Lipase synthesis of short-chain flavour thioesters in solvent-free medium

This study demonstrates from a kinetic (initial rate) and thermodynamic (equilibrium yield) point of view the ability of an immobilised lipase, Lipozyme(TM), to catalyse the synthesis of short-chain flavour thioesters such as thioethyl thiobutyl and thiohexyl propionate, butyrate and valerate. The influence of different parameters such as temperature, acid/thiol molar ratio or water content was studied for the synthesis of thiobutyl valerate. Optimisation of the esterification of valeric acid and butanethiol was achieved and some optimal reaction conditions were defined. At 60 degrees C, with an acid/thiol molar ratio of about 1/5, in a free-solvent medium, the conversion yield of valeric acid in thioester was higher than 40% in the presence of 100 gl(-1) molecular sieves and a ratio of immobilised enzyme/molecular sieves of about 1. Comparison between thioesterification and transthioesterification shows that higher initial rates were obtained in the latter case when the acid/thiol molar ratio was 1/1. However, at equilibrium esterification remained more efficient. Finally another immobilised Lipase, Novozym(TM) was assayed. The esterification and transesterification reactions were accelerated but the thioester concentrations at equilibrium were the same as those obtained with Lipozyme(TM).