Scanning electron microscopic studies of lipase-catalysed esterification catalysis for the synthesis of stearoyl lactate and p-cresyl laurate

The state of three lipases, two from Rhizomucor miehei and one from porcine pancreas, employed in the esterification reactions leading to the preparat ion of food additive esters were investigated by scanning electron microsco py (SEM). The lipases employed in the synthesis of stearoyl lactic acid and p-cresyl laurate in 10 ml solvent at 40-60 degreesC in shake-flask experim ents and 150 ml in non-polar solvents at 50-60 degreesC in bench-scale leve l experiments were compared. All three lipases, which were subjected to hig h temperatures and non-polar solvents for a prolonged period of incubation of 72-120 h, showed decrease in the 'compactness' when compared to unused l ipase. The presence of buffer preserved the activity and compactness and th e absence of the same reduced the amount of enzyme per unit area on the sup port. R. miehei lipase samples subjected to reaction in presence of 0.0004 ml of 0.1 M buffer/mg enzyme preparation at different pH values (4.0-9.0) s howed a decrease in compactness of the enzyme on the surface which correlat ed to an increase in esterification activity. An increase in volume of buff er (0.0002-0.003 ml/mg enzyme preparation) in the reaction mixture at pH 7. 0 showed a decrease in compactness and also a reduction in activity. The st udies indicate that a compromise between pH and volume of buffer can lead t o variation in the extent of adsorption, distribution and activity, enablin g the achievement of maximum conversions in the esterification reactions.