Evaluation of the Properties of NaC/PVA/CA Composite Films for Grape Packaging

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper uses sodium caseinate (NaC) and polyvinyl alcohol (PVA) as matrices, adding different concentrations (2.5%, 5%, 15%) of caffeic acid (CA) to prepare composite films via the solution casting method. The films were characterized by FTIR, mechanical testing, water vapor transmission rate (WVTR), and oxygen transmission rate (OTR). The NaC/PVA/5CA film was selected for a packaging preservation experiment of 'Crimson Seedless' table grapes, measuring weight loss, color, soluble solids, sugar composition, and changes in O2/CO2 within the packaging during 48 days of storage. The research is interesting, but the current version has many shortcomings and requires further revision.
1. Key parameters for mechanical testing are not clearly provided. The authors state that tests were conducted according to ASTM D882, but the crosshead speed, initial grip separation, and sample conditioning conditions (temperature, humidity) are not reported. Furthermore, Figure 3 only presents average tensile strength and elongation at break without providing full stress-strain curves, making it impossible to assess yield behavior, elastic modulus, or genuine changes in toughness. Representative stress-strain curves and detailed test conditions should be supplied.
2. The testing conditions for water vapor and oxygen barrier properties are critically missing. WVTR and OTR measurements are highly dependent on temperature and relative humidity, but the authors only refer to ASTM D6701 and ISO 15105-1 without reporting the actual test temperature (e.g., 23°C, 38°C) and relative humidity (e.g., 50%, 90%). Without this information, the data are not comparable across studies. Please provide the specific temperature and humidity conditions used.
3. The choice of caffeic acid concentrations (2.5%, 5%, 15%) lacks preliminary experimental or literature justification, and 15% is extremely high for biopolymer composite films. In the FTIR spectra, the 15% sample exhibits characteristic peaks of free caffeic acid molecules (812 cm⁻¹ and 776 cm⁻¹), indicating phase separation or aggregation, yet the authors provide no direct microstructural evidence (e.g., SEM cross‑sectional images, DSC thermograms, or XRD patterns) to confirm the extent of phase separation. Such characterization should be supplemented.
4. In the grape storage experiment, only NaC/PVA and NaC/PVA/5CA packaging treatments were selected, while films containing 2.5% and 15% caffeic acid were not included. The authors conclude that 5% is the optimal formulation, yet they did not verify the performance of other concentrations (especially 15%, which may cause negative effects) through actual fruit storage trials. This selective screening lacks methodological transparency. Please explain why only the 5% formulation was tested for packaging, or acknowledge this limitation.
5. The analysis of color change mechanisms lacks direct measurement of anthocyanin content. The authors attribute the higher hue angle (h°) observed in the NaC/PVA/5CA‑treated group to copigmentation or antioxidant modulation between caffeic acid and anthocyanins, but they did not measure total or individual anthocyanin contents. Speculating about intermolecular interactions based solely on L*a*b* parameters is insufficient. HPLC or pH‑differential method data for skin anthocyanin content at different storage times should be supplemented.
6. The gas composition data show that after 48 days, the CO₂ concentration reached as high as 30.28% while O₂ dropped to 5.84%. Such high CO₂ levels may cause physiological damage to table grapes, including browning, off‑flavors, or rachis decay. The authors only mention “may also pose a risk” in the discussion, but they do not report any actual sensory or quality deterioration indicators (e.g., off‑flavor scores, rachis browning index) in the results or conclusions. Please provide whether observable physiological injuries occurred under these high CO₂ conditions.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The work is interesting. The following issues should be solved:

1. What is the source of caffeic acid and how was integrated into films, should be given explanation even in abstract.
2. The table with the sample information, what was included in their preparation, should be given too.
3. The picture of film preparation should be included in the Material and Methods part.
4. The picture of texture analysis, used extension, should be shown too.
5. I do not see the reason why tables are in the supplement material.
6. The commas in tables should be replaced by dots.
7. Table S1 and S2 are not necessary the statistical differences should be present as upper script in tables.
8. The authors should give an idea how their experiment should be improved in the future. The perspective can be given in the conclusion part.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Article: Evaluation of the Properties of NaC/PVA/CA Composite Films for Grape Packaging

 

Review

 

General Comments

This manuscript investigates the development of glycerol-plasticized sodium caseinate/polyvinyl alcohol (NaC/PVA) composite films incorporating different concentrations of caffeic acid (CA) (0-15% w/w) via solution casting. The study aims to evaluate the effect of CA on the structural, mechanical, barrier, and postharvest performance of the films for grape packaging. The results indicate a concentration-dependent behavior, with 5% CA identified as the optimal formulation, showing improved water vapor barrier properties, balanced mechanical performance, and a positive effect on reducing weight loss and delaying sugar accumulation in grapes.

The manuscript presents a well-structured experimental study with consistent results identifying 5% CA as the optimal formulation. However, several aspects require improvement. The interpretation of FTIR results appears overstated without complementary evidence supporting intermolecular interactions. In addition, there is an inconsistency between the abstract and conclusions regarding oxygen barrier properties that should be clarified. The discussion on postharvest performance could be further strengthened by including additional parameters such as microbial stability or oxidation, which would enhance the relevance for active packaging applications. Finally, some conclusions overstate the mechanism (e.g., metabolic regulation) without sufficient evidence. Strengthening these aspects would significantly improve the scientific rigor and impact of the manuscript. Therefore, I consider that the manuscript may be suitable for publication after minor revision.

 

Specific Comments

 

Below are my specific comments on the manuscript:

1. L97: The claim of scalability appears overstated, as the films were produced by solution casting. This should be moderated or better justified in terms of industrial feasibility.
2. There is an inconsistency between the abstract and the reported OTR results. This should be corrected to ensure alignment throughout the manuscript.
3. The abbreviation “NaC” is uncommon in the literature for sodium caseinate. More widely accepted abbreviations include “SC”. The authors are encouraged to adopt a standard terminology to improve clarity and consistency with existing literature.
4. Figure 1 is placed in the methodology section, but it shows the resulting films. It would be more appropriate to include it in the results section.
5. The control sample (SC/PVA) should be clearly included in Figures 3, 4 and 5. Its absence makes it difficult to properly compare the effect of caffeic acid incorporation and evaluate the relative changes in the measured properties.
6. Figure 3 shows a clear trend in mechanical behavior; however, several aspects should be improved. The control sample (SC/PVA) is not included, making it difficult to assess the actual effect of caffeic acid. The use of separate axes or independent plots is recommended to improve clarity. Finally, the unit for tensile stress should be corrected from “Mpa” to “MPa”.
7. Section 3.1: The FTIR results suggest possible intermolecular interactions between CA and the SC/PVA/Glycerol matrix; however, they are not sufficient to confirm strong hydrogen bonding or physical crosslinking. The authors should be cautious with the claim of crosslinking, as FTIR alone does not provide direct evidence of network formation. In addition, the disappearance or weakening of bands at 2941 and 1328 cm^–1 is interpreted as evidence of maximum polymer-additive interaction at 5% CA. This conclusion appears speculative and should be moderated. Terms such as “strong molecular interactions”, “compact network”, “integrated network structure”, and “maximum interaction level” should be toned down unless supported by complementary characterization.
8. The tensile test was performed according to ASTM D882, but Young’s modulus results are not reported. This parameter is essential to evaluate stiffness and support the interpretation of structural interactions. The authors should include it or justify its absence.
9. Section 3.4: The OTR values are reported as cm³/m²·24 h·0.1 MPa, which differs from the standard expression typically used (atm). The reported decrease in OTR at 5% CA is relative to the 2.5% formulation and not to the control film. This should be clearly stated to avoid misleading interpretation.
10. The film thickness is not reported or discussed. This parameter is critical for interpreting barrier and mechanical properties, as OTR and WVTR are strongly dependent on thickness. The authors should include thickness measurements and clarify whether the results were normalized accordingly.
11. Section 3.9: The observed decrease in O₂ and increase in CO₂ are attributed to fruit respiration within the packaging. However, since the system is fully sealed, these changes are expected in a closed environment and should not be solely attributed to the film properties. The statement that the packaging system effectively restricted gas exchange is not fully supported, as the closed configuration itself inherently limits gas exchange with the external environment.
12. Section 3.10: the observed differences in sugar composition can be largely explained by water loss and concentration effects, as well as normal respiration-related processes. Therefore, the interpretation of a direct modulation of metabolic activity by the packaging appears overstated and should be moderated.

 

Final recommendation: Minor revision.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript can be accepted