Development, RSM-Based Optimization, and Characterization of a Unique Edible Composite Film by Incorporating Clove Essential Oil Based on Sodium Alginate and <i>Aloe vera</i> Gel to Preserve the Quality of Blueberries

Cui, Zhuoyu; Li, Yang; Feng, Xin; Hu, Zexi

doi:https://doi.org/10.1155/2023/3578799

Journal of Food Processing and Preservation

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Abstract Introduction Materials and Methods Results and Discussion Conclusion Data Availability Additional Points Conflicts of Interest Authors’ Contributions Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 3578799 | https://doi.org/10.1155/2023/3578799

Development, RSM-Based Optimization, and Characterization of a Unique Edible Composite Film by Incorporating Clove Essential Oil Based on Sodium Alginate and Aloe vera Gel to Preserve the Quality of Blueberries

Zhuoyu Cui,¹Yang Li,²Xin Feng,¹and Zexi Hu²

Academic Editor: Rui M.S. Cruz

Received19 Aug 2022

Revised28 Nov 2022

Accepted14 Dec 2022

Published29 Mar 2023

Abstract

In recent years, edible composite films have attracted more attention instead of plastic packaging in the field of food packaging. Combined with clove essential oil (CO), the edible composite film based on sodium alginate (SA) and Aloe vera gel (AG) was developed. The aim of this study was to explore the effect of SA, AG, and CO on the barrier and mechanical properties of the composite films and the impact of the edible composite film on the freshness of blueberry fruits. Response surface methodology (RSM) based on single factor experiments were used to get the optimum formula with the best property. The results showed that the optimal formula was SA 15.6 gL^-1, AG 90 gL^-1, and CO 10 gL^-1, SA, AG, and CO had significant effect on the characteristics of the films, also, the crossed-linking action of SA and AG was further confirmed by characterization, while CO significantly influenced barrier and mechanical property and improved the antibacterial activity of the film. The composite film kept great hardness, less weight loss, more soluble solid, and more vitamin C of blueberries, proving the bacteriostatic activity of AG and CO. The results suggest that the edible composite film has an enormous potential in extending the shelf-life of blueberries with this formula.

1. Introduction

With the improvement of living standards, more and more people realized that food safety is vital [1]. Food is susceptible to spoilage because spoilage germs were provided an almost ideal habitat to grow and breed during storage. Packaging technologies that extend the shelf life of food by separating it from its surroundings while reducing the interaction of spoilage factors may be one of the best ways to keep food fresh. Plastic packaging without a rich recycling rate is hard to degrade and therefore leads to serious environmental pollution [2]. Nowadays, people pay more attention to protecting the environment, so nontoxic, harmless, and degradable edible packaging films have received extensive attention [3].

Currently, many studies that concern the role of edible packaging films in the regulation of food preservation have been performed. However, current research on composite films is limited, and there is still a wide range of material selection. Sodium alginate (SA) is a natural polysaccharide obtained by extracting iodine and mannitol from brown algae, which has a length of biological activities and is health beneficial [4]. SA has good gel properties and film-forming properties [5]. The sodium alginate film has high transparency and relatively good mechanical properties, but its application is hindered by its poor antibacterial effect [6]. Aloe vera is a medicinal plant of the Liliaceae family, with antibacterial, antioxidant, and anti-inflammatory. Aloe vera gel (AG) is a complex mixture of vitamins, minerals, amino acids, organic acids, and phenolic compounds [7]. It has good film-forming properties and can prevent microbial contamination [8]. It is usually used as an active packaging for food, studies have shown that compared with uncoated papaya, the soluble solids and disease degree of papaya coated with AG were reduced by 3% and 29%, respectively, which significantly extended the shelf life of papaya, but it is vital to improve mechanical properties [9]. To overcome these shortcomings, the addition of other biopolymer and natural additives with SA has become one of the effective strategies to improve properties. Several studies have tried to improve the quality of alginate films by adding gum tragacanth, gallnut extract and so on [10, 11]. However, SA and AG have no obvious antioxidant effect. Thus, it is necessary to add antioxidant agents to improve the antioxidant properties of the film. Thus far, SA and AG composite films containing natural plant essential oils are rarely explored. Clove is a traditional medicinal plant. Clove essential oil (CO) is a natural plant essential oil extracted from clove buds through a series of processes. It has strong antibacterial and antioxidant properties [12]. Therefore, it is necessary to research SA and AG composite film containing CO.

Blueberries are rich in nutrients and are one of the fruits with the most antioxidants. The International Food and Agriculture Organization lists them as one of the top five healthy foods for humans and the third generation of fruits in the world. They protect eyesight, inhibiting tumors, and are anti-inflammatory [13]. In addition, it also has the effect of delaying aging, improving memory, and enhancing immunity. However, the blueberry peel is thin and juicy [14], which is prone to mechanical damage and disease during transportation, resulting in the loss of nutrients and water [15], which destroys the quality of fresh blueberry fruit, resulting in significantly lower market demand for fresh blueberry fruit than the supply, affecting the blueberry economic development [16]. Therefore, it is necessary to develop effective postharvest techniques to maintain the quality of blueberries. So far, various preservation methods have been used for improving the shelf-life of blueberries, mainly including low temperature, irradiation, controlled atmosphere, and coating. The coating preservation has attracted considerable attention due to its simple, convenient, low cost, and environmentally friendly [17].

Because of the importance and perishable properties of blueberries in commercial agriculture, the goal of this work was to create a new edible composite film made of SA and AG that was infused with CO to preserve blueberries. Using SA, AG, CO, and glycerin as film-forming substances, through single-factor trials and response surface analysis, the best film-forming procedure for the composite film was discovered in order to optimize film-forming performance. Microstructure and chemical structure, the effects of composite films on the hardness, weight loss, soluble solids content, and vitamin C content of blueberries during storage were evaluated.

2. Materials and Methods

2.1. Materials and Chemicals

Blueberries were purchased from Loong’s Blueberry Picking Garden (Heilongjiang, China). SA was purchased from Fuchen Biotech Co., Ltd. (Tianjin, China). Glycerol was obtained from Xilong Scientific Co., Ltd. (Guangdong, China) and used as a plasticizer. The fresh Aloe vera leaves were collected from Northeast Forest University (Heilongjiang, China). All other chemicals were of analytical grade.

2.2. Preparation of Aloe vera Gel and Films

According to the method of Parven et al. [9, 18, 19], Aloe vera that had reached maturity was removed from the plant and cleaned with distilled water. Using a knife, the outer surface of leaves was peeled back to reveal an underlying matrix made up of colorless hydro parenchyma tissues. To eliminate suspended particles, the resulting matrix was first homogenized in a blender (Pro 500; Whirlpool Corporation, Benton Harbor, MI, USA) and then filtered through a sieve. The gel was pasteurized in a beaker on a hot plate for 35 minutes at 65°C while being constantly stirred, and it was then quickly cooled to 25°C. Keep it for later use.

The films were prepared by tape casting method [20]. 1.3 g of SA was dissolved in 100 mL distilled water and stirred in a water bath at 45°C (600 rpm, 90 min) until transparent and viscous, and prepared AG solution with concentration 60 gL^-1 stirred in a water bath at 45°C (600 rpm, 20 min), then 10 mL SA solution and 10 mL AG solution were blended and stirred together in a water bath at 45°C (600 rpm, 30 min). Then, 0.146 g of glycerol (20%, on a dry basis of the weight of SA and AG) was added and moderately stirred at 45°C (600 rpm, 20 min), and 5 mL CO solution diluted with Tween-80 at 1.0% was added with concentration 10 gL^-1, and the resulting dispersion was subjected to further mixing at 45°C (600 rpm, 30 min), and then removed air bubbles for 60 min using an ultrasonic equipment (KQ2200E, Anyuan Instrument Co., LTD, Suzhou, Jiangsu, China). The mixed solution was cast on a plastic plate and dried using electric thermostatic drying oven (101-3A, Taisite Instrument Co., Ltd, Tianjin City, Tianjin, China) at 50°C for 12 h, and then the films were removed from the plastic plate and conditioned at 25°C and 50% relative humidity before running further tests. Different concentrations of the composite films were prepared as the same above method.

2.3. Single Factor Experiment

Referring to the preexperiment results, the single factor experiment design was shown in Table 1 and made films according to Section 2.2.

2.3.1. Measurement of Water Vapor Permeability (WVP)

Concerning the cup method with a little change [21], WVP was measured. Briefly, the film was sealed after being cut into a circle with a diameter of 40 mm on top of a glass container containing 30.0 g of anhydrous calcium chloride (CaCl₂) desiccant getting 0% relative humidity of internal environment. The glass bottle was placed at a constant temperature and relative humidity at 25°C and 70% for 24 h and its weight were recorded at the beginning and end. The measurements were repeated five times for each film type. WVP was determined using where is the water vapor permeability of the film (g·mm·m^-2·h^-1·kPa^-1); is the mass of the glass bottle (g); is the thickness of the film (mm); is the permeation area (m²); is the time of permeation (h); and is the difference in pressure between the sides of the glass bottle (kPa).

2.3.2. Mechanical Properties

In accordance with the standard method [22], the mechanical properties of the films were determined by a texture analyzer (CT3-10 K, Brookfield, Middleboro, Mass., America). Each film was retested five times in the longitudinal direction. Film strips measuring were cut into strips, which were mounted individually on the grips of the equipment [23]. We started with a gauge length of 20 mm and tensioned the film at a speed of 1.0 mm/s. Several properties were measured, including maximum tensile strength (TS) (MPa) and elongation at break (E) (%). The measurements were repeated five times for each film type. TS was determined using where is tensile strength (MPa); indicates the maximum force at which films can be tensed; is the thickness of the film (mm); and is the width of the film (mm).

was determined using where stands for elongation (%); is the length of the edible composite film after breaking (m); and is the initial length of the film (m).

2.4. Response Surface Methodology Experiment

Based on the data range obtained from single factor experiment results, Box-Behnken response surface design with three factors and three levels was shown in Table 2, the concentration of SA, the concentration of AG and the concentration of CO were independent variables. , , and were response values. It is powerfully showed that the preparation of the edible composite film could reasonably predict the optimum formulation through three significant factors, including SA, AG, and CO. The aim was to minimize while maximizing and . Based on the principle of Box-Behnken design, the optimal preparation conditions were determined by three-factor three-level response surface.

2.5. Characterization of Films

2.5.1. The Microstructure of Films

Scanning electron microscopy (SEM) (JEOL-JSM 6510, Jeol, Akishima-shi, Tokyo, Japan) was used to record the surface morphology of the film. Films with different compositions were cut to a size of , and the film samples were fixed on a metal table with conductive glue and sprayed with gold. According to Ran Zhao et al. [24], the cross-section structure of the film was studied at a magnification of under 3 kV.

2.5.2. Chemical Structure of Films

The chemical structure of films was conducted using a Fourier-transform infrared (FTIR) spectroscopy (Spotlight 400, PerkinElmer Inc., Waltham, Mass., USA) according to the method of Sorde & Ananthanarayan [25] with a little change. This was calculated by crushing 100 mg of KBr and 4 mg of the film sample to a fine powder in a mortar and pestle. The model was scanned between 500-4000/cm.

2.6. Determination of the Quality of Blueberries

2.6.1. Blueberries Materials and Treatments

The control check (CK) group had only blueberries without coating; the second group was coated with the edible film containing SA (SA group); the third group was covered with the edible film containing SA and CO (SA/CO group); and the last group was covered with the edible film containing SA, CO, and AG (SA/CO/AG group). All groups were stored in a refrigerator at 4°C for later use.

2.6.2. Measurement of Hardness

The hardness of the blueberries was measured by a fruit hardness meter (GY-2, Hange Biotechnology Co., Ltd, Shanghai City, Shanghai, China) according to the instruction manual. It was worth noting that the scale mark was adjusted to zero before testing to ensure numerical accuracy. The hardness of blueberries was repeated three times for each group and measured every other day, and the result was recorded.

2.6.3. Measurement of Weight Loss

The weight of each group of blueberries was recorded every 2 days [26] and repeated three times for each group. The weight loss ratio was calculated by using the where is weight loss ratio (%), and are the initial and final weight of the blueberries at every group with 2 days intervals (g).

2.6.4. Measurement of Soluble Solids Content

The soluble solids of blueberries were measured by Brix Meter (HY618, Lichen, Shanghai City, Shanghai, China) [26]. Before testing, the scale mark was adjusted to zero with distilled water. The blueberries were grounded in an ice bath in a mortar, and two drops of grinding fluid were drawn with a pipette to the test panel, the data was recorded and repeated three times for each group.

2.6.5. Measurement of Vitamin C Content

On account of the method of Xu et al. ([27]), the vitamin C content was measured by titration with 1 mol/mL potassium-iodate solution. 10.0 g blueberries were weighed into a mortar, and then a small amount of 2% hydrochloric acid was added, grinded them into pulp in an ice bath, transferred them to a 100 mL volumetric flask, fixed the volume to the scale line with 2% hydrochloric acid, mixed for 10 min, and refrigerated the filtrate at 4°C for later use. Putted them in triangular bottles, respectively, and titrated them with potassium iodate solution. During titration, shook the triangular bottles continuously until the light blue color did not fade. The volume of potassium iodate solution was recorded, and titrated with hydrochloric acid solution as blank in the same way. The vitamin C content was expressed as mg per 100 g of fresh blueberries.

The vitamin C content was calculated by using the where is the content of vitamin C (mg/100 g); is total volume of blueberry extract (mL); is the volume of KIO₃ solution consumed when titrating blueberries (mL); is the volume of KIO₃ solution consumed when titrating blank group (mL); is 1 mL 1 mmol/L KIO₃ solution equivalent to vitamin C (); is the volume of blueberry solution taken during titration (mL); and is total volume of blueberry solution (mL); m is the weight of blueberries (g).

2.7. Statistical Analysis

Data processing was analyzed by Excel 2016 and Design-expert 8.0.6; graph plotting was performed with Origin 8. Statistical analysis by the analysis of variance (ANOVA) was done by SPSS 16. All tests were repeated at least three times and the results were expressed as deviation. The differences among means were considered significant.

3. Results and Discussion

3.1. Analysis of Single Factor

3.1.1. Effect Single Factor Experiment on WVP

WVP is one of the most crucial properties of edible films. Low WVP of films will contribute to reduce or avoid the passage of water from food to surrounding environment. The effect of a single factor experiment on the results of WVP is shown in Figure 1. Improvements in the barrier properties were obtained where the WVP was decreased with increasing the SA content, and there was a significant decrease () from 10 gL^-1 to 16 gL^-1, which was due to the gradual accumulation of sodium alginate molecules and the continuous increase of intermolecular forces. Moreover, the barrier properties of the film were significantly improved () when the concentration of CO increased from 6 gL^-1 to 18 gL^-1. This is due to the hydrophobic characters of the oil. In contrast, Rojas-Graü et al. [28] found that the incorporation of oregano oil did not influence the WVP of the alginate-apple puree edible films. When the concentration of AG increased from 30 gL^-1 to 90 gL^-1, it can be observed that the value of WVP significantly decreased (); because the increased crosslinking interactions between AG and SA chains which reduce the availability of the hydrophilic groups on SA thus retarding the interactions between SA and water molecules. Previous research [29] has shown that this reduction of WVP was attributed to the interactions between SA and AG which decrease the availability of the hydrophilic groups of sodium alginate and thus hinder them to interact with water. When the AG concentration reaches 120 gL^-1, the value of WVP increased due to the increase of hydrophilic molecules in AG and the enhancement of water permeability.

3.1.2. Effect Single Factor Experiment on TS and E

Mechanical property, mainly including TS and E, is a vital factor for packaging as the integrity of films should be maintained during the logistics process. As seen from Figure 2, TS was significantly increased as SA content increased () from 10 gL^-1 to 13 gL^-1, while had a significant decrease () from 13 gL^-1 to 19 gL^-1. An increase in the value of TS was detected upon increasing the content of AG. This was due to the intermolecular hydrogen bonds of sodium alginate were more likely to be formed and the increased crosslinking interactions between SA and AG [29]. AG was ductile to some extent, and increased the value of () from 30 gL^-1 to 90 gL^-1. The increase of CO in films resulted in a nonsignificant decrease () in TS but the momentous increase in () from 2 gL^-1 to 10 gL^-1, coincided with previously found in the literature [30]. The essential oil also serves as a plasticizer that leads increasing the mobility of the chain and the ductility flexibility of the films on the one hand and shortens the cohesive forces of the polymer on the other hand [31]. At the same time, the addition of CO into the film can inhibit the intermolecular interaction between SA chains, resulting in less TS. However, the addition of CO at 10 gL^-1 resulted in a diminution in E which might be on account of the aggregation of some oil droplets. Rodríguez et al. [32] reported that an obvious synergistic effect appeared between the surfactant and glycerol at high concentrations, so more plasticizer may reduce the mechanical resistance.

Therefore, based on the above analysis, the SA of 16 gL^-1, the AG of 90 gL^-1, and the CO of 10 gL^-1 were selected for the subsequent response surface optimization test.

3.2. Analysis of Response Surface Methodology

The experimental results of response surface methodology were shown in Table 3.

3.2.1. Analysis of Variance of Regression Equation

It could be seen from Table 4 that developed models were statistically significant for all responses. The value of the models of , , and were, respectively, 81.25, 5260.92, and 1281.46. All the values were satisfied to , showing that the three regression models were highly significant. The values of the lack of fit were 0.6249, 0.3363, and 0.1427, respectively (), which were not significant, meaning that the model fits were performed well. The correlation coefficient value of model was 0.9905, model was 0.9999, and model was 0.9994, which all revealed that the models fitted well to the test. Analysis and prediction of , , and E could achieve by the models. Comparing the values of the three factors SA (a), AG (b), and CO (c), it can be concluded that was significantly influenced by varying the amount of SA. Besides, SA had an important effect on while AG showed a major effect on . It could be concluded that the concentration of SA and AG were the main factor determining the physical properties of the edible composite film within the concentration range selected, different results were reported in previous studies [33] because of the addition of CO, which played a part.

3.2.2. Response Surface Interaction Analysis and Result Optimization

Response surface analysis charts were designed to determine the effect of related variables and obtain the optimal formula as shown in Figure 3. It could be seen that the interaction between SA and AG had the most significant effect on the of the film; the concentration of AG largely determined the of the film, the interactions between SA and AG or SA and CO were not significant for the of the film; the concentration of SA played an important role in the of the film, which were consistent with the results of the single factor experiment.

(a)

(b)

(c)

(d)

(e)

(f)

3.2.3. Prediction and Verification of Optimal Conditions

The optimum consequence of different formulas was obtained under condition of 15.6 gL^-1 of SA, 92.8 gL^-1 of AG and CO concentration of 10.2 gL^-1. The theoretical values of responses thus obtained were of 0.4235 g·mm/(m²·h·kPa), of 12.09 MPa, and of 15.44%. In order to confirm the adequacy of the model, triplicate experiments were performed in the same responses under the optimized conditions to compare the experimental values with the predicted results of the model. The experimental results of , , and were g·mm/(m²·h·kPa), MPa, and . The experimental values obtained highly were similar with the predicted results, so the model generated was reasonable to describe the behavior of , , and .

3.3. Characterization Analysis of Films

3.3.1. Effects of Different Components on the Microstructure of Films

Since the AG was light green in color and the CO was canary yellow (Figure 4(a)), so when incorporated in the film, they reduced the overall whiteness index and transparency of the film. Figure 4(b) show the scanning electron microscope images of films with different compositions. It can be seen from Figure 4(b) A–C that the films of different components uniform and smooth showing a homogenous film network. The addition of CO and AG did not destroy the structure of the film but made the internal arrangement more compact. As seen from Figure 4(b) D–F, SA/CO and SA/CO/AG have a porous network structure, the structure of the cross section of films is an indication of the cross linking between various functional groups of SA and AG components, and cross linking was enhanced with the incorporation of CO.

(a)

(b)

(c)

3.3.2. Effects of Different Components on Chemical Structure of Films

Infrared spectroscopy can reflect the interaction between the substrates. The Figure 4(c) showed the FTIR spectra of films with different compositions. The FTIR spectrum of SA film showed that the O-H stretching vibration of intramolecular and intermolecular hydrogen bond formation at 3325 cm^-1, due to the association of hydrogen bonds between hydroxyl groups, reduced the wavenumber of the infrared absorption peak to form a characteristic peak; 2934 cm^-1 and 2879 cm^-1 was the C-H stretching vibration in SA, AG, and glycerin; 1560 cm^-1 and 1506 cm^-1 were stretching vibration and COO^- symmetric stretching vibration [34]; 1031 cm^-1, 942 cm^-1, and 883 cm^-1 were the C-O stretching vibration and the O-C stretching vibration in the anhydroglucose rang. At the same time, after adding CO and AG, the intensity of the absorption peak between 750 and 850 cm^-1 was changed, and part of the bond energy was enhanced, which would lead to changes in the mechanical properties of the indicator, which may be due to the occurrence of ortho-substitution [35]. When the composition of the film changed, the overall change trend of the spectrum was not obvious, so it was speculated that there was a benign interaction between the components of the substrate. Except for the slight difference in the peak position and peak intensity, there was no noticeable change in the FTIR spectrum, which meant that SA, CO, and AG had good compatibility, forming a dense network structure.

3.4. Qualities Analysis of Blueberries

3.4.1. Analysis of Hardness

It can be seen from Figure 5(a) that during the storage of blueberries, both untreated blueberries and blueberries treated with edible packaging film showed a trend of increasing first and then decreasing fruit hardness.

(a)

(b)

(c)

(d)

Loss of hardness in fruits is largely associated with the degradation of cell wall polysaccharides, which eventually leads to soften of fruit. Due to the high moisture content of fresh blueberries, the fruit was full and firm. After the second day, as the blueberries matured, the water content of the blueberries gradually lost, resulting in a gradual decrease in the hardness. The hardness of the treated blueberries was higher than that of the untreated blueberries. There were significant differences () between treated and untreated blueberries in all of the hardness from the fourth day, and until day 14 of storage the hardness values for SA coating showed no significant differences () from the SA/CO coating, in addition, the hardness of SA/CO/AG treated blueberries was significantly higher () than other groups. It was indicated that hardness of uncoated blueberries decreased from N to less than N after 14 storing days. The blueberry hardness was significantly improved when coated with SA coating, which was N after 14 days. In addition, when AG and CO were incorporated to the SA system, the hardness of blueberries was further increased with the hardness retention of approximately N after 14 days of storage with SA/CO/AG. The higher remain of hardness in coated blueberries indicated that the coating formulation observably inhibited the enzymatic and metabolic activities limiting water losses, which leaded to prevent a decline in intercellular adhesion. [36]. Several studies have been reported higher fruit hardness by sodium alginate-based coatings in apple [6], winter jujube [37], sweet cherry [38], etc. While AG and CO has good bacteriostatic and antioxidant properties, our examination was in agreement with the previous blueberry reported results [39].

3.4.2. Analysis of Weight Loss

It could be seen from Figure 5(b) that the changes of weight loss rate of blueberries with the storage time for four different treatment conditions (CK group, SA group, SA/CO group, and SA/CO/AG group), regardless of treatments conditions, weight loss in blueberries continued to increasing with the storage period but the increment rate was slower in coating blueberries. Weight loss in single SA or SA/CO treatments was a significant increase () as compared to composite SA/CO/AG coating and significant differences () were shown between uncoated and coated blueberries. The weight loss rates of CK group, SA group, SA/CO group, and SA/CO/AG group were , , , and , at the fourteenth day storage, respectively. Lower weight loss in SA coated blueberries could be due to the film formed by SA on blueberries surface against the water and gas diffusion thus restricting the transpiration and respiration [40]. AG enhance the hydrophobicity by cross-linking with alginate chain in the composite film [29], that further reduced WVP of the film, also, CO was hydrophobic and enhanced cross-linking, so the results suggested that the SA/CO/AG presented the best efficacy.

3.4.3. Analysis of Soluble Solid Content

The content of soluble solids can seriously affect the taste of blueberries. It could be seen from Figure 5(c) that the effects of different treatments on during the storage of blueberries initially increased and then decreased, and the soluble solids content of the coated blueberries was higher than the CK. The increase of the soluble solids content was contributed to the transpiration of water and the solute of cell sap increased with the loss of water in early storage [40]. Due to the continuous consumption of soluble solids by blueberry respiration as a substrate, the content continued to decreasing with prolongs storage time. The content of soluble solids was %, , , and , respectively, in the fourteenth day. There were significant differences between the CK group and other groups, and at the end of storage (day 14) were no significant differences () in the soluble solid content found between the SA/CO group and SA/CO/AG group. It was deduced that the respiration and metabolism of blueberries life activities were effectively prevented in the case of the experimental group. Total sugars were consumed slowly and delayed the reduction of soluble solids contents.

3.4.4. Analysis of Vitamin C Content

It can be seen from Figure 5(d) that the vitamin C content of blueberries treated with packaging films of different components is constantly decreasing, the results indicated that the SA/AG/CO coating slowed down the loss of vitamin C content compared with another three groups (CK group, SA group, SA/CO group). Vitamin C content of blueberries observed between SA/CO/AG group and other groups were significantly different () after 14 days of storage. It still retained the vitamin C content of mg/100 g on the fourteenth day. These changes could be putted down to the low diffusion of oxygen, slowed down respiration rates of blueberries, and decreased enzyme activity, which delayed the oxidation of vitamin C. Furthermore, AG and CO have excellent antioxidant properties and can alleviate the oxidation of vitamin C and inhibit bacterial contact and reproduction [41]. Ali et al. [42] have found that AG coating has enormous potential to maintain higher antioxidant enzymes activities due to the inhibition of commodity senescence, which can effectively alleviate the oxidative decomposition of vitamin C, thus greatly improving the quality of blueberries.

4. Conclusion

In this work, a sodium alginate/Aloe vera gel-based edible composite film combined with clove essential oil was developed for the first time, which has excellent comprehensive properties, including barrier property, mechanical properties, and antifungal activity, in order to improve the freshness of blueberries. Based on the single factor, a response surface method was established to optimize the preparation process of the composite film. The optimal formulation of the composite film was 15.6 gL^-1 SA, 90 gL^-1 AG, and 10 gL^-1 CO, and the model adapted well with the experimental values and was fitted for the preparation of composite films. The results showed that SA and AG had good cross-linked structure, and CO had a significant effect on the property of the films, including barrier, mechanical and bacteriostatic properties. The freshness of blueberries coated were improved, and the film by incorporating CO based on SA and AG coating could more effectively keep the quality of blueberries during storage and prolong the shelf life of blueberries, maintaining greater hardness, more soluble solid content, more vitamin C content, and less weight loss, which indicated that AG and CO had good bacteriostatic properties and preservation effect. The edible composite film created in this work could be a potential strategy for maintaining the quality of blueberries after harvest and could be applied to other fruits.

Data Availability

Research data are not shared.

Additional Points

Novelty impact statement. A composite edible film based on sodium alginate and Aloe vera gel by incorporating clove essential oil was developed and characterized. (i) Aloe vera gel and clove essential oil improved the properties of the film. (ii) The formula of the composite film was optimized. (iii) The coating preserved the postharvest quality of blueberries and prolonged shelf life.

Conflicts of Interest

The authors have declared no conflicts of interest for this article.

Authors’ Contributions

Zhuoyu Cui was assigned in conceptualization, investigation, methodology, and writing—original draft. Yang Li performed the conceptualization, funding acquisition, project administration, supervision, and writing—review and editing. Xin Feng was appointed in formal analysis and validation; writing—review and editing. Zexi Hu was tasked in investigation and methodology.

Acknowledgments

The authors would like to thank the Natural Science Foundation of Heilongjiang Province (LH2021C016).

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Copyright © 2023 Zhuoyu Cui et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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