-type changes, but the changes of microwaved sample was in the range of rapid heating sample and slowly heating sample. The rapid heating caursed by the thermal effect of microwave resulted in a tighter arrangement of double helices in crystalline layer and the destructive of the transition layer and the reduction of the thickness of transition layer.On the whole, the microwave heating did not change the structure of the potato starch, but changed the magnitude of the structure changes of starch. The special effects of microwave were the main factors that led to these structural changes. |
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