Preparation of cross-linked enzyme aggregates of lipase from Aspergillus niger : process optimization, characterization, stability, and application for epoxidation of lemongrass oil

Cross-linked enzyme aggregates (CLEAs) of lipase were prepared after fractional precipitation with 40–50% ammonium sulfate and then cross-linking with glutaraldehyde. The process variables for the preparation of lipase-CLEAs such as glutaraldehyde concentration, cross-linking period, and initial pH of medium were optimized. The optimized conditions for the preparation of lipase-CLEAs were 25 mM/80 min/pH 7.0, and 31.62 mM/90 min/pH 6.0 with one factor at a time approach and numerical optimization with central composite design, respectively. Lipase-CLEAs were characterized by particle size analysis, SEM, and FTIR. Cross-linking not only shifted the optimal pH and temperature from 7.0 to 7.5 and 40–45 to 45–50 °C, but also altered the secondary structure. Lipase-CLEAs showed an increase in K m by 7.70%, and a decrease in V max by 16.63%. Lipase-CLEAs presented better thermostability than free lipase as evident from thermal inactivation constants ( t 1/2 , D and E d value), and thermodynamic parameters ( E d , Δ H °, Δ G °, and Δ S °) in the range of 50–70 °C. Lipase-CLEAs retained more than 65% activity up to four cycles and showed good storage stability for 12 days when stored at 4 ± 2 °C. They were successfully utilized for the epoxidation of lemongrass oil which was confirmed by changes in iodine value, epoxide value, and FTIR spectra. Graphic abstract