Spectrum Analysis of Crystalline Structure of Crop Starches

Abstract

Crop starches have A-type, B-type, and C-type crystallinity, and C-type crystallinity is the combination of both A-type and B-type crystallinity. In this paper, spectrum characteristics and relative crystallinity of starches from different plants were investigated with X-ray powder diffraction (XRD) and 13C cross-polarization magic-angle spinning nuclear magnetic resonance (13C CP/MAS NMR). The results indicated that rice, potato and pea starches showed typical A-type, B-type, and C-type XRD spectra respectively. Water chestnut starch showed a CA-type XRD spectrum, which was a C-type closer to A-type. Kudzu starch showed a CB-type XRD spectrum, which was a C-type closer to B-type. The relative crystallinity of starch from XRD was ob-tained using the Jade 5.0 software and the curve mapping method. The results of two methods showed significant difference and had no correlation. The crystallinity with the curve mapping method was more reliable. The spectra of 13C CP/MAS NMR from different crop starches showed similar characteristics, and had four regions of C1, C4, C2, 3, 5 and C6, while the difference of the spectra among different starches was from C1 region. In C1 region, A-type starches of waxy and normal maize showed three peaks, B-type starch of potato showed two peaks, the transgenic resistant starch rice line (TRS) starch, which was a CA-type crys-tallinity, showed three inconspicuous peaks, the acid-modified TRS starch with CB-type crystalline showed two peaks and the amorphous starch had no peaks. The 13C CP/MAS NMR spectra were peak fitted by using the PeakFit 4.12 software. The relative crystallinity and the percentage of double helix content in starches were calculated. The double helix content was higher than the relative crystallinity. The crystallinity obtained from 13C CP/MAS NMR was higher than that from XRD. These results would be very useful for the application of XRD and 13C CP/MAS NMR to the analysis of crystalline structure of crop starches.