Investigation and Enhancement of the Antioxidant Compound Recovery of Pyrus communis Peel

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript by Bentoulla et al. provides a comprehensive examination of a multi-factor extraction system to enhance the antioxidant activity of pear fruit peel (Pyrus communis). The methodology and findings are promising, yet several areas require clarification and improvement before this manuscript can be considered for publication in Waste.

My specific comments and suggestions:

1.     All abbreviations used in the abstract should be defined upon their first mention. Please check throughout the manuscript.

2.     Line 40: Provide recent statistics on the global production of P. communis

3.     Line 52-55: The study on the antioxidant properties of P. communis is not new and has been investigated previously. What is the gap in knowledge that the authors want to address in the present study?

4.     Line 96-98: The selection of these parameters to be investigated and the range selected for each parameter should be justified.

5.     Line 217: A detailed explanation is required.

6.     Line 242-256: To improve readability and provide a clearer comparison, I suggest adding a table that compares the antioxidant activity of pear fruit peel obtained in this study with findings from other relevant studies.

7.     Line 244: As the study mentioned here utilized methanol, it is important to consider that variations in antioxidant activity might also be attributed to the solvent used. This should be acknowledged and discussed.

8.     Line 248-249: Incomplete sentence

 

9.     Line 251: Species name should be italicized

Author Response

The manuscript by Bentoulla et al. provides a comprehensive examination of a multi-factor extraction system to enhance the antioxidant activity of pear fruit peel (Pyrus communis). The methodology and findings are promising, yet several areas require clarification and improvement before this manuscript can be considered for publication in Waste.

We would like to thank the reviewer for the insightful comments about our manuscript.

My specific comments and suggestions:

1. All abbreviations used in the abstract should be defined upon their first mention. Please check throughout the manuscript.

All abbreviations in the Abstract section are now mentioned upon their first mention, as recommended.

2. Line 40: Provide recent statistics on the global production of P. communis

Recent statistics on the global production of P. communis were added in the Introduction section, as recommended.

3. Line 52-55: The study on the antioxidant properties of P. communis is not new and has been investigated previously. What is the gap in knowledge that the authors want to address in the present study?

The innovation of this study is now presented in the Introduction section.

4. Line 96-98: The selection of these parameters to be investigated and the range selected for each parameter should be justified.

A wide range in the values of these parameters was employed because it is well known that polarity and temperature have a significant impact on bioactive compound recovery in the extraction process. The manuscript has been revised in section 2.4.; however, an extended discussion of the impact of each parameter was done in section 3.1., which is now indicated in the manuscript.

5. Line 217: A detailed explanation is required.

As the solubility increases with the increase of temperature and the dielectric constant decreases with the increase of temperature, it can be deduced that the temperature plays a more important role than the dielectric constant in the dissolution process of ascorbic acid. To that end, the solubility of ascorbic acid could be more a matter of temperature than solvent composition in a binary hydroalcoholic mixture, as it is now explained in the manuscript. To establish this statement, a new citation is added.

6. Line 242-256: To improve readability and provide a clearer comparison, I suggest adding a table that compares the antioxidant activity of pear fruit peel obtained in this study with findings from other relevant studies.

A new table is now inserted in the manuscript comparing our findings to other relevant studies. However, we opted to compare the total polyphenols yielded due to the common measuring units (mg GAE/g) to maintain uniformity and provide a clear picture to the readers.

7. Line 244: As the study mentioned here utilized methanol, it is important to consider that variations in antioxidant activity might also be attributed to the solvent used. This should be acknowledged and discussed.

The antioxidant activity as determined by various tests can be strongly impacted by the solvent used to extract components from plants. Because of its polarity, ethanol is frequently used to extract a variety of compounds, including those with antioxidant properties. However, the antioxidant activity can vary depending on the solvent utilized since different solvents can dissolve different sets of molecules. This diversity should be considered when comparing studies or attempting to replicate findings. Consequently, we have investigated ethanol concentration in water (C_EtOH, % v/v) at three levels: 0%, 50%, and 100% v/v. The aim was to determine the optimal concentration (via RSM) that would yield a pear peel extract rich in bioactive compounds and exhibit significant antioxidant activity.

8. Line 248-249: Incomplete sentence

The sentence “Regarding antioxidant activity, the authors stated that both DPPH (3.25–5.72 mg AAE/g dw) and FRAP (2.15–4.37 mg AAE/g dw) had almost a two-fold range” has been completed.

9. Line 251: Species name should be italicized

Giovanni variety has been italicized, as suggested by the reviewer.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript of Bentoulla et al. investigates the extraction of polyphenols from pear peels via the addition of ethanol and assesses their subsequent antioxidant properties. The extraction of natural compounds from several biobased feedstocks is a well-researched topic. Therefore, the authors should emphasize the key novelty of this work more explicitly. Ethanol extraction by various means has been performed on numerous feedstocks. Therefore, please emphasize the novelty of the current work, also in relation with the current state of the art regarding the antioxidants from pear peels. The approach of the authors is well executed, the article is well written and the applied methodology merits publication in Waste. However, after reading the manuscript I have some major and minor comments which ought to be addressed before publication:

-          Following sentence is out of place in the introduction section, please remove: “Determination of polyphenolic compounds was carried out using high-performance liquid chromatography coupled with a diode array detector.”

-          Please limit key words to the true key words of this manuscript. For instance, “pareto plot” should not be a key word.

-          The introduction section should be extended with the current state of the art regarding the bioactives from pear (/residues). Has any work already been done in this area? Please state finding from the literature and emphasize the novelty of this research.

-          Do the authors have any idea regarding the (bio-) chemical composition of the pear (residues/waste)?

-          What kind of reactor was applied? The authors mention hotplate and elevated temperatures. Was the reactor closed to prevent evaporation of ethanol? What was the pressure at elevated temperatures?

-          For the statistical analysis: the authors mention triplicates for the analyses. However, what is the standard deviation of the extraction itself? Did the authors also perform the extraction experiments in triplicate? If yes, what were the standard deviations? If not, what is the standard deviation of a typical experiment?

-          Table 3 suggests that all variables and their interactions are deemed significant? Is this the case? Did the authors consider removing insignificant terms? Did the authors consider external validation via random points (within the model’s ranges) and check for the response?

-          How much of the solid matter of the pear peel remained  after extraction? Did the authors consider any use cases for the residual solid matter? What is its composition?

-          Once an abbreviation is used in the text along its full name please use the abbreviation. This is not the case now (for instance PLS, but not limited to this example).

-          Figure 5: there seems to be a lack of good separation of the polyphenols based on the chromatogram.  How can the quantity therefore be determined?

-          What do the authors actually mean with quantity expressed as %? Is this based on total peak area? Please elaborate.

-          Please extend the conclusion part with quantitative key results of the study.

 

Author Response

The manuscript of Bentoulla et al. investigates the extraction of polyphenols from pear peels via the addition of ethanol and assesses their subsequent antioxidant properties. The extraction of natural compounds from several biobased feedstocks is a well-researched topic. Therefore, the authors should emphasize the key novelty of this work more explicitly. Ethanol extraction by various means has been performed on numerous feedstocks. Therefore, please emphasize the novelty of the current work, also in relation with the current state of the art regarding the antioxidants from pear peels. The approach of the authors is well executed, the article is well written and the applied methodology merits publication in Waste. However, after reading the manuscript I have some major and minor comments which ought to be addressed before publication:

We would like to acknowledge the reviewer for the insightful feedback regarding our manuscript.

-Following sentence is out of place in the introduction section, please remove: “Determination of polyphenolic compounds was carried out using high-performance liquid chromatography coupled with a diode array detector.”

The specific sentence has been deleted from the Abstract, as suggested by the reviewer.

- Please limit key words to the true key words of this manuscript. For instance, “pareto plot” should not be a key word.

The manuscript has been stripped of several keywords and is now composed of only the most critical ones.

- The introduction section should be extended with the current state of the art regarding the bioactives from pear (/residues). Has any work already been done in this area? Please state finding from the literature and emphasize the novelty of this research.

Recent information about the bioactive compounds contained in pear flesh and peel has been added in the Introduction section, as suggested.

- Do the authors have any idea regarding the (bio-) chemical composition of the pear (residues/waste)?

We have explored the biochemical composition of pear residues and waste. Pear bio-waste, including peels, seeds, and leaves, is rich in various bioactive compounds. These bioactive compounds are now stated in the Introduction section, providing a clearer picture of the chemical composition of pear waste.

- What kind of reactor was applied? The authors mention hotplate and elevated temperatures. Was the reactor closed to prevent evaporation of ethanol? What was the pressure at elevated temperatures?

Section 2.4. has been enhanced with further details regarding the extraction method. A 50-mL Duran flask was used to insert the mixture, and it was tightly sealed to prevent evaporation during the extraction process using a stirring hotplate. To calculate the pressure within a sealed flask at a certain temperature, the ideal gas law is commonly employed. However, in the absence of precise measurements for the quantity of gas and the assumption of ideal behavior, an exact pressure cannot be determined. For precise outcomes, it's necessary to account for the characteristics of the particular gas mixture and any vapors that may be present.

- For the statistical analysis: the authors mention triplicates for the analyses. However, what is the standard deviation of the extraction itself? Did the authors also perform the extraction experiments in triplicate? If yes, what were the standard deviations? If not, what is the standard deviation of a typical experiment?

Each extract was prepared in triplicate, and every extract underwent three analyses, yielding a total of nine (3 × 3) measurements. The data are reported as mean values with the standard deviation.

- Table 3 suggests that all variables and their interactions are deemed significant? Is this the case? Did the authors consider removing insignificant terms? Did the authors consider external validation via random points (within the model’s ranges) and check for the response?

Table 3 indicates that all variables and their interactions are considered significant, as the statistical analysis, such as ANOVA, yielded p-values lower than 0.05. This suggests that the variables and their interactions significantly impact the outcome. Insignificant terms were not removed. Additionally, external validation with new, independent data points within the model's range was conducted to verify its predictive accuracy. The model validation process is also reported in section 2.4 and the validation statistics are presented in Table S1.

- How much of the solid matter of the pear peel remained after extraction? Did the authors consider any use cases for the residual solid matter? What is its composition?

The residual solid matter from pear peels post-extraction was discarded as it fell outside the scope of this study. The present study aimed to use a multifactor extraction system to produce antioxidant-rich extracts from an important agro-industrial waste, P. communis peels. Yet, this residual solid matter holds potential for various applications. It can be used as a substrate in solid-state fermentation to generate bioactive compounds with antioxidant and antimicrobial properties. Additionally, it can be utilized for animal feed, composting, and biofuel production. The residual solid matter is composed of fibers, proteins, and other bioactive compounds, such as hydrolyzable and condensed tannins, that were not extracted.

-Once an abbreviation is used in the text along its full name please use the abbreviation. This is not the case now (for instance PLS, but not limited to this example).

Every abbreviated term is now revised in the manuscript. Since their first mention, every term is now mentioned with its abbreviation, even in Tables and Figures, conforming to the reviewer’s recommendation.

- Figure 5: there seems to be a lack of good separation of the polyphenols based on the chromatogram. How can the quantity therefore be determined?

The quantification of polyphenols in a chromatogram, despite challenges in separation, can be reliably determined by the peak area and resolution. As detailed in section 3.5 of the manuscript, these parameters were found to be sufficient for both identifying and quantifying the polyphenolic components. This suggests that the methodology used is robust enough to ensure accurate analysis of the polyphenolic content, even when the chromatographic separation is not ideal.

- What do the authors actually mean with quantity expressed as %? Is this based on total peak area? Please elaborate.

It is now clearly indicated in both section 3.5 and Table 8 caption that the percentage quantity refers to the total identified polyphenols in optimal pear peels.

- Please extend the conclusion part with quantitative key results of the study.

The Conclusion section has been revised correspondingly, including key findings of the study.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors addressed most of the comments raised by the reviewer, and therefore, the reviewer accepts this manuscript for publication in Waste.