*3.1. Weight Loss*

Figure 2 shows that the application of an edible coating of gelatin composite enriched with black tea extract can effectively reduce the percentage of weight loss in minimally processed watermelon stored at ±4 ◦C for 13 days. The samples not treated with an edible coating (control) had a significantly higher percentage of weight loss (*p* < 0.05) than the samples treated with an edible coating (Figure 1). Weight loss is generally solely attributed to water loss, although the loss of some other components may also contribute to this problem. Nevertheless, except for water loss, the contribution from other components is considered negligible. This loss of water decreases the turgor and firmness of fruits. It can cause acceleration of surface depression and deformation of the produce. Water loss is associated with several other changes occurring in fruits and can act as a trigger to initiate these changes [23]. The edible coatings cover the surface layer of the fruit and inhibit the processes of respiration, transpiration, and syneresis [21,24,25]. This finding is evidence for the benefits of applying the edible coating to fresh-cut watermelon pieces, mainly due to the formation of a polymeric barrier that can reduce the water loss from fresh-cut samples, as is found with other fruits [3,22].

Based on Figure 2, the higher the concentration of black tea extract added, the lower the percentage of weight loss. The addition of black tea extract as a natural bioactive compound can act as a cross-linking agent in edible coating components to increase the barrier against water vapor because gelatin is sensitive to water vapor when used as the sole constituent of edible coatings [24]. Although there was a decrease in the percentage of weight loss, there was no significant difference (*p* > 0.05) in the addition of 0.25–0.75% black tea extract. However, there was an increase in weight loss with the addition of 1% black tea extract. This could be caused by the addition of phenolic compounds to a certain level that can reduce the gel properties of gelatin. Phenolic compounds are more likely to interact with gelatin as an aggregate, causing an irregular gel structure [20,24].

Based on Figure 2, the concentration of black tea extract, storage time, and the interaction between the two treatments had a significant effect (*p* < 0.05) on the percentage of minimally processed watermelon weight loss during storage. Our results showed that there was no significant difference in the percentage of weight loss (*p* > 0.05) in the treatment with the addition of 0.25% black tea extract or with 0.50% and 0.75% on the 13th day. Therefore the treatment of adding black tea extract with a concentration of 0.25% to the edible coating

*Polymers* **2022**, *13*, x FOR PEER REVIEW 6 of 16

solution of the gelatin composite was the best in reducing the percentage of weight loss of watermelons processed for at least 13 days of storage at a temperature of ±4 ◦C.

**Figure 2.** Effect of an edible coating of gelatin composite enriched with black tea extract on weight loss (%) of minimally processed watermelon during storage. The value shown is the average of the three experimental replications. A–D Values followed by the same capital letters showed no significant difference (*p* > 0.05) between treatments on the same day. a–d Values followed by the same noncapital letters showed no significant difference (*p* > 0.05) between storage times in the same treat-**Figure 2.** Effect of an edible coating of gelatin composite enriched with black tea extract on weight loss (%) of minimally processed watermelon during storage. The value shown is the average of the three experimental replications. A–D Values followed by the same capital letters showed no significant difference (*p* > 0.05) between treatments on the same day. b–e Values followed by the same non-capital letters showed no significant difference (*p* > 0.05) between storage times in the same treatment.

#### ment. *3.2. Texture*

Based on Figure 2, the concentration of black tea extract, storage time, and the interaction between the two treatments had a significant effect (*p* < 0.05) on the percentage of minimally processed watermelon weight loss during storage. Our results showed that there was no significant difference in the percentage of weight loss (*p* > 0.05) in the treatment with the addition of 0.25% black tea extract or with 0.50% and 0.75% on the 13th day. Therefore the treatment of adding black tea extract with a concentration of 0.25% to the edible coating solution of the gelatin composite was the best in reducing the percentage of weight loss of watermelons processed for at least 13 days of storage at a temperature of ±4 °C. *3.2. Texture*  Figure 2 shows the results of texture analysis (firmness) on minimally processed watermelon coated with an edible coating of a gelatin composite enriched with black tea extract. The higher the Fmax (Newton) value, the harder the sample texture. Based on Figure 2, the longer the storage time, the lower the Fmax value, but the edible coating treatment Figure 3 shows the results of texture analysis (firmness) on minimally processed watermelon coated with an edible coating of a gelatin composite enriched with black tea extract. The higher the Fmax (Newton) value, the harder the sample texture. Based on Figure 3, the longer the storage time, the lower the Fmax value, but the edible coating treatment can reduce the loss of firmness. Firmness is one of the most important quality attributes for all fruits, although it may be of more critical importance to some fruits (such as berries) than others. Tissue softening is thus a severe concern that affects visual quality and reduces the shelf life of fruits, especially in fresh-cut products [23]. The control sample had a significantly lower firmness (*p* < 0.05) than the sample treated with an edible coating. The hardness of the watermelon samples was minimally reduced during storage, but the samples treated with an edible coating of sodium alginate had a significantly higher hardness (*p* < 0.05) than the control. This is because during storage there are still metabolic processes in fruit, such as respiration and transpiration, as well as the activity of pectinase enzymes such as polygalacturonase and pectin methylesterase, which can convert insoluble pectin into water-soluble pectin so that the fruit texture becomes soft [3,22].

can reduce the loss of firmness. Firmness is one of the most important quality attributes for all fruits, although it may be of more critical importance to some fruits (such as berries) than others. Tissue softening is thus a severe concern that affects visual quality and reduces the shelf life of fruits, especially in fresh-cut products [23]. The control sample had a significantly lower firmness (*p* < 0.05) than the sample treated with an edible coating. The hardness of the watermelon samples was minimally reduced during storage, but the samples treated with an edible coating of sodium alginate had a significantly higher hardness (*p* < 0.05) than the control. This is because during storage there are still metabolic processes in fruit, such as respiration and transpiration, as well as the activity of pectinase enzymes such as polygalacturonase and pectin methylesterase, which can convert insoluble pectin into water-soluble pectin so that the fruit texture becomes soft [3,22]. Based on Figure 3, the higher the concentration of black tea extract added, the greater Based on Figure 3, the higher the concentration of black tea extract added, the greater the Fmax value, but there was a decrease with the addition of 1% black tea extract. The addition of black tea extract can act as a cross-linking agent that can increase hardness at certain concentrations. The hardness of the fruit can be related to the percentage of weight loss because the higher the percentage of weight loss (loss of water), the softer the texture of the fruit. The incorporation of tea extract into coating formulations for fruits and vegetables may have a remarkable effect on the firmness of the tissue. The presence of some compounds in tea extract may have some activity that promotes alterations in firmness. In addition, the use of calcium lactate also plays a role in maintaining fruit texture, because calcium ions can strengthen cell walls. It can be seen in Figure 3 that on days 10 and 13 there was no significant difference in hardness (*p* > 0.05) with the addition of 0.50% and 0.75% black tea extract.

the Fmax value, but there was a decrease with the addition of 1% black tea extract. The addition of black tea extract can act as a cross-linking agent that can increase hardness at certain concentrations. The hardness of the fruit can be related to the percentage of weight

0.75% black tea extract.

**Figure 3.** Effect of edible coating of gelatin composite enriched with black tea extract on the hardness (Newton) of minimally processed watermelon during storage. The value shown is the average of the three experimental replications. A–D Values followed by the same capital letters showed no significant difference (*p* > 0.05) between treatments on the same day. a–e Values followed by the same non-capital letters showed no significant difference (*p* > 0.05) between storage times in the same **Figure 3.** Effect of edible coating of gelatin composite enriched with black tea extract on the hardness (Newton) of minimally processed watermelon during storage. The value shown is the average of the three experimental replications. A–E Values followed by the same capital letters showed no significant difference (*p* > 0.05) between treatments on the same day. a–e Values followed by the same non-capital letters showed no significant difference (*p* > 0.05) between storage times in the same treatment.

loss because the higher the percentage of weight loss (loss of water), the softer the texture of the fruit. The incorporation of tea extract into coating formulations for fruits and vegetables may have a remarkable effect on the firmness of the tissue. The presence of some compounds in tea extract may have some activity that promotes alterations in firmness. In addition, the use of calcium lactate also plays a role in maintaining fruit texture, because calcium ions can strengthen cell walls. It can be seen in Figure 3 that on days 10 and 13 there was no significant difference in hardness (*p* > 0.05) with the addition of 0.50% and

#### treatment. *3.3. Color*

*3.3. Color*  3.3.1. Lightness

3.3.1. Lightness Visual appearance and color are important quality criteria that directly influence the customer's perception of quality and are two of the most crucial quality attributes in fruits. The color of fruits particularly changes in their storage; when transformed into fresh-cut products they become highly prone to developing a browning discoloration. This discol-Visual appearance and color are important quality criteria that directly influence the customer's perception of quality and are two of the most crucial quality attributes in fruits. The color of fruits particularly changes in their storage; when transformed into fresh-cut products they become highly prone to developing a browning discoloration. This discoloration in fruits is thus a critical issue that may render the product unacceptable to the consumers [23].

oration in fruits is thus a critical issue that may render the product unacceptable to the consumers [23]. Figure 4 shows the results of the brightness analysis (L\*) of minimally processed watermelon coated with an edible coating of a gelatin composite enriched with black tea extract. Based on Figure 3, the surface brightness value of the processed watermelon samples minimally decreased during storage. This can be caused by the activity of the PPO (polyphenol oxidase) enzyme which causes a decrease in brightness on the surface of the fruit flesh and tends to brown [25]. Browning reactions can occur because oxygen reacts directly with the polyphenol compounds catalyzed by the polyphenol oxidase enzyme to form a brown melanin compound. Oxygen can react directly with polyphenol compounds if some cells or tissues are open due to wounds [26]. Enzymatic browning diminishes visual appearance, makes undesirable changes in flavor, and causes a loss of nutrients in fresh/fresh-cut fruits and vegetables. Such changes make the product less desirable to the Figure 4 shows the results of the brightness analysis (L\*) of minimally processed watermelon coated with an edible coating of a gelatin composite enriched with black tea extract. Based on Figure 3, the surface brightness value of the processed watermelon samples minimally decreased during storage. This can be caused by the activity of the PPO (polyphenol oxidase) enzyme which causes a decrease in brightness on the surface of the fruit flesh and tends to brown [25]. Browning reactions can occur because oxygen reacts directly with the polyphenol compounds catalyzed by the polyphenol oxidase enzyme to form a brown melanin compound. Oxygen can react directly with polyphenol compounds if some cells or tissues are open due to wounds [26]. Enzymatic browning diminishes visual appearance, makes undesirable changes in flavor, and causes a loss of nutrients in fresh/fresh-cut fruits and vegetables. Such changes make the product less desirable to the consumers. The development of browning degrades the original color of the product [2]. In the control, however, the decrease in brightness value tends to be greater than the sample treated with the edible coating because the edible coating can reduce the contact between oxygen and the polyphenol compounds. Gelatin and chitosan, as components of edible coatings, have good barrier properties against oxygen [4,27].

Based on Figure 4, on the first day, the higher the concentration of black tea extract added, the darker the color of the black tea extract solution used as an edible coating and therefore the lower the brightness value of the sample. However, during storage, the edible coating treatment enriched with black tea extract was better able to maintain brightness than the control. It can be seen in Figure 4 that on day 13 there was no significant difference in brightness value (*p* > 0.05) in the control treatment or in the edible coatings with the

addition of 0% and 1% black tea extract. The application of an edible coating incorporated with green tea has been applied to fresh lettuce. The treatments prevented ascorbic acid and carotenoid loss. Green tea has also been incorporated into the coating solution applied to peach slices to enhance their shelf life [2]. ple treated with the edible coating because the edible coating can reduce the contact between oxygen and the polyphenol compounds. Gelatin and chitosan, as components of edible coatings, have good barrier properties against oxygen [4,27].

consumers. The development of browning degrades the original color of the product [2]. In the control, however, the decrease in brightness value tends to be greater than the sam-

*Polymers* **2022**, *13*, x FOR PEER REVIEW 8 of 16

**Figure 4.** Effect of edible coating of gelatin composite enriched with black tea extract on the color (L\*) of minimally processed watermelon during storage. The value shown is the average of the three experimental replications. A–C Values followed by the same capital letters showed no significant difference (*p* > 0.05) between treatments on the same day. a–d Values followed by the same non-capital letters showed no significant difference (*p* > 0.05) between storage times in the same treatment. **Figure 4.** Effect of edible coating of gelatin composite enriched with black tea extract on the color (L\*) of minimally processed watermelon during storage. The value shown is the average of the three experimental replications. A–C Values followed by the same capital letters showed no significant difference (*p* > 0.05) between treatments on the same day. a–d Values followed by the same non-capital letters showed no significant difference (*p* > 0.05) between storage times in the same treatment.

Based on Figure 4, on the first day, the higher the concentration of black tea extract added, the darker the color of the black tea extract solution used as an edible coating and therefore the lower the brightness value of the sample. However, during storage, the edible coating treatment enriched with black tea extract was better able to maintain brightness than the control. It can be seen in Figure 4 that on day 13 there was no significant difference in brightness value (*p* > 0.05) in the control treatment or in the edible coatings with the addition of 0% and 1% black tea extract. The application of an edible coating incorporated with green tea has been applied to fresh lettuce. The treatments prevented ascorbic acid and carotenoid loss. Green tea has also been incorporated into the coating The results of the statistical analysis showed that the concentration of black tea extract, storage time, and the interaction between the two treatments had a significant effect (*p* < 0.05) on the brightness of the watermelon flesh that was minimally processed during storage. Further Duncan's test results showed that there was no significant difference in brightness (*p* > 0.05) with the addition of 0.25% and 0.50% black tea extract treatment on day 13. Therefore, the addition of black tea extract with a concentration of 0.25% in the edible coating solution of gelatin composite is the best treatment for maintaining the brightness of the color of watermelons processed for at least 13 days of storage at a temperature of ± 4 ◦C.

#### solution applied to peach slices to enhance their shelf life [2]. 3.3.2. a (Degree of Redness)

The results of the statistical analysis showed that the concentration of black tea extract, storage time, and the interaction between the two treatments had a significant effect (*p* < 0.05) on the brightness of the watermelon flesh that was minimally processed during storage. Further Duncan's test results showed that there was no significant difference in brightness (*p* > 0.05) with the addition of 0.25% and 0.50% black tea extract treatment on day 13. Therefore, the addition of black tea extract with a concentration of 0.25% in the edible coating solution of gelatin composite is the best treatment for maintaining the brightness of the color of watermelons processed for at least 13 days of storage at a temperature of ± 4 °C. 3.3.2. a (Degree of Redness) Figure 5 shows the results of the analysis of the reddish color (a\*) of minimally processed watermelon coated with an edible coating of a gelatin composite enriched with black tea extract. Based on Figure 4, the red color of all the samples of minimally processed Figure 5 shows the results of the analysis of the reddish color (a\*) of minimally processed watermelon coated with an edible coating of a gelatin composite enriched with black tea extract. Based on Figure 5, the red color of all the samples of minimally processed watermelon experienced a significant decrease (*p* < 0.05) during the storage period. Lycopene is a red pigment in watermelon that has the ability to be an antioxidant; so, it is sensitive and easily damaged when exposed to light and oxygen during storage [1]. Samples without treatment (control) had a higher intensity of red color than samples treated with an edible coating. This can be caused by the red pigment in watermelon dissolving during the drying process in an edible coating solution. There was a significant difference (*p* < 0.05) for each addition of black tea extract concentration to the red color of minimally processed watermelon samples. Black tea contains theaflavin and thearubigin compounds, which give the tea a brown color so that the higher the concentration of black tea extract added, the more theaflavin and thearubigin compounds are present, which will affect the appearance of color in minimally processed watermelon samples [14].

**Figure 5.** Effect of edible coating of gelatin composite enriched with black tea extract on the color (a\*) of cut watermelon during storage. The value shown is the average of the three experimental replications. A–F Values followed by the same capital letters showed no significant difference (*p* > 0.05) between treatments on the same day. a–e Values followed by the same non-capital letters showed no significant difference (*p* > 0.05) between storage times in the same treatment. **Figure 5.** Effect of edible coating of gelatin composite enriched with black tea extract on the color (a\*) of cut watermelon during storage. The value shown is the average of the three experimental replications. A–F Values followed by the same capital letters showed no significant difference (*p* > 0.05) between treatments on the same day. a–e Values followed by the same non-capital letters showed no significant difference (*p* > 0.05) between storage times in the same treatment.

watermelon experienced a significant decrease (*p* < 0.05) during the storage period. Lycopene is a red pigment in watermelon that has the ability to be an antioxidant; so, it is sensitive and easily damaged when exposed to light and oxygen during storage [1]. Samples without treatment (control) had a higher intensity of red color than samples treated with an edible coating. This can be caused by the red pigment in watermelon dissolving during the drying process in an edible coating solution. There was a significant difference (*p* < 0.05) for each addition of black tea extract concentration to the red color of minimally processed watermelon samples. Black tea contains theaflavin and thearubigin compounds, which give the tea a brown color so that the higher the concentration of black tea extract added, the more theaflavin and thearubigin compounds are present, which will

affect the appearance of color in minimally processed watermelon samples [14].

#### 3.3.3. Degree of Yellowness 3.3.3. Degree of Yellowness

Figure 6 shows the results of the analysis of the yellowish color (b\*) on minimally processed watermelon coated with an edible coating of a gelatin composite enriched with black tea extract. Based on Figure 6, the yellow color in all the samples of minimally processed watermelon tends to decrease during the storage period. This can be seen in Figure 5, where, on the first day, the higher the concentration of the addition of black tea extract, the lower the intensity of the yellow color. On the first day, the sample without treatment (control) had a higher yellow color intensity than the sample treated with an edible coating, but there was a significant decrease (*p* < 0.05) during storage so that the control had the lowest yellow color intensity on the 13th day. However, on day 13 there was no significant difference (*p* > 0.05) between the control and the treatment with the addition of 1% black tea extract. Figure 6 shows the results of the analysis of the yellowish color (b\*) on minimally processed watermelon coated with an edible coating of a gelatin composite enriched with black tea extract. Based on Figure 6, the yellow color in all the samples of minimally processed watermelon tends to decrease during the storage period. This can be seen in Figure 6, where, on the first day, the higher the concentration of the addition of black tea extract, the lower the intensity of the yellow color. On the first day, the sample without treatment (control) had a higher yellow color intensity than the sample treated with an edible coating, but there was a significant decrease (*p* < 0.05) during storage so that the control had the lowest yellow color intensity on the 13th day. However, on day 13 there was no significant difference (*p* > 0.05) between the control and the treatment with the addition of 1% black tea extract. *Polymers* **2022**, *13*, x FOR PEER REVIEW 10 of 16

**Figure 6.** Effect of edible coating of gelatin composite enriched with black tea extract on the color (b\*) of minimally processed watermelon during storage. The value shown is the average of the three experimental replications. A–E Values followed by the same capital letters showed no significant difference (*p* > 0.05) between treatments on the same day. a–e Values followed by the same non-capital letters showed no significant difference (*p* > 0.05) between storage times in the same treatment. **Figure 6.** Effect of edible coating of gelatin composite enriched with black tea extract on the color (b\*) of minimally processed watermelon during storage. The value shown is the average of the three experimental replications. A–E Values followed by the same capital letters showed no significant difference (*p* > 0.05) between treatments on the same day. a–e Values followed by the same non-capital letters showed no significant difference (*p* > 0.05) between storage times in the same treatment.

Figure 7 shows the results of the pH analysis on minimally processed watermelon

value compared to the sample with the edible coating treatment, and the higher the black tea extract added to the edible coating solution, the lower the pH of the sample. This could be caused by the use of 1% acetic acid to dissolve chitosan as a component of the edible coating composites and by the presence of organic acids in tea, such as malic acid, citric

**Figure 7.** The effect of edible coating from gelatin composite enriched with black tea extract on the pH of watermelon was minimally processed during the storage period. The value shown is the average of the three experimental replications. A–D Values followed by the same capital letters showed

acid, oxalic acid, and succinic acid, which affect the pH of the sample [15].

*3.4. pH* 

#### *3.4. pH 3.4. pH*  Figure 7 shows the results of the pH analysis on minimally processed watermelon

Figure 7 shows the results of the pH analysis on minimally processed watermelon coated with an edible coating of a gelatin composite enriched with black tea extract. Based on Figure 7, on the first day the sample without treatment (control) had the highest pH value compared to the sample with the edible coating treatment, and the higher the black tea extract added to the edible coating solution, the lower the pH of the sample. This could be caused by the use of 1% acetic acid to dissolve chitosan as a component of the edible coating composites and by the presence of organic acids in tea, such as malic acid, citric acid, oxalic acid, and succinic acid, which affect the pH of the sample [15]. coated with an edible coating of a gelatin composite enriched with black tea extract. Based on Figure 7, on the first day the sample without treatment (control) had the highest pH value compared to the sample with the edible coating treatment, and the higher the black tea extract added to the edible coating solution, the lower the pH of the sample. This could be caused by the use of 1% acetic acid to dissolve chitosan as a component of the edible coating composites and by the presence of organic acids in tea, such as malic acid, citric acid, oxalic acid, and succinic acid, which affect the pH of the sample [15].

**Figure 6.** Effect of edible coating of gelatin composite enriched with black tea extract on the color (b\*) of minimally processed watermelon during storage. The value shown is the average of the three experimental replications. A–E Values followed by the same capital letters showed no significant difference (*p* > 0.05) between treatments on the same day. a–e Values followed by the same non-capital letters showed no significant difference (*p* > 0.05) between storage times in the same treatment.

*Polymers* **2022**, *13*, x FOR PEER REVIEW 10 of 16

**Figure 7.** The effect of edible coating from gelatin composite enriched with black tea extract on the pH of watermelon was minimally processed during the storage period. The value shown is the average of the three experimental replications. A–D Values followed by the same capital letters showed **Figure 7.** The effect of edible coating from gelatin composite enriched with black tea extract on the pH of watermelon was minimally processed during the storage period. The value shown is the average of the three experimental replications. A–D Values followed by the same capital letters showed no significant difference (*p* > 0.05) between treatments on the same day. a–e Values followed by the same non-capital letters showed no significant difference (*p* > 0.05) between storage times in the same treatment.

There was a decrease in pH in all the treatments during the storage period. This was presumably due to the activity of the microorganisms that can convert sugar into organic acids. It can be seen in Figure 6 that the decrease in pH in samples treated with an edible coating enriched with the addition of black tea extract was not as sharp as in the control, because chitosan and black tea, as components of edible coatings, have antimicrobial activity, and the combination of gelatin and chitosan can increase the antimicrobial properties of chitosan [22]. Fruits are a naturally existing source of several organic acids such as Vitamin C. Vitamin C (ascorbic acid) degradation occurs during storage because it is highly sensitive and is lost upon exposure to heat, light, oxygen metals, and enzymes. Coating treatments have been shown to reduce the loss of organic acids such as ascorbic acid in fresh/fresh-cut fruits. Usually, ascorbic acid content decreases with the storage of fruits, irrespective of whether the fruit is covered with a coating or not, but the extent of the loss may be less in fruits that are coated [23].
