**1. Introduction**

It is hoped that the consumption of sufficient fruits and vegetables during the current pandemic can have many functional effects on human health [1,2]. In Indonesia, fruit consumption is less than vegetable consumption. One of the reasons for the low consumption of fruit is the necessity to peel it before consumption. Therefore, there is the potential to develop minimally processed fruit because it only involves washing, peeling, cutting, packaging, and storing the fruit at low temperatures to maintain the freshness and nutritional content [2]. An example of minimally processed fruit that is often found in the market is watermelon (*Citrullus lanatus*).

Watermelon is very popular with the public because of its sweet taste and high water content, which give it a freshness when consumed. Watermelon is rich in vitamins A, B6, C, and K and antioxidants, which are very good for maintaining a healthy body [1,3]. Currently, many watermelons are sold in a minimally processed form. Minimally processed watermelon, in the form of slices without the skin, is more in demand by the public

**Citation:** Salsabiela, S.; Sukma Sekarina, A.; Bagus, H.; Audiensi, A.; Azizah, F.; Heristika, W.; Manikharda; Susanto, E.; Munawaroh, H.S.H.; Show, P.L.; et al. Development of Edible Coating from Gelatin Composites with the Addition of Black Tea Extract (*Camellia sinensis*) on Minimally Processed Watermelon (*Citrullus lanatus*). *Polymers* **2022**, *14*, 2628. https://doi.org/10.3390/ polym14132628

Academic Editor: Evgenia G. Korzhikova-Vlakh

Received: 9 June 2022 Accepted: 22 June 2022 Published: 28 June 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

because of the convenience and practicality of consuming it. However, minimally processed watermelon easily loses weight through the evaporation of its water content, the growth of spoilage microbes, and the several enzymatic reactions that cause changes in the texture, color, taste, and nutritional content [3]. One way to maintain quality and freshness and extend the shelf life of minimally processed watermelon is to coat the pieces of fruit using a coating solution that is safe for consumption, commonly called edible coating [4,5].The edible coating can maintain quality and extend the shelf life of minimally processed fruit. The requirements for the components that can be used as edible coatings include those that do not affect the smell and taste of the food used and are easy to obtain, easy to digest, and non-toxic [5,6]. The edible coating could also contain several polymers from volatile or non-volatile parts [7]. Edible coating is used for the coating of fresh-cut fruits and vegetables such as watermelon. The sample is immersed in the film-forming solution, and it creates a protective coating directly on the surface of food such as minimally processed fruits [8]. The polymers that are usually used for edible coating could be from polymers of protein, carbohydrates, and lipid derivatives [8,9]. Gelatin is one of the ingredients that is often used as a component of edible coatings. One type of fish that is effective in producing gelatin is tuna. Gelatin from tuna fish skin as a food by-product has a good gel strength and viscosity [8,9]. Several studies have succeeded in combining gelatin with chitosan as a composite edible coating. Chitosan is one of the polysaccharides that is widely used as a constituent of edible coatings because it has antimicrobial properties. The combination of gelatin and chitosan can increase the antimicrobial properties of chitosan [10,11].

The addition of natural extracts can also improve the physical and functional properties of edible coatings, especially natural extracts that are rich in bioactive compounds [12]. Tea (*Camellia sinensis*) is a plant that has both antimicrobial and antioxidant activity [13]. Black tea contains tannins such as thearubigins, which have antimicrobial activities. On the other hand, several polyphenolic compounds, such as catechins, theaflavins, and methyl jasmonate, also have antimicrobial and antioxidant activity [14,15]. The fermentation process in black tea converts polyphenolic compounds (catechins and their derivatives) into theaflavins and thearubigins so that the content of catechin compounds decreases. However, many studies have shown that the antioxidant activity of black tea is comparable to that of green tea [12,14,15]. The theaflavins in black tea have the same antioxidant potential as the catechins in green tea [14,16]. The theaflavins in black tea extract are more effective in capturing free radicals than the catechins found in green tea. Therefore, based on the description above, in our study the aim was to determine the effect of using tuna skin gelatin composite and chitosan enriched with the addition of black tea extract as an edible coating on minimally processed fruits, e.g., fresh-cut watermelon. The samples were stored at ±4 ◦C for 13 days at low-temperature storage (±4 ◦C), and the physicochemical characterization and the fungal decay of the minimally processed watermelon was evaluated.

#### **2. Material and Methods**

#### *2.1. Materials*

The ingredients used in this study include red seeded watermelon 7–8 days after harvest (Berkat fruit shop, Yogyakarta, Indonesia), black tea (Tong Tji Super), yellowfin tuna (*Thunnus albacares*) skin (Omah Tuna, Yogyakarta, Indonesia), chitosan with a 20–100 mPas viscosity and a min 94% degree of acetylation (Phy Edumedia, Malang, Indonesia), glycerol (Progo Mulyo, Yogyakarta, Indonesia), aquades (Progo Mulyo, Yogyakarta, Indonesia), sodium hydroxide (Merck, Darmstadt, Germany), sulfuric acid (Merck, Darmstadt, Germany), citric acid (Merck, Darmstadt, Germany), acetic acid (Merck, Darmstadt, Germany), methanol (Merck, Darmstadt, Germany), and DPPH (2,2-diphenyl-1-picrylhydrazyl) (Sigma Aldrich, Singapore).

#### *2.2. Extraction of Gelatin from Yellowfin Tuna Fish Skin 2.2. Extraction of Gelatin from Yellowfin Tuna Fish Skin*

The extraction of the gelatin began with the separation of the tuna skin from the scales, bones, and nonskin components, followed by the cutting of the tuna skin to a size of ±5 × 5 cm. The tuna skin was soaked in NaOH solution with a 0.2% ratio of 1:6 (*w*/*v*) for 2 h and neutralized to pH 6–7 with water. The fish skin was then immersed in a 0.2% H2SO<sup>4</sup> solution at a ratio of 1:6 (*w*/*v*) for 2 h, washed to neutralize, soaked in citric acid (C6H8O7) 0.1%, with a ratio of 1:6 (*w*/*v*) for 2 h, and washed to neutral pH (6–7). Continued extraction was performed with distilled water in a 60 ◦C water bath shaker for 6 h at a ratio of 1:3 (*w*/*v*) and filtered twice before drying at a temperature of 50 ◦C for 24 h [17]. The gelatin sheet was then milled to obtain the gelatin powder [17]. The specification of gelatin viscosity is approximately 25 cPs. The extraction of the gelatin began with the separation of the tuna skin from the scales, bones, and nonskin components, followed by the cutting of the tuna skin to a size of ± 5 × 5 cm. The tuna skin was soaked in NaOH solution with a 0.2% ratio of 1:6 (*w*/*v*) for 2 h and neutralized to pH 6–7 with water. The fish skin was then immersed in a 0.2% H2SO4 solution at a ratio of 1:6 (*w*/*v*) for 2 h, washed to neutralize, soaked in citric acid (C6H8O7) 0.1%, with a ratio of 1:6 (*w*/*v*) for 2 h, and washed to neutral pH (6–7). Continued extraction was performed with distilled water in a 60 °C water bath shaker for 6 h at a ratio of 1:3 (*w*/*v*) and filtered twice before drying at a temperature of 50 °C for 24 h [17]. The gelatin sheet was then milled to obtain the gelatin powder [17]. The specification of gelatin viscosity is approximately 25 cPs.

#### *2.3. Preparation of Edible Coating Solution 2.3. Preparation of Edible Coating Solution*

The process of applying the edible coating solution onto the watermelon (*Citrullus lanatus*) was achieved using a layer-by-layer technique, with three different types of edible coating solutions. The first solution was a gelatin–chitosan composite solution and glycerol; the second solution was a black tea extract at five concentration variations (0%; 0.25%; 0.50%; 0.75%; and 1%); and the third solution was a 2% calcium lactate solution. The process of applying the edible coating solution onto the watermelon (*Citrullus lanatus*) was achieved using a layer-by-layer technique, with three different types of edible coating solutions. The first solution was a gelatin–chitosan composite solution and glycerol; the second solution was a black tea extract at five concentration variations (0%; 0.25%; 0.50%; 0.75%; and 1%); and the third solution was a 2% calcium lactate solution.

#### *2.4. Coating of Edible Coating Solution on Minimally Processed Watermelon (Citrullus lanatus) 2.4. Coating of Edible Coating Solution on Minimally Processed Watermelon (Citrullus lanatus)*

The process of applying the edible coating solution to the minimally processed watermelon (*Citrullus lanatus*) was carried out using the layer-by-layer immersion technique. First, a sample of watermelon that had been minimally processed with a size of ±(3 × 3 cm) was immersed in a 2% calcium lactate solution for two minutes, then drained and dried for two minutes. Next, the sample was immersed in a gelatin–chitosan composite solution and glycerol for two minutes, then drained and dried for two minutes. After that, the sample was again immersed in a 2% calcium lactate solution for two minutes, then drained and dried for two minutes. Furthermore, the packaging was carried out in plastic cups for samples treated with 0% black tea extract, while the other samples were immersed in a black tea extract solution with different concentrations, namely 0.25%, 0.50%, 0.75%, and 1% each for two minutes, then drained and dried for two minutes. After that, the sample was again immersed in a 2% calcium lactate solution for two minutes, then drained and dried for two minutes. Then, the sample was put into a plastic cup and stored in the refrigerator at a temperature of ± 4 ◦C. The sample was stored for 13 days of storage, and the physicochemical characterization and fungal decay evaluation were investigated at 4, 7, 10, and 13 days of storage. The following is a schematic of the edible coating solution coating process on watermelons that are minimally processed with the layer-by-layer technique. Further processes of the layer-by-layer technique can be seen in Figure 1. The process of applying the edible coating solution to the minimally processed watermelon (*Citrullus lanatus*) was carried out using the layer-by-layer immersion technique. First, a sample of watermelon that had been minimally processed with a size of ± (3 × 3 cm) was immersed in a 2% calcium lactate solution for two minutes, then drained and dried for two minutes. Next, the sample was immersed in a gelatin–chitosan composite solution and glycerol for two minutes, then drained and dried for two minutes. After that, the sample was again immersed in a 2% calcium lactate solution for two minutes, then drained and dried for two minutes. Furthermore, the packaging was carried out in plastic cups for samples treated with 0% black tea extract, while the other samples were immersed in a black tea extract solution with different concentrations, namely 0.25%, 0.50%, 0.75%, and 1% each for two minutes, then drained and dried for two minutes. After that, the sample was again immersed in a 2% calcium lactate solution for two minutes, then drained and dried for two minutes. Then, the sample was put into a plastic cup and stored in the refrigerator at a temperature of ± 4 °C. The sample was stored for 13 days of storage, and the physicochemical characterization and fungal decay evaluation were investigated at 4, 7,10, and 13 days of storage. The following is a schematic of the edible coating solution coating process on watermelons that are minimally processed with the layer-by-layer technique. Further processes of the layer-by-layer technique can be seen in Figure 1.

**Figure 1.** Layer-by-layer technique of edible coating.

#### *2.5. Weight Loss Analysis*

Weight loss analysis was carried out gravimetrically, namely by calculating the difference in weight before and after storage [18,19]. The measurement of the sample weight was carried out using an analytical balance every three days for 13 days of storage.

$$\text{Weight loss} \left( \% \right) = \frac{\left( \text{initial weight (grams)} - \text{final weight (grams)} \right)}{\left( \text{initial weight (grams)} \right)} \times 100\% \tag{1}$$

#### *2.6. Texture Analysis*

The texture parameter observed was hardness. Texture analysis was performed using a Universal Testing Machine (UTM) with a pre-load of 0.02 N; a pre-load speed of 50 mm/min; and a test speed of 50 mm/min. The analysis of sample hardness was interpreted by the maximum force (Fmax) required to pierce 30% of the sample height. Higher Fmax (Newton) values indicate that the texture of the sample is becoming harder [17].

### *2.7. Color Analysis*

Color intensity analysis was carried out using a Minolta CR-400 Chroma Meter (Konica Minolta, Inc., Tokyo, Japan). The sample was placed on top of the chroma meter sensor, then a light was fired at the part to be measured so that the values of L (lightness), a (green-red chromaticity), and b (yellow-blue chromaticity) would appear on the chroma meter display [17].

#### *2.8. pH Analysis*

The pH analysis was performed using a pH meter previously calibrated with standard buffer solutions of pH 4 and 7. A total of 10 g of the sample was mashed using a blender and then centrifuged for one hour at a speed of 1000× *g* at 4 ◦C until separation between the natant and the supernatant occurred. The supernatant was used for pH analysis. To measure the pH value, the pH meter probe was immersed in the sample supernatant to obtain the pH value directly [20].

#### *2.9. Total Dissolved Solids Analysis*

Total dissolved solids analysis was performed using an Atago Master-53M refractometer (Atago Co., Ltd., Fukuoka, Japan). Sample preparation for the total dissolved solids analysis was carried out as in step 2.8. One to two drops of the sample supernatant at room temperature were placed on the prism of the refractometer, then the Brix percentage was read through the eyepiece of the refractometer [21].

#### *2.10. Antioxidant Activity Analysis*

A total of 15 g of watermelon samples were mashed using a blender for 2 min, then centrifuged for one hour at a speed of 1000× *g* at 4 ◦C until the separation between the natant and the supernatant occurred. Then, the natant or solid particles resulting from the centrifugation were filtered using a vacuum filter.

For the analysis of the antioxidant activity, a sample solution with a concentration of 100,000 ppm was made by dissolving 2.5 g of the sample, which had been filtered with a vacuum filter in 25 mL of methanol, then stirred with a magnetic stirrer for 90 min. After that, the samples were filtered with filter paper; then, 1 mL of each was taken and put into a test tube. Each sample had 7 mL of 0.1 mM DPPH solution added and homogenized using a vortex. The samples were then incubated for 30 min in the dark. After that, the absorbance value was measured using a UV-Vis spectrophotometer at a wavelength of 517 nm.

The antioxidant activity of the sample is interpreted in terms of the percentage of radical scavenging activity, namely the ability of the sample to capture free radical compounds. The more free radical compounds that can be captured, the more the antioxidant activity content in the sample [17].

$$\text{DPPH radical saving activity} \left(\% \right) = \frac{\left(\text{A blank} - \text{A sample}\right)}{\left(\text{A blank}\right)} \times 100\% \tag{2}$$

where A = Absorbance.

#### *2.11. Fungal Decay*

Determination of fungal decay was performed by observing the presence or absence of the fungi that grow on the surface of the sample, then calculating the percentage of the sample that was overgrown with fungus (Equation (3)). The sample is considered damaged if there is fungal mycelium on its surface. The results of these observations are expressed as the percentage of samples contaminated with fungi [22].

$$\text{Fungal contamination (\%)} = \frac{(\text{Number of samples contained with fungus})}{(\text{Total number of samples})} \times 100\% \tag{3}$$

#### *2.12. Statistical Analysis*

The statistical analysis was performed using Minitab v. 19 statistical software (State College, PA, USA). Analysis of variance (ANOVA) and Tukey's pairwise comparisons were carried out using a level of 95% confidence. The experiments were performed at least in triplicate, and the data were reported as mean ± standard deviation.

#### **3. Results and Discussion**
