EFFECT OF THE THERMODYNAMIC WATER ACTIVITY AND THE REACTION MEDIUM HYDROPHOBICITY ON THE ENZYMATIC-SYNTHESIS OF ASCORBYL PALMITATE

The influences of the thermodynamic water activity (a(w))() and the re action medium hydrophobicity on the synthesis of ascorbyl palmitate, i n the 2-methyl-2-butanol as a solvent, in the presence of the immobili zed Candida antarctica lipase as a biocatalyst, were investigated. The effect of moisture on the equilibrium and the rate of the transesteri fication reaction, involving palmitic acid methyl ester and ascorbic a cid, was evaluated through measurements made in reaction media, which were initially pre-equilibrated in controlled water activity atmospher e. The catalytic activity of the enzyme as well as the production were significantly affected by the pre-fixed water activity level, best re sults being obtained for the lowest a, values tested. An increase in t he acyl donor/ascorbic acid molar ratio (R) up to 9 was demonstrated t o promote the activation of the lipase, leading to an ascorbyl palmita te production of 19 g l(-1) after a reaction time of 5 h. That perform ance was not reached in a reaction medium of equal log P level, where methyl palmitate was partly substituted by a hydrophobic solvent. Hydr ophobic property of the bulk organic phase interacting with the cataly st was suggested to be a necessary but insufficient factor promoting l ipase activation. (C) 1998 Elsevier Science B.V. All rights reserved.