OBSERVATION BY SOLID-STATE C-3 CP MAS NMR-SPECTROSCOPY OF THE TRANSFORMATIONS OF WHEAT-STARCH ASSOCIATED WITH THE MAKING AND STALING OF BREAD

Wheat starch and wheat starch gels were characterised by C-13 CP MAS NMR spectroscopy. It was found that rotating-frame relaxation rates were significantly different for NMR resonances attributable to crystalline and amorphous regions in the wheat starch. Thus C-13 CP MAS NMR sub-spectra of crystalline and amorphous regions before and after gelation could be constructed from spectra obtained at different spin-locking times. Our results indicated that the amorphous regions in the native starch were unaltered by gelation and retrogradation, and we conclude, therefore, that they correspond to the branching regions of the amylopectin component. Upon gelation the amount of crystallinity decreased and a large proportion of the starch became mobile. Since the mobile components of the starch gel were not observed by CP MAS NMR, the kinetics of starch retrogradation could be determined by observation of the increase in crystallinity over a period of time. Starch gels were found to contain three distinct components: amorphous regions, crystalline regions, and liquid-like regions.