Enzymatic Oxidation of Ca-Lignosulfonate and Kraft Lignin in Different Lignin-Laccase-Mediator-Systems and MDF Production

Markus Euring,Ursula Kües,Martin Rühl,Kolja Ostendorf

doi:10.3389/fbioe.2021.788622

Abstract

Laccase-mediator-oxidized lignin offers replacement for conventional chemical binders to produce fiberboards. Compared to the previously reported laccase–mediator system (LMS), a lignin-laccase-mediator-system (LLMS) has an advantage in that it requires much shorter fiber-enzyme incubation time due to significantly increased redox reactions. However, the cost of regularly applying laccase on an industrial scale is currently too high. We have employed CcLcc5 from cultures of the basidiomycete Coprinopsis cinerea as a novel basi-laccase (a CAZy subfamily AA1_1 laccase) in medium-density fiberboard (MDF) production, in comparison to the commercial formulation Novozym 51003 with recombinantly produced asco-laccase MtL (a CAZy subfamily AA1_3 laccase-like multicopper oxidase from the ascomycete Myceliophthora thermophila). With the best-performing natural mediator 2,6-dimethoxyphenol (DMP), unpurified CcLcc5 was almost as good as formulated Novozym 51003 in increasing the molecular weight (MW) of the technical lignins tested, the hydrophilic high-MW Ca-lignosulfonate and the hydrophobic low-MW kraft lignin (Indulin AT). Oxygen consumption rates of the two distantly related, poorly conserved enzymes (31% sequence identity) with different mediators and lignosulfonate were also comparable, but Indulin AT significantly reduced the oxidative activity of Novozym 51003 unlike CcLcc5, regardless of the mediator used, either DMP or guaiacol. Oxygen uptake by both laccases was much faster with both technical lignins with DMP than with guaiacol. In case of lignosulfonate and DMP, 20–30 min of incubation was sufficient for full oxygen consumption, which fits in well in time with the usual binder application steps in industrial MDF production processes. LLMS-bonded MDF was thus produced on a pilot-plant scale with either crude CcLcc5 or Novozym 51003 at reduced enzyme levels of 5 kU/kg absolutely dry wood fiber with lignosulfonate and mediator DMP. Boards produced with CcLcc5 were comparably good as those made with Novozym 51003. Boards reached nearly standard specifications in internal bond strength (IB) and modulus of rupture (MOR), while thickness swelling (TS) was less good based on the hydrophilic character of lignosulfonate. LLMS-bonded MDF with Indulin AT and DMP performed better in TS but showed reduced IB and MOR values.

Highlights

After cellulose, lignin ranks second as the most abundant renewable organic macromolecule on Earth
Unpurified C. cinerea Lcc5 (CcLcc5) and Myceliophthora thermophila (MtL) in the Novozym 51003 formulation performed well with 10% lignosulfonate, 10 mM mediator DMP, and only 5 kU enzyme applied per kilogram of atro wood fiber in medium-density fiberboard (MDF) production on a pilot plant scale
The values obtained in LLMS with the massively available hydrophobic Indulin AT were slightly inferior but show a potential for use as a material in a green MDF binder system with either enzyme and the natural mediator DMP

Summary

Introduction

Lignin ranks second as the most abundant renewable organic macromolecule on Earth. The largest amount of technical lignins (∼85%) results from alkaline kraft pulping with “white liquor” of NaOH and Na2S mixed with hot water (155–175°C), i.e., hydrophobic kraft lignin with an increased content of methoxyl groups and βO-4-bonds and 2%–3% organically bound sulfur It is present in the “black liquor” generated during fiber pulping through dissolution of lignin via ionization of phenolic groups, cleavage of β-aryl ether linkages, and condensation reactions. Kraft lignin is recovered from the “black liquor” by acid precipitation, which protonates phenols in the lignin under conformational changes that reduce electrostatic repulsion and hydrophilicity of lignin molecules and end up in lignin flocculation Another 10% of technical lignins, i.e., hydrophilic lignosulfonates, originate from sulfite processing with SO2 and alkali metal or alkaline earth hydroxides. Sulfur-free technical lignins with low molecular weight (MW) and higher homogeneity are obtained from organosolv pulping of lignocellulosic biomass as by-products of the second generation of bioethanol production (Ekielski and Mishra, 2021; Melro et al, 2021)

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Results

Conclusion