Extraction of Grape Juice: Impact of Laboratory-Scale Pressing Methods on the Chemical Composition

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript by Shanshiashvili et al. studied laboratory-scale pressing methods for the chemical composition of grape juice.  Please see my comments to improve the manuscript.

-Line 114-115, please provide the purity of these reagents.

-Line 122-123, please provide more details of the vineyards, such as location, agronomy conditions, plant growing conditions.

-Line 125-126, please provide more information on the fruit sample. Were all the fruit sterilized? How to remove the pedicel/vines/seeds. Were all the fruit peeled?

-Line 135-139, what was the degree of vacuum?

-Regarding pressing methods, what were the criteria for preparing juice? Is it based on the water/juice content of the residue? How is the approximate preparation time estimated?

-Line 160, the samples were peeled? Was the fruit frozen sample?

-Table 2, please provide the juice pictures in the table.

Author Response

The manuscript by Shanshiashvili et al. studied laboratory-scale pressing methods for the chemical composition of grape juice.  Please see my comments to improve the manuscript.

Comment 1: Line 114-115, please provide the purity of these reagents.

Response 1: As the reviewer suggested, the purity was added to the reagents. “Folin-Ciocalteu reagent, gallic acid (99.85 % purity), sodium carbonate (99.99 % purity) were purchased from Merck (St Louis, MO, USA)’’. (lines 116-117).

Comment 2: Line 122-123, please provide more details of the vineyards, such as location, agronomy conditions, plant growing conditions.

Response 2: We provided more information about the location in lines 127-132: “The grape samples are grown in a limited area (latitude: 45.68-45.54; longitude 9.91-10.20) where the 8 vineyards are located (Figure 1). All the vines are pruned using the classic Guyot system.” Moreover, Figure 1 was added with the shape of Franciacorta area where the sampled vineyards are pointed.

Comment 3: -Line 125-126, please provide more information on the fruit sample. Were all the fruit sterilized? How to remove the pedicel/vines/seeds. Were all the fruit peeled?

Response 3: The grape samples were not sterilized and peeled prior the pressing (line 156-157). The grape berries were manually destemmed before the preparation that juice was collected without seeds and skins. We specified the latter aspect for manual (line 160) and vacuum (line 166) pressing methods.

 

Comment 4: -Line 135-139, what was the degree of vacuum?

Response 4: The degree of vacuum was approximately 1mbar (lines 164-165).

 

Comment 5: -Regarding pressing methods, what were the criteria for preparing juice? Is it based on the water/juice content of the residue? How is the approximate preparation time estimated?

Response 5: The yield of grape juice ranged 60-65% for all the methods applied in order to compare the results obtained. Consequently, the preparation time was fixed accordingly to the juice yield (line 155-157). 

 

Comment 6: -Line 160, the samples were peeled? Was the fruit frozen sample?

Response 6: The grape samples were not peeled or frozen before the homogenization. Grape samples were kept for 4 °C overnight. (line 149)

 

Comment 7: -Table 2, please provide the juice pictures in the table.

Response 7: Unfortunately, we are not able to provide the pictures of the experimental juices since we did not take any pictures during the sample preparation. We observed the major visual difference for the juice samples made with the juicer because of its turbidity just after the sample preparation. Nonetheless, most of the parameters were determined on clear samples being centrifuged before the analysis. We believe this lacking would not represent a limit of our paper.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The aim of the reviewed article was to determine the effects of different pressing methods on the general chemical parameters (sugars, pH, titratable acidity) as well as on phenolic-related indexes and antioxidant capacity, for the laboratory-made juice obtained from and Pinot blanc over two vintages. The authors used four techniques to obtain juice from the grape fruits mentioned above, namely: manual pressing, vacuum pressing, using a juicer and a small screw press.

 

The literature introduction is written correctly and provides an introduction to the justification for undertaking the research.

The description of the research methods does not raise any major objections.

 

It should be emphasized, however, that the research methods used are mainly titrimetric and spectrophotometric methods. The authors did not attempt to determine individual polyphenolic compounds using chromatographic methods, which would certainly be an added value of this study.

The results are described correctly.

 

Discussing the results obtained with the literature is not the article's strong point. The research methods used in the article are routine and well-known enough, and the composition of the raw material itself and the juice obtained from it are so well described in the literature that one could attempt to compare the data obtained with the literature data.y the discussion of the obtained results with the literature is exhaustive.

 

 

 

Below are my detailed comments.

Line 128: The text and Table 1 describe four juicing techniques, not five methods as the authors suggest.

 

Lines 127-155: Why did the authors not use the same juicing time conditions for each of the four pressing methods used?

 

Line 160: Use “10 g” instead of “ten g”.

Line 161: In the expression "20±2°C" there should be a space between the number and the unit.

Lines 161, 164: “min.” instead of “minutes”.

Line 238: Molar concentration is expressed using the units mol/L or M. Therefore, the unit M/L (mM Trolox eq./L) is not correct.

Lines 407-408 and 443-444: Why are only 3 pressing methods included in Tables 3 and 4? Why is the pressing method using a juicer omitted? Appendix 1 only contains information on the chemical composition of lab-scale Chardonnay and Pinot blanc juice samples obtained by juicer in 2022 vintage. What about the 2023 vintage? The article should include a commentary on this pressing method. If the authors have for some reason not included the juicer results in Tables 3 and 4, the reader should at least be referred to appendix 1.

Author Response

The aim of the reviewed article was to determine the effects of different pressing methods on the general chemical parameters (sugars, pH, titratable acidity) as well as on phenolic-related indexes and antioxidant capacity, for the laboratory-made juice obtained from and Pinot blanc over two vintages. The authors used four techniques to obtain juice from the grape fruits mentioned above, namely: manual pressing, vacuum pressing, using a juicer and a small screw press. 

The literature introduction is written correctly and provides an introduction to the justification for undertaking the research.

The description of the research methods does not raise any major objections.

Comment 1: It should be emphasized, however, that the research methods used are mainly titrimetric and spectrophotometric methods. The authors did not attempt to determine individual polyphenolic compounds using chromatographic methods, which would certainly be an added value of this study.

Response 1: We agree with the reviewer; surely, the qualitative analysis of phenolics can add value to this research. However, the assessment of both general chemical parameters and phenolic-related indexes in grape juice samples obtained with different lab-scale pressing method was carried out for the first time. In accordance to the reviewer’s comment, we specified this in lines 572-576: “It should be considered that quantitative analyses of phenols were carried out; qualitative assessment of phenolics may explain more accurately the differences among the pressing methods. However, this is the first investigation related to the monitoring of both general chemical parameters and phenolic-related indexes in grape juice samples obtained with different lab-pressing methods and addressed to the sparkling wine production.”

Comment 2: The results are described correctly.

Discussing the results obtained with the literature is not the article's strong point. The research methods used in the article are routine and well-known enough, and the composition of the raw material itself and the juice obtained from it are so well described in the literature that one could attempt to compare the data obtained with the literature data and the discussion of the obtained results with the literature is exhaustive.

Response 2: We are conscious some of the analytical methods applied are well-known. Nonetheless, the Raman technique, allowing to obtain the fingerprint, was quite limited for fruit juices as well as on their classification, and particularly the classification of juices from different pressings. 

 

Below are my detailed comments.

Comment 3: Line 128: The text and Table 1 describe four juicing techniques, not five methods as the authors suggest.

Response 3: Amended (line 154).

 

Comment 4: Lines 127-155: Why did the authors not use the same juicing time conditions for each of the four pressing methods used?

Response 4: The time of pressing depends on the method applied itself. The manual and in-bag crushing of the berries required less time in comparison to the juicer or the small screw press due to the impact of the working mode of the systems. However, the juice yield was comparable among methods, so we can compare the data obtained (lines 155-157).  

 

Comment 5: Line 160: Use “10 g” instead of “ten g”.

Response 5: The sentence was modified as follow: “The homogenized samples (10 g) were added with 10 mL of hydrochloric solution…” Lines (190-191).

 

Comment 6: Line 161: In the expression "20±2°C" there should be a space between the number and the unit.

Response 6: Amended (line 191).

 

Comment 7: Lines 161, 164: “min.” instead of “minutes”.

Response 7: ‘Amended (lines 161, 167, 173, 181, 192, 194, 254).

 

Comment 8: Line 238: Molar concentration is expressed using the units mol/L or M. Therefore, the unit M/L (mM Trolox eq./L) is not correct.

Response 8: We would apologize for this fault. The measurement unit was adjusted throughout the text (lines 270)

 

Comment 9: Lines 407-408 and 443-444: Why are only 3 pressing methods included in Tables 3 and 4? Why is the pressing method using a juicer omitted? Appendix 1 only contains information on the chemical composition of lab-scale Chardonnay and Pinot blanc juice samples obtained by juicer in 2022 vintage. What about the 2023 vintage? The article should include a commentary on this pressing method. If the authors have for some reason not included the juicer results in Tables 3 and 4, the reader should at least be referred to appendix 1.

Response 9: Comments related to the juice extraction with juicer were reported in lines 409-422. Specifically: “However, the production of juice samples by the juicer was not carried out in vintage 2023. This is because the Principal Component Analysis (PCA) showed the juice samples obtained by the juicer with both Chardonnay and Pinot blanc grapes resulted clearly clustered (Figure 2) indicating these juice samples differed from the juice samples produced by the other methods investigated (manual, vacuum and small screw press). ... The clustering was mainly due to the total phenol index (TPI), color index and antioxidant capacity for Chardonnay grapes (Figure 2B), and total FLVs and TPI for the Pinot blanc (Figure 2D). Moreover, higher pH values for the juice sample obtained from juicer (Table S1) can be attributable to skin disruption that can favor the extraction of potassium leading to the precipitation of tartaric acid and the consequent pH drop.”

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript "Extraction of grape juice: impact of laboratory-scale pressing methods on the chemical composition" presents the impact of grape juice extraction methods on its biochemical parameters. The research assumption and the way it was implemented are correct. However, from the scientific point of view and for better readability of the manuscript, there is a need for some correction. Comments on the manuscript are included in the attached file.

Comments for author File: Comments.pdf

Author Response

Comment 1: Line 121-126 Inaccurate description of sampling from what area the samples were taken and in what model the sampling was performed. Were 8-10 kg collected in total or from one vineyard? Provide GPS locations of the Vineyards. Provide general meteorological data for 2022 and 2023.

Response 1: As the reviewer suggested, further information was reported. Specifically: “Chardonnay (CH) and Pinot blanc (Pb) grape samples (around 8-10 kg each) were taken from the sorting belt in order to have a homogenous grapple sample. The sampling was carried out in 6 different wineries for Chardonnay grape and 2 wineries for Pinot blanc grape of the Franciacorta area (Italy Lombardy) in two consecutive harvests, specifically in 2022 and 2023. The grape samples are grown in a limited area (latitude: 45.68-45.54; longitude 9.91-10.20) where the 8 vineyards are located (Figure 1). All the vines are pruned using the classic Guyot system.” (lines 123-129).

With regards to the meteorological data, we implemented the paper specifying the determination of certain parameters, mentioned in lines 134-149: “An agrometeorological analysis of the two seasons was based on the Corte Franca weather station (Brescia Province Agrometeorological Network), representative of the Franciacorta PDO environmental conditions. 2022 and 2023 were compared to the reference time-series 1997-2023. The analysis focused on the comparison between the time-series and the experimental seasons, with reference to:

-             Tavg – mean yearly temperature

-             HD – number of hot days (Tmax>32°C)

-             Ptot – total yearly precipitation and number of rainy days

-             Pgs – precipitation the growing season (April-August)

-             RDtot – yearly rainy days

-             RDgs – rainy days along the the growing season (April-August).

Based on the comparison with the historical data, values exceeding 1 standard deviation from the average value, are considered anomalous. Grapes were collected at the harvest date chosen by the winemakers, approximately 7-20 August for vintage 2022 and 15-31 August for vintage 2023, accordingly to their suitability for the sparkling wine production.” The results were reported in Results and Discussion section in lines 308-347; Table 1 was included in the text.

 

Comment 2: Line 123-124 What were the harvests dates?

Response 2: As the reviewer suggested, “Grapes were collected at the harvest date chosen by the winemakers, approximately 7-20 August for vintage 2022 and 15-31 August for vintage 2023, accordingly to their suitability for the sparkling wine production (around 19°Brix).” (lines 147-149). These dates are in accordance to the Consorzio per la tutela del Franciacorta indicating the harvest at about 19° Brix.

 

Comment 3: Line 124 What was the acidity like?

Response 3: The data concerning the titratable acidity of the grape juices are presented below in the paper (Tables 4 and 5). For this reason, we did not include this data in the description of the grape samples.

 

Comment 4: Line 128 Table 1 lists 4 methods

Response 4: Amended (line 154).

 

Comment 5: Line 160-161 Describe what kind of solution it was

Response 5: The composition of hydrochloric ethanol was described in Section 2.1: ‘’The hydrochloric ethanol was prepared as follows: ethanol/water/hydrochloric acid 116 37% 70/30/1 (v/v/v)’’. (lines 118-119).

 

Comment 6: Line 166-167 Model wine solution what is it?

Response 6: The composition of hydrochloric ethanol was described in Section 2.1: ‘’The model wine solution contained tartaric acid 5.0 g/L, ethanol 12% (v/v) with pH 3.2 adjusted with sodium hydroxide’’. (lines 120-121).

 

Comment 7: Line 170 The topic has already been so researched that recording the spectrum in the given range is unnecessary, since the measurement was made at 280 nm anyway.

Response 7: We would prefer to leave the registration of the absorbance range (700-230 nm) because the method considered the derivative of the peak at 280 nm.

 

Comment 8: Line 194 The recommended measuring range for absorbance is 0.2 to 0.8, not 1.

Response 8: We are grateful with the reviewer; we adjusted the maximum reading of the absorbance as 0.8±0.05 AU (line 224).

 

Comment 9: Line 205 Reading was performed on what? Specify model I understand that other spectrophotometric measurements were performed on the same device.

Response 9: Spectrophotometer model was reported (line 235-236).

 

Comment 10: Line 210 Describe the interpretation of the color index, usually the CIE Lab scale is used to measure color.

Response 10: We agree that the CIELab is used for measuring the color. However, the absorbance reading at 420 nm (color index) is considered a suitable measurement for determining the browning for must and wine from white grape. The absorbance at 420 nm represents an overall measurement of specific consequences of oxidation reactions (Gabrielli et al., 2021, Food Chemistry 348, 128922; doi: https://doi.org/10.1016/j.foodchem.2020.128922). Thus, differences of the absorbance value at 420 nm among the extraction methods assayed towards higher values indicate a major oxidation of the must.  

 

Comment 11: Line 214 Briefly state the conversion method used

Response 11: The turbidity was estimated by means of the absorbance reading at 750 nm that was converted in NTU through the correlation reported by Goodner (2009). (lines 245-247). 

 

Comment 12: Line 217 Please include PPO data in Tables 4 and 3.

Response 12: As the reviewer suggested, the data was reported Tables 4 and 5 as well as in Table S1. 

 

Comment 13: Line 231 The recommended measuring range for absorbance is 0.2 to 0.8, not 1.

Response 13: The absorbance mentioned in the text was related to the diluted solution of DPPH. Once the reaction was carried out, lower absorbance values were found being included in the linearity of Lambert-Beer law. 

 

Comment 14: Line 231 Measurement at 516 nm is recommended

Response 14: The protocol applied for the determination of PPO activity is based on the oxidation of catechol leading to the browning of the medium, specifically the grape juice samples. For this reason, we measured the absorbance at 400 nm.

 

Comment 15: Line 240-248 What parameters were studied using Raman spectroscopy

Response 15: We recorded the Raman spectra corresponding to the reaction of the matter when irradiated by a given wavelength at 785 nm, modifying the vibrational or rotary energy of the molecules present in the matter from an inelastic scattering. Thus, the resulting spectra depends on the composition of the matter (Yang, D., & Ying, Y. 2011, Applied Spectroscopy Reviews 46, 539-560; doi: https://doi.org/10.1080/05704928.2011.593216). (lines 272-276).

 

Comment 16: Line 257 The work uses extensive statistical analysis, including Random Forest (RF) analysis and Support Vector Machine (SVM), i.e. elements and techniques of machine learning in data exploration. The techniques used are intended to increase the scientific value of the manuscript. However, the question arises whether this makes sense in the development of the presented data. Because the amount of processed data is low, and moreover, the data obtained are related to results whose repeatability due to numerous interfering factors (precipitation, temperature, soil type or its moisture) is questionable. Such computational techniques make sense in models with a smaller number of interfering factors. I suggest introducing a more pictorial representation of the dynamics of changes in the results obtained for 2022 and 2023 using a heat map, a package that is also present in R Sudio.

Response 16: The heterogeneity of grapes, due to many factors like those you cited, is a reality and is inherent to the product analyzed. The fact that we analyze juices limits this bias since we use a certain number of berries, which are supposed to be representative of the parcel. Since we compared different juices from different batches of grapes harvested at the same time and being representative of the material allowed us to such data treatments. To check the repeatability of the results, we did the same experimental procedures and analyses for two vintages and in different wineries to determine the impact of the kind of lab scale pressings. The objective was to determine if the type of lab scale pressing impacted the quality of the juice obtained. If the data are well classified in their type of press, that means that the product is highly impacted by the processes. The classification methods used in this work only aim to reach that objective and are very easy to understand by looking at the data. And the data treatments used are compatible with small amounts of data (refs). A pictorial representation could have been interesting, but it is only possible with the physicochemical data and not with Raman data (Gabrielli et al., 2024, Chemosensors 9, 71; doi: https://doi.org/10.3390/chemosensors9040071; Khiari et al., 2024, Journal of Food Measurement and Characterization, 1-14 doi: https://doi.org/10.1007/s11694-024-03036-1; Gabrieli et al., 2023, Journal of the Science of Food and Agriculure 10, 452-462; doi: https://doi.org/10.1002/jsf2.150). We think that using the same method for the 2 data set is better and stronger since the spectra could contain more information than the targeted analyses.

 

Comment 17: Table 2: Provide statistically significant differences between means. In column headings, add in brackets the abbreviations used in the text. (Original comment: Table 2 Podać różnice między  średnimi statystycznie istotne. W nagłówkach kolumn dopisać w nawiasach stosowane  w tekście skróty)

Response 17: As the reviewer suggested, significant differences were added in the tables. Abbreviations were also added in Table 2.

 

Comment 18: Table 3: In the column headings, add the abbreviations used in the text in brackets. (Original comment: Table 3. W nagłówkach kolumn dopisać w nawiasach stosowane w tekście skróty)

Response 18: The abbreviations were added in Table 3 as well as in supplementary table S1.   

 

Comment 19: Table 4. In the column headings, add the abbreviations used in the text in brackets. (Original comment: Table 4. W nagłówkach kolumn dopisać w nawiasach stosowane w tekście skróty)

Response 19: The abbreviations are added to the tables. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors improved the manuscript, it can be accepted in the present form.

Reviewer 2 Report

Comments and Suggestions for Authors The text of the article has been corrected and is suitable for publication.