Structural Characterization of Starch Networks in the Solid State by Cross-Polarization Magic-Angle-Spinning Carbon-13 NMR Spectroscopy and Wide Angle X-Ray Diffraction

The changes in structure and morphology following network formation of pregelatinized starch were studied in the solid state by cross-polarization magic-angle-spinning ¹³C NMR spectroscopy and wide angle X-ray diffraction. Two types of mechanisms for network formation were employed, complexation with calcium and covalent cross-linking with epichlorohydrin. The spectra lines of the amylose network prepared by the calcium procedure were very narrow, and fine splitting was observed, indicating the formation of helical structure. The spectra lines of the network formed following the addition of epichlorohydrin were very broad and almost structureless, indicating that the resulting network is highly disordered and largely amorphous. These observations are supported by wide angle X-ray diffractograms. Changes in C-6 chemical shift was observed following starch complexation with calcium, whereas following the reaction with epichlorohydrin, only minor changes in spectra pattern were observed. The amylose network formed by the calcium procedure is of physical character, whereas a covalently cross-linked network was formed by the reaction with epichlorohydrin. These differences in structure have been observed to manifest themselves in the enzymatic degradation rates of these networks.