A H-2 NMR LINESHAPE STUDY OF -CH3 GROUP-DYNAMICS IN PECTIN GELS

Deuterium-labelled (methyl-d(3)) pectin was prepared by esterifying a lemon peel pectin with methanol-h. A pure powder sample and multicompo nent model confectioneries containing the labelled pectin and various amounts of water were studied by solid-state H-2 NMR spectroscopy. In the pure sample the broad H-2 NMR lineshape indicates that the only fa st motion occurring on the NMR timescale is reorientation of the methy l-d3 group about its C-3 axis. In contrast, in systems of 50% water co ntent the H-2 NMR lineshape is much narrower, indicating significant m otional averaging. As the percentage water in the confectionery system s is reduced to <40% the main effect is the appearance of a broad comp onent in the spectrum associated with a more static motional environme nt. As the water content decreases further the proportion of labelled pectin in this region increases and the amplitude of the motions decre ases, until for systems of <10% water content motions are now slow on the NMR timescale, except for the methyl-d(3) group reorientation abou t its C-3 axis, and H-2 NMR lineshapes are identical to those observed in the pure powder sample. The evolution of the two component linesha pe indicates that the removal of successive amounts of water does not simply 'stiffen' the entire system but rather initially restricts moti on in certain regions and eventually 'crystallizes' distinct domains w ithin the sample. (C) 1998 Elsevier Science Ltd. All rights reserved.