Olive Mill Wastewater as Renewable Raw Materials to Generate High Added-Value Ingredients for Agro-Food Industries

Round 1

Reviewer 1 Report

The manuscript provides an interesting review of the possibilities of using OMWW. However, there are important issues that need to be taken into account.

It is important for the authors to point out that the continuous two-phase extraction system is the most widely used worldwide, and in Spain alone it is used in more than 90% of olive oil mills. It is also important that they state the ratio of hot water to be added in the malaxation in the case of the three-phase system.

The distribution of phenols in the wastewater from the three-stage extraction system is usually much lower than the 52% reported by the authors in the manuscript, which is why it is necessary to provide more bibliographical data to support this high percentage, as it is normal for the wastewater phenol content to be between 30-40% of the total phenol content of the olives.

Lines 107-110. The most commonly used techniques for obtaining hydroxytyrosol are chromatographic techniques, which are much more widely used than membrane filtration. Therefore, they should be taken into account in a review article.

The legend of figure one does not read correctly, the quality of the figure should be improved.

Point 4.3. The authors should change the wording to OMWW as feed, and only talk about liquid effluent and not solid effluent if the review is to focus on liquid effluent, so they should change the first paragraph of this section which seems to mention solid effluent because of its high fat content.

Line 442. Oleuropein is found in very low concentration already in the OMWW as it is rapidly hydrolysed and transformed into secoiridoid derivatives during malaxation. And in the case of oleocanthal, everything that is formed from oleuropein passes into the oil and practically nothing remains in the water because it is a very hydrophobic compound. The compounds that are also derived from the hydrolysis of oleuropein and that have shown the greatest antimicrobial activity are the elenolic acid derivatives, and these are present in significant quantities in OMWW, unlike oleuropein and oleocanthal. Authors should therefore include these compounds and take these considerations into account.

In the conclusions, the authors' views on the future of the three-phase extraction system are missing, as the two-phase extraction system is gradually becoming more widespread and replacing the three-phase extraction system. It is in Italy that the changeover is taking place more slowly, but nevertheless the trend is clear.

Author Response

Referee 1

The manuscript provides an interesting review of the possibilities of using OMWW. However, there are important issues that need to be taken into account.

 

It is important for the authors to point out that the continuous two-phase extraction system is the most widely used worldwide, and in Spain alone it is used in more than 90% of olive oil mills. It is also important that they state the ratio of hot water to be added in the malaxation in the case of the three-phase system.

Thank you for your suggestion, we added this point (see lines 66 and 70).

 

The distribution of phenols in the wastewater from the three-stage extraction system is usually much lower than the 52% reported by the authors in the manuscript, which is why it is necessary to provide more bibliographical data to support this high percentage, as it is normal for the wastewater phenol content to be between 30-40% of the total phenol content of the olives.

Thank you for your suggestion. In line 105 of the Introduction section, we have reported the distribution of phenols taking into account some of the latest papers (Di Nunzio et al, 2020 and the review of Caporaso et al., 2019) without considering any estraction system in particular. Furthermore, even rilevant differences could occour in OMMW obtained trough different processing systems. But in this case, we have not differentiated among OMWs obtained from different systems.

 

Lines 107-110. The most commonly used techniques for obtaining hydroxytyrosol are chromatographic techniques, which are much more widely used than membrane filtration. Therefore, they should be taken into account in a review article.

Thank you for the suggestion, we changed the sentences as you could see at lines 110-111.

 

The legend of figure one does not read correctly, the quality of the figure should be improved.

Thank you for suggestion, we repleaced the figure.

 

Point 4.3. The authors should change the wording to OMWW as feed, and only talk about liquid effluent and not solid effluent if the review is to focus on liquid effluent, so they should change the first paragraph of this section which seems to mention solid effluent because of its high fat content.

Thank you for the suggestion. We changeged the sentence (see line 378) and deleted lines 381-384 because they referred to solid olive byproducts.

 

Line 442. Oleuropein is found in very low concentration already in the OMWW as it is rapidly hydrolysed and transformed into secoiridoid derivatives during malaxation. And in the case of oleocanthal, everything that is formed from oleuropein passes into the oil and practically nothing remains in the water because it is a very hydrophobic compound. The compounds that are also derived from the hydrolysis of oleuropein and that have shown the greatest antimicrobial activity are the elenolic acid derivatives, and these are present in significant quantities in OMWW, unlike oleuropein and oleocanthal. Authors should therefore include these compounds and take these considerations into account.

Thank you for your detailed clarification. We have rearranged the sentence in lines 442-443. However, in our experience, we have found oleuropein also in fresh OMWW.

 

In the conclusions, the authors' views on the future of the three-phase extraction system are missing, as the two-phase extraction system is gradually becoming more widespread and replacing the three-phase extraction system. It is in Italy that the changeover is taking place more slowly, but nevertheless the trend is clear.

We agree with your comment. We have consequently modified the conclusions.

 

Referee 2

The article "Olive mill wastewater as renewable raw materials to generate high added-value components for the agri-food industry" presented for review is interesting and the topic discussed is well-chosen.

The authors described all the potential uses of the olive mill wastewater and added the necessary information that could also provide a good summary of current trends in the food industry.

I only have two "cosmetic" comments

Please change the resolution of picture 1 now it is too small - I can't see the letters describing the charts please provide the title of point 6 - it is missing from the manuscript.

Thank you for improving the layout of our manuscript. We have modified the figure according to your suggestions. Moreover, the title of point 6 was changed. 

Reviewer 2 Report

The article "Olive mill wastewater as renewable raw materials to generate high added-value components for the agri-food industry" presented for review is interesting and the topic discussed is well-chosen.

The authors described all the potential uses of the olive mill wastewater and added the necessary information that could also provide a good summary of current trends in the food industry.

I only have two "cosmetic" comments

Please change the resolution of picture 1 now it is too small - I can't see the letters describing the charts

please provide the title of point 6 - it is missing from the manuscript.

Author Response

Referee 1

The manuscript provides an interesting review of the possibilities of using OMWW. However, there are important issues that need to be taken into account.

 

It is important for the authors to point out that the continuous two-phase extraction system is the most widely used worldwide, and in Spain alone it is used in more than 90% of olive oil mills. It is also important that they state the ratio of hot water to be added in the malaxation in the case of the three-phase system.

Thank you for your suggestion, we added this point (see lines 66 and 70).

 

The distribution of phenols in the wastewater from the three-stage extraction system is usually much lower than the 52% reported by the authors in the manuscript, which is why it is necessary to provide more bibliographical data to support this high percentage, as it is normal for the wastewater phenol content to be between 30-40% of the total phenol content of the olives.

Thank you for your suggestion. In line 105 of the Introduction section, we have reported the distribution of phenols taking into account some of the latest papers (Di Nunzio et al, 2020 and the review of Caporaso et al., 2019) without considering any estraction system in particular. Furthermore, even rilevant differences could occour in OMMW obtained trough different processing systems. But in this case, we have not differentiated among OMWs obtained from different systems.

 

Lines 107-110. The most commonly used techniques for obtaining hydroxytyrosol are chromatographic techniques, which are much more widely used than membrane filtration. Therefore, they should be taken into account in a review article.

Thank you for the suggestion, we changed the sentences as you could see at lines 110-111.

 

The legend of figure one does not read correctly, the quality of the figure should be improved.

Thank you for suggestion, we repleaced the figure.

 

Point 4.3. The authors should change the wording to OMWW as feed, and only talk about liquid effluent and not solid effluent if the review is to focus on liquid effluent, so they should change the first paragraph of this section which seems to mention solid effluent because of its high fat content.

Thank you for the suggestion. We changeged the sentence (see line 378) and deleted lines 381-384 because they referred to solid olive byproducts.

 

Line 442. Oleuropein is found in very low concentration already in the OMWW as it is rapidly hydrolysed and transformed into secoiridoid derivatives during malaxation. And in the case of oleocanthal, everything that is formed from oleuropein passes into the oil and practically nothing remains in the water because it is a very hydrophobic compound. The compounds that are also derived from the hydrolysis of oleuropein and that have shown the greatest antimicrobial activity are the elenolic acid derivatives, and these are present in significant quantities in OMWW, unlike oleuropein and oleocanthal. Authors should therefore include these compounds and take these considerations into account.

Thank you for your detailed clarification. We have rearranged the sentence in lines 442-443. However, in our experience, we have found oleuropein also in fresh OMWW.

 

In the conclusions, the authors' views on the future of the three-phase extraction system are missing, as the two-phase extraction system is gradually becoming more widespread and replacing the three-phase extraction system. It is in Italy that the changeover is taking place more slowly, but nevertheless the trend is clear.

We agree with your comment. We have consequently modified the conclusions.

 

Referee 2

The article "Olive mill wastewater as renewable raw materials to generate high added-value components for the agri-food industry" presented for review is interesting and the topic discussed is well-chosen.

The authors described all the potential uses of the olive mill wastewater and added the necessary information that could also provide a good summary of current trends in the food industry.

I only have two "cosmetic" comments

Please change the resolution of picture 1 now it is too small - I can't see the letters describing the charts please provide the title of point 6 - it is missing from the manuscript.

Thank you for improving the layout of our manuscript. We have modified the figure according to your suggestions. Moreover, the title of point 6 was changed. 

Round 2

Reviewer 1 Report

All changes and suggestions have been taken into account by the authors and have been duly responded to and accepted, substantially improving the quality of the manuscript. The revision is very complete and needs no further changes.