Can Lignin Wastes Originating From Cellulosic EthanolBiorefineries Act as Radical Scavenging Agents?

Caroline Vanderghem,A Nicolas Jacquet,A and Aurore RichelA,B

AUnit of Biological and Industrial Chemistry, Gembloux Agro-Bio Tech,

University of Liege, Passage des Deportes 2, B-5030 Gembloux, Belgium.BCorresponding author. Email: a.richel@ulg.ac.be

Lignin is a co-product from the biorefinery and paper industries. Its non-energetic valorisation remains a field of extensiveresearch and development. In this perspective, this study was undertaken to evaluate the radical scavenging ability ofselected herbaceous lignins. These lignins, extracted from either Miscanthus (Miscanthus� giganteus) or switchgrass

(Panicum virgatum L.), were selected as benchmarks for this study based on their chemical structure and averagemolecular weight. These technical lignins, which are side-products in the bioethanol production process, displayed amoderate antioxidant activity as evaluated by the 1,1-diphenyl-2-picrylhydrazil free radical scavenging test system.

A correlation between the radical scavenging properties and the molecular features is proposed and discussed. Infraredspectroscopy was employed as a straightforward qualitative prediction tool for assessing the radical scavenging capacity.

Manuscript received: 15 February 2014.

Manuscript accepted: 13 March 2014.

Published online: 8 May 2014.

Introduction

Native lignin is the second most abundant biopolymer on

earth and represents 15–30% by weight of the lignocellulosicbiomass. Lignin is a heterogeneous cross-linked polymercomposed of phenylpropane units (guaiacyl, syringyl, or

p-hydroxyphenyl) linked together by a panel of specific ether orcarbon–carbon bonds. Nowadays, large amounts of lignins andlignin-based effluents are available and originate from either the

pulp and paper manufacturing industries or the production ofbioethanol from lignocellulose.[1] Typically, these lignins arededicated to energy-related applications by combustion. How-ever, in recent years, novel areas for high added value applica-

tions have emerged and involve the use of lignin as performanceproducts (e.g. polymer additives, binders), individual low-molecular weight compounds, and speciality chemicals (e.g.

surface-active agents).[2,3] Particularly, owing to its polyphenolic-like structure, lignin is gaining progressive interest as aneffective radical scavenger for food and non-related food

applications. As an illustration, incorporation of lignin intosynthetic polymer materials is expected to increase the thermalstability and inhibit photo-oxidation of the resultingmaterials.[4]

Perennial grasses such asMiscanthus (Miscanthus� giganteus)and switchgrass (Panicum virgatum L.) find huge interest asbiomass crops.[5,6] Switchgrass is primarily used for cellulosicethanol production in the United States, whereas Miscanthus is

predominantly used in Europe. Practically, removal of ligninfrom these herbaceous raw materials using physical, mechani-cal, and/or chemical processes is a prerequisite for bioethanol

production.[7] However, these processes alter the native ligninarchitecture by selective rupture of specific internal bonds,generating heterogeneous lignin materials with variable mole-

cular weights and structures. Specifically, chemical processes

involving either acid or alkali agents are attractive from aneconomical point of view.[8] As demonstrated, acid processes

preferentially depolymerise lignin by rupture of ether bondsbetween constitutive phenylpropane units. Conversely, alkalinechemicals methods have no effect on ether bonds that remain

intact but are detrimental to carbon–carbon linkages.[9–11]

In the perspective of valorisation, the radical scavengingability of selected Miscanthus and switchgrass lignins arising

from bioethanol production wastes is herein evaluated andcorrelated with their structural features and extraction processconditions.

Experimental

All chemicals and standards were purchased from commercialsuppliers and used as received. Model lignin compounds

were ferulic acid (I), p-coumaric acid (II), caffeic acid (III),coniferyl alcohol (IV), 2-(4-hydroxyphenyl)ethanol (V), and2-hydroxyphenethyl alcohol (VI).

Lignin Materials

Three distinct lignins were extracted from Miscanthus�giganteus using a published protocol.[12] After acid treatment(formic acid/acetic acid/water) of the Miscanthus samples for

3 h at either 808C or 1078C, lignins FAL80 (‘Formic AceticLignin’ obtained at 808C) and FAL107 (‘Formic Acetic Lignin’obtained at 1078C) were recovered. Lignin AL (‘Ammonia

Lignin’) was obtained after ammonia soaking for 6 h at 608C.Lignins SG-107 and SG-AL were extracted from Panicum

virgatum L. feedstock (switchgrass), using the same respective

acid process at 1078C and aqueous ammonia soaking.

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Lignin Characterisation by Nuclear MagneticResonance (NMR)

NMR analyses of samples containing 75mL lignin dissolvedin 0.75mL [D6]DMSO were conducted on a Varian Unity600MHz spectrometer at 298K.

Adiabatic broadband {13C-1H} 2D heteronuclear (multiplic-ity-edited) single quantum coherence (g-HSQCAD) analyseswere performed by applying a Varian ChemPack 4.0 pulsesequence with 2048 collected complex points for the 1H dimen-

sion and with a relaxation delay of 5 s. The number of transientswas 32, and 256 time increments were recorded in the13C dimension. The 1JC–H used was 140Hz. The Gaussian

apodisation function was applied in both dimensions.

Lignin Characterisation by Fourier TransformInfrared (FTIR) Spectroscopy

The lignin sampleswere used in powder form and FTIR analyseswere performed in triplicate on a Bruker IFS 48 spectrometerusing KBr pellets. FTIR spectra were recorded in the 4000–

400 cm�1 range, using 64 scans and a resolution of 4 cm�1 foreach run.

Lignin Molecular Weight Determination

The lignins molecular weights (Mw) were determined by size-

exclusion chromatography in tetrahydrofuran (THF). ThreeStyragel columns (HR1, HR2, and HR3) (Waters) were con-nected in series to anHPLC system (Agilent Technologies, 1200

series) equipped with a differential refractometre (BI-DNDC/GPC, 620 nm). The lignin samples were dissolved in THF at aconcentration of 1.0mgmL�1 and the solution was filtered

through a 0.45-mm filter. Then, 100mL of the filtered solutionwas injected into the HPLC system and THF was used as theeluent at a flow rate of 1mLmin�1. A calibration curve was

constructed using polystyrene standards (Sigma–Aldrich).

Carbohydrates Content in Lignin Samples

The residual carbohydrates composition in the extracted ligninswas determined by gas chromatography after hydrolysis and

derivatisation using the alditol acetate protocol.[13]

Total Phenolic Content in Lignin Samples

The total amount of soluble phenolics was estimated using twomethods: the Folin–Ciocalteu assay and direct measurement ofthe absorbance at 280 nm.[14] The total phenolic content wasdetermined as mg equivalents of gallic acid per g dry matter

(GAE), based on the calibration curves obtained from the gallicacid standards. For the Folin–Ciocalteu assay, 100 mL ligninextract in dioxane was added to 500 mL Folin reagent and 2mL

Na2CO3 (20wt-% in water). The absorbance was measuredusing an Ultrospect 4000 (Pharmacia BioTech) spectropho-tometer at 750 nm after 30min at room temperature. A mixture

of dioxane, water, and reagent were used as a blank. For thedirect measurement at 280 nm (commonly used in the foodsector), the lignin samples were adequately diluted and the

absorbance at 280 nm was measured. Each experiment was runin duplicate. Results are expressed in arbitrary units.[15]

Radical Scavenging Assay

The radical scavenging activity of the lignins and model lignincompounds was evaluated according to the protocol fromSanchez-Moreno adapted by Dizhbite et al. in organic media

with slight modifications.[16,17] 1,1-Diphenyl-2-picrylhydrazil

(DPPH) was used as the radical source. Typically, 0.1mL of thesample in dioxane/water (90 : 10, v/v) was mixed with 3.9mLDPPH (6.1� 10�5M) at 258C. The absorbance of the solutionwas immediately measured at 515 nm and the decrease inabsorbance was monitored until a steady-state was reached. Thepercentage of DPPH remaining at the steady-state stage wasplotted as a function of the molar ratio of the sample and DPPH

(Rm). The sample concentration required to decrease the initialDPPH concentration by 50% was estimated from the graph anddesignated as EC50. The radical scavenging activity was asses-

sed as a measure of the radical scavenging index (RSI), definedas RSI¼ 1/(EC50). The smaller the EC50, the higher the radicalscavenging activity is.

Results and Discussion

Two-dimensional NMR analyses provide straightforwardqualitative information about the chemical structure of the iso-lated lignin compounds. In particular, the nature of constitutive

phenylpropane units and intramolecular bonds can be identifiedfrom two distinct zones of the NRM spectra namely between dH7.2–5.6 and dC 140–110 for the aromatic units and dH 4.0–2.4and dC 90–55 for the inter-unit linkages (Fig. 1). Complete

signal assignment was achieved according to previously pub-lished data and is listed in Tables 1 and 2.[12] Themain structuralfeatures of alkali and acid lignins from Miscanthus (FAL107,

FAL80, and AL) and switchgrass (SG-107 and SG-AL), asdetermined by two-dimensional 1H-13C NMR analysis in [D6]DMSO, are presented in Fig. 2.[6] The lignin samples recovered

after the acid process under harsh conditions (FAL107 andSG-107) display typical b-5 bonds, whereas the lignin samplesrecovered after the alkaline treatment (AL and SG-AL) exhibitb-O-4 linkages. FAL80, recovered under milder acid condi-

tions, displays statistically as many b-5 as b-O-4 bonds. Mis-

canthus and switchgrass lignins feature guaiacyl (G), syringyl(S), and p-hydroxyphenyl (H) units at a relative composition of

,50/40/10.[5] The data from Tables 1 and 2 show that the ligninsamples recovered after the acid process (FAL107, FAL80, andSG-107) display coumarate esters end-groups, whereas the

alkaline-treated lignin samples (AL and SG-AL) display cin-namyl alcohols end-groups. Based on this finding, the plausiblemolecular models of the lignins investigated in this study are

shown in Fig. 2.Lignins samples are usually described as sources of antiox-

idants, owing to the effect of their phenolic structures onoxygen-containing free radicals.[18] The use of DPPH as a

reactive free radical is a widely recognised method for estimat-ing antioxidant activity by measuring EC50 values and associ-ated radical scavenging indexes (RSI). From a methodological

point of view, the DPPH assay is recommended for hydroxyl-containing compounds.Moreover, this test is performed at roomtemperature, thus avoiding any risks of thermal degradation. In

this context, this DPPH assay is recognised as a very versatiletool for evaluating the radical scavenging ability of a range ofnatural compounds.

Influence of Structural Features of LigninModel Compounds

Several lignin model compounds I–VI were selected for this

study based on their commercial availability and similitude withlignin structural features. Of particular interest, p-coumaric acid(II) and coniferyl alcohol (IV) are effective models that

B C. Vanderghem, N. Jacquet, and A. Richel

respectively represent the acid and alkali extracted lignins end-groups. The concentration of the phenolic compounds requiredto decrease the initial DPPH concentration by 50%, i.e. EC50,

was used to quantify the radical scavenging (RS) activity. As anexample, Fig. 3a displays the percentage of DPPH remaining asa function of time for the reaction involving ferulic acid and

Fig. 3b shows the percentage of DPPH remaining at the steady-state stage as a function of ferulic acid content.

Even if they are structurally analogous, phenolic compounds

I–VI display relatively different radical scavenging abilities.The RS activity depends on the O–H (phenol) bond dissociationenergy and resonance delocalisation of the phenol radical that isrelated to inductive and steric effects arising from substituents

on the aromatic ring.[6] From results collected experimentally inTable 3, we observe that relatively structurally similarp-coumaric acid, ferulic acid, and caffeic acid have different

RS behaviours. For caffeic acid, the presence of two hydroxyl

groups on the cinnamic acid structure enhances the RSI value.Comparatively, the substitution of the hydroxyl group at the

meta position by a methoxy group leads to a partial decrease inthe RS ability. Complete disappearance of a specific group in themeta position (p-coumaric acid) significantly enhances EC50,leading to a low radical scavenging index. Reduction of ferulic

acid to coniferyl alcohol induces a slight increase in theRS ability (decrease of EC50). Additionally, the position ofthe –OH substituent on the aromatic cycle has an impact on

Table 1. Signal assignment in 2D-NMR HSQC spectra of acid-

extracted lignin samples FAL107, FAL80, and SG-107

dH [ppm] dC [ppm] Assignment

7.40 145.8 Ca in etherified ferulic acid

7.44 130.7 C2, C6 in esterified p-coumaric acid

6.74 119.5 C6 in guaiacyl units

6.75 (H5) 116.2 C3, C5 in esterified p-coumaric acid

6.23 114.3 C5 in guaiacyl units

6.96 111.6 C2 in guaiacyl units

6.66 104.3 C2, C6 in syringyl units

3.71 83.1 Polysaccharide contamination

4.49 81.1 Not assigned

3.98 80.0 Residual mono-/oligo-/polysaccharide

(carbohydrate-lignin complexes)3.63 77.5

4.75 71.7

4.24a 65.2 Cg in b-5 substructures

4.29a 64.8

4.12a 63.8 Cg in sinapyl/coniferyl/p-coumaryl

alcohol end-groups4.22a 63.4

4.05a 63.2

3.69 56.2 –OCH3 in syringyl and guaiacyl units

aProtons referred to CH2 in the HSQC spectrum.

F2 [ppm] F2 [ppm]

3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.44.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.44.0

90

85

80

75

70

65

60OMe

OMe55

(a) (b)

90

85

80

75

70

65

60

alcoholend-groups

Carbohydrates

55

β-5β�O-4

Cγ

Fig. 1. Magnified 2D-NMR HSQC spectra of (a) SG-107 and (b) SG-AL lignins extracted from switchgrass showing the inter-unit linkages region and

associated peaks assignment.

Table 2. Signal assignment in 2D-NMR HSQC spectra of alkali-

extracted lignin samples AL and SG-AL

dH [ppm] dC [ppm] Assignment

7.40 145.8 Ca in etherified ferulic acid

7.44 144.8

7.44 130.7 C2, C6 in esterified p-coumaric acid

6.74 119.5 C6 in guaiacyl units

6.75 (H3) 117.8 C3, C5 in esterified p-coumaric acid

6.75 (H5) 116.2

6.92 115.7 Cb, C5 in etherified ferulic acid

6.66 115.2

6.23 114.3 C5 in guaiacyl units

6.96 111.6 C2 in guaiacyl units

6.66 104.3 C2, C6 in syringyl units

6.61 103.8

4.07 86.5 Cb in b-O-40 linked to a S unit

4.24 84.4 Cb in b-O-40 linked to a G unit

3.79 80.7 Residual carbohydrates

3.48 75.9

3.23 74.4

3.01 73.1

4.78 72.4 Ca in b-O-40 linkages3.85b 63.6 Cg in b-O-40, with Ca¼O

3.14b 63.6

3.68b 63.3 Cg in phenylcoumaran substructures

3.43b 62.3 Cg in cinnamyl (sinapyl/coniferyl)

alcohol end-groups4.05b 62.0

3.19 60.4 Cg in b-O-40 linkages3.58 60.4

3.69 56.2 –OCH3 in syringyl and guaiacyl units

Lignin and Antioxidant C

EC50 values, as illustrated for 2-(4-hydroxyphenyl)ethanol and2-hydroxyphenethyl alcohol. In the ortho position, reaction with

DPPH is more efficient in contrast to that of the para positionwhere EC50 is higher.

Radical Scavenging Ability of Technical Lignins

The kinetics of the DPPH reaction with the lignin samples werestudied according to the same experimental protocol. As

exemplified, Fig. 4 shows the change in DPPH concentration asa function of time for Miscanthus lignin FAL107 isolated afteracid treatment at 1078C.

Using these graphical results, it is possible to estimate the

EC50 and radical scavenging index values of all lignin samples.The resulting radical scavenging properties were correlated withthe total phenolic contents estimated by two experimental

methods (Table 4). As observed, the radical scavenging abilityof the Miscanthus lignins is higher than that of the switchgrasscounterparts with RS indexes ranging from 0.125 to 0.253 for

Miscanthus and from 0.025 to 0.054 for switchgrass. Ligninsrecovered after a harsh acid process at 1078C (FAL107 andSG-107) feature higher RSI than the lignins extracted afterammonia soaking (AL and SG-AL). This shows that the

fractionation process imparts antioxidant properties to thelignin. FAL 107 exhibits the highest RSI index (0.253), withan apparent radical scavenging ability similar to that of

p-coumaric acid, whereas SG-AL displays the lowest RSI value

(0.025). Using the Folin–Ciocalteu colorimetric method, thetotal phenolic content is estimated between 82.59 and 115.86mg

equivalents of gallic acid per g dry lignin for the Miscanthus

extracts series, and between 74.00 and 99.65 for the switchgrassextracts. The standard error for the Folin–Ciocalteu assay is

,5%. In general, lignin fractions with a high RSI value show ahigh phenolic content as well. Many authors suggest that asatisfactory linear trend can be obtained between such para-

meters, however, the data did not fit the trend for some of thestudied lignins (Fig. 5).[19] Similarly, the linear relationshipbetween the total phenolic content estimated by the Folin–Ciocalteu method and the absorbance measured at l¼ 280 nm

is not clear. A plausible explanation is the presence of residualreducing carbohydrates (mainly xylose and glucose) in some ofthe technical lignin extracts obtained (mainly FAL80, AL, and

SG-AL), with total carbohydrate contents ranging from 5.8 to13.7wt-%. We believe that these reducing sugars might inter-fere with the redox Folin–Ciocalteu assay, particularly at low

phenol levels. Moreover, carbohydrate contamination wasshown to alter the antioxidant capabilities of samples becausecarbohydrates formed hydrogen bonds with the lignin phenolicgroups.[20] No accurate correlation between the ligninmolecular

weight and their antioxidant activity is herein established. Theconformation of the lignin in solution (relating to the formationof intra- and intermolecular hydrogen bonds) is also another

point of discussion and could explain this non-linear change in

β-5

S unit

G unit

H unit

Acid process(FAL107, SG-107)

Ammonia process(AL, SG-AL)

β-O-4

HO O

HO O

O

OH OH

OMe

OR

OMe

OH

OOO

HO

OMe

MeO OR

OMe

Fig. 2. Structural features of lignins used in this study. The structure of sample FAL80 (not shown) is an intermediate

between both structures.

00 15 30 45 60 75 90 105 120

Rm � 0.789Rm � 0.394

Rm � 0.0394

00 0.2 0.4 0.6 0.8

20

40

60

80

100

20

40

60P

erce

ntag

e of

DP

PH

rem

aini

ng [%

]

80

100(a) (b)

Per

cent

age

DP

PH

[%]

Time [min] Molar ratio (Rm)

Fig. 3. (a) Change in DPPH concentration as a function of time at different molar ratios Rm of ferulic acid. (b) Percentage of DPPH remaining at the steady-

state stage as a function of Rm.

D C. Vanderghem, N. Jacquet, and A. Richel

RSI as a function of the phenolic content. For instance, weobserve that the RSI value of SG-107 is lower than that of

Miscanthus samples AL and FAL-80 although the phenoliccontent of SG-107 is higher.

FTIR spectra of the extracted lignin samples show distinctivefeatures, mainly between 2000 and 400 cm�1, evidencing that

both the nature and content of some lignin functional groups areaffected by the extraction process (Fig. 6).[21] Particularly, theband at 1720 cm�1, corresponding to carbonyl/carboxyl bond

stretching, is well developed for the lignin samples obtained

under acid conditions (FAL107, FAL80, and SG-107) asopposed to the samples extracted under alkaline conditions

(AL and SG-AL).Although the intensity of these bands differs, typical aroma-

tic skeleton vibrations are detected at 1600, 1515, and1424 cm�1, withC–Hdeformations and aromatic ring vibrations

at 1463 cm�1. A characteristic band at 1370 cm�1, correspond-ing to the phenolic group is not fully detected in the ligninsample obtained after ammonia soaking (AL), but is well

defined for FAL107 and FAL80. Additionally, well-defined

0 0

10

20

30

40

50

60

70

80

90

100

0 20

Rm � 1.050

Rm � 0.210

Rm � 1.050

40

Time [min] Molar ratio (Rm)60 80 0 0.2 0.4 0.6 0.8 1.0 1.2

10

20

30

40

50

60

Per

cent

age

DP

PH

[%]

Per

cent

age

of D

PP

H r

emai

ning

[%]

70

80

90

100(a) (b)

Fig. 4. (a) Change in DPPH content as a function of time at varying molar ratios Rm of Miscanthus lignin FAL107. (b) Percentage of DPPH

remaining at the steady-state stage as a function of Rm.

Table 3. Radical scavenging activity (EC50 and RSI) for model lignin compounds I–VI

Compound Structure EC50 [M] RSI [M�1]

Ferulic acid (I)MeO

O

HO

OH 0.0239 41.774

p-Coumaric acid (II)

HO

O

OH 3.7379 0.267

Caffeic acid (III)

HO

HO

O

OH 0.0081 123.762

Coniferyl alcohol (IV)

MeO

HO

OH0.0197 50.787

2-(4-Hydroxyphenyl)ethanol (V)HO

OH

3.0071 0.332

2-Hydroxyphenethyl alcohol (VI)OH

OH

1.2271 0.815

Lignin and Antioxidant E

bands at 1165 and 1035 cm�1 in the alkaline samples AL andSG-AL are observed and respectively correspond to the OH

stretching of secondary and primary alcohols. On the other hand,a peak at 1655 cm�1 can be identified in the spectrum ofswitchgrass lignin SG-107. Based on previous studies, this band

is related to the presence of quinonic structures owing to theoxidation of some aromatic rings during the acid process.[21]

This observation leads us to conclude that the use of the same

extraction process on two distinct raw materials produces twodistinct types of lignin samples, featuring different structuralfeatures and different antioxidant abilities. Moreover, as sug-gested by Pan et al., and although theDPPH trappingmechanism

is not clearly understood, the presence of high amounts ofaliphatic alcohols has a detrimental effect on radical scavengingindexes.[22] This hypothesis also confirms the lower RSI values

obtained for alkali lignins when compared with those obtainedfor lignins subjected to acidic cracking conditions.

At this stage, it remains difficult to conclude that lignins

wastes obtained from cellulosic ethanol biorefineries are power-ful antioxidant molecules. The quest for establishing an accuraterelationship between RSI values and experimental data remainsopen as the antioxidant capability is also affected by independent

factors such as the molecular weights, and the presence ofreducing carbohydrates, steric, and electronic effects around thephenolic substituents. Technical lignins obtained under acidic

conditions possess RSI values similar to that of p-coumaric acid,especially when extracted fromMiscanthus at elevated tempera-tures (1078C). The antioxidant ability of these lignins could havesignificant practical implications for, notably, their inclusion asbiomaterials in biological media or additives in polymers.

Conclusion

Although many authors suggest that lignin wastes arising frombiorefinery can act as powerful scavenging agents, these

Table 4. Radical scavenging activity (EC50 and RSI) and total phenolic content of lignin samples

Lignin Molecular weight

[gmol�1]

EC50

[M]

RSI

[M�1]

Phenolic content

(Folin–Ciocalteu)

Phenolic content

(Absorbance at 280 nm)

Miscanthus

FAL107 2000 3.9584 0.253 115.86 0.61

FAL80 1700 7.9976 0.125 82.59 0.49

AL 3100 5.8586 0.170 90.54 0.50

Switchgrass

SG-107 1600 18.5874 0.054 99.65 0.50

SG-AL 500 39.4796 0.025 74.06 0.53

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0

20

40

60

80

100

120

140

0 0.1 0.2 0.3 Phe

nolic

con

etnt

(A

bsor

banc

e at

280

nm

)

Phe

nolic

con

tent

(F

olin

–Cio

calte

u)

Radical scavenging index (RSI)

Folin–Ciocalteu A (280 nm)

Fig. 5. Relationship between the radical scavenging index and total

phenolic content determined by two methods.

2000 1800 1600 1400 1200

Wavenumber [cm�1]

Wavenumber [cm�1]

1000 800 600 400

2000 1800 1600 1400 1200 1000

SG-107

1655

--

800 600 400

1165

--

1370

--

1035

--

1370

--

1370

--

--17

20

Tran

smitt

ance

[a.u

.]Tr

ansm

ittan

ce [a

.u.]

AL

FAL80

FAL107

Fig. 6. FTIR spectra of selected technical lignin samples in the 2000–

400 cm�1 range.

F C. Vanderghem, N. Jacquet, and A. Richel

observations require further validation. Lignins arising from

biorefinery wastes display a moderate radical scavengingcapability based on the use of the free radical 1,1-diphenyl-2-picrylhydrazyl (DPPH). The influence of the extraction process

conditions is demonstrated. Herein, technical lignins extractedunder acidic conditions displayed radical scavenging propertiessimilar to those of p-coumaric acid. The presence of residualcarbohydrates and aliphatic hydroxyls in alkaline lignin samples

is expected to alter antioxidant properties.

Acknowledgements

This work was financially supported by the Walloon Region (TECHNOSE

Excellence Research Program, project number 716757). The authors thank

Mr Eric Groignet for technical assistance.

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