Influence of water activity and water content on sugar esters lipase-catalyzed synthesis in organic media

Lipase-catalysed ester bond synthesis in organic media was directed by thermodynamic water activity of the reaction mixture. The effects of water activity and water content were investigated with fructose palmitate synthesis catalysed by Candida antarctica lipase. Conversion yield, initial rate of synthesis and reaction selectivity were analysed. Initial water activity of the reaction medium was fixed at a desired value by pre-equilibration with saturated salt solutions. C. antarctica lipase activity depended strongly on initial water activity value. Conversion yield and initial rate decreased with the increase of water activity. Whatever the aw value, fructose monopalmitate was only synthesised. The best results were achieved for a reaction medium with an initial water activity less than 0.07. In these conditions, 28.5% of fructose was acylated in fructose monopalmitate with an initial rate of 4.9 g l−1h−1. To improve the process, the effect of the water was determined by drying the medium with molecular sieves. Conversion yield of fructose and initial rate of fructose monopalmitate synthesis were raised, respectively, to 73.4% (32 g l−1) and 10.1 g l−1 h−1. However, the use of dried medium affected the selectivity of the reaction. In these conditions, both mono- and di-fructose palmitate were synthesised (16.7 g l−1).