H-1-NMR ON LINE MONITORING OF WATER ACTIVITY DURING LIPASE-CATALYZED ESTERIFICATION

H-1-NMR spectroscopy is used to determine simultaneously the water act ivity (a(w)) and the time course of an esterification reaction catalys ed by a lipase. Chemical shifts signals of hydroxylic hydrogens in fas t exchange (i.e the average hydroxylic signal of acid, alcohol and wat er) varies with water activity and ester content. Calibration curves h ave been established from model media composed of the substrates and v arious ester contents, at different water activities, in order to mimi c a reaction medium. One relationship is established between water act ivity, hydroxylic hydrogen signal chemical shift and ester content. In order to estimate the water activity evolution as a function of time, this last relationship is applied to the hydroxylic hydrogen chemical shift measured in a reaction medium where the Rhizomucor miehei lipas e in a powder form is suspended in the liquid substrates. This alterna tive way of determining the water activity based on hydroxylic hydroge n chemical shift. presents some advantages over more classical means, i.e. time saved and inaccuracies avoided by monitoring without handlin g the sample. (C) 1998 Elsevier Science B.V, All rights reserved.