Phospholipid-stabilised water-in-oil microemulsions: A study of ester synthesis by Humicola lanuginosa lipase

Abstract

In a model reaction, octyl decanoate was synthesised from n-octanol and decanoic acid using Humicola lanuginosa lipase entrapped in hydrated lecithin reversed micelles in n-heptane. The surfactants used were pure phosphatidylcholine (PC) from egg yolk, a partially purified lecithin preparation from dried egg yolk (EY-PC) containing about 60% PC, and two from soybean (SB-PC), one containing about 40% PC the other containing about 48% PC. Reaction took place readily in all systems. The initial rate of reaction in the different phospholipid systems was compared to a similar system stabilised by sodium bis (2-ethylhexyl) sulphosuccinate (AOT) and was superior in every case. The rate of ester formation was higher in the systems stabilised by soybean lecithin compared to the system stabilised by pure phosphatidylcholine. For the system stabilised by pure PC containing 700 mM water, phase separation into an emulsion occurred at an ester concentration of around 75 mM using initial substrte concentrations of 100 mM. In contrast, the systems stabilised by the other lecithin preparations were stable microemulsions at reaction equilibrium (about 95% conversion). The effects of enzyme concentration, initial water concentration, water activity, surfactant concentration, initial substrate concentration and temperature were studied in a system stabilised by soybean lecithin. The initial rate of reaction was proportional to the enzyme concentration, and was broadly independent of the surfactant concentration. Esterification activity tended to zero at very low water activity ( a w) and water content. Maximal esterification activity was expressed above ω 0 ≈ 10 ( ω 0 = [water] [PC] ) where a w≈0.8. An apparent activation energy of 20 ± 2 kJ mol −1 was determined over the temperature range 10–60°C, which is consistent with previous determinations for lipases obtained using alternative microemulsion systems stabilised by synthetic surfactants such as AOT.