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Article

Modified Hydrothermal Pretreatment Conditions Enhance Alcohol Solubility of Lignin from Wheat Straw Biorefining

by
Tor Ivan Simonsen
,
Demi Tristan Djajadi
and
Sune Tjalfe Thomsen
*
Department of Geosciences and Natural Resource Management, University of Copenhagen, Frederiksberg C, 1958 Copenhagen, Denmark
*
Author to whom correspondence should be addressed.
Biomass 2025, 5(2), 23; https://doi.org/10.3390/biomass5020023
Submission received: 19 February 2025 / Revised: 9 April 2025 / Accepted: 15 April 2025 / Published: 24 April 2025
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Abstract

:
Lignin-rich residues from lignocellulosic biorefineries remain underutilized, limiting their economic viability. This study demonstrates how modifying hydrothermal pretreatments with temperatures and additives enhances the lignin-rich residue’s solubility in alcohol, a key step toward its valorization in biofuel and material applications. Effective carbohydrate removal greatly enhanced the residue’s alcohol solubility, supporting both saccharification and lignin utilization. Notably, a 5% hydrogen peroxide treatment doubled the residue’s alcohol solubility, reaching ~40%, while maintaining similar saccharification yields. Low concentrations of surfactants and oxidizers enhanced the alcohol solubility independently of the saccharification yield, while alkali improved both. These findings highlight that minor pretreatment adjustments, such as low-concentration additives, can optimize lignin’s utilization in biorefineries, while maintaining a high carbohydrate conversion

1. Introduction

The economic and sustainable viability of lignocellulosic biorefineries depends on fully utilizing biomass, yet lignin-rich residues from ethanol production are mostly discarded or incinerated [1]. A potential pathway for lignin valorization is through alcohol solvation, which enables various applications, such as stable dispersions for marine fuel [2,3], hydrogels [4], wood impregnation [5], and lignin nanoparticles [6]. Integrating such lignin valorization technologies into bioethanol production could enhance the resource efficiency and the economic viability of biorefineries. However, the effectiveness of these technologies will significantly benefit from achieving a high alcohol solubility of the residual lignin.
Alcohol solubility is primarily governed by its molecular weight, polarity, and purity [7]. Low-molecular-weight lignins with a reduced structural complexity and minimal crosslinking tend to dissolve more readily in alcohol-based solvents. Additionally, lignin with a higher proportion of hydroxyl and ether functional groups exhibits improved interactions with polar solvents, like ethanol and methanol. Purity also plays a crucial role, as lignins with minimal carbohydrate contamination, such as organosolv, soda, and kraft lignins, generally show superior solubility due to their reduced content of residual hemicellulose and other non-lignin components [8,9,10,11].
In contrast, lignin residues from a hydrothermal pretreatment (HTP) followed by enzymatic saccharification generally exhibit a lower alcohol solubility compared to organosolv, soda, or kraft lignins [12]. This is largely related to their higher molecular weight, reduced purity, and strong interactions with carbohydrates through lignin–carbohydrate complexes (LCCs), which hinder solvent accessibility. HTP is a widely used method in lignocellulosic biorefineries, employed both in research and at a commercial scale by companies like Beta Renewables (now Versalis), Inbicon (now Ørsted), and Raízen. It involves treating biomass with hot compressed water or steam at elevated temperatures (typically 170–220 °C) to enhance enzymatic saccharification by breaking down hemicellulose and modifying the lignin’s structure [13,14,15,16]. However, this process introduces several structural changes that impact lignin’s alcohol solubility. (A) The cleavage of β-O-4′ linkages releases alcohol-soluble phenolic compounds, which can inhibit enzymatic activity [15,17]. (B) Under severe pretreatment conditions, lignin condensation reactions lead to an increased molecular weight through C-C and C-O bond formation, reducing the solubility in organic solvents [14,16]. (C) Lignin migrates to the surface of the biomass, forming hydrophobic particles around the cellulose microfibrils, making a barrier to enzymatic saccharification, and repels polar solvents like ethanol and methanol, further limiting the solubility [17,18,19].
In addition, the presence of residual LCCs in lignin-rich residues following enzymatic saccharification presents another significant challenge for solubility. These covalent lignin–carbohydrate bonds lead to larger, structurally complex lignin fragments that are inherently less dispersible in ethanol and methanol [20,21].
To improve sugar yields during enzymatic saccharification, HTP is chemically catalyzed with dilute acids, bases, oxidizers, surfactants, or other agents [22]. While the impact on carbohydrates and their subsequent saccharification has been well documented, the effects on lignin are less understood.
Dilute acids enable hemicellulose dissolution and induce the cleavage of β-O-4′ ether bonds, leading to pronounced depolymerization with increased pretreatment severity, but also result in more condensed, crosslinked lignin with low solubility [23]. In contrast, alkaline-catalyzed HTP improves enzymatic saccharification through the effective cleavage of ester and ether bonds in lignin, leading to its depolymerization and decreased condensation reactions, and thus increases alcohol solubility in aqueous systems [24,25,26].
Whereas acid and alkaline catalysts directly alter the pH and influence lignin’s structure, other catalysts, such as surfactants, oxidizers, and capping agents, may enhance saccharification yields without significantly affecting the pH [27,28,29]. Among these catalysts, surfactants have been used in HTP to enhance enzymatic saccharification while reducing lignin’s aggregation and increasing alcohol solubility [30]. This has been confirmed in multiple studies, with Tween 80 often showing the highest saccharification gains [31,32,33]. Furthermore, surfactants can increase the hydrophilicity of the lignin-rich residue’s surface, potentially improving its solubility in alcohol-based systems [31].
Alternatively, oxidizers can be used to catalyze the cleavage of β-O-4′ linkages and the hydroxylation of lignin, breaking lignin structures into smaller, less hydrophobic molecules. Additionally, they disrupt lignin–carbohydrate complexes (LCCs), breaking covalent bonds between lignin and hemicellulose, which could further enhance the solubility [34,35]. Unlike oxidizers, capping agents, such as sodium borohydride, phenol, and simple alcohols, are used to stabilize lignin fragments by preventing their re-condensation and polymerization under hydrothermal conditions [36]. Although their effects during HTPs are not well studied, it is hypothesized that they react with reactive lignin intermediates, “capping” them and preventing their recombination into larger, less soluble molecules [37,38]. Similarly, adding low concentrations of formic acid during pretreatment has been shown to modify lignin by introducing formyl groups, reducing its molecular weight, and altering its solubility properties [39].
While these modifications highlight promising strategies for lignin valorization, a more comprehensive understanding of how each pretreatment condition influences alcohol solubility is necessary to optimize lignin utilization in biorefinery applications.
This study investigates how industrially relevant modifications to HTP affect alcohol solubility of the residual lignin fraction, as well as the total sugar recovery, demonstrating that targeted adjustments can simultaneously enhance lignin valorization and enzymatic saccharification yields—a dual-optimization approach that has not previously been explored for lignin-rich residues. Specifically, we examine the effects of temperature, a surfactant, an oxidizing agent, an organic acid, an alkali, and a capping agent. By analyzing alcohol solubility in ethanol and methanol at the ambient temperature alongside enzymatic conversion, we aim to provide actionable insights for integrating lignin valorization into existing biorefinery processes.

2. Materials and Methods

2.1. Pretreatments in PARR Reactor

The pretreatments were conducted using a 2 L Hastelloy C276 Parr reactor, controlled by a Parr 4848 reactor controller. Eight HTPs were performed on 60 g of dry wheat straw mixed with 600 mL of demineralized water at temperatures ranging from 170 to 210 °C, using various chemical additives, as specified in Table 1. HTP at 190 °C was used as a base condition to test additives as it is a well-known optimal temperature [14,18,22,30,40]. The reactor operated at a rotary speed of 150 rpm. Following the pretreatment, the solid and liquid fractions were stored at −20 °C for later enzymatic saccharification. The chemicals used were Tween 80 (Sigma-Aldrich, St. Louis, MO, USA), 30% (w/w) H2O2 (Sigma-Aldrich), 99% analytical grade formic acid (Thermo Scientific, Waltham, MA, USA), sodium hydroxide (NaOH) pellets (Merck, Darmstadt, Germany), and 99% phenol (Honeywell Fluka, Fisher Scientific, Waltham, MA, USA).

2.2. Enzymatic Saccharification

Sodium azide was added to the pretreated material slurry to a final concentration of 0.1% (w/v) to prevent microbial activity. The pH was adjusted to 5.0 using H2SO4 or NaOH, followed by at least 30 min of free-fall mixing to ensure stability. Enzymatic saccharification was conducted on the pH-adjusted whole slurry without dilution in two stages. The first stage followed the previously published conditions of 50 °C for 72 h in a free-fall tumbler rotating at 10 rpm, with a moderate enzyme loading [41]. Cellic© CTec2 (Novozymes, Hørsholm, Denmark) was added at 0.024 g enzyme per g of dry matter (DM) [42]. pH readjustments to 5.0 were made every 24 h to counteract acidification from the deacetylation of hemicellulose. After 72 h, subsamples were taken for monosaccharide quantification using HPLC. In the second stage, additional enzymes were added to a total of 0.1 g enzyme per g DM, and the saccharification continued for 7 days, after which subsamples were again collected for monosaccharides quantification. All the enzymatic saccharification experiments were conducted in triplicate to ensure reproducibility.

2.3. Washing of Solids After Enzymatic Saccharification

Post saccharification, the remaining solid residue was washed to remove the produced sugars. The residues were centrifuged at 4223× g for 10 min, the supernatants were decanted, and the solids were resuspended in 100 mL of demineralized water. These washing steps were repeated five times, and thereafter the remaining lignin-rich residues were dried at 105 °C. The structure of the dried solids was subsequently loosened by ~5 s of blade milling.

2.4. Analysis of Dissolved Monosaccharides

Monosaccharides were quantified using an Ultimate 3000 High-Performance Liquid Chromatography (HPLC) system (Dionex, Sunnyvale, CA, USA) equipped with a Phenomenex Resex ROA column. The column temperature was set to 80 °C, and a mobile phase consisting of 5 mM H2SO4 was employed at a flow rate of 0.6 mL min−1. The samples were diluted in the mobile phase and filtered through a 0.45 μm nylon filter before injection to remove particulate matter and ensure optimal injection conditions.

2.5. Determination of Structural Carbohydrates and Lignin

Strong acid hydrolysis was conducted as an adapted version of the standard method outlined by Sluiter et al. [43]. Then, 0.0600 g of dry biomass sample was weighed into a 20 mL pressure-proof glass tube (Ace glass, Vineland, NJ, USA). Triplicates were prepared for each pretreatment. Subsequently, 0.6 mL of 72% w/w H2SO4 was added to each vial, followed by thorough vortex mixing. Then, the glass tubes were immersed in a water bath set at 30 °C for 1 h with manual agitation. After the incubation period, 16.8 mL of MilliQ water was added to each glass vial. The vials were capped and inverted to ensure thorough mixing. Subsequently, the tubes were autoclaved at 121 °C for 1 h. Following hydrolysis, aliquots of each sample were adjusted to a neutral pH using a concentrated Na2CO3 solution. The samples were then filtered through a 0.45 μm filter and monomeric sugars were quantified using HPLC, as described above. The lignin content was determined as being acid-insoluble Klason lignin.

2.6. Ash Content Analysis

The ash content was measured gravimetrically by placing a 5.0 g oven-dried sample in dried ceramic crucibles, covering them with punctured aluminum foil, and heating at 550 °C for 3 h in a muffle furnace. The ceramic crucibles were then transferred to a desiccator and weighed after cooling to the ambient temperature.

2.7. ATR-FTIR

The samples were analyzed using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. For this analysis, we employed a Perkin Elmer FTIR Spectrum-3 spectrometer (PerkinElmer, Waltham, MA, USA) equipped with a Universal ATR Sampling Accessory. Each treatment was measured in five technical replications. The spectral range was set between 4000 and 400 cm−1 with a spectral resolution of 4.0 cm−1. Each spectrum was derived from 64 scans, with 128 scans being used for the background spectrum. The spectra were subsequently corrected for the baseline drift using Asymmetric Least Squares Smoothing (ALS). The spectra were normalized using the peak area of the 1595–1615 cm−1 region, corresponding to the lignin aromatic skeletal vibration [44,45].

2.8. Alcohol Solubility

The lignin’s solubility in the solvents was assessed by preparing a mixture of 10 wt% water and 90 wt% alcohol, using either 96% (v/v) ethanol (VWR Chemicals, Radnor, PA, USA) or 100% methanol (VWR Chemicals). The lignin samples were combined with the solvent in a 2 mL Eppendorf tube at a solid-to-liquid ratio of 1:5 (w/w) based on the lignin dry weight, following Cabrera [2]. The lignin–alcohol mixtures were stirred at 700 rpm for 1 h at the ambient temperature, then centrifuged at 1000× g for 10 min. The supernatants were separated and dried in a vacuum oven at 40 °C until reaching a constant weight. The fractionation yield (wt%) was determined by calculating the dry weight ratio of the lignin in the supernatant to the initial Klason lignin concentration. The solubility tests were performed in triplicate, and the average yield and standard deviation were reported.

2.9. Statistical Analysis

The open-source software “R” (version 4.4.0) was utilized for the statistical computing. The analysis of variance (AOV) function was employed for conducting a one-way ANOVA. Tukey’s multiple comparisons of means, with a 95% family-wise confidence level, were performed based on the Studentized range statistic and Tukey’s “honest significant difference” method (using the Tukey HSD function in R).

3. Results and Discussions

3.1. Effects of Pretreatment Modifications on Enzymatic Saccharification Yield

The impact of the HTP modifications was first evaluated based on enzymatic carbohydrate conversion and the composition of residual fractions. After 7 days (including a second enzyme addition at day 3), higher temperatures (210 °C), and the addition of Tween 80, formic acid, NaOH, and phenol increased the saccharification yield compared to the Control (Figure 1). To our knowledge, this is the first study comparing the effects of such a diverse range of pretreatment additives on both enzymatic digestibility and lignin’s alcohol solubility in a unified experimental setup.
The highest increases in saccharification yield, between 21% and 24%, were observed for adding formic acid and NaOH, which is likely due to enhanced hemicellulose removal and the disruption of LCCs, thereby improving the enzymatic accessibility to cellulose, which is in accordance with previously published results [46,47]
The increased temperature (210 °C) and the addition of phenol showed only moderate increases in saccharification (7–8%). Higher temperatures promote hemicellulose solubilization [48], but might also lead to partial lignin condensation, limiting further enzymatic accessibility to cellulose [49]. The effect of adding phenol as a HTP modification has, to our knowledge, not been previously reported. The slight positive effect observed here could be related to phenol–lignin interactions, potentially preventing some lignin condensation or modifying the surface properties of lignin in a way that enhances enzyme accessibility. However, the underlying mechanisms require further investigation.
The surfactant Tween 80 increased saccharification by 4.5%, which is consistent with its known role in reducing lignin aggregation and improving cellulose accessibility. However, its effect in this study was smaller than previously reported [33], likely due to the higher enzyme loading applied here, which may have reduced the relative impact of the surfactant.
In contrast, a reduction in the saccharification yield was observed when the pretreatment temperature was lowered to 170 °C (Low) or when H2O2 was added, resulting in decreases of 15.5% and 8.5%, respectively. Surprisingly, despite previous studies showing oxidants can enhance sugar yields [28], H2O2 here reduced saccharification. The lower temperature likely reduced hemicellulose removal, leaving a higher residual carbohydrate content that may have hindered enzyme penetration. The lower yield when applying H2O2 is more surprising, since oxidizing agents previously have been shown to increase yields [28]. One could hypothesize that any residual H2O2 could potentially affect the enzymatic activities at the saccharification stage due to its oxidative activity. However, thorough washing is considered to render this effect negligible. Instead, the observed decrease in saccharification could be attributed to oxidative modifications in lignin during pretreatment, potentially increasing crosslinking or altering lignin–carbohydrate interactions in a way that reduced enzyme accessibility.
The relationship between the saccharification yield and the Klason lignin content followed a general trend, where increased saccharification was associated with a higher proportion of Klason lignin in the residue. This was confirmed by FTIR measurements, where the 1037 cm−1 peak, corresponding to C-C and C-O bond stretching in cellulose, was more pronounced in samples with higher saccharification yields [49,50] (Appendix A, Figure A1, Figure A2 and Figure A3). This trend became particularly evident at higher pretreatment temperatures (170–210 °C), where the glucan and xylan contents decreased with increasing severity, a well-known pattern observed in hydrothermal pretreatment studies [13,14,15] (Figure 2).
This was further supported by FTIR measurements, which showed an increased cellulose signal intensity in the samples with higher saccharification (Figure A1). The increase in the Klason lignin content was also seen under the formic acid and NaOH treatments, which both led to enhanced saccharification. This suggests that these treatments not only improve enzymatic hydrolysis but also remove a substantial proportion of carbohydrates, resulting in residues with a higher relative lignin content.
Since residual carbohydrates are largely insoluble in alcohol, their presence is expected to reduce the overall alcohol solubility of the residue. Consequently, the lignin-rich residues from the 210 °C, formic acid, and NaOH treatments, which exhibited the highest Klason lignin content (63–79%) and the lowest residual carbohydrate content, were anticipated to show the greatest alcohol solubility.

3.2. Effects of Pretreatment Modifications on Alcohol Solubility

To evaluate how the HTP modifications affected alcohol solubility, the lignin-rich residues’ solubility in ethanol and methanol was measured. The three treatments that resulted in the highest Klason lignin content, i.e., the higher temperature (High), formic acid, and NaOH, increased the alcohol solubility by approximately 90–100% per gram of sample, relative to the Control (Figure 3A), supporting the expectation that a higher lignin purity enhances the solubility of the residue.
However, the addition of Tween 80 and H2O2 also enhanced the residues’ alcohol solubility, despite having lower or similar enzymatic saccharification yields and Klason lignin contents. This suggests that the Klason lignin content alone does not fully determine alcohol solubility in alcohol and aligns with previous studies showing that alcohol solubility in organic solvents is influenced by its molecular weight, polarity, and the degree of lignin–carbohydrate interactions [3,6,51,52,53,54,55,56].
To better understand solubility trends independent of the residue composition, we normalized the solvation yield to the Klason lignin content of each sample (Figure 3B). This approach decoupled the solubility from the yield–related concentration effects and allowed for the evaluation of intrinsic changes in lignin chemistry.
Notably, H2O2 resulted in a twofold increase in alcohol solubility, an effect that remained even after normalization to the Klason lignin content. In contrast, when normalized, the solubility improvement seen at 210 °C was reduced after normalization, suggesting that the effect of H2O2 on alcohol solubility is independent of the Klason lignin content. This is likely a result of oxidative delignification and hydroxylation that reduces lignin’s molecular weight and increases its polarity, making it more soluble in alcohol [57,58]. Similarly, NaOH is known to disrupt LCCs and depolymerize lignin [24,25], facilitating alcohol solvation.
Although the mechanism by which Tween 80 increases alcohol solubility remains less understood, its ability to alter lignin surface properties, potentially increasing hydrophilicity, may reduce non-productive enzyme binding and facilitate better dispersion in alcohol-based systems [30,33].
These findings demonstrate that alcohol solubility is governed not only by its purity or carbohydrate content but also by its chemical structure and polarity, underscoring the importance of pretreatments tailored for specific downstream applications.
This study is, to our knowledge, the first to systematically assess alcohol solubility of lignin-rich residues at the ambient temperature following modified hydrothermal pretreatments and saccharification. Consequently, comparative benchmarking against the existing literature is not possible.

4. Conclusions

Given the growing interest in alcohol-based lignin valorization, industrial-scale implementation should prioritize strategies that balance carbohydrate conversion with alcohol solubility. This study demonstrates that efficient carbohydrate removal during HTP and enzymatic saccharification enhances lignin’s alcohol solubility, indicating that carbohydrate conversion and lignin valorization are not necessarily in conflict. While a higher Klason lignin content generally correlated with increased solubility, treatments with H2O2, Tween 80, and NaOH improved the solubility through additional mechanisms that were independent of carbohydrate removal. These findings suggest that beyond the Klason lignin content, improved solubility may also arise from pretreatment-induced changes in lignin’s structure, including depolymerization, reduced condensation, and an altered lignin redistribution, making lignin more accessible to the alcoholic solvent.
Our evaluation highlights H2O2 and surfactants as particularly promising additives due to their ability to significantly enhance alcohol solubility while maintaining enzymatic saccharification yields. However, other factors need to be considered, including the process’s safety, the potential impacts on downstream fermentation, and the operational complexity—elements that must be carefully balanced in a biorefinery setting to ensure economic and technical viability.
Future research should investigate the structural changes responsible for increased solubility—particularly in treatments involving H2O2—and assess their effects on downstream processing and integration. Taken together, these findings support the development of dual-optimization strategies that align carbohydrate conversion and lignin valorization, paving the way for more efficient and sustainable lignocellulosic biorefineries.

Author Contributions

Conceptualization, T.I.S., D.T.D. and S.T.T.; methodology T.I.S., D.T.D. and S.T.T.; formal analysis, T.I.S. and S.T.T.; data curation, T.I.S. and S.T.T.; writing—original draft preparation, T.I.S.; writing—review and editing, T.I.S., D.T.D. and S.T.T.; visualization, T.I.S.; supervision, D.T.D. and S.T.T. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledge the financial support of the Danish Energy Agency for the project “CLEO, a Carbon-Neutral Fuel for the Maritime Sector”, funded through “The Energy Technology Development and Demonstration Program” (Grant number 64020-1101).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets generated during this study are available from the corresponding author on reasonable request.

Acknowledgments

Britta Skov is acknowledged for her for her invaluable contributions in the laboratory.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
ALSAsymmetric Least Squares Smoothing
ATR-FTIRattenuated total reflectance-Fourier transform infrared
HPLCHigh-Performance Liquid Chromatography
HTPhydrothermal pretreatment
LCClignin–carbohydrate complex

Appendix A

Appendix A.1. FTIR Spectra

Biomass 05 00023 g0a1
Figure A1. FTIR spectra of pretreated and saccharified wheat straw. The 1037 cm−1 peak, indicated by the dashed line, is related to the CC and CO stretch of cellulose, according to Wiley and Atalla (1987) [50].
Figure A1. FTIR spectra of pretreated and saccharified wheat straw. The 1037 cm−1 peak, indicated by the dashed line, is related to the CC and CO stretch of cellulose, according to Wiley and Atalla (1987) [50].
Biomass 05 00023 g0a1

Appendix A.2. Enzymatic Saccharification Yields

Biomass 05 00023 g0a2
Figure A2. Enzymatic saccharification yields of xylan after 3 and 7 days. The percentages are given in w/w, based on the initial xylan content.
Figure A2. Enzymatic saccharification yields of xylan after 3 and 7 days. The percentages are given in w/w, based on the initial xylan content.
Biomass 05 00023 g0a2

Appendix A.3. Enzymatic Saccharification Yields of Glucan After 3 Days

Biomass 05 00023 g0a3
Figure A3. Enzymatic saccharification yields of glucan after 3 days. The percentages are given in w/w, based on the initial glucan content.
Figure A3. Enzymatic saccharification yields of glucan after 3 days. The percentages are given in w/w, based on the initial glucan content.
Biomass 05 00023 g0a3

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Figure 1. Enzymatic saccharification yields after 7 days. Percentages are given in w/w based on the initial glucan content. Saccharification yields after 3 days can be found in Appendix A, Figure A3. Small letters [a–e] indicate statistically significant differences between treatments. The dotted line represents the saccharification yield under standard conditions.
Figure 1. Enzymatic saccharification yields after 7 days. Percentages are given in w/w based on the initial glucan content. Saccharification yields after 3 days can be found in Appendix A, Figure A3. Small letters [a–e] indicate statistically significant differences between treatments. The dotted line represents the saccharification yield under standard conditions.
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Figure 2. Compositional analysis after pretreatment followed by 7 days of enzymatic saccharification. Values are given in w/w of total sample. Small letters [a–c] indicate statistically significant differences in Klason lignin content between treatments.
Figure 2. Compositional analysis after pretreatment followed by 7 days of enzymatic saccharification. Values are given in w/w of total sample. Small letters [a–c] indicate statistically significant differences in Klason lignin content between treatments.
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Figure 3. Solvation yields. (A) Alcohol solubility, measured as g/g sample. (B) Alcohol solubility, measured when normalized by Klason lignin content. Small letters [a–f] indicate statistically significant differences between treatments. The dotted line represents alcohol solubility under standard conditions.
Figure 3. Solvation yields. (A) Alcohol solubility, measured as g/g sample. (B) Alcohol solubility, measured when normalized by Klason lignin content. Small letters [a–f] indicate statistically significant differences between treatments. The dotted line represents alcohol solubility under standard conditions.
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Table 1. List of pretreatments.
Table 1. List of pretreatments.
NameTemperatureAdditiveEffect
Control190 °C-Control
Low temp.170 °C-Reduced temperature
High temp.210 °C-Increased temperature
Formic acid [38]190 °C0.1 M Formic acidAcid
NaOH190 °C1% (w/v) NaOHAlkaline
H2O2 [39]190 °C5% (w/w) H2O2Oxidant
Tween 80 [32]190 °C3% (w/w) Tween 80 *Surfactant
Phenol190 °C1% (w/v) PhenolCapping agent
* 3% w/w is based on dry matter wheat straw.
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Simonsen, T.I.; Djajadi, D.T.; Thomsen, S.T. Modified Hydrothermal Pretreatment Conditions Enhance Alcohol Solubility of Lignin from Wheat Straw Biorefining. Biomass 2025, 5, 23. https://doi.org/10.3390/biomass5020023

AMA Style

Simonsen TI, Djajadi DT, Thomsen ST. Modified Hydrothermal Pretreatment Conditions Enhance Alcohol Solubility of Lignin from Wheat Straw Biorefining. Biomass. 2025; 5(2):23. https://doi.org/10.3390/biomass5020023

Chicago/Turabian Style

Simonsen, Tor Ivan, Demi Tristan Djajadi, and Sune Tjalfe Thomsen. 2025. "Modified Hydrothermal Pretreatment Conditions Enhance Alcohol Solubility of Lignin from Wheat Straw Biorefining" Biomass 5, no. 2: 23. https://doi.org/10.3390/biomass5020023

APA Style

Simonsen, T. I., Djajadi, D. T., & Thomsen, S. T. (2025). Modified Hydrothermal Pretreatment Conditions Enhance Alcohol Solubility of Lignin from Wheat Straw Biorefining. Biomass, 5(2), 23. https://doi.org/10.3390/biomass5020023

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