Solid-like components in carbohydrate gels probed by NMR spectroscopy

Abstract

Solid-like components in maltodextrin and inulin model gels were probed with NMR spectroscopy. 1H NMR cross-relaxation experiments yielded quantitative information about the ageing process during which the immobile fraction of the polymer increased. The increase of immobile protons was correlated with crystallization in the gels. From the growth rate of the amount of solid-like component in inulin and maltodextrin gels different kinetics of ageing were deduced. In the case of inulin, the fraction of immobilized polymer was reduced at lower polymer concentration which was correlated with the observed decrease in the gel strength. Firming of the inulin gel took longer when starting from totally dissolved polysaccharide material, as was the case with solutions prepared at temperatures exceeding 82°C. However, neither the kinetics of solidification, nor the fraction of immobilized polymer at equilibrium appeared to be influenced by the temperature at which the starting solution had been prepared. Additionally, molecular mobility of maltodextrin chains and interaction with water in the gel was investigated by 13C 1D and 2D solid-state NMR. The molecular mobility of the polymer chains was studied as a function of storage time and storage temperature and turned out to be higher in gels stored at higher temperatures. Water binding was probed with the WISE experiment. Lower molecular mobility of polysaccharide chains as well as stronger interaction with water in gels stored at lower temperature was attributed to the formation of a 3D polymer network accompanied by partial crystallization.