*3.5. Listeria monocytogenes Analysis*

The growth of *L. monocytogenes* on the fresh-cut potatoes treated with chitosan EC and chitosan EC containing cinnamon oil was evaluated (Figure 6). The number of *L. monocytogenes* on the fresh-cut potatoes treated with chitosan-based EC and containing cinnamon oil was reduced approximately 1 log cfu/g compared with that on the control at the initial day. That reason is that *L. monocytogenes* inoculated on the surface of fresh-cut potatoes may have been removed after soaking with the chitosan-based EC treatment [80]. *L. monocytogenes* was reduced 2.17 log cfu/g on fresh-cut potatoes treated with chitosan-based EC at 16 days (*p* < 0.5). It indicated that chitosan EC exhibits antimicrobial activity against *L. monocytogenes*. This result is in agreement with those of other research studies in which the chitosan-based film reduced 2 log cfu and 1.3 log cfu against *E. coli* and *L. monocytogenes*, respectively [81]. *L. monocytogenes* was reduced at 1.94, 2.44, and 2.92 log cfu/g in chitosanbased EC containing 0.2, 0.4, and 0.6% cinnamon oil, respectively, compared with that in the control group during storage time (*p* < 0.05). In this study, higher antibacterial activity was shown in EC-added cinnamon oil (the decrease is 2.92 log cfu/g) than only EC (the decrease is 2.17 log cfu/g) against *L. monocytogenes*. Therefore, the combination of EC and essential oils shows a synergistic effect of antibacterial activity against *L. monocytogenes*. The volatile component of cinnamon oil may have strong antibacterial activity. The cinnamon oil destroyed membrane phospholipids and permeability of cell membranes, eventually

causing the cytoplasm to leak and the cell to die [54]. The cinnamaldehyde in cinnamon oil is an aromatic aldehyde that can reduce the survival of foodborne pathogens through the inhibition of amino acid decarboxylase activity [82]. Some studies reported that cinnamon incorporated into polymer-based films (fish gelatin films, chitosan gelatin blend films, polymer film) has enhanced the antimicrobial effect of the film [83–85].

**Figure 5.** Population of naturally occurring microorganisms on fresh-cut potatoes coated with chitosan-based EC containing cinnamon oil. (**A**) Total plate counts; (**B**) yeast and mould counts; (**C**) total coliform counts; (**D**) lactic acid bacteria counts. Control: uncoated; EC: edible coating; Cin: cinnamon oil. Bars represent means ± SD (*n* = 3, *p* < 0.05).

**Figure 6.** Reduction in *Listeria monocytogenes* on fresh-cut potatoes coated with chitosan-based EC containing cinnamon oil. Control: uncoated; EC: edible coating; Cin: cinnamon oil. Bars represent means ± SD (*n* = 3, *p* < 0.05).

### **4. Conclusions**

We demonstrated that the use of chitosan EC containing cinnamon oil maintained the quality, reduced the deterioration, and thus extended the shelf life of fresh-cut potatoes. Chitosan EC containing 0.2% cinnamon oil reduced the degree of browning and delayed the weight loss and softening of the fresh-cut potatoes. Moreover, the addition of cinnamon oil increased the antibacterial activity of chitosan EC against naturally occurring microorganisms and LM.

Accordingly, the effective antibacterial activity of chitosan EC incorporated with cinnamon oil indicates its potential and extended application in fresh-cut fruits and vegetables preservation. It can further be applied to other types of fresh-cut fruits and vegetables, owing to the characteristics of cinnamon oil as a GRAS compound and being easily obtainable. Higher concentrations of cinnamon oil can inhibit a wide range of microorganisms. However, the EO components may impact on the quality such as colour, firmness, taste, and odour of the coated fruit. Therefore, chitosan EC incorporated with lower concentrations of cinnamon oil may be the optimum formula for maintaining the quality of fresh-cut potatoes. It also provides a strategy for designing new preservation agents and achieving the ultimate goal of commercialization of food products.

**Author Contributions:** Conceptualization, K.F. and W.H.; methodology, S. and K.F.; software, S. and K.F.; validation, W.H.; investigation, Y.L.; resources, W.H.; data curation, L.W. and C.Y.; writing—original draft preparation, S.; writing—review and editing, W.H. and K.F.; supervision, W.H.; project administration, K.F. and W.H.; funding acquisition, S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by Zhuhai College of Science and Technology Innovation Capability Cultivation Project (Grant No. 2020XJCQ018), Doctor Promotion Program of Zhuhai College of Science and Technology, and Young Innovative Talents Project of "Innovation and Improving School Project" of Education Department of Guangdong Province (Grant No. 2019KQNCX197).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data used to support the findings of this study are available from the corresponding author upon request.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


**Ignasius Radix A. P. Jati \*, Erni Setijawaty, Adrianus Rulianto Utomo and Laurensia Maria Y. D. Darmoatmodjo**

Department of Food Technology, Widya Mandala Surabaya Catholic University, Jl. Dinoyo 42-44, Surabaya 60265, Indonesia

**\*** Correspondence: radix@ukwms.ac.id

**Abstract:** *Aloe vera* is widely used to manufacture medicinal products, cosmetics, and hair treatments. The polysaccharide components in *A. vera* gel can be used as ingredients for edible films or coatings. The edible film can also be applied to fresh fruits and vegetables using the coating principle. Tomatoes are one of the fruit commodities that can be maintained in terms of quality during storage using an edible coating. This study aims to determine the effect of an edible coating made from *A. vera* on tomatoes' physical, chemical, and organoleptic properties during storage. The *A. vera* gel was prepared and used for coating the tomatoes, and the tomatoes were then stored for twelve days. The analysis was conducted every three days, and a comparison with non-coated tomatoes was performed for tomatoes' physicochemical and organoleptic properties. The results show that the application of A. vera as a coating agent could prolong the shelf life of tomatoes, as described in the ability to decrease moisture content and weight loss. The coated tomatoes had lower titratable acidity value, pH, and total soluble solid contents than the non-coated tomatoes. From the organoleptic test, the non-coated tomatoes were preferred by the panelists for color, but the glossiness, skin appearance, and texture of the coated tomatoes were preferred. The coating process could maintain the hardness of tomatoes and prevent the production of phenolic compounds, flavonoids, and lycopene; thus, the antioxidant activity could be conserved.

**Keywords:** tomato; *Aloe vera*; edible coating; storage; postharvest

### **1. Introduction**

*Aloe vera* is a Liliaceae family plant extensively distributed in the Middle East and Africa. This plant is widely grown in tropical and subtropical areas, including Indonesia. Its resistance to dry conditions is because of its ability to absorb and store water for a longer time. Therefore, *A. vera* can live in drought and extreme dry conditions [1]. *A. vera* is widely used to manufacture medicinal products, cosmetics, and hair treatments [2]. Meanwhile, on a small scale, it is also processed for food products such as nata de *A. vera*, drinks, and snack mixes. However, the utilization of *A. vera* is limited to food products because it naturally tastes bitter when consumed [3].

The most significant component of *A. vera* gel is water (99.20%). The remaining solids consist of carbohydrates, monosaccharides comprising mainly glucomannan and small amounts of arabinan and galactan, and polysaccharides such as D-glucose, D-mannose, arabinose, galactose, and xylose [4]. According to Gupta et al. [5], the active chemical components contained in *A. vera* are vitamins, minerals, lignin, saponins, salicylic acid, and amino acids, which could act as antimicrobials and antioxidants.

The presence of polysaccharide components in *A. vera* gel can be used as an ingredient for edible films or coatings. Polysaccharide components can provide hardness, density, quality, viscosity, adhesiveness, and gelling ability [6]. An edible film or coating is a thin layer made of hydrocolloids (proteins, polysaccharides, and alginates), lipids (fatty acids, glycerol, and wax), and emulsifiers that function as coatings of or packaging for food

**Citation:** Jati, I.R.A.P.; Setijawaty, E.; Utomo, A.R.; Darmoatmodjo, L.M.Y.D. The Application of *Aloe vera* Gel as Coating Agent to Maintain the Quality of Tomatoes during Storage. *Coatings* **2022**, *12*, 1480. https://doi.org/10.3390/coatings 12101480

Academic Editors: Lili Ren and Stefano Farris

Received: 10 September 2022 Accepted: 3 October 2022 Published: 6 October 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/).

products and at the same time can be directly consumed [7]. The main goal of developing edible films or coatings is to create an environmentally friendly packaging or protector for food and food products to replace plastic or other harmful substances to extend the product's shelf life. In addition, the advanced research of edible film and coating allows them to become carriers of beneficial compounds such as vitamins, minerals, antioxidants, and antimicrobials. As a result, the film or coating are able to actively protect the food and food products from damage [8]. Moreover, the edible film and coating can also carry preservative agents, flavoring agents, and colorants to extend the shelf life, enhance the flavor, and improve the appearance of food and food products [9]. Some food products that often found using edible packaging are candy, chocolate, sausage, dried fruit, and bakery products [10].

The edible film can also be applied to fresh fruits and vegetables using the coating principle. An enormous percentage of postharvest losses, especially for fruits and vegetables, is a major challenge in developing countries to ensuring food security status [11]. In contrast to edible films that are in a solid layer form when used to wrap food products, edible coatings are applied in a liquid state to coat fruits or vegetables by dipping or spraying. The coating agent will then dry and form a thin layer that protects the product. As a result, the edible coating can extend the shelf life of fresh fruits and vegetables because it decreases the contact with oxygen, as well as the respiration rate, and generally affects the metabolism of fruits and vegetables, thereby preventing the spoilage of fruits [12]. In addition, the presence of an edible coating also inhibits the transpiration of water vapor from the commodity to the environment, reducing the risk of wilting and weight loss and minimizing the vulnerability to insects or other animals, known as postharvest losses [13]. Due to their functionality and environmentally friendly nature, research on edible coatings has been increasing rapidly, especially characterization based on different materials and formulation, for example the use of starch, soy protein isolate, carboxymethyl cellulose, alginate, chitosan, agar, chlorine, ascorbic acid as an antioxidant, pectin, and essential oil coatings, and their application on food and food products, such as strawberries, blueberries, apples, and several types of cut fruit [14].

Tomatoes (*Solanum lycopersicum* Mill.) are one of the fruit commodities that can be maintained in terms of quality during storage using the edible coating. Tomato, as a climacteric fruit, is susceptible to postharvest damage [15]. The skin and flesh of the fruit are soft, increasing the risk of physical damage due to friction and impact. Wounds on the surface of the fruit skin will trigger damage due to the increase in respiration rate and the growth of microbes, thus accelerating spoilage [16]. Proper storage for tomatoes at 10 ◦C could extend the shelf life by 14 days. Meanwhile, tomatoes which are stored at room temperature (25 ◦C) undergo a rapid quality decrease on the fifth day of storage [17]. Research on the application of edible coatings on tomatoes has been reported [18–20], generally using various starch and hydrocolloids. However, limited research is available on the edible coatings made from *A. vera* to maintain the physical, chemical, and organoleptic qualities of tomato during storage. Therefore, this study aims to determine the effect of an edible coating made from *A. vera* on tomatoes' physical, chemical, and organoleptic properties during storage.
