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Review

Lignin as a Natural Antioxidant: Chemistry and Applications

by
Hasan Sadeghifar
1,* and
Arthur J. Ragauskas
2
1
R&D Laboratory, ASR Group, Boca Raton, FL 33431, USA
2
Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996, USA
*
Author to whom correspondence should be addressed.
Macromol 2025, 5(1), 5; https://doi.org/10.3390/macromol5010005
Submission received: 24 November 2024 / Revised: 24 December 2024 / Accepted: 28 January 2025 / Published: 31 January 2025
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Abstract

:
The growing interest in renewable and natural antioxidants has positioned lignin as one of the most significant bioresources for sustainable applications. Lignin, a polyphenolic biomolecule and a major by-product of chemical pulping and biorefinery processes, is abundant and widely accessible. Recent advancements in lignin modification, fractionation, and innovative biorefinery techniques have expanded its potential applications, particularly as a natural antioxidant. This review explores the underlying chemistry of lignin’s antioxidant activities, from model compounds to technical lignin resources, and examines its current applications. Additionally, we highlight the influence of lignin’s chemical structure and functional groups on its antioxidant efficacy, emphasizing its promising role in the development of practical and sustainable solutions.

Graphical Abstract

1. Introduction

Lignocellulosic biomass contains 15–30% lignin as a natural polyphenolic polymer [1,2]. The main role of lignin in plant structure is mechanical support and antimicrobial activities against biological degradation [3,4,5]. Chemically, lignin is mainly a polymer of methoxylated phenylpropanoids (p-coumaryl alcohol (H), coniferyl alcohol (G), and sinapyl alcohol (S) precursors) and their derivatives connected with different ether and non-ether bonding types [1,6,7].
The ratio of each precursor is mainly dependent on the type of plant (softwood, hardwood, and nonwood) and the extraction procedure employed. Commercially available lignin is a by-product of pulp and paper industries, which is known as technical lignin. Around 70 Mt of technical lignin is estimated to be available annually from wood pulping and the emerging cellulosic ethanol industries [8]. Due to technical challenges, the complexity of lignin, and the lack of non-degradative methods to extract pure, unaltered lignin, it is hard to define the exact structure of lignin in plants. A model of lignin’s chemical structure, its monolingual precursors, and different linkages that are proposed [9] based on extracted lignin are presented in Figure 1. More than 50% of inter-unit linkages in native lignin are of the β-O-4 ether type and play a main role in the pulping and biorefining delignification process [5,10,11,12]. Other significant linkages are β-5, β-β, 5-5, and 4-O-5.
The polymer structure, degree of branching, and internal cross-linking of lignin are related to its monomer composition [6,13]. The methoxyl groups in monolignols clearly influence the overall cross-linking in the lignin structure. Due to higher amounts of sinapyl units with two methoxyl groups in hardwood lignin, this type of lignin exhibits minimal internal cross-linking. The same considerations result in grasses yielding lignin with more cross-linked structures than other lignin classes due to the presence of p-hydroxyphenyl alcohol with no methoxyl groups.
In the plant cell wall, monolignol polymerization occurs via radical polymerization with a variety of inter-unit linkages being formed, including β-O-4, β–β, β-5, 5–5 and 4-O-5 structural motifs (Figure 1). The β-O-4 linkage is typically the most abundant in all plant sources and has a dominant role in the de-polymerization (or pretreatment) of lignin in different chemical processes. Depending on the plant source and the method of extraction, the β-O-4 content of isolated lignin is different [3,7]. The focus of this process is on the valorization of carbohydrates instead of lignin. Therefore, lignin as a by-product of this process undergoes a high degree of chemical modification. Kraft pulping using NaOH and Na2S and soda pulping using NaOH are two main pulping process [14]. Organosolv pulping, which uses a mixture of water and an organic solvent, is another pulping and biorefinery process. Table 1 shows the content of lignin precursors, bonding types, and molecular weights of different technical lignins [3]. This information is important for a better understanding of lignin’s antioxidant properties.
Kraft pulping is the main commercial lignin source; around 85–95% of the lignin from biomass is fragmented and dissolved in this process. The black liquor known as the dissolved fraction of the pulping process has approximately 12–18% solid content [2]. The black liquor also contains dissolved carbohydrates, extractive materials, salts, and ash. The main goal of kraft lignin purification is to remove carbohydrate residues and ash/metals from black liquor.
Commercially available lignin recovery processes include LignoBoost [15], LignoForce [16] and Sequential Liquid-lignin Recovery and Purification (SLRP) [17], which are based upon black liquor acidification, separation, washing, and dewatering. The LignoBoost process is the main source of alkali lignin in the market. The LignoBoost process at the pulp mill in Plymouth, NC, USA, is the main installed line. The basic process steps of the LignoBoost system are illustrated in Figure 2 [18]. The process utilizes kraft black liquor, which then undergoes a pH reduction to ~10 using carbon dioxide gas in the first step of the process. The precipitated lignin, known as cake, is reslurred with a sulfuric acid solution with pH in the range of 2–4. Further reduction in pH to around 1.5 yields a final product with less ash. Precipitated lignin after the second step of acidification goes to a second filter press followed by washing with diluted acid and water. The final product is a relatively pure lignin with 50–60% solid content. The LignoForce process is like the LignoBoost process with an oxidation step before acidification and a final wash with sulfuric acid and water.

2. Antioxidants

Applying a minor number of antioxidants can inhibit or quench free radical reactions. Oxidation is one of the primary factors for many diseases related to biological components like aging and cancer [19,20]. Using antioxidant supplements to prevent and cure diseases has become an important medical treatment in the last few decades. On the other hand, oxidation is a major issue for many industrial processes, and the application of synthetic antioxidants is a key component to control this process [21].
Generally, antioxidant reagents can be divided into enzymatic and non-enzymatic compounds. For example, human plasma has a wide range of enzymatic antioxidant components like glutathione peroxidase and catalase [19]. Non-enzymatic antioxidants are typically composed of small molecular organic chemicals such as α-tocopherol (TOH), ascorbic acid, ubiquinol, and β-carotene [22]. Lignin is a non-enzymatic antioxidant additive. Reactive oxygen species (ROS) are the main source of radicals in many biological systems (Table 2) [23,24].
Non-enzymatic antioxidants are categorized into natural and synthetic antioxidants. Vitamins C, E, and A and bioflavonoids (flavonol, flavone, carotenoids, and hydroxycinnamates) are the main types of natural antioxidants. The primary synthetic antioxidants are butylated hydroxy anisole (BHA), butylated hydroxy toluene (BHT), nordihydro guaretic acid (NDGA), and propyl gallate (PG) [25].
Several assays have been reported for antioxidant activity measurement. ORAC (oxygen radical absorbance capacity), FRAP (ferric reducing antioxidant power), ABTS (2,2′-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid)), DPPH (2,2-diphenyl-1-picrylhydrazyl), and FC/TPC (Folin–Ciocalteu assay/total phenolic assay) are the main reported methods [26,27,28,29].
Depending on the applied method to measure antioxidant activity, oxidation generators, and substrates, the antioxidant properties can be different for a given material [26]. In general, antioxidant assays can be divided in two mechanisms: (1) radical quenching by hydrogen atom donation of the antioxidant or hydrogen atom transfer (HAT) mechanism and (2) the single electron transfer (SET) mechanism, where the antioxidant’s ability to transfer one electron to reduce any compound is used, as shown in Equations (1) and (2), respectively. Both of these radical quenching mechanisms usually occur in parallel.
HAT: X· + AH → XH + A
SET: X· + AH → X − + AH+
Note: X· is the radical; AH is the antioxidant resulting in protonated radical XH and an antioxidant radical, or anion X and radical cation AH+.
For polyphenols, such as lignin, the antioxidant activity and mechanism depend on the medium, solvent, and testing substrate. For example, for using lignin in food, cosmetics, or biomedicine as an antioxidant, the ORAC assay is the best, since peroxyl radicals ROO· are more relevant to in vivo conditions than DPPH− and ABTS+ radicals [30,31]. For other lignin applications and general antioxidant measurement, the DPPH assay and the Folin–Ciocalteu method are extensively employed for lignin antioxidant activities [31,32]. It is important to mention that the Folin–Ciocalteu method is primarily used to quantify total phenolic content and correlate it with antioxidant activity based on the abundance or types of phenolics present.

3. Chemistry of Lignin Antioxidant Properties

The phenolic structure of lignin provides a readily available means of quenching radicals and yielding a stabilized phenoxy radical [31,33,34,35,36,37]. However, to be an effective antioxidant capable of trapping highly reactive peroxyl radicals (R-O-O•), the phenolic antioxidant has to contain other functional groups to hinder the phenoxyl radical (ArO•) and an ether oxygen positioned on the aromatic ring to stabilize the phenoxyl radical (Ar-O•) by stereo-electronic effects [31,38]. Conjugated double bonds in the lignin structure provide more stabilization of the phenoxy radicals through extended delocalization, except for a conjugated carbonyl group that has a negative effect on lignin’s antioxidant activity [39].
Ph-OH has been reported up to 3.5 mmol Ph-OH/g in different lignins [3,35,40]. The phenoxyl radical in the lignin structure can stabilize by presence of ortho-methoxy in the phenolic structure and C=C bonds in side chains (Figure 3) [33,39].
The absolute rate constant for inhibition (kinh) for the lignin models was investigated in comparison with synthetic BHT antioxidants [38]. Figure 4 and Table 3 indicate the antioxidant activities, Kinh, and stoichiometric factor of lignin models in comparison with BHT.
The results demonstrated that the antioxidant activity of all studied lignin models surpassed that of butylated hydroxytoluene (BHT). This superior performance is attributed to the role of ortho-methoxy groups and aliphatic side chains in lignin models, which enhance radical stabilization. The antioxidant properties of lignin are influenced not only by the presence of phenolic hydroxyl (OH) groups but also by other functional groups in the lignin structure that can either enhance or diminish its effectiveness.
For instance, as shown in Table 2, isoeugenol (3) and coniferyl alcohol (4) exhibit approximately twice the antioxidant activity of 4-propylguaiacol (1) and eugenol (2). This increased activity is attributed to the presence of conjugated double bonds in compounds (3) and (4), compared to the saturated propyl group in (1) and the distant double bond in (2) [34,36,38,39]. Conversely, model (5) demonstrated lower antioxidant activity due to the presence of a carbonyl group, where the electropositive carbon in the CHO group reduced the molecule’s radical stabilization capacity.
Syringyl-type lignin (S-type), containing a dimethoxy compound (6), exhibited higher antioxidant activity than guaiacyl (G-type) lignin (2), which has only one methoxy group. This is likely due to the additional resonance stabilization provided by the dimethoxy structure. Furthermore, lignin dimers (7 and 8) and the tetramer (9) showed superior antioxidant properties compared to monomeric lignin models. Notably, model (7), featuring an ortho-methylene bridge between two eugenol units, enhanced antioxidant activity threefold compared to eugenol (2). This effect was even more pronounced in the lignin tetramer model (9) [41].
These findings highlight the critical role of lignin’s structural features in determining its antioxidant efficacy, paving the way for the development of lignin-based antioxidants with tailored properties.
The same results on lignin model’s antioxidant activity were reported using the DPPH radical assay [31] (Table 4) in agreement with other reports [36,39,42].
Non-etherified OH phenolic groups play a significant role in enhancing their radical-scavenging ability. The presence of ortho- and para-substituted methoxy groups, along with a saturated propane chain and a propanoid chain containing hydroxyl groups, contributes to its antioxidant properties. On the other hand, certain structural features reduce lignin’s antioxidant activity. Carbonyl substitution in the propanoid side chain drastically decreases its radical-scavenging activity. A carbon double bond conjugated with an aromatic ring results in lower ARP values compared to a double bond located between the outermost carbon atoms.
The content of G- and S-type lignin and -CH2 groups in the lignin side chain indicated a positive effect on its antioxidant activity, while the presence of O-containing groups inside chains (mainly -CHO and -CO groups) has a negative effect on lignin’s antioxidant activity [35,36,43].
Correlation coefficients between lignin’s antioxidant activity and its structural parameters using DPPH and ABST radicals have been reported (Table 5). Interestingly, lignin’s antioxidant activity was different when using different radicals (DPPH• vs. ABTS•+) applied for the test. For example, the absence of o-substitution in an aromatic ring molecule significantly decreased its AoA in the DPPH test, while the observed value increased in the ABTS+ test [35,36,44]. For example, the absence of o-substitution in an aromatic ring molecule significantly decreased its AoA in the DPPH test, while this number increased in the ABTS+ test.

4. Antioxidant Activity of Technical Lignin

The antioxidant activity of different lignins extracted from hardwoods by the alkaline process and pyrolysis treatment indicated that the stability of the aroxyl radicals strongly depends on the unpaired electron delocalization properties [32,36,43,45,46,47]. The formation of poly-conjugated clusters in extracted lignin macromolecules also improved electron delocalization and antioxidant activity. Based on an EPR study, it seems that the extent of conjugation of paramagnetic clusters in extracted lignin has a major role in antioxidant activity. Phenolic and methoxyl functional groups’ contents’ influence is of lesser importance. The presence of carbohydrates indicated a negative effect on lignin’s antioxidant activity.
Lower-molecular-weight fractions of lignin with a larger size in the poly-conjugated system indicate higher antioxidant activity. The correlations between lignin parameters and their antioxidant activity using DPPH• and ABST•+ were developed, as shown in Equations (1) and (2) [43]. As discussed previously, the antioxidant test method is important for lignin antioxidant measurement. Lignin’s structure and its functional groups behave differently in terms of antioxidant activity depending on the test methods.
Y1DPPH = −3.08 + 0.043 × X1 + 0.018 × X2 − 0.018 × X3 − 0.014 × X6 + 0.24 × X7
Y2ABST = 1.46 − 0.013 × X3 − 0.015 × X4 + 0.050 × X5 − 0.018 × X6 + 0.32 × X7
where Y1 is the AoA in the DPPH· test (number of the deactivated radicals per phen-OH). Y2 is the AoA in the ABTS·+ test (number of deactivated radicals per OHphen). X1 is the relative content of G + S phenols (%). X2 is the relative content of the phenylpropane units with -CH2 groups (%). X3 is the relative content of the phenylpropane units with oxygen on side chains (%). X4 is the relative content of carbohydrates (%). X5 is the (ArC1 + ArC2)/ArC3 proportion. X6 is the size of π-conjugated systems (the number of atoms), and X7 is the amount of OCH3 per C9.
Unlike lignin model compounds, the aliphatic hydroxyl groups in the side chains of extracted lignin samples did not exhibit a positive effect on their antioxidant activity [31,36,48]. A comparative analysis of aspen acid-soluble, birch alkaline, and spruce alkaline lignins revealed that variations in aliphatic-OH content did not significantly influence their antiradical power (ARP) (Table 6) [31,49]. Notably, between two lignin samples with similar phenolic-OH content, the sample with a higher -OCH3 content demonstrated significantly greater ARP.
Additionally, lignin samples with higher molecular weights showed reduced radical-scavenging activity. This diminished antioxidant performance is attributed to several factors, including the heterogeneity of isolated lignin, the presence of non-lignin components, polydispersity in molecular weight and functional groups [39,50], and the presence of inorganic cations [51,52,53,54].
These findings highlight the complex interplay of structural and compositional factors in determining the antioxidant activity of extracted lignin, emphasizing the need for precise characterization and optimization in applications.
ArOH content in lignin is a key parameter in the antioxidant activity of lignin, and other parameters like molecular weight and functional groups can increase or reduce the antioxidant power. In general, lignin with higher molecular weight indicates lower antioxidant properties [39].
Despite the high antioxidant activity of lignin’s model products [31,33], extracted lignin antioxidant activity is lower than synthetic antioxidant activity [49,55,56], mainly due to its structural complexity, high degree of polymerization, and heterogeneity.
Different methods have been introduced to improve lignin’s antioxidant activity. Lignin fractionation to separate lower-molecular-weight fractions containing higher PhOH and less structural complexity is reported to collect lignin with higher antioxidant activity, as mentioned in different references in this review [57]. The fractionation method is an expensive process, and the higher-molecular-weight fraction with low antioxidant activity needs to be managed as a waste product, or other applications need to be determined for it.
Recent publications on the lignin nanoparticle indicated that lignin particle size impacts its antioxidant activities [58,59,60,61]. Lignin nanoparticles with a mean particle size of 0.144 ± 0.03 µm, prepared using an antisolvent process, indicated 12.4 times more solubility than the parent lignin with better antioxidant activities [58]. Lignin particle size is an important factor for tuning PhOH accessibility as the main factor of antioxidant activity for a given lignin sample [62].
Enzymatic treatment of alkali lignin with laccase has been reported to increase its antioxidant activities through increasing Ph-OH content and decreasing the lignin molecular weight [63]. Lignin Ph-OH increased through demethylation of lignin by the enzyme. Catalytic modifications of lignin [64,65,66,67] are reported to boost the antioxidant activities of lignin through breaking down the molecular structure and increasing the Ph-OH content.
In a recent report, the antioxidant property of flavone-bound lignin as a by-product of biorefinery sugar production processes from wheat straw was investigated [68]. Separated compounds were shown to contain more than 50 low-molecular-weight fragments of lignin and ligno-carbohydrate complexes, which exhibited strong antioxidant properties (around 50% equivalent of Trolox as a strong synthetic antioxidant).

5. Lignin as an Antioxidant in the Lignin–Polymer Composite

In the last few decades, lignin has been examined as an additive for resins, adhesives, polymer blends [69,70,71,72,73,74,75,76], copolymers [21,77,78], 3D-printing polymers [79], biobased composite films [80,81], biodiesel [82,83], and many other applications. A high content of lignin was applied as an additive in synthetic polymers in the form of unmodified [84] or derivatized lignins [85] for the last three decades. Usually, the addition of a high content of lignin to synthetic polymers tends to decrease the mechanical performance of the resulting composite. Incorporation of lower amounts of lignin has been investigated more recently to take advantage of lignin as a photo and thermal stabilizer. Lignin is a polar polymer with poor solubility in apolar [86] materials such as polyolefins, which can limit their reactivity with radicals. Good solubility, mobility, and low volatility are important factors for a good antioxidant as a stabilizer for a synthetic polymer [74]. Lignin’s antioxidant activity and its chemical structure (molar mass, functionality, cross-linking density) are not consistent and are dependent, in part, on its botanical origin and extraction process. Lack of homogeneity is the main challenge for lignin’s application in bio-composite materials.
In general, the presence of functional groups like Ph-OH, C=C bonds, and carbonyl and other groups in lignin’s structure and its polymer structure is the main factor that provides oxidative, thermal, and light stability characteristics in blends with synthetic polymers [72]. The ability of lignin to act as a radical scavenger and to inhibit or stabilize oxidation reactions during high-temperature processing of synthetic polymers has been frequently explored. The power of this effect is dependent on the origin of the lignin as well as its extraction process, chemical structure, heterogeneity, and purity [87,88,89,90,91,92].
Oxidation Induction Time (OIT) is one of the most widely used methods for evaluating the effect of antioxidants on the thermal stability of composites at high temperatures. In a common procedure, a composite sample containing an antioxidant is heated from ambient temperature to a maximum temperature (e.g., 180 °C) at a rate of 5–20 °C/min under a nitrogen flow of 50 mL/min [72,74,87]. The heat flow is then recorded as a function of time under isothermal conditions at the maximum temperature. A higher OIT value indicates improved thermal stability of the composite or polymer. Typically, the addition of lignin or other antioxidants enhances polymer thermal stability and increases OIT. For instance, the OIT of neat polypropylene increased from 8.6 min to 30 min when lignin was incorporated into the polymer [74] (Figure 5). Lignin improves thermal stability by capturing radicals generated from the macromolecular polymer chains through its phenolic groups, thereby controlling thermal degradation. Moreover, OIT shows a positive correlation with the content of non-condensed Ph-OH and total Ph-OH, while it has a negative correlation with molecular weight, aliphatic OH, and condensed Ph-OH [72,74,93].
The effect of kraft lignin fractionation on its antioxidant properties and thermal stability in blends with polyethylene has been reported [49,72]. Fractionated lignin with different molecular weights from low to high was extruded with polyethylene from 1 to 5% loading. Lignin fractions with smaller molecular weight and higher Ph-OH indicated higher antioxidant ability with better polyethylene thermal stability. Blending 5% of non-fractionated kraft lignin with PE increased the maximum heat flow temperature from 215 °C in pure PE to 250 °C. The addition of the same amount of low-molecular-weight kraft lignin increased this temperature up to 260 °C. Blending 5% fully methylated kraft lignin with PE indicated less of an effect on the blend’s OIT (220 °C).
In another report [87], the effect of industrial kraft lignin on polypropylene induction time is reported. Kraft lignin was first dissolved in p-dioxine/water (9:1 v/v) and blended in 1 wt% with polypropylene. The blend was dried and then extruded at 170–190 °C. Blending lignin with PP increased the induction time of virgin PP from 30 min to more than 670 min depending on different lignins. The authors concluded that lignin’s effect on polymer stabilization at high temperatures depends more on lignin’s compatibility and solubility with the polymer (i.e., low molecular weight and low OH content) than the increase in its antioxidant reactivity.
The effect of kraft lignin’s molecular mass and phenolic OH content on the thermo-oxidation resistance and compatibility with polypropylene (PP) has been reported [93]. Polypropylene containing 1% wt butylated lignin displayed higher OIT compared to that of neat PP. Compared with neat PP (OIT = 6.2 min), the OIT of PP/lignin increased to ~8.0 min. Phenolic –OH in kraft lignins (5.79 mmol/g), like synthetic phenolic antioxidants, acts as a scavenger of radicals generated during the thermal process. The results indicated that despite the reduction in phenolic –OH, after butylation, the OIT increased with the degree of lignin butylation, from ~8.0 min (KL, phenolic–OH: 5.79 mmol/g) to 10.3 min (modified lignin, phenolic–OH: 2.12 mmol/g). The authors explained this effect based on the reduction in butylated lignin’s surface energy and increasing its hydrophobicity and compatibility with PP (SEM observations). Despite 1 wt% butylated lignin indicating good compatibility with PP, this level of lignin loading limits antioxidant performance. Notably, increasing butylated lignin loading in PP up to 5 wt% improved the OIT value up to ~16 min, which is comparable to that reported for antioxidants used commercially.
Blending unmodified, etherified, and esterified alkali lignin nanoparticles (LNPs) with poly (vinyl alcohol) (PVA) at various loading levels (up to 10 wt%) has been reported using a casting approach [94]. Modified lignin was evenly dispersed in the PVA matrix with no macroscopic phase separation in the nanocomposite films. PVA containing modified lignin revealed an overall increase in dimensional stability and reduced moisture adsorption. The modified lignin–PVA composite was a promising candidate for applications requiring high antioxidant potential and improved performance in light absorbance. The modified lignin–PVA composite exhibits increased thermal stability compared with pure PVA. The first maximum thermal degradation temperature (Tm1) of PVA was measured at 251.2 °C, whereas it was 280.3 °C for the PVA composite at a concentration of 10 wt%. PVA film with 10 wt% of non-modified lignin particles displayed 83.8% DPPH radical scavenging. This number for the modified lignin composite was 98.2%. Neat PVA had no antioxidant activity.
The use of 1–3 wt% lignin nanoparticles as an antioxidant additive in polyvinyl alcohol/chitosan (PVA/Ch) hydrogels was investigated [95,96]. The composites were prepared through a freeze–thaw procedure. A synergic effect of chitosan and lignin nanoparticles was claimed to improve the antioxidant ability of substances. Using PVA/chitosan as a control sample also indicated a level of antioxidant ability lower than when mixed with lignin. The addition of 1–3 wt% lignin in a PVA/chitosan hydrogel indicated 74% increased antioxidant activity based on the DPPH antioxidant test. The obtained results suggested the possible use of the produced PVA/chitosan hydrogels incorporating lignin nanoparticles in many different sectors, such as drug delivery, food packaging, and wound dressing.
Nanocelluloses with and without residual lignin isolated from wheat straw was reported as a reinforcing agent in poly(vinyl alcohol) films. The researchers evaluated different properties of the film, including its antioxidant properties. The antioxidant capacity of the films increased to 10% using lignin-containing nanocellulose. Increasing the lignin content of the nanocellulose was shown to increase its antioxidant capabilities. The presence of lignin in nanocelluloses produces films with higher antioxidant capacity.

6. Conclusions

Antioxidants are essential in food preservation, healthcare, and various industrial processes. The growing demand for natural antioxidants, particularly in food and preservative applications, underscores the need for reliable methods to study and evaluate their efficacy. Several chemical assays have been developed to assess antioxidant activity, focusing on radical-scavenging ability, reducing power, and inhibition of oxidation. Choosing the appropriate method is crucial to ensure the accuracy and validity of antioxidant activity measurements. Lignocellulosic biomass, a renewable resource, contains approximately 15–30% lignin, a natural polyphenolic polymer. Lignin is primarily obtained as a by-product of pulping and biorefinery processes. Structurally, lignin consists of methoxylated phenylpropanoid units derived from p-coumaryl alcohol (H), coniferyl alcohol (G), and sinapyl alcohol (S) precursors. Its antioxidant properties stem primarily from the presence of phenolic hydroxyl groups, which play a critical role in its radical-scavenging activity. However, lignin’s overall antioxidant performance can vary depending on its structural features and functional groups. For example, lignin structures with conjugated double bonds on the aliphatic side chains exhibit greater antioxidant capacity compared to those with saturated side chains. Additionally, dimeric and tetrameric phenolic models have demonstrated superior antioxidant performance. The antioxidant properties of lignin are strongly influenced by specific structural components, including non-etherified phenolic hydroxyl groups, ortho-methoxy groups (-OCH3), α-CH2 groups, aliphatic carbonyl groups, and extended π-conjugated systems.
Lignin’s application as an antioxidant and a thermal stabilizer in high-temperature polymer processing is well documented in industrial settings. Its solubility and compatibility with synthetic polymers significantly affect its stabilization performance. Notably, lignin fractions with lower molecular weight and reduced total hydroxyl content (including aliphatic and phenolic hydroxyls) demonstrate superior compatibility and blend morphology, leading to enhanced antioxidant activity in polymer blends. In contrast, lignin’s high polydispersity can negatively impact on its antioxidant efficiency due to the limited solubility of high-molecular-weight chains, which prevents the dispersion of active low-molecular-weight species within the polymer matrix.

Funding

This research received no external funding.

Conflicts of Interest

Hasan Sadeghifar was employed by the ASR group. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Lignin precursors and linkages and a model of a lignin polymer [10].
Figure 1. Lignin precursors and linkages and a model of a lignin polymer [10].
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Figure 2. The LignoBoost process as a debottleneck and part of the recovery system.
Figure 2. The LignoBoost process as a debottleneck and part of the recovery system.
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Figure 3. Effect of ortho-methoxy group and C=C bond on phenoxyl radical stabilization in lignin structure (drawn and edited from (top) [38] (Reproduced with permission of ACS publication, 2025) and (bottom) [39]). (Reproduced with permission of ACS publication, 2025).
Figure 3. Effect of ortho-methoxy group and C=C bond on phenoxyl radical stabilization in lignin structure (drawn and edited from (top) [38] (Reproduced with permission of ACS publication, 2025) and (bottom) [39]). (Reproduced with permission of ACS publication, 2025).
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Figure 4. Structures for the model lignin phenolic compounds. Drawn and edited from [38]. (Reproduced with permission of ACS publication, 2025).
Figure 4. Structures for the model lignin phenolic compounds. Drawn and edited from [38]. (Reproduced with permission of ACS publication, 2025).
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Figure 5. Effect of lignin antioxidant on polyethylene stability during high-temperature processing (redrawn from ref. [72]).
Figure 5. Effect of lignin antioxidant on polyethylene stability during high-temperature processing (redrawn from ref. [72]).
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Table 1. Content of functional groups, monolignols, and molecular weights in different technical lignins: Soda p-100 (straw glass), alcell (mixed hardwood), organosolv wheat straw, os poplar, and os spruce [3].
Table 1. Content of functional groups, monolignols, and molecular weights in different technical lignins: Soda p-100 (straw glass), alcell (mixed hardwood), organosolv wheat straw, os poplar, and os spruce [3].
CategoriesPropertiesDelignification Process
Induline KraftSoda p-1000AlcellOS-WOS-POS-S
Functional groups, mmol/1 g lignin, calculated by 31P NMRAliphatic-OH1.791.311.261.041.270.871.43
5-Substituted OH1.311.731.681.241.831.21
Guaiacyl OH1.30.730.580.920.581.44
P-Hydroxyphenyle OH0.160.40.110.380.180.08
Total Ph-OH1.772.863.32.542.592.73
COOH0.330.80.220.210.070.06
Molecular weightMw429032702580196021802030
Mn530620600450570420
Polydispersity (PD)8.15.24.34.43.84.9
a: Precursors, S, G, H (Molar percentage (S + G + H = 100)
b: Linkage (Number per 100 aromatic units (S + G)		B-O-4 a6.13.45.34.30.10.00
B-5 a0.30.000.84.51.83.3
B-B a1.000.72.80.11.10.2
Sinapyl alcohol (S) % b0.00506339530.00
Coniferyl Alcohol (H) % b9739375847100
P-Comaryl alcohol (P) % b3110.0030.000.00
S/G ratio0.001.31.70.71.20.00
H/G ratio0.000.30.000.10.000.00
Table 2. List of ROS.
Table 2. List of ROS.
SymbolName
RS●Thiyl radical
O2●Superoxide anion radical
OH●Hydroxyl radical
RO●Alkoxyl radical
ROO●Peroxyl radical
P●Hydrogen peroxide
LO●Lipid hydroperoxide
LOO●Lipid peroxyl radical
NO●Nitric oxide radical
NO2Nitrogen dioxide radical
Table 3. Antioxidant activities, Kinh, and stoichiometric factor of lignin models and BHT (measured in styrene/chlorobenzene during peroxidation thermally initiated by AIBN at 30 °C). This table is redesigned and matched with each model in Figure 4.
Table 3. Antioxidant activities, Kinh, and stoichiometric factor of lignin models and BHT (measured in styrene/chlorobenzene during peroxidation thermally initiated by AIBN at 30 °C). This table is redesigned and matched with each model in Figure 4.
Lignin ModelKinh a, M−1 s−1 × 10−4K, Relativen
(1) R = -CH2-CH-CH33.071.691.68
(2) R = -CH2-CH=CH23.331.831.62
(3) R = -CH=CH-CH37.193.951.64
(4) R = -CH=CHCH2OH7.744.251.69
(5) R-CH=CH-CHO3.5821.68
(6)7.454.091.73
(7)9.765.363.19
(8)8.454.483.25
(9)14.17.756.38
BHT1.8211.84
a: Inhibition rate constants determined from linear plots of ∆[O2]t versus −Ln(1 − t/τ) where the slope is Kinh/Kp × [R-H] and Kp = 41 M−1 s−1. n: stoichiometric factor, the number of peroxyl radicals per molecule of antioxidant.
Table 4. Antiradical power and amount of reduced DPPH by different species of lignin model [31].
Table 4. Antiradical power and amount of reduced DPPH by different species of lignin model [31].
Lignin ModelAntiradical Power, ARP aAmount of Reduced b DPPH•
1. R1 = CH2-CH-CH3, R2 = HMacromol 05 00005 i0013.51.75
2. R1 = -CH2-CH=CH2, R2 = H42
3. R1 = -CH=CH-CH3, R2 = H2.21
4. R1 = -CH=CH-CH2OH, R2 = H4.22.1
5. R1 = -CH=CH-CHO, R2 = H1.90.97
6. R1, R2 = H2.61.3
7. R1 = -CHO-CH2-CH3, R2 = H0.20.1
8. R1 = -CO-CH2-CH3, R2 = -OCH30.50.2
9. R1 = -CHOH-CH2-CH3, R2 = H31.5
a Antiradical power of one mole of a lignin-related compound. b The number of 1,1-diphenyl-2-picrylhydrazyl moles reduced.
Table 5. Correlation coefficients between lignin structural parameters and AoA based on test results using DPPH· and ABTS·+ [36,43]. (Reproduced with permission of ACS publication, 2025).
Table 5. Correlation coefficients between lignin structural parameters and AoA based on test results using DPPH· and ABTS·+ [36,43]. (Reproduced with permission of ACS publication, 2025).
Structural ParameterPearson’s Correlation Coefficient (n = 50, α = 0.05)
DPPH•ABTS•+
G + S phenol, %0.52−0.20
CH2 in the α-position of the side chain, %0.530.33
Oxygen containing group (α-C=O) in the side chains, %−0.42−0.25
Number of atoms in the π-conjugated system−0.40−0.50
Carbohydrates, %−0.210.61
OCH3 per C90.140.01
Mw (g/mol)0.04−0.08
Table 6. Characterization of radical-scavenging activity of lignins depending on their chemical structure and physicochemical parameters [31,49].
Table 6. Characterization of radical-scavenging activity of lignins depending on their chemical structure and physicochemical parameters [31,49].
LigninWoodOH Phen. %OH Aliph. %OCH3, %MwAPR aK2 b, M−1 s−1
Acid-solubleAspen5.13.321.519801.58600
AlkalineSpruce5.03.515.122000.52000
Birch4.06.917.429901.05600
Aspen3.65.218.131001.16100
EthanolAspen2.04.525.018700.62600
MWLSpruce3.18.415.375000.2300
a: Antiradical power; b: the rate constant of the 1,1-diphenyl-2-picrylhydrazyl decay.
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Sadeghifar, H.; Ragauskas, A.J. Lignin as a Natural Antioxidant: Chemistry and Applications. Macromol 2025, 5, 5. https://doi.org/10.3390/macromol5010005

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Sadeghifar H, Ragauskas AJ. Lignin as a Natural Antioxidant: Chemistry and Applications. Macromol. 2025; 5(1):5. https://doi.org/10.3390/macromol5010005

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Sadeghifar, Hasan, and Arthur J. Ragauskas. 2025. "Lignin as a Natural Antioxidant: Chemistry and Applications" Macromol 5, no. 1: 5. https://doi.org/10.3390/macromol5010005

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Sadeghifar, H., & Ragauskas, A. J. (2025). Lignin as a Natural Antioxidant: Chemistry and Applications. Macromol, 5(1), 5. https://doi.org/10.3390/macromol5010005

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