Enzymatic Processes to Unlock the Lignin Value

Abstract

Main hurdles of lignin valorization are its diverse chemical composition, recalcitrance, and poor solubility due to high-molecular weight and branched structure. Controlled fragmentation of lignin could lead to its use in higher value products such as binders, coatings, fillers, etc. Oxidative enzymes (i.e., laccases and peroxidases) have long been proposed as a potentially promising tool in lignin depolymerization. However, their application was limited to ambient pH, where lignin is poorly soluble in water. A Finnish biotechnology company, MetGen Oy, that designs and supplies industrial enzymes, has developed and brought to market several lignin oxidizing enzymes, including an extremely alkaline lignin oxidase MetZyme® LIGNO™, a genetically engineered laccase of bacterial origin. This enzyme can function at pH values as high as 10–11 and at elevated temperatures, addressing lignin at its soluble state. In this article, main characteristics of this enzyme as well as its action on bulk lignin coming from an industrial process are demonstrated. Lignin modification by MetZyme® LIGNO™ was characterized by size exclusion chromatography, UV spectroscopy, and dynamic light scattering for monitoring particle size of solubilized lignin. Under highly alkaline conditions, laccase treatment not only decreased molecular weight of lignin but also increased its solubility in water and altered its dispersion properties. Importantly, organic solvent-free soluble lignin fragmentation allowed for robust industrially relevant membrane separation technologies to be applicable for product fractionation. These enzyme-based solutions open new opportunities for biorefinery lignin valorization thus paving the way for economically viable biorefinery business.

KnowVolution of a Fungal Laccase toward Alkaline pH

“…Within the paper and pulp industry, several million tons of lignin are produced per year as a by‐product . Lignin is the most abundant renewable source of aromatic polymers in nature .…”

“…One of the main challenges of lignin valorization lies in its diverse structure and poor solubility. Lignin is insoluble in water and alcohol, but soluble in alkaline solutions . In nature, depending on their origin, laccases are involved in lignin degradation and biosynthesis .…”

“…In terms of lignin valorization, depolymerization processes employing laccases were considered on an industrial level . Notably, most of the reported laccases are not yet active under application conditions for lignin depolymerization (pH≥9.0) …”

Enzyme biotechnology in degradation and modification of plant cell wall polymers

“…These challenges may be overcome by modifying the biorefinery process and removal of the depolymerization products of lignin during the enzyme treatment. Alkaline reaction conditions that maintain lignin solubility appear to be beneficial to reaction efficiency (Hämäläinen et al ). Salvachúa et al () applied enzyme mixtures produced by different lignolytic fungi in depolymerization of lignin from a mild biorefinery process in presence of aromatic metabolizing bacterium, which acted as a ‘microbial sink’ for the released aromatics, converted them to the desired product and simultaneously reduced the repolymerization.…”

Lignin as Alternative Reinforcing Filler in the Rubber Industry: A Review

“…A huge amount of lignin by-product is produced by the paper and pulp industries, and about ∼98% of that is usually burned to generate energy or just disposed of in landfill, hence posing a major disposal problem (Mohan and Karthikeyan, 1997;Zhang and Chuang, 2001; Kumar et al, 2009;Doherty et al, 2011;Saake and Lehnen, 2012;Laurichesse and Avérous, 2014;Lang et al, 2018). Therefore, the development of new technologies is crucial to explore the functionality of this precious by-product further, not only for use in low-and medium-value applications but in high-value applications as well (Hämäläinen et al, 2018). Many applications that have been explored by researchers, for example, as adhesive (Gosselink et al, 2004), stabilizing agents (De Paoli and Furlan, 1985), reinforcing agents (Kumaran and De, 1978;Setua et al, 2000;Benko et al, 2014), superabsorbent hydrogels, and phenolic resin (Suhas et al, 2007;Kumar et al, 2009;Hu et al, 2011).…”