Valorization of ‘Rossa di Tropea’ Onion Waste through Green Recovery Techniques of Antioxidant Compounds

Round 1

Reviewer 1 Report

Review of sustainability-1640866

This is a nice study about the extraction of antioxidants, phenolic compounds, etc. from the waste of a special onion, “Rossa di Tropea” onion, that has trademarks. The authors use conventional, ultrasound-assisted, and microwave-assisted extraction processes (in a separate ways). The extracts were characterized using HPLC, ABTS, DPPH, etc. There are some issues that have to be addressed before acceptance for publication, as follows:

1. In order to show that this report brings improvement to the field of extraction, please add benchmarking table, showing the performance of similar processes in extracting antioxidants, and please classify them in terms of the concentration of the extracted antioxidant, the type of extraction process, time, temperature, etc.
2. When comparing the results in the tables, there is a concerning issue, where the results of antioxidant assay using ABTS and DPPH sometimes are contradicting, where the highest antioxidant content (or the lowest) obtained using ABTS is sometimes not the highest for that of DPPH. Please write the reason(s) with you need two methods to get the antioxidant data.

As we pay attention to the results comparing water extracts vs hydroalcoholic extracts, it is clearly seen that the ones from water/alcohol mixture are higher than that of the pure water extract, since organic molecules from the sample like the alcohol as an organic solvent. However, the same set of data when measured using ABTS vs. DPPH sometimes does not result in a similar trend.

Maybe (not compulsory, but can be tried) reporting of only ABTS or only DPPH results might be considered.

3. Title: Please write “onion waste” with all lowercase letters.
4. Please check the guidelines for making a table, for Table 1, Table 3, and Table 5.
5. Line 235, caption of Table 1, you missed an asterisk (*) in the table footnote for p<0.05.
6. Scientific names must be written in italic. Location: lines 81, 478, 488, 496, 521, 526, 528, 537, 559, 560-561, 566, 569, 570, 576, 583, 589, 593, 596.
7. Line 27: …green extractions… --> with lowercase g.
8. Line 38: Change “on-ion” to be “onion”.
9. Line 62: Change “ex-traction” to be “extraction”.
10. Line 54-55: A report about the preservation of onion by using drying (as well as the modeling of the drying process) that contributes to the lowering of the environmental impact of food waste disposal can be found here: Compaore et al, Drying Technology 35 (2017) 500-508 DOI: 10.1080/07373937.2016.1192189
11. Line 43-66: Please separate this long paragraph to be 3-4 shorter ones.
12. Line 89-91: This is a short paragraph, please merge with the next one.
13. Line 100-101: This is a short paragraph, please merge with the next one.
14. Line 142-143: This is a short paragraph, please merge with the next one.
15. Line 160: cyanidiNE, not cyandiNG
16. Line 165: cyanidiNE, not cyandiNG
17. Line 181-182: This is a short paragraph, please merge with the previous one.
18. Table 7: How to obtain Table 7? Regression equations need set of data, which are not displayed. Please add the details in supplementary information.
19. Line 413, caption of Table 8: Please change “characterisation” to be “characterization”, since American English is more dominant in this manuscript, than that of British English.
20. Line 449: Please change “valorisation” to be “valorization”, since American English is more dominant in this manuscript, than that of British English. In fact, the title of this manuscript is “Valorization of…”
21. Table 8: Please justify the result of the conventional extraction of quercetin yields around 25x higher than that of the ultrasound or microwave extraction in this study. The difference is too striking. The yield ratio of other components are 0.5-3x only, fairly reasonable.
22. Line 518-520: Please add place, date, month year of this conference publication.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript “1640866” deals with the valorization of ‘Rossa di Tropea’ onion waste through green recovery techniques of antioxidant compounds. The topic is interesting and relevant, however major changes are needed to make this manuscript acceptable for publication in Sustainability journal.

General Comments:

According to the authors, the main aim of the present study was the extraction of valuable compounds from onion solid waste, through sustainable extraction techniques (conventional, ultra-sound and microwave-assisted extraction) and food grade extraction solvent, such as water and hydroalcoholic mixture. Sustainability journal covers environmental, cultural, economic, and social sustainability of human beings, providing an advanced forum for studies related to sustainability and sustainable development. Therefore, there are some important issues, which need to be addressed:

1) It is a well-developed study, with satisfactory scientific quality. However, the authors should clarify the study novelty, as well as described it clear in manuscript, since there are others studies covering the same aim as following:

- Onion Peel: Turning a Food Waste into a Resource, DOI: 10.3390/antiox10020304;

- Onion (Allium cepa L.) peels: A review on bioactive compounds and biomedical activities, DOI: 10.1016/j.biopha.2021.112498.

2) The introduction needs some work to better set up the stage for the aims of the study. It will help the author to better explain their results. For instance:

- Green Ultrasound-Assisted Extraction of Bioactive Compounds from Button Mushrooms, Potatoes, and Onion Peels: https://doi.org/10.1021/acsfoodscitech.1c00153.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Review of sustainability-1640866-v2

The authors have provided clear and acceptable arguments to address all the previously raised issues. The manuscript can be accepted for publication.

Note: Please write scientific names in italic for the new references in line 659 and line 662. This can be amended during the proofreading stage. Thanks.

Reviewer 2 Report

The authors properly agreed with the reviewer's observations, as well as correctly changed all the mistakes improving the article quality. Therefore, now I recommend this article for publication in Sustainability journal.