Review Reports - Comprehensive Review of Recent Trends in the Use of Deep Eutectic Solvents for the Valorization of Secondary Lignocellulosic Biomass

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript is well-written, and I suggest acceptance after minor revisions. The review on DES is quite interesting. Please find my detailed comments and suggested revisions below:

Lines 39–40: This line repeats the content in Lines 32–33.
Lines 39–52: The text largely repeats previously mentioned information. Please revise this section to remove redundancy and include some new information.
Lines 95–96: Please add more references (excluding review articles) for each type of pretreatment discussed.
Line 117: Please provide a reference to support the claim of lower energy usage.
Line 164: As this is a review paper, Table 2 contains relatively few references. Please try to add 2–3 more studies. Additionally, for each study, indicate the optimized conditions in bold.
Line 193 / Section 5: Please create a hypothesized figure illustrating how conventional lignocellulosic biomass reacts. For example, start with the production of compounds such as phenolics, categorize each type of phenolic, followed by dissolved lignin (please organize the order accordingly). In comparison, prepare a figure showing how these compounds are produced or react when DESs are used.
- In this section, you mainly focused on phenolic compounds. However, as you know, dissolved/soluble lignin is also a major inhibitory factor. Even if previous studies are limited, please include more discussion on this aspect.
Figure 1: Please improve the overall quality and clarity

Author Response

The manuscript is well-written, and I suggest acceptance after minor revisions. The review on DES is quite interesting. Please find my detailed comments and suggested revisions below:

Thank you for your objective feedback, your comments helped us to improve the manuscript.

Lines 39–40: This line repeats the content in Lines 32–33.

The repeated lines 32-33 were removed.

Lines 39–52: The text largely repeats previously mentioned information. Please revise this section to remove redundancy and include some new information.

We agree, we found the redundancy also after your remark. The section was revised and some new information included.

Lines 95–96: Please add more references (excluding review articles) for each type of pretreatment discussed.

More references were added and review articles were excluded.

Line 117: Please provide a reference to support the claim of lower energy usage.

A reference was provided to support the claim.

Line 164: As this is a review paper, Table 2 contains relatively few references. Please try to add 2–3 more studies. Additionally, for each study, indicate the optimized conditions in bold.

Thank you once again for your valuable feedback. Additional studies have been incorporated into the table accordingly, and the optimized conditions are highlighted in bold.

Line 193 / Section 5: Please create a hypothesized figure illustrating how conventional lignocellulosic biomass reacts. For example, start with the production of compounds such as phenolics, categorize each type of phenolic, followed by dissolved lignin (please organize the order accordingly). In comparison, prepare a figure showing how these compounds are produced or react when DESs are used.

The hypothesized figure (Figure 1) was created and included in the manuscript

In this section, you mainly focused on phenolic compounds. However, as you know, dissolved/soluble lignin is also a major inhibitory factor. Even if previous studies are limited, please include more discussion on this aspect.

Thank you for your valuable comment. We agree that soluble lignin and its derivatives can act as significant inhibitory factors. However, in Section 5, lignin is discussed as a target compound for extraction and subsequent valorization rather than as a source of inhibitors. At the same time, to reflect the importance of this aspect, a paragraph has been added to Section 4 (Pretreatment) discussing the possible formation of soluble lignin fragments during delignification with DES and their potential inhibitory effects on subsequent enzymatic and microbial processes.

Figure 1: Please improve the overall quality and clarity

The figure has also been improved for better quality and clarity, with enhanced resolution.

As a new figure has been added following your suggestion above, the original Figure 1 has been renumbered as Figure 2 in the revised manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

The work is a review on the use of deep eutectic solvents for biomass valorization. The manuscript must be improved in order to be published.

The review is quite limited, failing to go into detail about the mechanisms by which the solvents act. It merely reports the results of other authors without adding anything of value to the discussion.

The wording of the entire document needs to be improved.

In the introduction, what is mentioned in lines 39-45 was already mentioned in the previous paragraph. Please avoid being redundant.

Table 2 should be on a horizontal page to improve its visualization. The same goes for Table 3.

Please fix the reference issue that occurs on line 258, which says "[NO_PRINTED_FORM]"

Other comments

On line 92, please add the period after reference 15.

On line 99, please add the period after reference 18.

On line 154, please remove the extra bracket at the end of the line.

On line 171, please add a comma after "lignin".

Author Response

The work is a review on the use of deep eutectic solvents for biomass valorization. The manuscript must be improved in order to be published.

Thank you for your valuable feedback. We have revised and improved the manuscript to enhance its clarity and content. The updated version includes expanded discussion, improved tables and figures, and additional information on scalability, economic aspects, and environmental impact of DES applications.

We appreciate your observation. In the revised version, we have added a detailed section that discusses the mechanisms by which solvents, especially deep eutectic solvents (DES), act during biomass processing.

The wording of the entire document needs to be improved.

We appreciate your concern regarding the wording of the manuscript. To address this, we have carefully revised the entire text to improve clarity, readability, and academic style.

In the introduction, what is mentioned in lines 39-45 was already mentioned in the previous paragraph. Please avoid being redundant.

The redundancy in lines 39–45 has been addressed, and the introduction was revised to remove repetition and improve clarity.

Table 2 should be on a horizontal page to improve its visualization. The same goes for Table 3.

Thank you for your observation. We would like to kindly note that the tables were organized by the editorial office. In our original submission, they were formatted on a horizontal (landscape) page.

Please fix the reference issue that occurs on line 258, which says "[NO_PRINTED_FORM]"

We have corrected the reference formatting problem on line 258 to ensure it now displays properly in the text.

Other comments

On line 92, please add the period after reference 15.

Thank you for pointing this out. The punctuation in that sentence has been corrected.

On line 99, please add the period after reference 18.

Thank you for your attention to detail. The formatting has been revised, and the sentence now ends correctly with a period.

On line 154, please remove the extra bracket at the end of the line.

Thank you for noting this. The extra bracket has been removed

On line 171, please add a comma after "lignin".

Thank you for highlighting this. A comma has been added after “lignin” to improve readability.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript "Comprehensive review of recent trends in the use of deep eutectic solvents for the valorization of secondary lignocellulosic biomass" addresses an interesting topic. However, in its current form, it can only be considered for publication after major review. In a subsequent submission, the authors could address the following aspects:

Introduction

Line 36–37: Include plant proteins as value-added products.

There is repetition of information between lines 32–38 and 39–44 (duplicate text):

Line 32–38: It is expected that global demand for biofuel will increase by 28 % in the upcoming years [2]. Lignocellulosic biomass has been considered as a sustainable alternative to the depleting fossil fuels to meet the increased demand for energy. Besides, the valorization of lignocellulosic biomass can give a wide range of other value-added products. The potential economic benefits of valorizing secondary lignocellulosic biomass, such as biofuels, bioplastics, fertilizers, phenolic compounds, flavonoids, pep- tides, polyphenolic antioxidants [1].

Line 39–44: It is expected that global demand for biofuel will increase by 28 % in the upcoming years [2]. Lignocellulosic biomass has been considered as a sustainable alternative to the depleting fossil fuels to meet the increased demand for energy. Besides, the valorization of lignocellulosic biomass can give a wide range of other value-added products. The potential economic benefits of valorizing secondary lignocellulosic biomass, such as biofuels, bioplastics, fertilizers, phenolic compounds, flavonoids, peptides, polyphenolic antioxidants, chitin, furfural, etc.

Section “Current DES Types, Composition, and Characteristics”

The text is very brief (only 1 paragraph), limited to explaining the type and interaction between HBD and HBA. To strengthen the comprehensive review, it would be important to expand the content, covering the composition and characteristics of the different DES, as well as examples of the most relevant physicochemical properties and typical combinations.

Section “Conventional Solvents vs DES in the Biomass Valorization and Biorefinery”

The contrast presented is basic and does not offer the critical or in-depth analysis expected of a comprehensive review. It is recommended to explore the challenges associated with the use of conventional solvents in more detail. Costs and environmental impacts have already been mentioned, but it would be useful to include issues such as: formation of fermentation inhibitors (acetic acid, furfural, HMF) in acid pretreatments; corrosivity and reduced reactor lifespan; recovery difficulties in solvents such as organosolvents and ionic liquids; and safety aspects (flammability, toxicity). At the same time, a more balanced discussion of the advantages and, especially, limitations of DES is suggested, as these have been little explored in the current text.

Tables 2 and 3: The tables could be improved, as they currently include only 8–9 studies.

Section “Applications of DES in the Extraction Process”

The content is too superficial for a comprehensive review article. The section merely introduces the problems of conventional solvents, mentions the application of DES in the extraction of various compounds, and highlights phenolics as the most studied. However, there is no critical analysis or contextualization of the table presented.

Seçtion “Challenges and Future Directions”

While it addresses relevant points, the discussion is overly general and limited. It is suggested that the content be expanded to make this section more robust and aligned with the character of a comprehensive review. Additional aspects that could be included are: scalability and economic viability (considering the real cost of DES constituents), life cycle analyses (LCA), and techno-economic analysis in comparisons with traditional solvents on a pilot/industrial scale.

Author Response

The manuscript "Comprehensive review of recent trends in the use of deep eutectic solvents for the valorization of secondary lignocellulosic biomass" addresses an interesting topic. However, in its current form, it can only be considered for publication after major review. In a subsequent submission, the authors could address the following aspects:

We sincerely thank you for the thoughtful evaluation and constructive feedback. We have carefully revised the manuscript in accordance with all the suggested aspects, making substantial improvements in content, structure, and clarity.

Introduction

Line 36–37: Include plant proteins as value-added products.

Thank you for the suggestion. Plant proteins have been added to the list of value-added products obtained from the valorization of secondary lignocellulosic biomass.

There is repetition of information between lines 32–38 and 39–44 (duplicate text):

Thank you for pointing this out. The repeated information between lines 32–38 and 39–44 has been carefully reviewed and removed to avoid redundancy. The section has been revised for smoother flow and clarity.

Line 32–38: It is expected that global demand for biofuel will increase by 28 % in the upcoming years [2]. Lignocellulosic biomass has been considered as a sustainable alternative to the depleting fossil fuels to meet the increased demand for energy. Besides, the valorization of lignocellulosic biomass can give a wide range of other value-added products. The potential economic benefits of valorizing secondary lignocellulosic biomass, such as biofuels, bioplastics, fertilizers, phenolic compounds, flavonoids, pep- tides, polyphenolic antioxidants [1].

Line 39–44: It is expected that global demand for biofuel will increase by 28 % in the upcoming years [2]. Lignocellulosic biomass has been considered as a sustainable alternative to the depleting fossil fuels to meet the increased demand for energy. Besides, the valorization of lignocellulosic biomass can give a wide range of other value-added products. The potential economic benefits of valorizing secondary lignocellulosic biomass, such as biofuels, bioplastics, fertilizers, phenolic compounds, flavonoids, peptides, polyphenolic antioxidants, chitin, furfural, etc.

Section “Current DES Types, Composition, and Characteristics”

Thank you for this valuable comment. We have expanded Section 2 to provide a more comprehensive overview of the composition and physicochemical characteristics of various DES.

Section “Conventional Solvents vs DES in the Biomass Valorization and Biorefinery”

Thank you for this valuable suggestion. In the revised manuscript, we have expanded the discussion of conventional solvent-based pretreatments. At the same time, we have also provided a more balanced evaluation of deep eutectic solvents (DES).

Tables 2 and 3: The tables could be improved, as they currently include only 8–9 studies.

Both tables have been expanded and updated. Additional relevant studies were incorporated to provide a more comprehensive overview of the literature and to enhance the depth of comparison across different biomass types, DES compositions.

Section “Applications of DES in the Extraction Process”

We appreciate the reviewer’s valuable observation. The section has been substantially revised to include a deeper critical analysis of the studies summarized in Table 3.

Seçtion “Challenges and Future Directions”

We appreciate your comment. The “Challenges and Future Directions” section has been expanded to include discussion on scalability, economic feasibility, and solvent recovery, as well as insights from recent techno-economic and life cycle assessments comparing DES with conventional solvents.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors presented

The authors presented a review of the recent use of deep eutectic solvents to revalorize biomass. After revising I found significant novelty in the document. However, the manuscript needs to improve in some areas before is suitable for publication

General observations

Please revise the manuscript to correct doble spaces, misplace of signs, absence of point at the end of sentences, references format, and publishing guidelines in general.

Please review one more time the English, there are some phrases that can be improve.

I suggest the authors to review, detect and correct information that appears doble alongside the manuscript, I put an example in one of the observations.

Please indicate the time use to elaborate the review, from which year and month until when. In that way the reader can visualize how recent the review is.

Observations

Line 32 and 39, The idea presented here is duplicated, please revise.

Line 47, Does the authors share a small definition on what they consider secondary biomass? If not, please elaborate.

Line 58 and 143, The phrase green solvents appear under “ “ after is being first declare, please correct alongside the manuscript.

Lines 153-154. “For example, without DES pretreatment, the sugar recovery was estimated at around 20 %, according to Singhvi, Chaudhari, and Gokhale” Can the authors elaborate more on this explanation, what type of biomass? Compared to what type of conventional solvents?

Line 258, Please check the phrase under [].

In the section Challenges and future directions, the authors don’t consider the cost of DES compared to conventional solvents a challenge? Please elaborate

Author Response

We sincerely thank you for recognizing the novelty and relevance of our work. We have carefully revised and improved the manuscript to address all the suggested points and enhance its overall quality. The updated version includes an expanded critical discussion, improved figures and tables, and additional analysis of the scalability, economic feasibility, and environmental impact of DES-based biomass valorization. We believe these revisions have made the manuscript suitable for publication.

General observations

Please revise the manuscript to correct doble spaces, misplace of signs, absence of point at the end of sentences, references format, and publishing guidelines in general.

We appreciate your careful observation. The manuscript has been thoroughly revised and formatted according to the journal’s publishing guidelines. We have corrected double spaces, punctuation errors, missing periods, misplaced symbols, and standardized the reference style to ensure consistency and compliance with the required format.

Please review one more time the English, there are some phrases that can be improve.

Thank you for your helpful remark. We have carefully rechecked and refined the English throughout the manuscript to improve clarity, grammar, and readability. The text has been edited for smoother phrasing and academic tone to ensure high linguistic quality.

I suggest the authors to review, detect and correct information that appears doble alongside the manuscript, I put an example in one of the observations.

Thank you for pointing this out. We have carefully reviewed the entire manuscript to identify and remove duplicated or repetitive information. The revised version now presents the content more clearly and concisely, avoiding unnecessary repetition.

Please indicate the time use to elaborate the review, from which year and month until when. In that way the reader can visualize how recent the review is.

Thank you for the suggestion. We have added a statement specifying the review window and preparation period at the end of the Introduction section.

Observations

Line 32 and 39, The idea presented here is duplicated, please revise.

We agree, we found the redundancy also after your remark. The section was revised and some new information included.

Line 47, Does the authors share a small definition on what they consider secondary biomass? If not, please elaborate.

Thank you. Following your remark, a definition has been added in lines 36–37 of the revised manuscript.

Line 58 and 143, The phrase green solvents appear under “ “ after is being first declared, please correct alongside the manuscript.

Thank you for your observation. The unnecessary quotation marks around the phrase green solvents have been removed throughout the manuscript. The term now appears consistently without quotation marks after its first introduction.

Thank you for the comment. The statement has been elaborated in the revised manuscript for clarity.

Line 258, Please check the phrase under [].

Thank you for the observation. The brackets [ ] have been removed in the revised manuscript.

In the section Challenges and future directions, the authors don’t consider the cost of DES compared to conventional solvents a challenge? Please elaborate

Thank you for the comment. This point has been addressed in the revised version.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The changes were made to the manuscript and it is now ready for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have revised the manuscript and significantly improved its quality. Therefore, the revised version is ready for acceptance.

Reviewer 4 Report

Comments and Suggestions for Authors

I can now recommend this manuscript for publication. The authors have addressed all the point in the previous version