Moisture Sorption Isotherms and Thermodynamic Properties of Biodegradable Polymers for Application in Food Packaging Industry

Round 1

Reviewer 1 Report

An interesting article in which the analysis of water adsorption of two samples of polymers containing starch is given. The GAB model was chosen to analyze the results. The publication touches on a very topical topic related to the use of biopolymers as food packaging. The results of the presented studies can be used to improve the quality of such packaging, which will help to expand the scope of its application.

 

However, there are a number of comments that will help improve the quality of the publication.

1. The materials and methods do deficiency describe the analyzed samples. What geometry did the samples have: films, granules, powder ..? You can add a photo.

2. In the results, line 195 - 240 and table 1. It is not clear what kind of material this is, are these paragraphs written on the basis of an analysis of the literature, or are the results of using different models by the authors? If these are the results, then why are references to the literature provided. If the authors thus wanted to point to the original source of the models, then it might be correct to present these links in Table 1.

3. The text uses a different style of references to literature.

4. Line. 206 “….(Lomauro et al. 1985)…” could not be found in the References.

5. Line. 211 “…monolayer…”, how was the monolayer defined? Add in materials and methods.

6. Line. 247 – 248 The authors talk about the chemical binding of water. This statement is not confirmed in the work. To do this, it is necessary to carry out at least an IR spectroscopy of the samples. IR spectroscopy would greatly improve the article.

7. Line. 366 The authors indicate the presence of micropores and mesopores, but the conclusion says that the material contains micropores and macropores p. 393. Which pores are present in the material? How was the porosity of the material determined, and the classification of the pores?

Author Response

Comment: An interesting article in which the analysis of water adsorption of two samples of polymers containing starch is given. The GAB model was chosen to analyze the results. The publication touches on a very topical topic related to the use of biopolymers as food packaging. The results of the presented studies can be used to improve the quality of such packaging, which will help to expand the scope of its application. However, there are a number of comments that will help improve the quality of the publication.

Response: The authors would like to thank Reviewer 1 for taking into account the scientific quality of our manuscript and considering it worthy of publication after some improvements. We answered all of your concerns and suggestions in order to make our manuscript worthy of publication. 

 

Comment: 1. The materials and methods do deficiency describe the analyzed samples. What geometry did the samples have: films, granules, powder ..? You can add a photo.

Response: Thank you for the observation. The geometry of samples is granules. The photos were added to the revised manuscript (Line 91).

 

Comment .2. In the results, line 195 - 240 and table 1. It is not clear what kind of material this is, are these paragraphs written on the basis of an analysis of the literature, or are the results of using different models by the authors? If these are the results, then why are references to the literature provided. If the authors thus wanted to point to the original source of the models, then it might be correct to present these links in Table 1.

Response: We greatly appreciate your comment. The materials are granules. The paragraphs were written based on the results that we obtained using different models (GAB, Halsey, Smith, Oswin, and Peleg models), as presented in Table 1. The column with the original source of the models was added. According with our results, the GAB and Peleg models were the best ones to describe the experimental adsorption data, considering the higher values of coefficient determination (R²) and the lowest values of average relative deviation (E). However, the Peleg model failed to describe the sorption isotherm at 50 °C for both samples since the values of E were higher than 10%, while the GAB model presented values of E lower than 10% for all isotherms. Also, the GAB model has been recommended by the European Project Group on COST 90 as the fundamental model to determine the moisture content present in the materials. The references were included (Table 1, Line 326).

 

Comment: 3. The text uses a different style of references to literature.

Response: Thanks for the observation. All the references were checked and corrected according to the guidelines for Polymer citation style.

 

Comment: 4. Line. 206 “….(Lomauro et al. 1985)…” could not be found in the References.

Reference: Thanks for the observation. The reference was included (Line 273).

 

Comment: 5. Line. 211 “…monolayer…”, how was the monolayer defined? Add in materials and methods.

Response: We appreciate this observation. The monolayer moisture content value, X_m, represents the monolayer moisture content on a dry basis, i.e. the water content corresponding to saturation of all primary adsorption sites by one water molecule [12]. The definition was added to the revised manuscript (Line 224).

 

Comment: 6. Line. 247 – 248 The authors talk about the chemical binding of water. This statement is not confirmed in the work. To do this, it is necessary to carry out at least an IR spectroscopy of the samples. IR spectroscopy would greatly improve the article.

Response: Thanks for the salient observation. Fourier-transform infrared spectroscopy (FTIR) technique was used to obtain an infrared spectrum of absorption of samples. FTIR spectra were collected on a Bruker INVENIO S FTIR spectrometer equipped with an attenuated total reflection (ATR) attachment using platinum ATR (A225/Q) with a diamond crystal plate as an accessory. For each sample, the infrared (IR) spectra were recorded in the range of 4000–500 cm⁻¹ at a resolution of 4 cm⁻¹ and 64 scans.

The results of this study are presented in the revised manuscript.  The attenuated total reflectance–Fourier-transform infrared spectroscopy (ATR-FTIR) technique was used to assess the molecular structure, chemical bonds, and changes that can occur during the moisture sorption isotherm of two starch-based materials (B16 and B20) (Figure 2). The starch molecular structure consists of amylopectin an amylose which have α-1,4- and α-1,6-linked glucose units, respectively [13]. The following peaks were identified in both samples: broad absorption bands between 3000 and 3600 cm⁻¹ is attributed to the complex vibrational stresses associated with the chemical bonds (free, inter- and intra-molecular) of hydroxyl groups [14]. Two weak peaks at 2951 cm⁻¹ and 2860 cm⁻¹ indicate the C–H stretching of the glucose ring, and the peak at 1720 cm−1 indicates the presence of C=O bonds. The peak at 1420 cm⁻¹ is associate with CH2 vibrations, 1330 cm⁻¹ (CH bending), 1270 cm⁻¹ (C–O–C stretching), 1150 cm⁻¹ (C−O−C bonds), ~1049 cm⁻¹ (C−O stretching) and 730 cm⁻¹ (bending modes of C–H bonds in aromatic rings). B16 and B20 samples showed absorption peaks in the same wavenumber regions, which reveals that the two samples possess similar functional groups. However, B16 showed higher peak intensities between 3000 and 3600 cm−1 than B20, indicating a molecular structure with more hydroxyl groups available to interact with polar water molecules when exposed to high relative humidity environments. B20 exhibited a higher intensity of peaks at 2951 cm−1 and 2860 cm−1 than B16, indicating more available C–H groups that were not much affected by the presence of moisture content. Starch is highly hygroscopic and the degree of moisture absorption is affected by the hydrogen bonds formed by the hydroxyl groups of the aminoglycoside units along the chain [15]. In this sense, the results of FTIR spectra can be used to adjust the amount of starch content in the polymer formulation in order to absorb a known quantity of moisture content, which is very essential for progress in starch-based materials for food packaging applications. The information was added to the revised manuscript (Lines 96 and 234).

 

Comment: 7. Line. 366 The authors indicate the presence of micropores and mesopores, but the conclusion says that the material contains micropores and macropores p. 393. Which pores are present in the material? How was the porosity of the material determined, and the classification of the pores?

Response:  Thanks for the salient observation. According to the IUPAC classification, pores are classified into the following groups: macropores >50 nm, mesopores in the range of 2.0 to 50 nm and micropores <2.0 nm (Line 443).

According to our results, the pore radius (Rp) for B16 ranged from 0.92 to 7.18 (temperature of 30 °C), 0.87 to 6.62 nm (temperature of 40 °C), and 0.83 to 6.57 nm (temperature of 50 °C), and for B20 ranged from 1.08 to 35.27 nm (temperature of 30 °C), 0.96 to 17.62 nm (temperature of 40 °C), and 0.76 to 12.93 nm (temperature of 50 °C), with moisture contents varying from 0.01 to 0.2 g water·g^-1 db (Table 2). Based on the IUPAC classification, we can conclude that the samples contain micropores and mesopores, and not macropores. The conclusion section was corrected, and the words "micropores and macropores" were changed to "micropores and mesopores". We are aware that the nitrogen adsorption isotherm technique is the most recommended and used technique to determine the pore size of samples. However, the results of the water adsorption isotherm are also used to determine the pore size of the samples. The size of the water molecules (0.2 nm) is smaller than the nitrogen molecule (0.43 nm), and some narrow pores are penetrated by water molecules, while the nitrogen molecules are unable to penetrate the whole existing pore space. In future studies, we will apply both techniques to compare the results. The above information was added to the revised manuscript (Lines 443 and 482).

Reviewer 2 Report

 

The manuscript “Moisture sorption isotherms and thermodynamic properties of biodegradable polymers for application in food packaging industry” is a good article and matches the scope of the journal. This study aimed to investigate the impact of two starch-based materials, B16 and B20, on moisture sorption isotherms at different temperatures. B16 contains a higher amount of starch (5% w/w) than B20. The researchers used thermodynamic functions to gain insights into water binding behaviors and energy requirements for moisture removal from the material surfaces.

I listed below some of my major concerns:

1.                  It would have been helpful if the study had provided more information about the preparation and characteristics of the B16 and B20 materials, such as particle size distribution, surface area, and morphology using N₂ adsorption-desorption isotherms to confirm their obtained results.

2.                  The study could have included additional analyses, such as Fourier-transform infrared spectroscopy (FTIR) or X-ray diffraction (XRD), to gain more insights into the structural and chemical changes that occur during moisture sorption.

3.                  The study only evaluated two starch-based materials, and it would be interesting to see how other types of starches or other biopolymers compare in terms of their moisture sorption behavior.

4.                  The study could have explored the effect of other factors that can influence moisture sorption behavior, such as pH, salt content,....

5.                  The language in the manuscript is generally clear, but there are some grammatical errors that need to be addressed.

6.                  It would be helpful if the authors provided a more detailed conclusion that summarizes the key findings and practical implications of the study.

Author Response

Comment: 1. The manuscript “Moisture sorption isotherms and thermodynamic properties of biodegradable polymers for application in food packaging industry” is a good article and matches the scope of the journal. This study aimed to investigate the impact of two starch-based materials, B16 and B20, on moisture sorption isotherms at different temperatures. B16 contains a higher amount of starch (5% w/w) than B20. The researchers used thermodynamic functions to gain insights into water binding behaviors and energy requirements for moisture removal from the material surfaces. I listed below some of my major concerns.

Response: The authors would like to thank Reviewer 1 for taking into account the scientific quality of our manuscript and considering it worthy of publication after some improvements. We answered all of your concerns and suggestions in order to make our manuscript worthy of publication. 

 

Comment:  1.  It would have been helpful if the study had provided more information about the preparation and characteristics of the B16 and B20 materials, such as particle size distribution, surface area, and morphology using N₂ adsorption-desorption isotherms to confirm their obtained results.

Response: Thanks for the suggestion. We have added a photo that shows the size and morphology of the granules used in this study (Figure 1). Regarding the suggestion for determining the N₂ adsorption-desorption isotherms, we agree with the reviewer's observation on the importance of this technique to determine the surface area and pore size of the samples. Unfortunately, at this moment, we have no possibility of performing the experiment. We are also aware that the nitrogen adsorption isotherm technique is the most recommended and used technique to determine the pore size of samples. However, the results of the water adsorption isotherm are also used to determine the pore size of the samples. The size of the water molecule (0.2 nm) is smaller than the nitrogen molecule (0.43 nm), and some narrow pores are penetrated by water molecules, while the nitrogen molecules are unable to penetrate the whole existing pore space. In future studies, we will apply both techniques to compare the results, as suggested in the section of conclusion (Line 482).

 

 

Comment: 2.  The study could have included additional analyses, such as Fourier-transform infrared spectroscopy (FTIR) or X-ray diffraction (XRD), to gain more insights into the structural and chemical changes that occur during moisture sorption.

Response: Thanks for the salient observation. Fourier-transform infrared spectroscopy (FTIR) technique was used to obtain an infrared spectrum of absorption of samples. FTIR spectra were collected on a Bruker INVENIO S FTIR spectrometer equipped with an attenuated total reflection (ATR) attachment using platinum ATR (A225/Q) with a diamond crystal plate as an accessory. For each sample, the infrared (IR) spectra were recorded in the range of 4000–500 cm⁻¹ at a resolution of 4 cm⁻¹ and 64 scans.

The results of this study are presented in the revised manuscript.  The attenuated total reflectance–Fourier-transform infrared spectroscopy (ATR-FTIR) technique was used to assess the molecular structure, chemical bonds, and changes that can occur during the moisture sorption isotherm of two starch-based materials (B16 and B20) (Figure 2). The starch molecular structure consists of amylopectin an amylose which have α-1,4- and α-1,6-linked glucose units, respectively [13]. The following peaks were identified in both samples: broad absorption bands between 3000 and 3600 cm⁻¹ is attributed to the complex vibrational stresses associated with the chemical bonds (free, inter- and intra-molecular) of hydroxyl groups [14]. Two weak peaks at 2951 cm⁻¹ and 2860 cm⁻¹ indicate the C–H stretching of the glucose ring, and the peak at 1720 cm−1 indicates the presence of C=O bonds. The peak at 1420 cm⁻¹ is associate with CH2 vibrations, 1330 cm⁻¹ (CH bending), 1270 cm⁻¹ (C–O–C stretching), 1150 cm⁻¹ (C−O−C bonds), ~1049 cm⁻¹ (C−O stretching) and 730 cm⁻¹ (bending modes of C–H bonds in aromatic rings). B16 and B20 samples showed absorption peaks in the same wavenumber regions, which reveals that the two samples possess similar functional groups. However, B16 showed higher peak intensities between 3000 and 3600 cm−1 than B20, indicating a molecular structure with more hydroxyl groups available to interact with polar water molecules when exposed to high relative humidity environments. B20 exhibited a higher intensity of peaks at 2951 cm−1 and 2860 cm−1 than B16, indicating more available C–H groups that were not much affected by the presence of moisture content. Starch is highly hygroscopic and the degree of moisture absorption is affected by the hydrogen bonds formed by the hydroxyl groups of the aminoglycoside units along the chain [15]. In this sense, the results of FTIR spectra can be used to adjust the amount of starch content in the polymer formulation in order to absorb a known quantity of moisture content, which is very essential for progress in starch-based materials for food packaging applications. The information was added to the revised manuscript (Lines 96 and 234).

 

Comment: 3. The study only evaluated two starch-based materials, and it would be interesting to see how other types of starches or other biopolymers compare in terms of their moisture sorption behavior.

Response: Thanks for the suggestion. We included other studies that reported moisture sorption behavior. Namely, Viollaz and Rovedo [29] reported sorption data for starch at four temperatures, obtaining the following GAB parameters: X_m values of 0.10221, 0.09828, 0.08573, and 0.08738; k values of 0.78696, 0.78848, 0.79277, and 0.71900; and C values of 28.4147, 18.9935, 18.0858, and 11.3984 for 2.7 °C, 20.2 °C, 40.2 °C, and 67.2 °C, respectively. Moghaddam et al.[30], produced blends of polylactic acid plasticized with acetyl tributyl citrate and thermoplastic wheat starch (TPS), reporting an increasing X_m value with an increased TPS ratio due to its hydrophilic nature. These authors also reported that the increase in TPS content favors the sample surface's ability to absorb more water molecules. The information was added to the revised manuscript (Line 307).

 

Comment: 4. The study could have explored the effect of other factors that can influence moisture sorption behavior, such as pH, salt content.

Response: Thanks for the suggestion. In this study, we focused on understanding the effect of starch content on the moisture sorption isotherm of two starch-based materials. In other words, our focus was on the final product, that is, two types of starch-based polymers in granule form with different starch contents. Yes, pH and salt content can influence moisture absorption behavior. The reviewer's suggestion is mainly associated with the preparation, production and applications stages of these polymers. Therefore, these suggestions will be considered in our future work in this field. 

 

Comment: 5. The language in the manuscript is generally clear, but there are some grammatical errors that need to be addressed.

Response: We appreciate this observation. Following your feedback, which we truly appreciate, our team has again revised the article to eliminate any misinterpretations.

 

Comment: 6.  It would be helpful if the authors provided a more detailed conclusion that summarizes the key findings and practical implications of the study.

Response: The section of conclusion was rewritten considering the reviewer's suggestions (Lines 463, 468, 474 and 482).

Round 2

Reviewer 2 Report

Accept in current form