Thermo-alkali-stable lipase from a novel Aspergillus niger: statistical optimization, enzyme purification, immobilization and its application in biodiesel production

Abstract

The influences of nutritional components affecting lipase production from the new Aspergillus niger using wheat bran as substrate were studied by employing Plackett-Burman and central composite statistical designs. Out of the 11 medium components tested, sucrose, KH₂PO₄ and MgSO₄ at final concentrations of 3.0, 1.0 and 0.5 g/L, respectively, were reported to contribute positively to enzyme production (20.09 ± 0.98 U/g ds). The enzyme was purified through ammonium sulfate precipitation followed by Sephadex G-100 gel filtration. Molecular mass of the purified lipase was 57 kDa as evident on SDS-PAGE. Different methods of immobilization were studied and the highest immobilization yield of 81.7 ± 2.18% was reported with agarose (2%) and the optimum temperature was raised from 45 to 50 °C. Immobilized lipase could retain 80% of its original activity at 60 °C after 1 hr of incubation, and was stable at pH values between neutral and alkaline pH. Lipase-catalyzed transesterification process of fungal oil resulted in a fatty acid methyl ester yield consisting of a high percentage of polyunsaturated fatty acids (83.6%), making it appropriate to be used as winter-grade biodiesel. The operational stability studies revealed that the immobilized lipase could keep 70% of its total activity after 5 cycles of the transesterification process.

Keywords: Aspergillus niger EM 2019; biodiesel production; experimental design; extracellular lipase; immobilization; solid-state fermentation.