Esterification reactions catalyzed by lipases in reverse micellar media.

Abstract

The objective of this thesis is to examine the esterification reactions of fatty acids catalyzed by lipases in reverse micellar media. This reaction is of great interest to industry in the synthesis, for example, of food and cosmetics emulsifiers and texturizers, flavors and fragrances, and complicated triglyceride oils. In addition, the use of lipases controls the reaction selectivity and reduces operating costs. Reverse micelles--dispersions of aqueous/polar material in lipophilic organic solvent formed by the action of surfactants--has been investigated here as a medium for hosting such reactions. It was chosen for application due to its low water content, ease of preparation, and possession of large amounts of interfacial area. In this thesis, the lipase-catalyzed esterification of fatty acids with various alcohols: monohydroxyl alkyl alcohols and polyols has been thoroughly investigated in reverse micellar media containing the surfactant Aerosol-OT (sodium bis-(2-ethylhexyl)sulfosuccinate). For monohydroxyl alcohols, conversions as high as 90% were obtained, while for the polyols glycerol and ethylene glycol, maximum conversions were 50-60%. For the reactions investigated, the lipase type employed specified the product distribution and dictated the rate(s) of formation for product(s), but had little effect on the thermodynamic equilibrium. Also, in the presence of substrates, remarkably good enzyme stability can be achieved; for example, lipase from Rhizopus delemar retained 80% of its original activity in three weeks. The physical nature of the media, as determined by light scattering and infrared spectroscopy, played a key role in determining the kinetics. For example, substrates present at the micellar interface were the ones most readily converted, indicating the site of reaction is at the interface. Also, the observed kinetics and enzyme activity retention were greatly enhanced by conditions of interfacial fluidity, which is promoted by the presence of short-tailed alcohols and employment of large-chained aliphatic oils. In addition, extraction techniques indicate at least 55% of original enzyme activity can be recovered from the reaction media; but, for recovery of products, an effective means to remove surfactant requires development.