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Article

Nanocomposite Coatings of Pectin and Oxide Zinc Nanoparticles to Increase Papaya Shelf Life

by
Joelma Saures dos Santos
1,
Caroline Cagnin
2,
Bheatriz Silva Morais de Freitas
3,
Richard Marins da Silva
1,
Glaydson Brasileiro Lopes de Jesus
1,
Celso Martins Belisário
1,
Mariana Buranelo Egea
1,
Josemar Gonçalves de Oliveira Filho
1,* and
Geovana Rocha Plácido
1
1
Department of Food Science, Instituto Federal Goiano, Rio Verde 75901-970, GO, Brazil
2
Department of Food Science, Universidade Estadual de Londrina, Londrina 86051-990, PR, Brazil
3
Department of Food Science, Universidade Federal de Santa Catarina, Florianópolis 88040-900, SC, Brazil
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(8), 990; https://doi.org/10.3390/coatings14080990
Submission received: 25 June 2024 / Revised: 26 July 2024 / Accepted: 31 July 2024 / Published: 6 August 2024
(This article belongs to the Special Issue Bio-Nanocomposite Coatings: A Potential Strategy to Improve the Overall Performance of Food Packaging Materials)
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Abstract

:
Nanocomposite coatings functionalized with antimicrobial nanoparticles could be a promising alternative for the postharvest preservation of fruits. This study aimed to develop nanocomposite coatings based on pectin incorporated with zinc oxide (NPZ) nanoparticles to preserve the postharvest quality of papaya fruits. The coatings were prepared using pectin (3%) and NPZ (0%–0.4%). The materials were characterized for water-related properties (water solubility and water vapor permeability) as well as physical, mechanical, morphological, rheological, and structural properties. The coatings were applied to papaya fruits, which were analyzed for weight loss, firmness, titratable acidity, and soluble solids over nine days of storage. Incorporating NPZ (0%–0.4%) did not affect the films’ water solubility and vapor permeability. However, films with NPZ exhibited lower mechanical properties than pure pectin films. Rheological behavior testing indicated that the pectin solution was a Newtonian fluid, whereas pectin solutions with zinc nanoparticles were non-Newtonian fluids. The pectin coating with 0.2% NPZ was the most effective in preserving the postharvest quality of papaya by reducing fruit weight loss and acidity content. Therefore, the developed coatings incorporated with NPZ showed promise for the postharvest preservation of papaya fruits.

1. Introduction

Papaya (Carica papaya L.) is an important fruit in tropical and subtropical regions. However, being climacteric, it ripens quickly and faces several postharvest challenges, such as inadequate temperature and microbial diseases, which reduce its shelf life [1]. Technologies such as ozone radiation and modified atmosphere packaging are traditionally used to mitigate these problems, but they have high costs and limitations [2].
With the growing demand for safe and sustainable solutions, there is a greater focus on developing technologies that extend the shelf life of food and use biodegradable products. Edible coatings, made primarily from natural materials such as polysaccharides, proteins, or lipids, are applied to the surface of fruits to protect them, maintain their quality, and prolong their shelf life. They act as barriers against volatile compounds, regulate atmospheric changes, and protect against microbial diseases [1].
Pectin is a polymer composed of galacturonic acid linked through α-1,4 bonds in which the carboxylic groups can be methyl-esterified to different extents. In some stretches, L-rhamnose alternates with D-galacturonic acid residues in α-(1,2) bonds [3]. Pectin is present in plant cell walls, and as a natural polymer, it is attractive for film production [4]. However, pectin coatings or films have a low water vapor barrier and moderate oxygen barrier properties, limiting their application in food packaging. An alternative used to improve the water barrier properties and mechanical resistance of coatings or films is the addition of compounds that increase the hydrophobicity of the matrix [5,6] and the use of crosslinking processes [4].
Inorganic nanomaterials such as zinc oxide nanoparticles are attracting much attention in the food, medicinal, and other industries. Compared with other nanomaterials, zinc oxide nanoparticles have unique features, such as being less toxic and biocompatible [7]. Additionally, they possess antimicrobial, antifungal, and UV-blocking properties and can enhance mechanical and water barrier properties [8]. ZnO nanoparticles (NPZs) have been applied in films owing to their safe chemical interaction with other components [7]. NPZ has been incorporated into various polymeric coatings or films to produce antimicrobial nanocomposite packaging [9]. Santhosh and Sarkar [10] described how an increase in ZnO nanoparticles in starch films reduced the hydrophilicity of the material and improved the mechanical properties. In addition, it contributed to lower microbial activity, prolonging the shelf life of tomato fruits stored with this material.
Therefore, this work aimed to develop nanocomposite coatings of pectin and oxide zinc nanoparticles to increase papaya shelf life. This study aimed to follow the current status of sustainable food packaging regulations, as indicated by Thapliyal et al. [11].

2. Materials and Methods

2.1. Material

Papaya fruits (Carica papaya L.) at similar maturity stages (green with a slight yellowish color) were selected and obtained from a wholesaler in Rio Verde City (Brazil). The fruits were standardized based on uniform shape, size, and color. The papayas were washed under running water to remove dirt and then sanitized with 2.5% w/v sodium hypochlorite for 15 min. Commercial citrus pectin (98% purity), glycerol (99% purity), and zinc oxide (ZnO) were obtained from Dinâmica Química Contemporânea Ltd.a® (Indaiatuba, Brazil).

2.2. Production of NPZ

Nanoparticles were produced by dissolving anhydrous zinc acetate in distilled water at a concentration of 0.3 M. The pH of the solution was adjusted to 10 using a 2 M NaOH solution. The obtained solution was irradiated for 1 h in a domestic microwave (Panasonic, 20 L, model CMA20BBBNA/220W), and the resulting product was filtered and washed with distilled water and ethanol until it was free of impurities. The residue was irradiated for another 1 h in a microwave oven and dried at room temperature (approximately 25 ± 2 °C) for 72 h.

2.3. Characterization of NPZ

NPZ was characterized using UV-Vis spectrophotometry, with preparations of suspensions of zinc oxide particles in 1% (w/v) deionized water. Absorption values were read on a spectrophotometer (Shimadzu UV-1601PC, Kyoto, Japan), as described by Porto et al. [11].
Field emission scanning electron microscopy (FEG-SEM) analysis was performed. The images were taken using a transmission electron microscope (JEOL JSM 7100F, Tokyo, Japan) operating at a resolution of 200 kV to take the images.

2.4. Nanocomposite Coating Preparation

After evaluating the experimental design, with modifications, the films were prepared, as described by Guerra, de Sousa, de Farias, Cappato, de Freitas, Romani, and Plácido [9]. A film-forming solution was prepared by dissolving 3 g of commercial citrus pectin in 100 g of distilled water, adding 30% of the plasticizing agent glycerol, and adding 0, 0.2, and 0.4 g of NPZ in referred pectin (referred to as the control, NPZ2, and NPZ4, respectively). The solution was magnetically stirred (SPLABOR, SP-10206/A, Presidente Prudente, Brazil) for 24 h for complete dissolution and then allowed to stand for 1 h to eliminate air bubbles. Subsequently, 60 mL of the film-forming solution was deposited in 14 cm diameter acrylic Petri dishes. The films were then dried in the dark at 25 ± 2 °C for 72 h.

2.5. Characterization of Films

2.5.1. Determination of Film Thickness

The thickness of the samples was studied using a digital micrometer with a resolution of 0.0001 mm (Qualitylabor, model MEP/Q, Sao Paulo, Brazil) by averaging the thickness at ten random points on the film [12].

2.5.2. Mechanical Properties

The film samples were cut to dimensions of 80 mm × 20 mm and stored in an environment with relative humidity (RH) at 55% (in a saturated magnesium nitrate solution) for 48 h at 25 °C. Subsequently, these were mounted on the pneumatic grips of the texturometer (INSTRON, model 3367, Norwood, MA, USA), and the test was conducted according to the method of the American Society for Testing and Material [12]. The distance between the claws was 50 mm, and the traction speed was 50 mm/min. The maximum force (MPa), elongation at break (%), and Young’s modulus (MPa) were determined. The analyses were performed in quintuplicates.

2.5.3. Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM; JEOL, JSM 7100F, Tokyo, Japan) was used to determine the microstructures of the films and compare the samples with different compositions. The samples were immersed in liquid nitrogen (for quick freezing), fractured using stainless-steel tongs, and then kept in a desiccator with calcium chloride (~0% RH) for drying for 48 h. The samples were analyzed using SEM with an electron acceleration voltage of 5 kV at magnifications of 100, 500, 2000, and 5000.

2.5.4. Water-Related Properties of Films

Water vapor permeability was determined using the gravimetric method at room temperature (25 ± 2 °C) according to ASTM E96-95 [13]. The film was attached to a test beaker containing calcium chloride capsules (0% RH), which was sealed with paraffin and placed in a desiccator at 75% RH (saturated sodium chloride solution). The weight of the calcium chloride capsules was monitored every 24 h for 7 d [14,15].
The solubility in water was determined using a gravimetric method. Samples of 2 cm in diameter were cut, placed for drying in an oven for 24 h at 105 °C, and then weighed. After drying, the samples were immersed in 50 mL of distilled water under constant and slow stirring at 25 °C for 24 h. The solution was filtered, the remaining material was dried in an oven at 105 °C, and the amount of non-solubilized dry matter was determined [16]. The solubility was calculated as the ratio of the initial mass subtracted from the final mass to the final mass.

2.5.5. Fourier Transform Infrared Spectroscopy (FTIR)

Samples were previously conditioned at 0% RH (calcium chloride) for 48 h at 30 °C. The analyses were performed in the mid-infrared region with Fourier transform, reaching wave numbers from 4000 to 400 cm−1 at a spectral resolution of 4 cm−1, with a Bruker Vertex 70 and an accessory platinum atr unit of 225. In total, 200 scans were performed for each sample.

2.5.6. Rheology of Filmogenic Solutions

The film-forming solutions were conditioned at 25 and 40 °C and analyzed at strain rates of 1–500 s−1 in cone-plate geometry using a rheometer (Physica, MCR101, Ostfildern, Germany) and texturometer (Texture Analyzer, TA-XT Plus, Surrey, UK).
The experimental values of the shear stress and strain rate were fitted with the rheological models of Newton, Power Law (Ostwald de Waele), Bingham, and Herschel–Bulkley (Table 1).
Statistica software (version 7.0) was used to process the experimental data, mathematical parameters for each model, statistical parameters (mean percentage deviations (p), and the coefficient of determination (R2)).

2.6. Application of Nanocomposite Coating Solutions on Papaya Fruits

Papayas were immersed in a coating-forming solution (P, NPZ2 and NPZ4) and classified as FC (fruits without coating), FP (fruits coated with P solution), FNPZ2 (fruits coated with NPZ2 solution) and FNPZ4 (fruits coated with NPZ4 solution) for 5 min, before being subjected to drying in B.O.D (Tecnal, San Sebastian, Spain) at a temperature of 20 °C ± 5 °C and humidity of 60% for subsequent physical–chemical evaluation. All analyses were carried out in triplicates. The nine-day shelf life was evaluated every 3 days at time 1 (beginning of storage), time 2 (after three days of storage), time 3 (after six days of storage), and time 4 (after nine days of storage).

2.6.1. Weight Loss

The mass loss analysis was carried out with an analytical balance, with four decimal places, weighing the coated and the control fruits during all the days the analyses were carried out.
The results were expressed as a percentage and obtained through the differences between the initial mass of the sample and the mass obtained in each storage period. The final mass represents the mass of the sample after 3, 6, and 9 days of fruit storage.

2.6.2. Firmness

Calbo and Nery [17] determined the firmness of the fruits using a pedestal planer on which a glass vat rested, and the weight of the vat (1107 kg) promoted the deformation in the region of contact with the fruit. The analysis was performed at three different points for each mango sample using a digital caliper to measure the length and width of each fruit.

2.6.3. Titratable Acidity and Soluble Solids

The samples were prepared by homogenizing them in a blender with distilled water in a 1:1 (m/m) ratio to assess titratable acidity and soluble solids. The total titratable acidity was determined by titration with a 0.1 mol/L NaOH solution [18], with the result expressed in % citric acid. A digital refractometer was used to determine total soluble solids, and the results were expressed in °Brix [19].

2.7. Statistical Analysis

From the data collected on the properties of the film and the shelf life of papaya, an analysis of variance (ANOVA) was carried out with a confidence interval of 95% (p < 0.05) using the software Statistica 7.0 (Statsoft®, Hamburg, Germany). Tukey’s test was used to evaluate the differences in means (p < 0.05) between the properties of the films produced and the conservation of papaya.

3. Results

3.1. Characterization of NPZ

UV-Vis spectroscopy demonstrated the highest absorption of NPZ at 380 nm, confirming their presence in the sample (Figure 1). This absorption peak is consistent with findings from other researchers who analyzed NPZ [20,21].
The XRD pattern demonstrates the crystallinity of the NPZ (Figure 2). It was possible to observe diffraction peaks at values 2θ 33.42°, 34.52°, 36.61°, 47.68°, 56.68°, 63.10°, 66.32° and 67.92°. These peaks are related to the structure of NPZ. A similar result was reported by Shaikhaldein, Al-Qurainy, Khan, Nadeem, Tarroum, Salih, Gaafar, Alshameri, Alansi, and Alenezi [20] and Raja et al. [18] so that the structure of NPZ is hexagonal wurtzite crystalline due to the peaks, as observed in this study. The NPZ sample showed impurities, with peaks smaller than 2θ 30° possibly associated with reaction byproducts or reagent residues.
The ZnO nanoparticles (NPZ) exhibited a granular and possibly hexagonal shape, as indicated by SEM images (Figure 3). The particle size analysis revealed an average size of 134.4 nm. Particles smaller than 100 nm were observed and classified as nanoparticles. Additionally, particles larger than 100 nm but not exceeding 153.5 nm were also present. These larger particles can be attributed to the formation of zinc oxide nanoparticle aggregates or particle growth during synthesis. The previous characterization of zinc oxide produced by this method showed that particles in this size range are expected, reinforcing the identification of these particles as zinc oxide. The presence of fine particles justifies the peaks related to impurities observed in Figure 2. The shape of the ZnO nanoparticles (NPZs) in this study was similar to that reported by Maher et al. [22].

3.2. Characterization of Film

3.2.1. Physical–Mechanical Properties

The incorporation of NPZ significantly increases film thickness, starting from a concentration of 0.4% (P4Z) (Table 2). Previous studies have reported similar behaviors involving pectin, agar, and zinc sulfide nanoparticle films [23] and pectin, cocoa, and NPZ films [24]. The increase in film thickness due to NPZ incorporation may be associated with an increase in the solids content of the films [25].
Adding NPZ reduced the tensile strength of the films from 19.81 to 6.78–8.69 (Table 2). This decrease in tensile strength can be attributed to discontinuities in the polymer matrix caused by NPZ [26,27], which is supported by SEM micrographs (as discussed later in Figure 4). Melo, Aouada, and Moura [24] also observed a similar decrease in tensile strength in pectin, cocoa, and NPZ films compared to pure pectin films.
Similar trends were observed for elongation and Young’s modulus, which were lower in the films containing NPZ than in the pure pectin film. Shankar et al. [28] demonstrated that adding NPZ to a gelatin matrix reduced tensile strength and increased elasticity.

3.2.2. SEM

The films had a continuous, regular, smooth structure without cracks (Figure 4). However, at higher magnification, the pectin film (Figure 4—P—on the right) showed some irregularities on its surface, possibly due to the slow drying of the films produced by molding. In contrast, the P2Z and P4Z films (Figure 4—P2Z and P4Z) presented a more irregular (rough) structure with NPZ aggregates, suggesting that the NPZs were not fully dispersed in the pectin polymer network. The presence of particle aggregates promoted discontinuities in the film structure, which impaired tensile, barrier, and optical properties [25].
Suyatma et al. [23] reported the good dispersion of NPZ in pectin films but confirmed that NPZ may agglomerate, increasing the film’s roughness. Ngo, Nguyen, Dang, Tran, and Rachtanapun [3] reported uniform pectin films with chitosan nanoparticles, which strong chemical interactions between the constituents can explain. Pectin and NPZ can interact through hydrogen bonds, but owing to the crystalline structure of NPZ, this interaction can be hampered depending on the environmental conditions.

3.2.3. Water-Related Properties of Films

The water solubility of the films decreased significantly with the incorporation of NPZ at a concentration of 0.4% (Table 3). Pantani et al. [29] reported that with the increase in NPZ content, the solubility of the film tended to decrease due to the strengthening of hydrogen bonds in the matrix; thus, free water molecules did not strongly interact with the nanocomposite films.
There were no statistically significant differences between the water vapor permeabilities of the pectin films with and without NPZ (Table 3). However, because water vapor permeability (WVP) is a function of both solubility and diffusivity [30], the lack of significant variation in WVP may be due to a simultaneous increase in water molecule diffusivity, resulting from the formation of a discontinuous phase between the nanofiller (NPZ) and the polymer matrix of the film [26], as demonstrated in the SEM images (Figure 4).

3.2.4. FTIR

FTIR analysis was used to identify the structural changes in citrus pectin after forming films with NPZ. The spectral bands of the citrus pectin film with 0.2% NPZ (P2Z) showed more significant chemical structural changes than in P4Z for P (Figure 5). The intensity of the higher peaks identified for (P2Z) compared with the others is indicative of the stretching of the C = O group of the non-ionized carboxylic group (methylated or protonated) and the low degree of esterification of commercial citrus pectin [27].
The sharp bands in all analyzed samples at approximately 3250 cm−1 were attributed to the OH stretching vibration and intermolecular hydrogen interactions between the polymers [27]. The bands between 2934 and 2938 cm−1 were attributed to CH elongation in the alkane groups of the polymers. The highlighted bands between 1412 and 1740 cm−1 were characteristic of the ester bond in the pectin molecule and could also be attributed to the stretching of the carbonyl group [27]. Roy and Rhim [26] reported that the bands around 1102 and 1000 cm−1 were related to the elongation of the C–O–C bonds in the saccharide structure of pectin. According to Jayarambabu et al. [31] and Etemadi [32], the peaks at 648 cm−1 correspond to the vibration of the bonds between oxygen and metal, that is, Zn–O stretching vibrations.

3.2.5. Rheology of Filmogenic Solutions

The strain rate of the pectin-only films is lower than those of the films with NPZ (Figure 6). The strain rates of the films at 25 °C (P1, P2Z1, and P4Z1) are higher than at 40 °C (P2, P2Z2, and P4Z2). For all cases studied, the shear stress continuously increased with strain rate. This relationship is non-linear, with downward concave curves characteristic of non-Newtonian fluids [33]. This effect can be associated with the collapse of the structure of the filmogenic solution caused by an increase in the deformation rate, resulting in the arrangement and greater alignment of the molecules in the solution [34].
The apparent viscosity decreased with an increasing strain rate for all samples except for P1 and P2 (Figure 7), which suggests that these films have characteristics of Newtonian fluids (the strain rate does not change the viscosity of the material) [35]. Samples P2Z1, P2Z2, P4Z1, and P4Z2 can be classified as pseudoplastic fluids because an increase in the deformation rate results in a decrease in the apparent viscosity, which is probably caused by the alignment of non-spherical particles, rupture or deformation of flocculated drops, the alteration of the spatial distribution by shear stress, and removal of solvent molecules bound to the droplets [36].
At the test temperatures (25 and 40 °C), the shear stress decreases with increasing temperature for all analyzed samples, confirming the effect of temperature on the system’s viscosity. This tendency has already been described by Marcotte et al. [37], who studied the rheological properties of several hydrocolloids with different concentrations at different temperatures. They reported that at high temperatures, the viscosity of the gums decreased, which implies lower stress values for fluid flow. A similar result was reported by Soto-Caballero et al. [38] for pectin hydrogels at 25 and 50 °C. These findings agree with Kumar and Mandal [39] and Matos et al. [40], indicating that colloidal dispersions exhibit shear-thinning behavior.
An analysis of the relationship between the apparent viscosity and strain rate does not allow for a complete characterization of the rheological behavior of the samples. For this purpose, the rheological parameters of the mathematical models of Newtonian and non-Newtonian fluids need to be obtained. Based on the analysis, we classified the films using the mathematical models that best describe them (Table 4). For films P1 and P2, the model of best fit (R2 = 1.000) is Newton’s model, confirming Newtonian fluid’s characteristics. For the P2Z1 and P2Z2 films, the model with the best fit was the Powel Law model (R2 of 0.997 for P2Z1 and 0.999 for P2Z2). The same model was considered for samples P4Z1 and P4Z2. The best response of this model contributed to the confirmation of pseudoplastic behavior.
According to Martínez-Padilla [41], the main rheological parameters of the Power Law model are a consistency index (K) and behavior index (n). The behavior index (n) defines the behavior of the fluids, and the consistency index (K) indicates the degree of resistance of the fluid during flow. As shown in Table 4, an increase in the NPZ content correlates with an increase in K, indicating an increase in the film’s consistency, as described by Soto-Caballero, Valdez-Fragoso, Salinas-López, Welti-Chanes, Verardo and Mújica-Paz [38] and Lastra Ripoll et al. [42]. The fluid behavior index values for P2Z1, P2Z2, P4Z1, and P4Z2 are less than 1 (n < 1), indicating non-Newtonian behavior with pseudoplastic fluid characteristics.

3.3. Preservation of Papaya Fruit Using the Pectin-ZnO Coating

3.3.1. Weight Loss

The weight loss increased in all treatments over nine days of storage, but the coated papayas showed reduced weight loss (10.02%–14.21%) compared to those in the control group (24.10%) (Table 5). Similar results were found by Oliveira Filho, Duarte, Silva, Milan, Santos, Moura, Bandini, Vitolano, Nobre, and Moreira [1], who observed a reduction in the mass loss of papayas from 24.85% to 5.78% after 15 days of storage with the application of a carnauba wax nanoemulsion coating with essential oils. According to Baldwin et al. [43], edible coatings mainly act as barriers in the fruit stomata, protecting them from moisture loss and reducing fruit transpiration.
The more significant mass loss observed in the FPNZ4 treatment (Table 5) may be related to the discontinuities in the film’s surface due to the presence of NPZ. This may have contributed to more significant gas and water vapor exchange due to the higher porosity of the films, as observed in Figure 4.
In most fresh vegetables, the maximum tolerated mass loss ranges between 5% and 10% so that the fruit does not show wilting and/or wrinkling on the surface [44], which indicates that, in this work, the FP and FPNZ2 treatments presented satisfactory results for the conservation of papayas to mass loss, tested over six and nine days of storage.

3.3.2. Firmness

The firmness of the papaya fruits decreased in all treatments over the nine days of storage (Table 6). The fruits from the FP treatment showed a higher loss of firmness relative to the initial firmness at approximately 70.37%, and the fruits from the FPNZ4 treatment showed a 62.10% decrease in firmness. The coated fruits FC and FPNZ2 showed smaller decreases in firmness at 44.44% and 45.39%, respectively. Similar behavior was observed by dos Passos Braga et al. [45] for papaya fruits coated with edible coatings formulated with chitosan and Mentha essential oils.
The lower efficiency of the FPNZ4 coating compared to FC and FPNZ2 may be related to the discontinuities in the surface of the film due to the higher concentration of NPZ in the pectin matrix. This may have resulted in greater gas exchange due to the higher porosity of the films, as observed in Figure 4, which maintained the activity of the enzymes responsible for degrading the cell and starchy structures. According to Meindrawan et al. [46], changes in firmness occur due to alterations in the cell and starchy structures present in fruits.

3.3.3. Titratable Acidity and Soluble Solids

The acidity of the fruits, expressed as a percentage of citric acid, was influenced by the treatment and storage time (Table 7). Faasema et al. [47] highlighted those fruits during postharvest and, consequently, maturation that suffered from a reduction in organic acids, mainly citric acid and malic acid, in the FPNZ2 and FC treatments. As expected, the FP and FPZN4 coatings increased fruit conservation due to their acting as a protective barrier around the fruit, modifying the atmosphere, reducing metabolic rates, and contributing to delayed ripening, as explained by Ebrahimi and Rastegar [48].
There was an increase in the soluble solids content for all treatments over the nine days of storage (Table 7). After six days of storage, the smallest increase in soluble solids was observed in the FPNZ2 and FPNZ4 treatments. However, this parameter had no significant difference after nine days of storage. The increase in total soluble solids (TSSs) is due to the accumulation of sugars, indicating the ripening of the fruit through the breakdown of more complex carbohydrates, which is in line with an increase in the fruit’s respiratory rate [49].

4. Conclusions

Incorporating NPZ (0%–0.4%) did not affect the films’ water solubility and vapor permeability. However, films with NPZ exhibited lower mechanical properties than pure pectin films. Rheological behavior testing indicated that the pectin solution was a Newtonian fluid, whereas pectin solutions with zinc nanoparticles were non-Newtonian fluids. Thus, pectin solutions with zinc nanoparticles may require adjustments in processing equipment to handle viscosity variations, especially in processes involving high shear rates.
The pectin coating with 0.2% NPZ was the most effective in preserving the postharvest quality of papaya by reducing fruit weight loss and acidity content. Thus, the developed coating material may be an alternative to preserve the quality of papaya fruits during postharvest.

Author Contributions

J.S.d.S.: Writing—Review and Editing, Writing—Original draft, Methodology, Investigation, Formal analysis, Data curation. C.C., B.S.M.d.F., R.M.d.S., G.B.L.d.J. and C.M.B.: Methodology, Investigation. M.B.E. and J.G.d.O.F.: Writing—Review and Editing, Methodology. G.R.P.: Writing—Review and Editing, Methodology, Funding acquisition, Conceptualization. All authors have read and agreed to the published version of the manuscript.

Funding

The APC was funding by IF Goiano (Process No. 23218.002184.2024-88).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors gratefully acknowledge the support received from CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and IF Goiano (Instituto Federal Goiano—Campus Rio Verde, Process No. 23218.002184.2024-88) for carrying out the research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Coatings 14 00990 g001
Figure 1. UV-Vis spectroscopy of zinc oxide nanoparticles (NPZs).
Figure 1. UV-Vis spectroscopy of zinc oxide nanoparticles (NPZs).
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Figure 2. X-ray diffraction of zinc oxide nanoparticles (NPZs).
Figure 2. X-ray diffraction of zinc oxide nanoparticles (NPZs).
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Figure 3. Scanning electron microscopy of zinc oxide nanoparticles (NPZs).
Figure 3. Scanning electron microscopy of zinc oxide nanoparticles (NPZs).
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Figure 4. Surface micrographs (1000×) of films consisting of pectin (P), pectin with 0.2% NPZ (P2Z), and pectin with 0.4% NPZ (P4Z).
Figure 4. Surface micrographs (1000×) of films consisting of pectin (P), pectin with 0.2% NPZ (P2Z), and pectin with 0.4% NPZ (P4Z).
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Coatings 14 00990 g005
Figure 5. FTIR spectra films consisting of pectin (P), pectin with 0.2% NPZ (P2Z), and pectin with 0.4% NPZ (P4Z).
Figure 5. FTIR spectra films consisting of pectin (P), pectin with 0.2% NPZ (P2Z), and pectin with 0.4% NPZ (P4Z).
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Coatings 14 00990 g006
Figure 6. Relationship between the shear stress and shear rate for pectin (P1), pectin with 0.2% NPZ (P2Z1), pectin with 0.4% NPZ (P4Z1) films at 25 °C (P2), pectin with 0.2% NPZ (P2Z2), and pectin with 0.4% NPZ (P4Z2) films at 40 °C.
Figure 6. Relationship between the shear stress and shear rate for pectin (P1), pectin with 0.2% NPZ (P2Z1), pectin with 0.4% NPZ (P4Z1) films at 25 °C (P2), pectin with 0.2% NPZ (P2Z2), and pectin with 0.4% NPZ (P4Z2) films at 40 °C.
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Coatings 14 00990 g007
Figure 7. Relationship between apparent viscosity and shear rate for pectin (P1), pectin with 0.2% NPZ (P2Z1), pectin with 0.4% NPZ (P4Z1) films at 25 °C (P2), pectin with 0.2% NPZ (P2Z2), and pectin with 0.4% NPZ (P4Z2) films at 40 °C.
Figure 7. Relationship between apparent viscosity and shear rate for pectin (P1), pectin with 0.2% NPZ (P2Z1), pectin with 0.4% NPZ (P4Z1) films at 25 °C (P2), pectin with 0.2% NPZ (P2Z2), and pectin with 0.4% NPZ (P4Z2) films at 40 °C.
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Table 1. Mathematical models applied for rheological analysis of filmogenic solutions.
Table 1. Mathematical models applied for rheological analysis of filmogenic solutions.
ModelModel Designation *
Newtonτ = µ × γ
Power Law (Ostwald-de-Waele)τ = K × γn
Binghamτ = τ0 + η∞ × γ
Herschel–Bulkleyτ = τ0 + K × γn
* Parameters: τ = shear stress (Pa); μ = Newtonian viscosity (Pa·s); γ = strain rate (1/s); K = consistency index (Pa·sn); n = flow behavior index; τ0 = initial shear stress (Pa); and η∞ = plastic viscosity (Pa·sn).
Table 2. The thickness and mechanical properties of the films consist of pectin (P), pectin with 0.2% NPZ (P2Z), and pectin with 0.4% NPZ (P4Z).
Table 2. The thickness and mechanical properties of the films consist of pectin (P), pectin with 0.2% NPZ (P2Z), and pectin with 0.4% NPZ (P4Z).
TreatmentThickness (µm)Tensile Strength (MPa)Elongation (%)Young Modulus (MPa)
P104.25 ± 17.68 a19.81 ± 1.65 c534.62 ± 1.78 c25.51 ± 1.28 b
P2Z121.21 ± 29.55 ab8.69 ± 0.56 b305.21 ± 2.32 b5.87 ± 1.23 a
P4Z157.12 ± 21.89 b6.78 ± 1.50 a254.21 ± 3.87 a7.82 ± 1.97 a
Results are expressed as the mean ± standard deviation. a,b,c Different lowercase letters in the same column indicate significant differences (p ≤ 0.05), according to Tukey’s test.
Table 3. Solubility and water vapor permeability of films consisting of pectin (P), pectin with 0.2% NPZ (P2Z), and pectin with 0.4% NPZ (P4Z).
Table 3. Solubility and water vapor permeability of films consisting of pectin (P), pectin with 0.2% NPZ (P2Z), and pectin with 0.4% NPZ (P4Z).
TreatmentWater Solubility (%)Water Vapor Permeability (g·mm/kPa·h·m2)
P35.50 ab ± 2.24 a8.89 ± 3.87 a
P2Z36.69 a ± 2.08 b9.19 ± 4.24 a
P4Z32.00 b ± 0.31 a11.49 ± 3.63 a
The results are expressed as the mean ± standard deviation. a,b Different lowercase letters in the same column indicate significant differences (p ≤ 0.05), according to Tukey’s test.
Table 4. Rheological parameters of Newton and Powel Law models for pectin (P1), pectin with 0.2% NPZ (P2Z1), pectin with 0.4% NPZ (P4Z1) films at 25 °C (P2), pectin with 0.2% NPZ (P2Z2), and pectin with 0.4% NPZ (P4Z2) films at 40 °C.
Table 4. Rheological parameters of Newton and Powel Law models for pectin (P1), pectin with 0.2% NPZ (P2Z1), pectin with 0.4% NPZ (P4Z1) films at 25 °C (P2), pectin with 0.2% NPZ (P2Z2), and pectin with 0.4% NPZ (P4Z2) films at 40 °C.
ParameterTreatment
P1P2
Newtonμ (Pa·s)0.0620.036
R21.0001.000
Error (P) (%)0.9861.446
P2Z1P2Z2
Power LawK (Pa·sn)18.96110.797
n0.4350.480
R20.9970.999
Error (P) (%)1.9531.360
P4Z1P4Z2
Power LawK (Pa·sn)20.04612.799
n0.4030.422
R20.9981.000
Error (P) (%)1.3560.688
K = consistency index; n = fluid behavior index; R2 = determination coefficient; and error (P) (%) = mean percentage deviation.
Table 5. Mass loss of uncoated and coated papaya fruits.
Table 5. Mass loss of uncoated and coated papaya fruits.
TreatmentStorage Days
369
FC2.29 ± 0.50 aB10.52 ± 0.31 aB24.10 ± 5.58 aA
FP3.43 ± 0.77 aB6.75 ± 0.94 bcB10.81 ± 0.66 cA
FPNZ22.94 ± 0.79 aB5.44 ± 0.32 cB10.02 ± 1.40 cA
FPNZ43.92 ± 0.67 aC8.63 ± 0.77 abB14.21 ± 1.39 bA
Different lowercase letters in the column show significant differences and uppercase letters indicate significant differences between rows using Tukey’s test (p < 0.05).
Table 6. Firmness of uncoated and uncoated papaya fruits.
Table 6. Firmness of uncoated and uncoated papaya fruits.
Treatment *Storage Days
0369
FC2.16 ± 0.67 aA1.84 ± 1.06 aA1.47 ± 0.38 aA1.20 ± 0.24 aA
FP2.43 ± 0.48 aA1.37 ± 0.58 aAB1.45 ± 0.64 aAB0.72 ± 0.12 aB
FPNZ21.63 ± 0.57 aA1.25 ± 0.48 aA1.20 ± 0.52 aA0.89 ± 0.13 aA
FPNZ42.19 ± 0.24 aA1.33 ± 0.26 aB0.87 ± 0.27 aB0.83 ± 0.15 aB
* Results in base 10−4. Different lowercase letters in the column show significant differences and uppercase letters indicate significant differences between rows using Tukey’s test (p < 0.05).
Table 7. Titratable acidity and soluble solids of uncoated and uncoated papaya fruits.
Table 7. Titratable acidity and soluble solids of uncoated and uncoated papaya fruits.
Titratable Acidity
TreatmentStorage Days
0369
FC0.068 ± 0.006 bA0.058 ± 0.002 cAB0.047 ± 0.008 aB0.058 ± 0.015 bA
FP0.070 ± 0.009 aA0.046 ± 0.009 aB0.068 ± 0.021 aAB0.071 ± 0.014 aA
FPNZ20.071 ± 0.003 bA0.040 ± 0.001 bB0.040 ± 0.011 aB0.043 ± 0.017 aB
FPNZ40.058 ± 0.002 aA0.058 ± 0.001 cA0.058 ± 0.010 aA0.068 ± 0.012 aA
Soluble Solids
TreatmentStorage Days
0369
FC7.47 ± 1.86 bAB7.85 ± 0.94 cAB8.68 ± 0.78 aB9.63 ± 0.61 aA
FP9.60 ± 0.11 aA10.35 ± 0.84 aA8.08 ± 0.46 aB9.50 ± 0.33 aA
FPNZ26.93 ± 0.50 bB9.22 ± 1.18 bA7.88 ± 0.58 bAB9.26 ± 0.85 aA
FPNZ49.62 ± 1.35 aA7.12 ± 0.41 cB7.78 ± 0.97 bB9.90 ± 0.28 aA
Different lowercase letters in the column show significant differences and uppercase letters indicate significant differences between rows using Tukey’s test (p < 0.05).
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dos Santos, J.S.; Cagnin, C.; de Freitas, B.S.M.; da Silva, R.M.; de Jesus, G.B.L.; Belisário, C.M.; Egea, M.B.; de Oliveira Filho, J.G.; Plácido, G.R. Nanocomposite Coatings of Pectin and Oxide Zinc Nanoparticles to Increase Papaya Shelf Life. Coatings 2024, 14, 990. https://doi.org/10.3390/coatings14080990

AMA Style

dos Santos JS, Cagnin C, de Freitas BSM, da Silva RM, de Jesus GBL, Belisário CM, Egea MB, de Oliveira Filho JG, Plácido GR. Nanocomposite Coatings of Pectin and Oxide Zinc Nanoparticles to Increase Papaya Shelf Life. Coatings. 2024; 14(8):990. https://doi.org/10.3390/coatings14080990

Chicago/Turabian Style

dos Santos, Joelma Saures, Caroline Cagnin, Bheatriz Silva Morais de Freitas, Richard Marins da Silva, Glaydson Brasileiro Lopes de Jesus, Celso Martins Belisário, Mariana Buranelo Egea, Josemar Gonçalves de Oliveira Filho, and Geovana Rocha Plácido. 2024. "Nanocomposite Coatings of Pectin and Oxide Zinc Nanoparticles to Increase Papaya Shelf Life" Coatings 14, no. 8: 990. https://doi.org/10.3390/coatings14080990

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