Oleaginous Yeast Biorefinery: Feedstocks, Processes, Techniques, Bioproducts

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors reviewed several aspects related to the valorization of oleaginous yeast biomass under a biorefinery model. Thus, the production of microbial oil coupled with the recovery of other valuable by-products plays a decisive role not only in cost reduction of yeast-derived biofuels but also in implementing circular economy concepts. This is an emerging and promising technology. It is highly discussed nowadays as well.

It is clear the scientific and technological value of the results reviewed in the work. However, the article has some issues that should be addressed, as outlined below.

1) Throughout the manuscript

The section numbers should be revised

2) Section “Lipid production by oleaginous yeasts”, line 320

Comments about the microbial oil production via de novo and ex novo pathways should be introduced after the “Introduction” section.

Concerning the ex novo lipid production, yeasts can also synthesize an emulsifier activity. Its potential role in the process should be described.

3) Section “Low-cost feedstock for oleaginous yeast production”, line 107.

As the main topic of the article is focused on the valorization of oleaginous yeast biomass (“all yeast biomass fractions”) under a biorefinery model, each low-cost feedstock should be introduced following this concept. Thus, beside microbial oil, some comments related to carotenoids, single cell proteins, etc. should be added.

3.1 Section “Wastewater”, line 125

Although in an emerging stage, the wastewater treatments are considered a production step rather than a simple bioremediation process. Thus, as the author presented chemical oxygen demand values, this aspect should be highlighted.

 

3.2 Section “Agri-Food industry wastes”

For example, this reference should be added:

Lakshmidevi et al (2021). Valorisation of molasses by oleaginous yeasts for single cell oil (SCO) and carotenoids production. Environmental Technology & Innovation, 21, 101281.

 

3.3 A new section including hydrophobic substrates should be added

4. Section “Yeast cultivation modes”, line 360

In my opinion, the cultivation modes should be presented considering the substrates characteristics (hydrophobic, hydrophilic, presence of heterogeneous/toxic components, etc). In addition, as it was mentioned before, the inclusion of yeast biomass fractions yields is also recommended.

Comments on the Quality of English Language

No comments

Author Response

We would like to thank the Experts for reviewing our paper so thoroughly and for providing us the opportunity to improve the presented work. We really appreciated the experts’ comments and proceeded according to every suggestion made. Overall, we believe that the changes have improved the paper and trust that it can now proceed to publication.

The additions and changes to the manuscript are inserted as track changes. 

Reviewer 1

The authors reviewed several aspects related to the valorisation of oleaginous yeast biomass under a biorefinery model. Thus, the production of microbial oil coupled with the recovery of other valuable by-products plays a decisive role not only in cost reduction of yeast-derived biofuels but also in implementing circular economy concepts. This is an emerging and promising technology. It is highly discussed nowadays as well.

It is clear the scientific and technological value of the results reviewed in the work. However, the article has some issues that should be addressed, as outlined below.

-Throughout the manuscript the section numbers should be revised

RE: The section numbers were thoroughly revised throughout the text.

2) Section “Lipid production by oleaginous yeasts”, line 320. Comments about the microbial oil production via de novo and ex novo pathways should be introduced after the “Introduction” section.

RE: Done. Lines 108-109.

- Concerning the ex novo lipid production, yeasts can also synthesize an emulsifier activity. Its potential role in the process should be described.

RE: The production of emulsifiers by oleaginous yeast has already been reported by the authors in the first submited version, in Table 1 and lines 641-642, mentioning the work of Kumar et al (2017) in which the authors used crude glycerol to produce emulsifiers.

3) Section “Low-cost feedstock for oleaginous yeast production”, line 107.

 - As the main topic of the article is focused on the valorisation of oleaginous yeast biomass (“all yeast biomass fractions”) under a biorefinery model, each low-cost feedstock should be introduced following this concept. Thus, beside microbial oil, some comments related to carotenoids, single cell proteins, etc. should be added.

RE: Section “Low-cost feedstock for oleaginous yeast production” concerns low-cost substrates, not yeast products. However, the authors note that the production of carotenoids by oleaginous yeasts has been highlighted in the text, being intensively referred throughout the first submitted version. Table 1 shows three references related to yeast carotenoid production, showing the yeast carotenoids contents; lines 156, 341, 474, 480, 596, 597, 615, 617, 625, 626, 628,649, 684, also mention yeast carotenoid production. Specifically, paragraph 612-633 describes the extraction and valorisation of the yeast carotenoid fraction. Proteins and carbon hydrates from oleaginous yeast biomass are refereed in lines 644 and 686; also, Table 3 shows Reference 91, mentioning the yeast biomass price if selling the whole yeast cell, including recovering value for the protein and carbohydrate, in the submitted version. The authors have now included in Table 1 a reference concerning the yeast biomass leftover containing proteins and carbohydrates (Reference [34]).

3.1 Section “Wastewater”, line 125

Although in an emerging stage, the wastewater treatments are considered a production step rather than a simple bioremediation process. Thus, as the author presented chemical oxygen demand values, this aspect should be highlighted.

RE: Done. Lines 174-176.

3.2 Section “Agri-Food industry wastes”. For example, this reference should be added:

Lakshmidevi et al (2021). Valorisation of molasses by oleaginous yeasts for single cell oil (SCO) and carotenoids production. Environmental Technology & Innovation, 21, 101281.

RE: Done, reference [22].

3.3 A new section including hydrophobic substrates should be added

RE: Section 2.4 “Volatile fatty acids” concerns hydrophobic substrates. The title has been changed accordingly.

4. Section “Yeast cultivation modes”, line 360

In my opinion, the cultivation modes should be presented considering the substrates characteristics (hydrophobic, hydrophilic, presence of heterogeneous/toxic components, etc). In addition, as it was mentioned before, the inclusion of yeast biomass fractions yields is also recommended.

RE: According to Literature, there are no connection between the substrate characteristics and the selection of the yeast cultivation mode. The yeast biomass fractions yields reported in Literature are shown in Table 1.

In conclusion, we hope that you will now consider our revised and improved manuscript suitable for publication in Fermentation.

Yours sincerely,

Teresa Lopes da Silva

Corresponding Author

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The presented review „Oleaginous Yeast Biorefinery: Feedstocks, Processes, Techniques, Bioproducts” prepared by Teresa Lopes da Silva 1*, Afonso Fontes 2, Alberto Reis3, Carla Siva4 and Francisco Gírio, refers to a topic concerning the world climate crisis and the need for renewable energy sources. It describes the recent advances in using oleaginous yeast biomass biorefineries as a potential renewable source of biofuels. This application is crucial for the economic and environmental sustainability of the whole process. The authors reported the potential feedstocks for yeast growth, as well as strategies to improve yeast biomass production, methods to extract and separate the various biomass fractions and products, and the final bioproducts resulting  from this biorefinery strategy.

The list of references is well updated. More than 80% of the publications are from the last 10 years. Almost 50% of the references are from the last 5 years.

The manuscript’s theme is relevant to the current needs of society, it is well written and detailed. It is worth being published in Fermentation Journal.

The paper work needs just a few corrections, listed below:

• Line 260 - Crude Glycerol – should be numbered as 2.3.
• Line 288 - Volatile Fatty acids – should be numbered as 2.4.
• Line 320 - Lipid production by oleaginous yeasts – should be numbered as 3.
• Line 355 - Oleaginous yeast biorefinery - should be numbered as 4.
• Line 476 - Downstream processing techniques - should be numbered as 5.
• Line 673 - Oleaginous yeast biorefinery technical economic assessment - should be numbered as 6.
• There should be line space between line 681 and 682.
• There should be line space between Table 3 and line 683.
• In the references list, all the references should be formatted according to the guidelines of the journal; there are some years missing.
• All other suggestions are colored in green in the PDF file with yellow notes.

As a reviewer, I state that I do not have conflicts of interest with the authors of this manuscript.

 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

In my opinion, the English language in the manuscript is quite good and needs only a few corrections.

Author Response

We would like to thank the experts for reviewing our paper so thoroughly and for providing us the opportunity to improve the presented work. We really appreciated the experts’ comments and proceeded according to every suggestion made. Overall, we believe that the changes have improved the paper and trust that it can now proceed to publication.

The additions and changes to the manuscript are inserted as track changes.

 

Reviewer 2

                                                                                                                                Comments and Suggestions for Authors

The presented review „Oleaginous Yeast Biorefinery: Feedstocks, Processes, Techniques, Bioproducts” prepared by Teresa Lopes da Silva 1*, Afonso Fontes 2, Alberto Reis3, Carla Siva4 and Francisco Gírio, refers to a topic concerning the world climate crisis and the need for renewable energy sources. It describes the recent advances in using oleaginous yeast biomass biorefineries as a potential renewable source of biofuels. This application is crucial for the economic and environmental sustainability of the whole process. The authors reported the potential feedstocks for yeast growth, as well as strategies to improve yeast biomass production, methods to extract and separate the various biomass fractions and products, and the final bioproducts resulting  from this biorefinery strategy.

The list of references is well updated. More than 80% of the publications are from the last 10 years. Almost 50% of the references are from the last 5 years.

The manuscript’s theme is relevant to the current needs of society, it is well written and detailed. It is worth being published in Fermentation Journal.

The paper work needs just a few corrections, listed below:

Line 260 - Crude Glycerol – should be numbered as 2.3.

RE: Done.

Line 288 - Volatile Fatty acids – should be numbered as 2.4.

RE: Done.

Line 320 - Lipid production by oleaginous yeasts – should be numbered as 3.

RE: Done.

Line 355 - Oleaginous yeast biorefinery - should be numbered as 4.

RE: Done.

Line 476 - Downstream processing techniques - should be numbered as 5.

RE: Done.

Line 673 - Oleaginous yeast biorefinery technical economic assessment - should be numbered as 6.

RE: Done.

There should be line space between line 681 and 682.

RE: Done.

There should be line space between Table 3 and line 683.

RE: Done

In the references list, all the references should be formatted according to the guidelines of the journal; there are some years missing.

RE: All the references were thoroughly revised.

All other suggestions are colored in green in the PDF file with yellow notes.

RE: All the suggestions have been inserted in the new version.

 

As a reviewer, I state that I do not have conflicts of interest with the authors of this manuscript.

 

In conclusion, we hope that you will now consider our revised and improved manuscript suitable for publication in Fermentation.

 

Yours sincerely,

Teresa Lopes da Silva

Corresponding Author

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

It is clear the scientific and technological value of the results reviewed in the work. According to title, abstract, and introduction sections, the authors should describe in detail the information related to the valorization of oleaginous yeast biomass under a biorefinery model. In important sections they fail to present the data following this concept. Thus, there are some issues that should be addressed, as outlined below.

For example:

1) Section 2, Low-cost feedstock for oleaginous yeast production.

-The authors write: “This section describes the low-cost feedstocks that have been successfully used to cultivate oleaginous yeasts, also having the potential of producing a range of bioproducts with commercial applications from the yeast biomass.”

Thus, beside microbial oil, some comments related to carotenoids, single cell proteins, etc. should be added. For example, in the section “2.1  Wastewater” this concept was followed. Both microbial lipids and carotenoids were described as bioproduct examples. Please revise the others subsections.

Bioproducts should be included in the text employing examples. In addition, the important table 1 should be more properly mentioned in the text.

2) Subsection “2.2 Agri-Food industry wastes”

The following reference was not properly cited. Please check.

Lakshmidevi et al (2021). Valorisation of molasses by oleaginous yeasts for single cell oil (SCO) and carotenoids production. Environmental Technology & Innovation, 21, 101281. (reference 22)

3) Both grammatical and format errors were detected. Captions of schemes should be added.

Comments on the Quality of English Language

Both grammatical and format errors were detected.

Author Response

Reviewer 1

It is clear the scientific and technological value of the results reviewed in the work. According to title, abstract, and introduction sections, the authors should describe in detail the information related to the valorization of oleaginous yeast biomass under a biorefinery model. In important sections they fail to present the data following this concept. Thus, there are some issues that should be addressed, as outlined below.

For example:

1) Section 2, Low-cost feedstock for oleaginous yeast production.

-The authors write: “This section describes the low-cost feedstocks that have been successfully used to cultivate oleaginous yeasts, also having the potential of producing a range of bioproducts with commercial applications from the yeast biomass.”

Thus, beside microbial oil, some comments related to carotenoids, single cell proteins, etc. should be added. For example, in the section “2.1  Wastewater” this concept was followed. Both microbial lipids and carotenoids were described as bioproduct examples. Please revise the others subsections.

Bioproducts should be included in the text employing examples. In addition, the important table 1 should be more properly mentioned in the text.

RE: The authors totally agree with the Referee. However, as stated in the Introduction  section, lines 96-99 “There are innumerous studies describing lipid production from oleaginous yeasts for biofuels [10-12]. However, there is still little information on the potential of oleaginous yeast biomass biorefinery, as an integrated process that uses low-cost feedstock to obtain a wide range of valuable bioproducts with commercial interest”. The authors have thoroughly searched in literature for bioproducts (other than lipids) production from oleaginous yeasts growing in low-cost substrates, and could not find more than those that are shown in Table 1. For instance, although Elfeky et al (Process 2020, 8, 140) studied the lipid and carotenoid co-production from Rhodotorula glutinis yeast, they used expensive synthetic media, which is out of the scope of the present review.

Therefore, the phrase “This section describes the low-cost feedstocks that have been successfully used to cultivate oleaginous yeasts, also having the potential of producing a range of bioproducts with commercial applications from the yeast biomass” has been replaced by: “This section describes the low-cost feedstocks that have been successfully used to cultivate oleaginous yeasts, showing the yeast bioproducts that were produced for each case.”

Nevertheless, the Authors highlight that there are several referents to yeast carotenoids (references 18, 20, 22, 30), proteins (reference 34) and carbohydrates (reference 344) are referred in the text. Table 1 is referred in the text in lines 116, 129, 164, 192, 247 and 622.

2) Subsection “2.2 Agri-Food industry wastes”

The following reference was not properly cited. Please check. Lakshmidevi et al (2021). Valorisation of molasses by oleaginous yeasts for single cell oil (SCO) and carotenoids production. Environmental Technology & Innovation, 21, 101281. (reference 22)

RE: The authors have now included this reference in Table 1, although the carotenoid content depicted by the two yeasts strains when grown on molasses was very low. Lines 188-192 and .

3) Both grammatical and format errors were detected. Captions of schemes should be added.

Re:  Grammatical and forma errors have been checked throughout the manuscript.