Kinetic study on the enzymatic esterification of octanoic acid and hexanol by immobilized Candida antarctica lipase B

This study investigates reaction kinetics of the esterification of octanoic acid and hexanol into hexyl octanoate, catalyzed by an immobilized Candida antarctica lipase (Novozym® 435). The product is considered natural and used as a fresh vegetable and fruity flavour additive in food, cosmetic and pharmaceutical products. The reaction is performed in n-decane as the solvent, to improve enzyme stability and to increase the reaction yield. The influence of substrate concentration on hexyl octanoate synthesis is investigated over a wide range up to 2 M. The observed bi-substrate inhibition pattern follows a Ping-Pong bi-bi mechanism with dead-end inhibition by both substrates and, based on the proposed model, the kinetic constants of the esterification reaction are estimated. These parameters are verified to be intrinsic – neither external nor internal mass transfer resistances are significant for the examined reaction system – and are essential to extend analysis to a large-scale process and for a wide range of operating conditions. The progress of the reaction is also observed and the kinetic model is validated by fitting experimental progress curves with two different concentrations of biocatalyst. Effects of biphasicity of the reaction system, inhibition by the ester produced and the influence of the reverse reaction have been also evaluated. Highlights Effects of substrate, catalyst amount and biphasic reaction medium were studied. Ping-Pong bi-bi model with twin substrate inhibition was used at initial rates. Possible ester inhibition and catalyst denaturation by substrates were investigated. Reverse reaction was considered and a reversible kinetic model was proposed. The intrinsic kinetic model was validated by experimental progress-curve data.