Solvent-free lipase-catalysed synthesis of saccharide-fatty acid esters: closed-loop bioreactor system with in situ formation of metastable suspensions

Abstract

Saccharide-fatty acid esters – biodegradable, biocompatible and non-ionic bio-based surfactants derived from inexpensive renewable agricultural sources, utilized in foods, cosmetics and pharmaceuticals, have been synthesized under solvent-free conditions using a closed-loop system operated under continuous recirculation consisting of a reservoir, a peristaltic pump and a packed-bed bioreactor containing immobilized Rhizomucor miehei lipase (RML). Metastable suspensions of saccharide crystals were formed through continuous stirring in the reservoir with an in-line filter of 180 µm normal size preventing larger crystals from clogging the tubing. The initial reaction medium consisted of oleic acid/fructose-oleic acid esters 95/5 w/w, with saccharide added periodically to the reservoir to replace consumed acyl acceptor substrate. The liquid-phase water concentration was retained at a previously determined optimal value of ∼0.4 wt % through free evaporation of the co-product, water, in the reservoir during the initial phase of the reaction, and N2(g) bubbling and vacuum pressure after 40 h of reaction. After 200 h, the reaction mixture contained 84 wt % ester, of which 90% of the ester consisted of monoester. Equivalently, the productivity was 0.195 mmolEster h− 1 gRML− 1. The resultant technical grade product can potentially be used directly, without further purification. A mathematical model based on mass balances and a zeroth-order kinetic model was successfully developed to predict the concentration of substrates (oleic acid and saccharide) in the reservoir during the time course of reaction.