polymer system. Currently, chemical products derived from lignin are still partly
oxygenated, such as phenol and vanillin.
The Norwegian company Borregaard, often referred to as the world's most
advanced integrated biorefinery based on lignocellulose, has a LignoTech branch,
focusing on the exploitation of lignin and lignosulfonates from softwood biomass
(primarily spruce) through pulping, sulfonation, and chemical processing. Borregaard is a leader in the production of vanillin as artificial vanilla flavor, which is
derivable through the oxidation of lignosulfonates from softwood trees. In addition,
Borregaard LignoTech produces additives for a large number of industrial and
commercial applications such as concrete mixtures, pesticide dispersants, textile
dyes, oil well drilling chemicals, batteries and ceramic products, road binders, and
feed pellet additives. In many applications, these products offer a direct replacement
of petroleum-based compounds with a renewable bio-based alternative.
The degradation of lignin in nature is primarily performed by specialized fungi
and bacteria [8, 16] that generate more simple aromatic breakdown products from
the complex lignin structures, making these accessible for further processing by
themselves and/or other microbes. Enzymes play a key role in that process as
further presented and discussed in Sect. 2. These aromatic low molecular weight
fragments are thereby channeled into the cellular central carbon metabolism, generating molecules like acetyl-CoA and pyruvate after cleavage of the aromatic ring
structures [8, 17], as further described in Sect. 5. For bio-based industrial conversion of the polysaccharide content of lignocellulose, the feedstock conversion is
generally preceded by a thermochemical pre-treatment step to open the complex
structure of the macro-polymer and render it more accessible for enzymatic
degradation [16]. Pre-treatments in use are, e.g., acid treatment, steam explosion,
hot water treatment, and ammonia treatments (Fig. 1). The pre-treatment used also
affects the structure and the properties of the derivable lignin fraction, and hence its
suitability for different valorization strategies and end-products. This, as well as the
formation of inhibitors like organic acids and furans like furfural and hydroxymethyl furfural (HMF) [18] also has consequences for the kind and the efficiency
of enzymatic processing of lignin.
2 Biocatalytic Depolymerization of Lignin
In nature, lignin is degraded by fungi and bacteria using mixtures of enzymes
from different classes, including oxidoreductases (peroxidases, laccases) and lyases
(etherases). These enzymes appear to assist the host organisms to degrade wood
materials (depolymerization) and gain access to smaller phenolic compounds that
can be further metabolized (mineralization) [19–21]. According to the eLignin
database [22], the majority of organisms that are able to degrade, or showing
activity on, natural or technical lignin have been isolated from soil and forest/wood
samples. This is a predictable outcome given the abundant processes of biomass
degradation taking place in such habitats. Another natural origin of microbial
444
G.-S. Nguyen et al.
oxygenated, such as phenol and vanillin.
The Norwegian company Borregaard, often referred to as the world's most
advanced integrated biorefinery based on lignocellulose, has a LignoTech branch,
focusing on the exploitation of lignin and lignosulfonates from softwood biomass
(primarily spruce) through pulping, sulfonation, and chemical processing. Borregaard is a leader in the production of vanillin as artificial vanilla flavor, which is
derivable through the oxidation of lignosulfonates from softwood trees. In addition,
Borregaard LignoTech produces additives for a large number of industrial and
commercial applications such as concrete mixtures, pesticide dispersants, textile
dyes, oil well drilling chemicals, batteries and ceramic products, road binders, and
feed pellet additives. In many applications, these products offer a direct replacement
of petroleum-based compounds with a renewable bio-based alternative.
The degradation of lignin in nature is primarily performed by specialized fungi
and bacteria [8, 16] that generate more simple aromatic breakdown products from
the complex lignin structures, making these accessible for further processing by
themselves and/or other microbes. Enzymes play a key role in that process as
further presented and discussed in Sect. 2. These aromatic low molecular weight
fragments are thereby channeled into the cellular central carbon metabolism, generating molecules like acetyl-CoA and pyruvate after cleavage of the aromatic ring
structures [8, 17], as further described in Sect. 5. For bio-based industrial conversion of the polysaccharide content of lignocellulose, the feedstock conversion is
generally preceded by a thermochemical pre-treatment step to open the complex
structure of the macro-polymer and render it more accessible for enzymatic
degradation [16]. Pre-treatments in use are, e.g., acid treatment, steam explosion,
hot water treatment, and ammonia treatments (Fig. 1). The pre-treatment used also
affects the structure and the properties of the derivable lignin fraction, and hence its
suitability for different valorization strategies and end-products. This, as well as the
formation of inhibitors like organic acids and furans like furfural and hydroxymethyl furfural (HMF) [18] also has consequences for the kind and the efficiency
of enzymatic processing of lignin.
2 Biocatalytic Depolymerization of Lignin
In nature, lignin is degraded by fungi and bacteria using mixtures of enzymes
from different classes, including oxidoreductases (peroxidases, laccases) and lyases
(etherases). These enzymes appear to assist the host organisms to degrade wood
materials (depolymerization) and gain access to smaller phenolic compounds that
can be further metabolized (mineralization) [19–21]. According to the eLignin
database [22], the majority of organisms that are able to degrade, or showing
activity on, natural or technical lignin have been isolated from soil and forest/wood
samples. This is a predictable outcome given the abundant processes of biomass
degradation taking place in such habitats. Another natural origin of microbial
444
G.-S. Nguyen et al.