Effect of water activity and solvent-ordering on intermolecular interaction of high-methoxyl pectins in various sugar solutions

Inter-macromolecular interaction of high-methoxyl pectins from citrus and apple in various sugar solutions (ribose, mannose, glucose, sucrose, trehalose, and maltose) was investigated through the analysis of the specific viscosity, ηsp, excluding the background effect of sugar solutions. With an increase in the sugar concentration, the water activity, AW, decreased and ηsp increased linearly with a decrease in AW for all the sugars tested, showing a change in the hydrogen-bonding among pectin molecules. The effect of AW on ηsp was also dependent on the type of the sugar coexisting. The different effect of sugars on the inter-macromolecular interaction was quantitatively evaluated by −dηsp/dAW, which showed a good correlation with aqueous solvent-ordering parameters for sugar solutions in the literature, such as the parameter α determined from the activity coefficient of water, the number of equatorial-OH group in sugar molecular structure, B-coefficients for spin–lattice and spin–spin relaxation in proton NMR, and B-coefficient of viscosity. This suggests that the different effect of sugars on the inter-macromolecular interaction is explained by their different effect on the hydrophobic interactions among pectin molecules through the change in the solvent-ordering.