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synthetic antioxidants such as BHA (IC 50 = 56.3 μg/ mL) and BHT (IC 50 = 38.2 μg/
mL). Qin et al. (2018b) designed a multi-enzyme mixture system for the isolation of
enzymatic hydrolysis lignin (EML) from Chinese quince fruits. EML isolated from
the fruits treated by Celluclast 1.5 L, Viscozyme L and Xylanase had the lowest
content of carbohydrate impurities (0.1%) and highest antioxidant activity among
the lignin fractions.
On the other hand, ultrafiltration, selective solvent fractionation and acid gradient precipitation have been shown to decrease lignin heterogeneity (dos Santos et al.
2014; Jääskeläinen et al. 2017). Ultrafiltration can efficiently change the molar mass
(Mn) distribution of the lignin fraction through membrane selection with suitable
cut-offs. Aminzadeh et al. (2018) fractionated a LignoBoost Kraft lignin (KL) fraction through a ceramic membrane with a Mw cut-off of 1 kDa. A lignin fraction
with an average Mw in the range of 450–500 Da was recovered after 100 hours of
ultrafiltration. Due to the partial solubility of lignin in various solvents, a selective
solvent fractionation can be developed to divide a fraction of lignin into subfractions, each with a narrow distribution of Mn. An et al. (2017) fractionated lignin
isolated from the corn straw residues after enzymatic hydrolysis pretreatment (EHP)
by sequential isolation with dichloromethane, ethyl acetate, and n-butyl alcohol.
The Mw and polydispersity of the lignin fractions obtained using the selective solvent fractionation method were lower than those of EHP. Acid gradient precipitation is another approach based on differences in solubility or Mw of lignins. Brenelli
et al. (2016) carried out an acid treatment of alkaline lignin and found that the
polydispersity of lignin was reduced to the lowest polydispersity value (1.21)
at pH 2.
The antioxidant activity of lignin can also be improved by chemical modification, such as the incorporation of organic scavengers (e.g. m-cresol, p-nitrophenol)
into its structure (Sa’don et al. 2017a, b), depolymerization by acid catalysis (An
et al. 2019), or hydrogenolysis (Zhao et al. 2018). These chemical modifications
contribute to the production of lower Mw fragments of lignin with high phenolic
hydroxyl content, and therefore, promote significant increases in antioxidant activity. Physical modifications of lignin such as gamma irradiation have also been
reported by Rajeswara et al. (2015). The gamma-irradiated lignin with diminished
Mw chains exhibited greater antioxidant activity than the unirradiated lignin
(Rajeswara et al. 2015).
It is worth noting that the antioxidant efficiency of a lignin fraction in polar and
non-polar phases may be different (Barsberg et al. 2014). Good solubility in the
reaction system is important for the high antioxidant activity of lignin.
5.4 Mechanism of Lignin Toxicity and Cell Damage
It is important to evaluate the toxicity of lignin for applications in cosmetics, food
packaging, foods, and pharmaceuticals. The cell cytotoxicity assay is one of the
most common in vitro bioassay methods to evaluate lignin toxicity (Ukelis et al.
2008). Ugartondo et al. (2008) assessed the cytotoxicity of four lignin fractions
5 Lignin as a Natural Antioxidant: Property-Structure Relationship and Potential…
synthetic antioxidants such as BHA (IC 50 = 56.3 μg/ mL) and BHT (IC 50 = 38.2 μg/
mL). Qin et al. (2018b) designed a multi-enzyme mixture system for the isolation of
enzymatic hydrolysis lignin (EML) from Chinese quince fruits. EML isolated from
the fruits treated by Celluclast 1.5 L, Viscozyme L and Xylanase had the lowest
content of carbohydrate impurities (0.1%) and highest antioxidant activity among
the lignin fractions.
On the other hand, ultrafiltration, selective solvent fractionation and acid gradient precipitation have been shown to decrease lignin heterogeneity (dos Santos et al.
2014; Jääskeläinen et al. 2017). Ultrafiltration can efficiently change the molar mass
(Mn) distribution of the lignin fraction through membrane selection with suitable
cut-offs. Aminzadeh et al. (2018) fractionated a LignoBoost Kraft lignin (KL) fraction through a ceramic membrane with a Mw cut-off of 1 kDa. A lignin fraction
with an average Mw in the range of 450–500 Da was recovered after 100 hours of
ultrafiltration. Due to the partial solubility of lignin in various solvents, a selective
solvent fractionation can be developed to divide a fraction of lignin into subfractions, each with a narrow distribution of Mn. An et al. (2017) fractionated lignin
isolated from the corn straw residues after enzymatic hydrolysis pretreatment (EHP)
by sequential isolation with dichloromethane, ethyl acetate, and n-butyl alcohol.
The Mw and polydispersity of the lignin fractions obtained using the selective solvent fractionation method were lower than those of EHP. Acid gradient precipitation is another approach based on differences in solubility or Mw of lignins. Brenelli
et al. (2016) carried out an acid treatment of alkaline lignin and found that the
polydispersity of lignin was reduced to the lowest polydispersity value (1.21)
at pH 2.
The antioxidant activity of lignin can also be improved by chemical modification, such as the incorporation of organic scavengers (e.g. m-cresol, p-nitrophenol)
into its structure (Sa’don et al. 2017a, b), depolymerization by acid catalysis (An
et al. 2019), or hydrogenolysis (Zhao et al. 2018). These chemical modifications
contribute to the production of lower Mw fragments of lignin with high phenolic
hydroxyl content, and therefore, promote significant increases in antioxidant activity. Physical modifications of lignin such as gamma irradiation have also been
reported by Rajeswara et al. (2015). The gamma-irradiated lignin with diminished
Mw chains exhibited greater antioxidant activity than the unirradiated lignin
(Rajeswara et al. 2015).
It is worth noting that the antioxidant efficiency of a lignin fraction in polar and
non-polar phases may be different (Barsberg et al. 2014). Good solubility in the
reaction system is important for the high antioxidant activity of lignin.
5.4 Mechanism of Lignin Toxicity and Cell Damage
It is important to evaluate the toxicity of lignin for applications in cosmetics, food
packaging, foods, and pharmaceuticals. The cell cytotoxicity assay is one of the
most common in vitro bioassay methods to evaluate lignin toxicity (Ukelis et al.
2008). Ugartondo et al. (2008) assessed the cytotoxicity of four lignin fractions
5 Lignin as a Natural Antioxidant: Property-Structure Relationship and Potential…