The Nature of Lignin and Implications for Its Technical Use as a Source for Biogenic Aromatics—A Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The review article is written on very basic level. Not many figures and tables were included, but many bulletpoints which is not appropriate for the review article and general scientific literature. I think that many possibilities exist how to improve the manuscript:

(1) Explain the novelty of your work compared to all previous work and review articles on lignin.

(2) How did you select the literature? Did you use the design of experiment and statistics? The literature for the review is typically collected withing the last 5 years prior the review. You should clearly explain it and show the diagrams or graphs.

(3) Explain the keywords for this review article and how did you select this specific topic? It does not make any sense to write a general review because there are so many review articles already collected. Can you select one topic that was not reviewed before? It could be lignin in biofermentation or purification procedures of lignin, etc.

(4) The chapter on chemical structure of lignocellulose is useless because there are books about it which describe it even better.

(5) The biodegradation nature of lignin is not well written. You should rather finish on positive note and also provide future scenarios how lignin will be used in the future. The key learnings of this review are also poor because they are described like in technical report. What are the main challenges and which solutions can you offer? 

(6) The key idea is missing in the review. You focus too much on general things and don't select one specific topic that was not investigated in other review articles. Firstly get this idea and describe in the introduction and then completely restructure your current review.

(7) Conclusion is also weak and does not show what is the value to readers by reading this article. Are there digital technologies which could help you to find solutions? Maybe some digital prediction could reduce the chemical challenges with lignin. Is there any key idea that could help to bring technical lignin on the market? Can you draw this idea as a part of the discussion?

Author Response

Thank you very much for taking the time to review and comment on our manuscript – we know it is a long text. Please find our detailed responses below. The corresponding revisions are highlighted in red in the re-submitted files.

 

Comments 1: The review article is written on very basic level. Not many figures and tables were included, but many bulletpoints which is not appropriate for the review article and general scientific literature. I think that many possibilities exist how to improve the manuscript:

Response 1: We used illustrations and tables where we considered them important and tried not to lengthen the text with unnecessary illustrations/tables. If the reviewer has a specific suggestion as to where an illustration/table would help improving the understanding, we will be happy to modify that. We do not understand why bullet points shouldn't be suitable for a review article. We use them as is customary to highlight or list things and consider them a very important tool for structuring.

 

Comments 2: (1) Explain the novelty of your work compared to all previous work and review articles on lignin.

Response 2: The following sentence has been added on page 2, l. 77. “The key novelty of this review article is its technical perspective on the updated state of knowledge about the nature of lignin in order to derive consequences for future process developments for the depolymerization of lignin from native lignocellulose.” To our knowledge, this has not been done before. If you are aware of similar articles, we would be pleased to learn about them.

 

Comments 3: (2) How did you select the literature? Did you use the design of experiment and statistics? The literature for the review is typically collected withing the last 5 years prior the review. You should clearly explain it and show the diagrams or graphs.

Response 3: We disagree with the reviewer on this point. There are reviews that provide an update on recent developments, where it makes sense to limit oneself to literature from the last five years. However, this review deals with the entire body of knowledge on a very comprehensive topic and examines it from a new perspective. Limiting the review to literature from the last five years would result in a significant loss of knowledge. Through comprehensive literature research on the identified, specific subtopics (sections) without restriction regarding the year of publication, a large number of sources were found and processed, and the information deemed relevant to the question/topic was selected.

 

Comments 4: (3) Explain the keywords for this review article and how did you select this specific topic?  Can you select one topic that was not reviewed before? It could be lignin in biofermentation or purification procedures of lignin, etc..

Response 4: This review article deals with “biomass”, reviews the “properties of native lignin” and derives the consequences for the “technical valorization of lignocellulose” with the goal of “lignin depolymerization”. Especially interesting in this regard are processes that try to avoid lignin condensation, typically designated as “lignin-first processes”. The keywords have been adjusted accordingly. We hope that this clarifies the idea of this review and convinces the reviewer that this is not just a “general review”.

 

Comments 5: (4) The chapter on chemical structure of lignocellulose is useless because there are books about it which describe it even better.

Response 5: The chapter provides a short summary of the topic and an update about recent developments. Especially, it is required as a basis for the technical perspective, which is the key element of this review (see novelty described in comment 2). We consider this chapter important and regret the reviewer's subjective assessment that it would be “useless.

 

Comments 6: (5) The biodegradation nature of lignin is not well written. You should rather finish on positive note and also provide future scenarios how lignin will be used in the future. The key learnings of this review are also poor because they are described like in technical report. What are the main challenges and which solutions can you offer?

Response 6: The whole section 6 is about the main challenges and solutions regarding the depolymerization of native lignin. The section about biodegradation is completely objective about the state of knowledge. Only when it comes to the question what conclusions can be drawn from this, it is certainly subjective where to focus one's attention. We summarize our conclusions under “Key findings and implications for technical lignin valorization by depolymerization” and our focus is on the technical implementation of depolymerization. Based on the objective facts, we conclude that there are major challenges regarding the technical implementation of purely biological lignin depolymerization to recover lignin monomeres. We are interested in hearing your reasons if you come to a different conclusion.

 

Comments 7: (6) The key idea is missing in the review. You focus too much on general things and don't select one specific topic that was not investigated in other review articles. Firstly get this idea and describe in the introduction and then completely restructure your current review.

Response 7: We hope that the key idea has become clearer to the reviewer now, after the previous responses and modifications.

 

Comments 8: (7) Conclusion is also weak and does not show what is the value to readers by reading this article. Are there digital technologies which could help you to find solutions? Maybe some digital prediction could reduce the chemical challenges with lignin. Is there any key idea that could help to bring technical lignin on the market? Can you draw this idea as a part of the discussion?

Response 8: As stated in the introduction of the article (p.2 , l. 80) , the topic of using technical lignin is excluded from this review. The article is about technical consequences with regard to process engineering, it is not about digital technologies, which would be another interesting topic for another article. The conclusion addresses the main challenges of the technical use and depolymerization of native lignin and summarizes the most important approaches and criteria for overcoming them. This is followed by a comparison with current process developments. We find it unfortunate that the reviewer considers this to be of no value. What topic-related points are missing to make the conclusion more valuable?

Reviewer 2 Report

Comments and Suggestions for Authors

The review article is a rather thorough and well-presented study concerning the native lignin structure and its implications for lignocellulosic biomass treatment. There are many other publications on this topic – but the rest of them do not speak the language of applied chemists. I significantly benefited from reading this manuscript. For the first time, I now have developed a clear understanding of how lignin is formed and why it has certain structural features. The manuscript provides convincing evidence that the “lignin-first” strategy is logical and most appropriate for biorefineries. Nonetheless, physical or mild physico-chemical methods rather than enzymatic treatments are key in extracting lignin, because the monolignol polymerization is a chemical, rather than biochemical, process. All of these conclusions in the manuscript are thoroughly supported by experimental and computational evidence.

The other pertinent topics, e.g., lignin interactions with carbohydrates in cell walls, its thermal degradation under varied conditions, re-polymerization during lignin extraction, and lignin solubility in various solvents and their combinations, are also covered thoroughly yet concisely. Furthermore, various topics are linked with each other, thus providing a comprehensive view. The short summaries provide by the authors are very helpful, too. The current state of development of the lignin-first treatments is also reviewed. Practitioners, i.e., applied chemists and chemical engineers, can readily use this paper for guidance in their work.

As to me, the only remaining work to be addressed by the authors is making their technical presentation more compatible with the norms of English language. Their English is very good – but there are some minor problems that make reading this manuscript a bit challenging. There are certain spots in the manuscript where the verbiage used is confusing and even ambiguous from a reader’s standpoint. This feature may reduce the impact of this paper if it is published as is or with some minimum adjustments. Let me use just the very first few sentences as an example.

The last clause in the first sentence of the Abstract is poorly connected with the rest of the sentence

In the first sentence of Introduction, the article (presumably, “a”) is missing in front of “decisive.”

In the second sentence of Introduction “could have provided” is rather ambiguous – it could be interpreted as if it could – but it did not. I suggest changing this part to “appears to provide” or something like that.

In the third sentence, remove dashes after “water” and “nutrients.”

The following sentence (“However, lignin cannot be regarded isolated”) makes no sense in English as written.

The next sentence (“It is part of the highly cross-linked composite material lignocellulose, which it is forming together with mainly two further carbohydrate polymers – cellulose (composed of glucose units) and hemicellulose (mainly composed of C5 sugars) [5]”) also needs attention, because a comma after “lignocellulose” is warranted and the use of Present Continuous verb tense is incorrect in the second clause – just say, in simple Present Tense, that it is formed as a composite of lignin with carbohydrates.

The word “learnings” in the headings for summaries used by the authors (“Learnings and implications for…”) is misleading and should be replaced.

I recommend the authors showing the manuscript to a native English speaker, asking him/her    to provide an honest critique combined with specific editing suggestions. In addition, the authors should break several long paragraphs into those of manageable size, avoid using contractions (e.g., “don’t”) and check the consistence of verb tenses throughout the manuscript.

Comments on the Quality of English Language

As to me, the only remaining work to be addressed by the authors is making their technical presentation more compatible with the norms of English language. Their English is very good – but there are some minor problems that make reading this manuscript a bit challenging. There are certain spots in the manuscript where the verbiage used is confusing and even ambiguous from a reader’s standpoint. This feature may reduce the impact of this paper if it is published as is or with some minimum adjustments. Let me use just the very first few sentences as an example.

The last clause in the first sentence of the Abstract is poorly connected with the rest of the sentence

In the first sentence of Introduction, the article (presumably, “a”) is missing in front of “decisive.”

In the second sentence of Introduction “could have provided” is rather ambiguous – it could be interpreted as if it could – but it did not. I suggest changing this part to “appears to provide” or something like that.

In the third sentence, remove dashes after “water” and “nutrients.”

The following sentence (“However, lignin cannot be regarded isolated”) makes no sense in English as written.

The next sentence (“It is part of the highly cross-linked composite material lignocellulose, which it is forming together with mainly two further carbohydrate polymers – cellulose (composed of glucose units) and hemicellulose (mainly composed of C5 sugars) [5]”) also needs attention, because a comma after “lignocellulose” is warranted and the use of Present Continuous verb tense is incorrect in the second clause – just say, in simple Present Tense, that it is formed as a composite of lignin with carbohydrates.

The word “learnings” in the headings for summaries used by the authors (“Learnings and implications for…”) is misleading and should be replaced.

I recommend the authors showing the manuscript to a native English speaker, asking him/her    to provide an honest critique combined with specific editing suggestions. In addition, the authors should break several long paragraphs into those of manageable size, avoid using contractions (e.g., “don’t”) and check the consistence of verb tenses throughout the manuscript.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors tried to integrate the comments in the first round. 

(1) The conclusion was not impoved. I assume that good and excellent review articles have 7-10 sentences as a conclusion. I suggest that you move the text and table into Discussion section or make a separated section from Conclusion that would come before it. It looks very heavy and distruptive with all italic questions. In the conclusion and recommendation you typically write the information in a concrete form without further questions.

(2) Can you also prepare a drawing of the main outcome of your article? 

(3) Can you explain prior biodegradation section why it is important to investigate in the current article?

(4) The quality of Figures 2 and 3 should be significantly improved. It looks with the broken lines time to time.

Author Response

Thank you again for taking the effort to review our manuscript and especially for responding so quickly. Our detailed responses are below and the corresponding revisions from Round 2 are highlighted in blue in the re-submitted files.

 

Comments 1: The conclusion was not impoved. I assume that good and excellent review articles have 7-10 sentences as a conclusion. I suggest that you move the text and table into Discussion section or make a separated section from Conclusion that would come before it. It looks very heavy and distruptive with all italic questions. In the conclusion and recommendation you typically write the information in a concrete form without further questions.

Response 1: We agree. In accordance with your suggestion, we have divided the last section into a discussion of the technical implications (section 6) and a brief conclusion (section 7), see pages 29 to 34.

 

Comments 2: Can you also prepare a drawing of the main outcome of your article?

Response 2: We believe that the derived implications, which are the main outcome, are best presented in concise text form, as is now the case in the new conclusion. At least, we cannot think of any way to present this graphically without the graphic mainly containing text. We hope you no longer feel the need for the graphic now that the concise conclusion is available, otherwise we are open to suggestions for graphical representation.

 

Comments 3: Can you explain prior biodegradation section why it is important to investigate in the current article?

Response 3: Yes, to make this clearer, we added the following sentence on page 26, l. 1046: “In the following, the biodegradation of lignocellulose will be reviewed in more detail to examine which lessons can be learned from nature and which implications for the technical degradation (with the aim of producing aromatic monomers) can be derived.”

 

Comments 4: The quality of Figures 2 and 3 should be significantly improved. It looks with the broken lines time to time.

Response 4: We have carefully reviewed both figures again and were unable to understand exactly what you are referring to and where there is a problem with broken lines. In Figure 2, we made the circle around the tetrahydrofuran structure slightly larger—perhaps you were bothered by the fact that the circle went through the structure? Otherwise, we need more specific instructions in order to make any changes.

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

All comments were implemented, but English could be significantly improved. I also recommend not to use the bullet points because it makes your article to look like a technical report.

Author Response

Thank you once again for your efforts. We are happy that we were able to address all comments appropriately. We had the entire text reviewed by an English teacher and hope that most linguistic ambiguities have now been resolved; according changes are marked in red. As discussed in round 1, we consider bullet points a very important tool for structuring and in this review, in particular, to highlight the derived implications. We removed the bullet points for listing the bond types on page 6 and for listing the main criteria on page 32, so that now, as a clear structure, only the derived implications are highlighted with bullet points