Parameters influencing hydrolysis kinetics of lipase in a hydrophilic membrane bioreactor

Abstract

The hydrolysis of lipids by Candida rugosa lipase was studied in a membrane reactor as a function of several reactor parameters: temperature, pH, Ca 2+, and Na + in the water phase. The initial hydrolysis rate of the free enzyme as a function of the temperature was shown to follow Arrhenius kinetics with an activation energy of 12.3 kJ mol −1 in the range of 25 to 55°C. Above 55°C the inactivation is too fast for an accurate determination of the initial rate. Literature data about the effect of temperature on the hydrolysis rate of C. rugosa are not consistent, possibly due to different measuring methods. The influence of temperature on the stability was investigated for the membrane reactor and for the free enzyme. A stabilizing effect of immobilization was observed in the membrane reactor as compared to the free enzyme. The immobilized enzyme was more stable by a factor of four; the activation energy for the inactivation reaction was found to be 201 kJ mole −1 for both free and immobilized enzyme. Combination of the relations for activity and stability rendered a model that described the activity and productivity as a function of temperature and time. The results of model calculations showed that for long operation times, like that used in a membrane reactor, the total productivity can increase with decreasing temperature. The optimal temperature is in the range of 25–30°C. However, a lower limit of the operating temperature is the melting point of the lipid substrate used. The pH optimum for the initial activity of free as well as membrane-immobilized lipase appeared to be 6.5–7.0. In the acidic range the immobilized enzyme had a higher activity than the free enzyme. Above pH 7 the membrane reactor was clogged by the formation of soaps. In the presence of CaCl 2 CaCl 2 (10 m m), the stability of the membrane-immobilized lipase increased by more than a factor 10. The effect of NaCl was less favorable: at low concentrations (0.1 m) no influence was observed, while at higher concentrations (1 m) the stability decreased by a factor of 20.