Lipase-catalyzed hydrolysis of milk fat in lecithin reverse micelles

Reverse micelles formed by soybean lecithin in isooctane were used as a reaction medium for the lipase-catalyzed hydrolysis of milk fat. The pH value for the maximum enzyme reaction rate was 6.5, which is similar to that in aqueous solutions and AOT reverse micelles. The optimum temperature for enzyme activity was 55°C with an activation energy of 7.5 kcal/mol. The optimum temperature was 20°C higher than that in aqueous solutions and AOT reverse micelles. The hydrolysis reaction does not appear to follow Michaelis-Menten kinetics and it is suggested that the reaction rates are diffusion controlled. The dependence of the reaction rate on the molar ratio of water to surfactant concentration (R value) showed a bell-shaped curve, with the maximum rate found at R=11. Enzyme activity varied little with surfactant concentration at a constant R value for up to 50 mmol dm^-3 lecithin. Higher surfactant concentrations led to a decrease of enzyme activity, suggesting possible inhibition effects from lecithin. The composition of free fatty acids released during the reaction, which is responsible for the flavour of lipolyzed milk fat, can be manipulated by controlling the reaction conditions in the reverse micellar system. To promote the release of short-chain fatty acids, a higher concentration of enzyme, a higher R value, or a modest reaction temperature (45°C) should be used.