Enzymatic conversion of lignin

Abstract

Lignin, a major byproduct of the pulp and paper industry, is not widely utilized as a potential source for valuable chemicals due to its complex and recalcitrant structure that needs to be changed into less intricate forms. This study investigates the enzymatic valorization of kraft lignin using laccase produced by the white-rot fungus Fomes fomentarius, isolated from Finland’s Repovesi National Park and cultivated on silver birch (Betula pendula) bark, an abundant forestry byproduct. Semi-solid-state fermentation conditions were optimized through central composite design, identifying pH 5, 4 g/L glucose, 5 g/L yeast extract, and 0.2 g/mL birch bark/medium ratio supplemented with 2 mM CuSO₄ as optimal for laccase production. Under these conditions, the 4-fold concentrated laccase reached an activity of 3533±153 U/L (specific activity of 4.48 ± 0.22 U/mg). Additionally, it was characterized for molecular weight, stability across pH and temperature ranges, and activity in different buffer systems, being its most stable form after 48 hours achieved in pH 6 for succinic buffer and pH 8 for citrate-phosphate buffer, both at 25 °C. Kraft lignin degradation was evaluated with and without mediators (ABTS and vanillin), with mediator-assisted systems significantly enhancing lignin modification and phenolic compound release. The results demonstrate that F. fomentarius laccase, produced from low-cost biowaste substrates, offers an efficient and sustainable biocatalytic route for lignin valorization, contributing to the development of circular bioeconomy strategies.