Stability analysis of Bacillus stearothermophilus L1 lipase immobilized on surface-modified silica gels

This work aims to investigate the effects of surface properties of carriers, and immobilization methods on the stability of a lipase. Factors affecting the lipase stability were divided into the reaction-independent factors (RIFs) and the reaction-dependent factors (RDFs). RIFs include the reaction conditions such as pH and temperature, whereas substrate limitation and product inhibition are examples of RDFs. Oil hydrolysis reactions were conducted at pH 10 and 50°C using free and immobilized lipase, which was derived from Bacillus stearothermophilus L1. Surface-modified silica gels were used as a carrier material for enzyme immobilization. Hydrophilic and hydrophobic silica gels were made by polyethyleneimine coating and silanization, respectively. Covalently bound lipase was found to be more stable than the lipase immobilized by physical adsorption. The effects of RIFs on the enzyme activity were less for the lipase immobilized on the hydrophilic silica gels than for the lipase on the hydrophobic silica gels regardless of immobilization methods. However, the opposite trend was observed for the effects of RDFs. Our study shows that the hydrophilicity and the hydrophobicity of carrier surfaces should be optimized in order to maximize the stability of immobilized lipase.