Evaluation of Direct Ultrasound-Assisted Extraction of Phenolic Compounds from Potato Peels
Round 1
Reviewer 1 Report
The topic of this study is very interesting and the results are presented appropriately. Is always very useful to find experimental studies comparing different technics.
After reviewing the manuscripts, I have some feedbacks:
1) Firstly, this reviewer recommend to check the English style in order to improve the meaning of some sentences (e.g. some “that’s” were added in the text with no reason)
2) After the analysis of the results, this reviewer noticed that the TPC obtained by DUAE and IUAE are very similar. Even considering the statistical analysis made by the authors were significant differences are found, if the final goal of this work is to after an alternative to the typical technics, the extraction capacity is so similar that this reviewer wondered if the posible higher technic difficulties or higher cost of DUAE would made worthy the effort. I highly recommend to add an analysis with a comparison of the technical viability of DUAE and IUAE, and an analysis of the economics of the process in order to unravel if DUAE is a feasible alternative to IUAE.
3) Line 199: Authors say that TPC peak at 45 min, but this reviewer cannot observe this in Fig 4. The TPC peak is centered at 30 min, which is not the point of the maximum temperature.
4) The conclusion made in line 202-204 is very daring. The differences in TPC observed are very small and the changes in temperature are also limited. At the same time, we have two different effects in this especific study: time of extraction and temperature. It is highly recommended to isolate the effects in order to assume a correct relation.
I do not if it is possible but just as a suggestion: maybe to maintain the solution after a continuous stirring would help to obtain more homogeneity and would help to improve the heat transfer into the vessel. This will help to mitigate the temperature increase caused by the ultrasound probe.
5) Fig 5: A relatively high temperature is obtained (70 ºC aprox.). This reviewer suggest to carry out an analysis of TPC and temperature with no ultrasound effect in order to isolate the temperature effect.
6) Fig 6: If the temperature also increase, we could not assume that the improvement observed is due to the increase in the amplitude.
7) 3.2.4. Effects of Solvent-to-Solid Ratio on TPC Yield: This should be study and discussed in more detail. Is only the temperature effect what made that TPC extraction increases?
8) Fig 8: Again, this reviewer highly recommend to analyse the effect of the temperature solely. It is not possible to correlate the TPOC extraction with other effects if temperature is also changing (specially if we do not know which is the real effect of the temperature)
9) Lines 308-309: “Our findings
308 suggest that the antioxidant activity in potato peel extract was primarily contributed by the phenolic
309 compounds, and this suggestion is supported by other reports.”
- Which phenolic compounds? It would be great to analyse the major compound of the mixture.
- ”...Supported by other reports...” References to these “other reports” should be added.
10) This reviewer suggest to add an analysis of TPC extraction with frequency. The higher the frequency, the higher the energy of the wave so, would not be this interesting?
Author Response
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Reviewer 2 Report
The paper describes the extraction of the phenolic contents from potato pells. Although the idea leading to utilisation of this "agriculture waste" is very interesting, mostly because (as authors rightly claim) potato peels contain valuable phenolic compounds with antioxidant activities. The idea is OK, what was confirmed by several papers published to date in the field including a review in Foods 2020, 9, 1598; doi:10.3390/foods9111598 . The performance of this particular paper is significantly weaker, and the paper in my opinion, does not bring new concepts it the field, moreover used in the paper methods give only coarse results. I understand that the Authors focused on differences in extraction processes; however, as endpoints of their experiments, effortless methods were used what significantly limits the possibility to form the right conclusions. For the estimation of the phenolic content in obtained extracts Folin–Ciocalteau method was used. This method gave as a result, "gallic acid equivalents". The study of reactivity of various compound classes towards the Folin-Ciocalteu reagent has been performed and showed by Everette et al J Agric Food Chem. 2010 Jul 28; 58(14): 8139–8144 therefore "gallic acid equivalents" provided by the Author say relatively nothing about contents of the extract. Even simple HPLC analysis showing differences between extractions could be very interesting for the readers and significantly improve the value of the paper. Similarly, DPPH assay has been performed in its the simplest version, which does not provide information about the reaction between phenolic antioxidants present in the tested extract and DPPH. The version allowing to calculate rate reaction constant could be definitively more informative https://doi.org/10.1006/fstl.1997.0240. In the present form, the paper provides only limited information.
Author Response
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Reviewer 3 Report
The manuscript is very interesting and may have a practical meaning regarding processing of the potato peel, which is a common agricultural waste. The biggest disadvantage of the study is lack of any chromatographic determinations regarding concentration of polyphenols in the obtained extracts. Studies on the content of polyphenolic compunds were based only on the TPC determination, which is rather a method evaluating the reducing power of the sample than a real content of polyphenols. Below I have pointed some errors or inaccuracies, which need to be corrected or explained:
- line 35 - "is a staple food after corn, rice, and wheat" - ahat does it mean "after"? The most popular? The most widely consumed?
- line 90 - TPC were performed in triplicates. But how many samples were extracted per each extraction?
- lines 306-309 - "Results showed that the antioxidant activities of the potato peel extract correlated with the quantity of TPC in potato peel extracts (p < .05; Fig. 9). Our findings suggest that the antioxidant activity in potato peel extract was primarily contributed by the phenolic compounds, and this suggestion is supported by other reports." It is rather obvious as TPC is very often described as one of the methods, which evaluate the antioxidant activity of the sample. Therefore results of TPC and DPPH are mostly well corelated. Please add some comment regarding this issue (including appropriate reference).
- lines 306-307 - data presented in Figure 9 do not show the results of TPC. Therefore I do not understand why the Authors wrote such a conclusion. Morover it is not clear to what extract these results are related to?
- Chapter 3.3 - I do not see any results of the antioxidant activity of the rest two extracts? It seems like the Authors perform DPPH scavenging activity of the DUAE extracts only. Morover if the Authors wanted to prove that direct ultrasound extraction do not reduce the amount of the polyphenols and the antioxidant activity of the extracts they should compare the results of the antioxidant power of DUAE and CSE extracts.
Author Response
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Round 2
Reviewer 1 Report
This reviewer thank authors for their effort. As mentioned, this is an innovative work, but it should be upgraded by a future work. I highly recommend to mention along the text that the results shown are preliminary and that they will be supported by additional experiments.
Author Response
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Reviewer 2 Report
The paper has been significantly improved and may be accepted now.
Author Response
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Reviewer 3 Report
The manuscript was significantly improved. However, as I have stated in my previous review report, the biggest disadvantage of the paper is lack of any chromatographic determinations. It would be very useful if at least major compounds in the extracts were identified. Therefore I suggest to change the title and add "Preeliminary studies". This will mean that more precise determinations are needed to fully evaluate applied methodology.
Still I do not understand Figure 9. What the description of axis "x" means? To what concentration it is related to? I suppose that it means GAE concentration in each extract. If so, it must be clearly indicated on the Figure 9.
lines 342-344 - "At a TPC the concentration of 50 μg GAE/mL, the potato peel extracts and TBHQ had higher scavenging activities than both BHA and BHT" - this sentence does not sound good and it should be corrected.
Author Response
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