ACTIVITY BY H-1 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY - APPLICATIONTO THE STUDY OF WATER EXCHANGE IN BIPHASIC MEDIA

The water activity (a(w)) of the liquid phase is investigated by means of H-1 NMR for both monophasic and biphasic systems. The chemical shi ft or the area of the signal of the hydroxylic hydrogens is compared t o calibration curves obtained from mixtures equilibrated at different water activities, thus allowing determination of the a(w) of the syste m. The chemical shift varies linearly as a function of the a(w) of the system. Through simple calculation the hydroxylic hydrogen concentrat ion obtained from the area gives the water concentration in the medium ; the sorption curve is thus obtained through NMR. The application of this method to a biphasic system composed of a liquid phase (an equimo lar mixture of acid-alcohol) and a solid phase (a lipase), equilibrate d separately at two different water activities, gives information on t he water exchange between both phases during the lag phase of the este rification reaction. Light and slow water exchanges are observed from the solid phase at high a(w) toward the liquid phase at low a(w). The lag phase of the esterification reaction is too short for the water ac tivity equilibrium to be reached before the reaction starts. (C) 1996 Academic Press, Inc.