Applications of Brewer’s Spent Grain Hemicelluloses in Biorefineries: Extraction and Value-Added Product Obtention

Round 1

Reviewer 1 Report

The manuscript-2283834 is associated with an extremely topical area of waste biotechnology. However, in this form it contains serious flaws, due to which it cannot be published in catalysts.

Main reasons

1.      The review is structured in such a way that the role of catalysts is poorly represented. This does not allow recommending for publication in this particular journal.

2.      The title of the review mentions the extraction of hemicelluloses and the production of products based on them, and this is a feature that the authors pay attention to in the processing of BSG. But in the abstract and conclusions, they limited themselves to mentioning these polysaccharides and listing the products. From which the purpose of the entire review becomes questionable.

3.      The review does not provide information on how the literature was searched. In addition, some sections do not explain why a particular literature reference was used. For example, in Table 1, 3 examples of BSG are used. Why?

4.      There are significant inconsistencies in the work. For example, in the title of Table 1, the first method for extracting hemicelluloses indicates treatment with dilute acid at high temperature. But as a product, not hemicelluloses, and not even hydrolysis products (as indicated in the description of the table), but bioethanol.

5.      Lack of unity of review. Some drawings or diagrams do not refer specifically to the object under study, but to lignocellulosic materials in general. Is it really necessary in a review about a certain by-product of breweries? It is necessary to take into account the differences in the spent grain from, for example, plant stems.

Authors are encouraged to fundamentally change the review, taking into account these comments, and try to publish it in another journal, for example, in Polysaccharides.

Author Response

Reviewer 1

Comments and Suggestions for Authors:

The manuscript-2283834 is associated with an extremely topical area of waste biotechnology. However, in this form it contains serious flaws, due to which it cannot be published in catalysts.

Response (R): The authors thank you for all the considerations.  Following the comments made by the Reviewers, we have improved the manuscript.

Main reasons

1. The review is structured in such a way that the role of catalysts is poorly represented. This does not allow recommending for publication in this particular journal.

R: We sincerely thank you for the comments, but after making significant modifications to the manuscript, we believe it is adequate for publication in Catalysts.

2. The title of the review mentions the extraction of hemicelluloses and the production of products based on them, and this is a feature that the authors pay attention to in the processing of BSG. But in the abstract and conclusions, they limited themselves to mentioning these polysaccharides and listing the products. From which the purpose of the entire review becomes questionable.

R: We thank you for the comment and have modified the abstract and conclusion sections to meet the reviewer's expectations.

3. The review does not provide information on how the literature was searched. In addition, some sections do not explain why a particular literature reference was used. For example, in Table 1, 3 examples of BSG are used. Why?

R: Information on how the literature search was carried out was added to the manuscript. The purpose of showing more than one reference about BSG in Table 1 was to corroborate the affirmation that this biomass has a quite variable composition in terms of its lignocellulosic structure.

4. There are significant inconsistencies in the work. For example, in the title of Table 1, the first method for extracting hemicelluloses indicates treatment with dilute acid at high temperature. But as a product, not hemicelluloses, and not even hydrolysis products (as indicated in the description of the Table), but bioethanol.

R: In Table 2, the "Final product obtained" column refers to the final product obtained from hemicelluloses in the paper, not just after hydrolysis. In this case, after solubilizing the hemicellulosic portion, a fermentation process was carried to obtain bioethanol.

5. Lack of unity of review. Some drawings or diagrams do not refer specifically to the object under study, but to lignocellulosic materials in general. Is it really necessary in a review about a certain by-product of breweries? It is necessary to take into account the differences in the spent grain from, for example, plant stems.

R: We thank you for this comment. Although the drawing does not refer specifically to BSG, the same processes can be applied, meaning that the scheme can also be used to refer to the processing of this biomass. Despite this, it seems reasonable to modify the figures to refer to the biomass used in this review directly.

Authors are encouraged to fundamentally change the review, taking into account these comments, and try to publish it in another journal, for example, in Polysaccharides.

R: Thanks for the contributions, but again, we believe this manuscript fits into the Catalysts' scope after some significant modification.

Author Response File: Author Response.pdf

Reviewer 2 Report

This review aims to summarize the main products obtained from brewer's spent grain (BSG), specifically from the hemicellulosic fraction, including the main strategies and technologies for extracting and benefiting this portion.

The review presents a comprehensive current status of the availability, composition, and cost of BSG and its possible derivatives. This  information  may be of interest to the reader of the Catalysts journal.

The review addresses a topic of high interest, it is current, and it is very well written.  However, the Catalsyts' reader might expect to obtain more information about the catalytic processes for obtaining and valorizing sugars derived from hemicellulose.

This reviewer suggests a major revision of it and adding data related to the numerous contributions of catalysts and, especially heterogeneous catalysts, in the topic.

Below I show some points that can be improved:

1- Please, add comments and references related to the use of heterogeneous catalysts in biomass pretreatment.

2- Line 166 "The main disadvantage of this pretreatment method is that it leads to the formation of toxic byproducts, such as 5-hydroxymethyl-furfural (HMF), furfural, and acetic acid (Figure 1), mainly related to the high process temperatures, that may interfere if the next step involves the use of microorganisms that do not tolerate the presence of these substances"... Both figure 1 and this comment present HMF, FUR or acetic acid, among others, as inhibitors. However, in the field of catalysis, these molecules are valious,  and some are considered chemical building blocks for biorefinery.

 

3- Table 2 shows some examples of different processes for the pretreatment of BSG. Which of them can be assisted by catalysts? It is suggested to broaden the discussion and include comments related to pyrolysis and catalytic torrefaction.

 

4- There are numerous bibliographical references, including some bibliographical reviews, related to catalytic processes for the recovery of sugars and their derivatives from the hydrolysis of hemicellulose. In this review, many of them were not taken into account. It is strongly suggested to expand the discussion of sections 3.2.1, 3.2.2., 3.2.3, and 3.2.6 since the catalytic systems that facilitate obtaining the detailed products have not been shown.

Author Response

Reviewer 2

Comments and Suggestions for Authors:

This review aims to summarize the main products obtained from brewer's spent grain (BSG), specifically from the hemicellulosic fraction, including the main strategies and technologies for extracting and benefiting this portion.

The review presents a comprehensive current status of the availability, composition, and cost of BSG and its possible derivatives. This  information  may be of interest to the reader of the Catalysts journal.

The review addresses a topic of high interest, it is current, and it is very well written.  However, the Catalysts' reader might expect to obtain more information about the catalytic processes for obtaining and valorizing sugars derived from hemicellulose.

This reviewer suggests a major revision of it and adding data related to the numerous contributions of catalysts and, especially heterogeneous catalysts, in the topic.

R: The authors thank you for the suggestions and have provided some essential modifications to attend to them.

Below I show some points that can be improved:

1- Please, add comments and references related to the use of heterogeneous catalysts in biomass pretreatment.

R: We thank you for the suggestions and added some comments and references related to using heterogeneous catalysts in biomass pretreatment.

2- Line 166 "The main disadvantage of this pretreatment method is that it leads to the formation of toxic byproducts, such as 5-hydroxymethyl-furfural (HMF), furfural, and acetic acid (Figure 1), mainly related to the high process temperatures, that may interfere if the next step involves the use of microorganisms that do not tolerate the presence of these substances"... Both figure 1 and this comment present HMF, FUR or acetic acid, among others, as inhibitors. However, in the field of catalysis, these molecules are valious, and some are considered chemical building blocks for biorefinery.

R: The authors thank you for this comment, but, in this particular case, the aim was to show these molecules as potential inhibitors of the fermentation process. Despite this, we agree that some clarification was lacking regarding the other roles they may play in a biorefinery. Understanding this, we have added some comments emphasizing the various uses of these molecules.

3- Table 2 shows some examples of different processes for the pretreatment of BSG. Which of them can be assisted by catalysts? It is suggested to broaden the discussion and include comments related to pyrolysis and catalytic torrefaction.

R: The authors thank you for the suggestion, but as far as we know, the use of these two techniques interferes with the recovery of the hemicellulosic portion present in the biomass after processing, and this is one of the main points of this review article. Despite that, we thank you for mentioning these methods that were, so far, little known to us.

4- There are numerous bibliographical references, including some bibliographical reviews, related to catalytic processes for the recovery of sugars and their derivatives from the hydrolysis of hemicellulose. In this review, many of them were not taken into account. It is strongly suggested to expand the discussion of sections 3.2.1, 3.2.2., 3.2.3, and 3.2.6 since the catalytic systems that facilitate obtaining the detailed products have not been shown.

R: The authors thank you for the suggestion; the discussion was expanded to include more information about the use of catalytic systems in some of those processes.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors deal with an interesting work like the review of brewer's spent grain hemicelluloses in biorefineries. The article is well-structured and the work that has been carried out is thorough. I do not have many concerns about this work, but I think that the authors could improve the cuality of this work with this piece of advice:

- Introduction section should have some "review parameters" like scopus trends (articles published per year about this subject, the main journals or scientific fields dealing with this subject). 

- Line 78. You could add an interesting article about drying process, like the following: 

https://doi.org/10.3390/proceedings2231467

- There are some typos or grammar mistakes, like in line 286, where "2" in CO2 should be subscript.

 

Author Response

Reviewer 3

Comments and Suggestions for Authors:

The authors deal with an interesting work like the review of brewer's spent grain hemicelluloses in biorefineries. The article is well-structured and the work that has been carried out is thorough. I do not have many concerns about this work, but I think that the authors could improve the quality of this work with this piece of advice:

- Introduction section should have some "review parameters" like scopus trends (articles published per year about this subject, the main journals or scientific fields dealing with this subject).

R: We thank you for this suggestion and have included some review parameters in the introduction.

- Line 78. You could add an interesting article about drying process, like the following: https://doi.org/10.3390/proceedings2231467

R: The authors thank you for the article suggestion and have added it to the text.

- There are some typos or grammar mistakes, like in line 286, where "2" in CO2 should be subscript.

R: We sincerely thank you for noticing this, the manuscript was grammatically reviewed to detect and solve those mistakes.

Author Response File: Author Response.pdf

Reviewer 4 Report

This study review the use of Brewer's spent grain (BSG) as a cheap raw material with its biomass containing significant amounts of hemicellulose, mainly constituted of xylose and arabinose monomers that can be technologically converted into value-added products such as xylooligosaccharides, xylitol, ethanol 2G, biofilms and furfural. The present review addresses the methods for extracting and processing hemicelluloses from BSG and the most relevant bioproducts that can be obtained from this biomass. This review contains informative and valuable contents worthy of publication. There are still some major points needed to be revised as follows.

a. The short name of BSG for brewer's spent grain should be used after its first abbreviation.

b.       Table 1, the ‘-‘ sign in Table should be addressed in footnote as whether no data in references or not detectable in this material?

c.       L144-145, ‘Brewer's spent grain can have more than 50 % hemicellulose in its composition, which consists mainly of xylose and arabinose monomers’, please check Table 1 that the hemicellulose in BSG might range only from 18.4%-29.9%. Which ones are correct?

d.       Fig.1, the arrows should be addressed in the text. The first layer of sugar monomers is OK. But the second layer of HMF, furfural seems not be produced via the sugar monomers. Between the second and third layer, the arrows show inhibitors, what are them? In the fourth layer, do you mean HMF can form levulinic acid and formic acid via some inhibitors?

e.       Define 2G ethanol the first appearance.

f.        More comparison and discussion should be addressed for those contents in Table 2.

g.       L91, double ‘lysine’?

h.       PE-2?

i.         3.2.3. 2G ethanol productions should be summarized in a Table or Figure as that of Fig.2 or Fig.3.

j.         L405, ‘using high constant temperatures (180-230 °C)’, constant temperature?

k.       Furan derivatives section could be organized into a Table for better reading.

Author Response

Reviewer 4

Comments and Suggestions for Authors:

This study reviews the use of Brewer's spent grain (BSG) as a cheap raw material with its biomass containing significant amounts of hemicellulose, mainly constituted of xylose and arabinose monomers that can be technologically converted into value-added products such as xylooligosaccharides, xylitol, ethanol 2G, biofilms and furfural. The present review addresses the methods for extracting and processing hemicelluloses from BSG and the most relevant bioproducts that can be obtained from this biomass. This review contains informative and valuable contents worthy of publication. There are still some major points needed to be revised as follows.

1. The short name of BSG for brewer's spent grain should be used after its first abbreviation.

R: This is already done in the document (L19, in the abstract section).

1. Table 1, the '-'sign in Table should be addressed in footnote as whether no data in references or not detectable in this material?

R: Thank you for this consideration. The '-' sign was addressed in the footnote as "data not available."

18. L144-145, 'Brewer's spent grain can have more than 50 % hemicellulose in its composition, which consists mainly of xylose and arabinose monomers', please check Table 1 that the hemicellulose in BSG might range only from 18.4%-29.9%. Which ones are correct?

R: Thank you for this consideration. Both data are correct. In Table 1, we summarized some studies to show that the composition of this biomass is quite variable, not considering those data as minimum or maximum values found in the literature. Reassessing this decision, it seems appropriate to include this reference to the Table in question so that there is no confusion in the interpretation.

1. Fig.1, the arrows should be addressed in the text. The first layer of sugar monomers is OK. But the second layer of HMF, furfural seems not be produced via the sugar monomers. Between the second and third layer, the arrows show inhibitors, what are them? In the fourth layer, do you mean HMF can form levulinic acid and formic acid via some inhibitors?

R: Thank you for the contribution, but we believe there was some confusion in understanding the figure in question. The arrows point directly to the de products, where the sugar monomers (glucose, xylose, etc.) are degraded to form pseudo-lignin precursors (HMF, furfural and phenolic compounds) that can further be degraded into weak acids (levulinic, formic and acetic acids), those last two categories acting as inhibitors for most microorganisms involved in the biorefinery processes (red contoured in the figure).

1. Define 2G ethanol the first appearance.

R: Thank you for the observation. We added "Second generation ethanol" to the text next to the first "2G ethanol".

2. More comparison and discussion should be addressed for those contents in Table 2.

R: The authors thank you for the suggestion; we added more information about some of the methods addressed in Table 2 and new insights about catalytic processes.

1. L91, double 'lysine'?

R: Thank you for noticing this; the doubled word was removed from the phrase.

1. PE-2?

R: PE-2 is referred to as a S. cerevisiae industrial strain, as explained in the text. More information can be found here: https://doi.org/10.3390/fermentation8120669

3. 3.2.3. 2G ethanol productions should be summarized in a Table or Figure as that of Fig.2 or Fig.3.

R: Thank you for this suggestion; we agree and have made a figure to illustrate the 2G ethanol process better.

1. L405, 'using high constant temperatures (180-230 °C)', constant temperature?

R: "180-230 °C" refers to the temperature range that can be used in the process. For example, 190 °C for 30 minutes, where the temperature remains constant during the reaction. An explanation was added to the manuscript for better understanding.

1. Furan derivatives section could be organized into a Table for better reading.

R: The authors thank you for this suggestion; we included a table with information about the use of catalysts in the furfural obtention.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The review has been modified by the authors, but significant rework is needed on the two main questions posed earlier:

1. The review is structured in such a way that the role of catalysts is poorly represented. This does not allow recommending for publication in this particular journal.

 

Despite the corrections made, the authors failed to demonstrate the place of catalysts in the processing of BSG, what is new in the review in this area? What are the achievements of the problem with chemical and bio- catalysts? This is shown especially poorly in the abstract, introduction and conclusion.

3. The review does not provide information on how the literature was searched. In addition, some sections do not explain why a particular literature reference was used. For example, in Table 1, 3 examples of BSG are used. Why?

 The changes made to this question in the text, unfortunately, did not improve the logic of the review. Need keyword alternatives in search queries. Despite the popularity of the term “biorefinery”, not all publications use it. You may have missed some valuable information. Besides, if we are talking about the extraction of hemicelluloses, why is this not reflected in the keywords? Why only one database (WoS) was used, Scopus could have been used as well. In general, there are recommendations for reviews, not mandatory, but very useful - https://www.bmj.com/content/372/bmj.n71.

Author Response

RESPONSE LETTER

Reviewer 1

The review has been modified by the authors, but significant rework is needed on the two main questions posed earlier:

1. The review is structured in such a way that the role of catalysts is poorly represented. This does not allow recommending for publication in this particular journal.

Despite the corrections made, the authors failed to demonstrate the place of catalysts in the processing of BSG, what is new in the review in this area? What are the achievements of the problem with chemical and bio- catalysts? This is shown especially poorly in the abstract, introduction and conclusion.

R: The authors sincerely thank you for the appointments made. We added new discussions and references regarding the suggestions and hoped the recent revisions could make the manuscript suitable for publication in Catalysts.

3. The review does not provide information on how the literature was searched. In addition, some sections do not explain why a particular literature reference was used. For example, in Table 1, 3 examples of BSG are used. Why?

 The changes made to this question in the text, unfortunately, did not improve the logic of the review. Need keyword alternatives in search queries. Despite the popularity of the term “biorefinery”, not all publications use it. You may have missed some valuable information. Besides, if we are talking about the extraction of hemicelluloses, why is this not reflected in the keywords? Why only one database (WoS) was used, Scopus could have been used as well. In general, there are recommendations for reviews, not mandatory, but very useful - https://www.bmj.com/content/372/bmj.n71.

R: The authors thank you for the comments and have improved the critical points addressed regarding how the literature was searched. We also expanded the search scope and hope these modifications clarify the abovementioned questions.

Author Response File: Author Response.pdf

Reviewer 2 Report

Some references related to the use of catalysis were added to the work. It still does not present a full screening of the use of heterogeneous catalysis in the pretreatment of biomass and the obtaining of some by-products. Despite this, I consider that with the new approach added to the abstract and introduction, this new version can be accepted.

The manuscript is missing some spaces or some small details that can be corrected after acceptance.

Author Response

All Reviewer suggestions/corrections were inserted in the revised version of our manuscript.

Author Response File: Author Response.pdf

Reviewer 4 Report

This MS is now acceptable.

Author Response

Thanks to the Reviewer for all your attention to our manuscript.

Author Response File: Author Response.pdf