**1. Introduction**

Fruits and vegetables are important sources of minerals, vitamins, and fibers, which are essential for human's well-being, and their consumption has been associated with several beneficial effects on human health. The demand for those benefits has considerably increased over the years due to consumer preference for natural products and changes in lifestyle [1]. In this sense, fruits and vegetables are an important component of the human diet.

After they are harvested, fruits and vegetables continue the respiration process, consuming O2 and releasing CO2 and water. Consequently, lipids, proteins, organic acids, and carbohydrates are metabolized and energy replacement is compromised, as the vegetable or fruit is separated from the mother plant [2]. Over time, quality characteristics such as color, flavor, weight, nutritional value, and bioactive compounds continue to deteriorate as a result of senescence [3]. The water released during the respiration process plays an important role in the postharvest quality of fresh fruits and vegetables and can result in loss of nutritional value, soft texture, sagging, wrinkling, and withering [4].

Although waxes were used to preserve citrus fruit in ancient China, it was not until the twentieth century that edible coatings based on emulsions were developed to preserve the quality of fresh fruits and vegetables [5]. These emulsions are typically formulated from oils (vegetable- or animal-derived), waxes (paraffin, carnauba wax, candelilla, or beeswax), and resins (shellac or wood rosin). Furthermore, polymer-based coating solutions can have additional functionality when formulated with plant essential oils having antimicrobial

**Citation:** de Oliveira Filho, J.G.; Miranda, M.; Ferreira, M.D.; Plotto, A. Nanoemulsions as Edible Coatings: A Potential Strategy for Fresh Fruits and Vegetables Preservation. *Foods* **2021**, *10*, 2438. https://doi.org/ 10.3390/foods10102438

Academic Editor: Like Mao

Received: 23 August 2021 Accepted: 6 October 2021 Published: 14 October 2021

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**Copyright:** © 2021 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/).

activity [6]. Due to their hydrophobic nature, oils and waxes have proven to be an efficient technology for fruits and vegetables preservation post-harvest, as they are able to minimize water loss and gas exchange and improve and/or preserve the physicochemical properties, such as color, firmness, fresh appearance, and microbial protection [7–10].

Recently, nanotechnology was introduced as a new tool for making coatings based on emulsions with improved properties and functionalities. Coatings are made of macro- or microemulsions (conventional) or nanoemulsions, for which the latter can be considered a conventional emulsion with very small particles. Droplets in nanoemulsions are on a nanoscale (particle radius less than 100 nm) dispersed in an aqueous solution [11]. This changes the physical properties of the coating by further reducing moisture migration, gas exchange, oxidative reactions, and suppressing pathogenic growth (microorganisms), product deterioration and enhancing control of physiological disorders [12]. In addition, coatings based on nanoemulsions have shown to be promising vehicles for several active compounds, such as oil-soluble vitamins, antimicrobials, flavors, and nutraceuticals, which may further contribute to maintenance of food product quality attributes [13].

Figure 1 shows a survey of published scientific manuscripts on nanoemulsions as edible coatings for fruits and vegetables. The number of studies on the topic has increased considerably over the past few years, demonstrating the scientific community's increased interest in the topic. However, studies concerning in vivo biological efficiencies are limited [14] and applications on fruits and vegetables are even fewer. Thus, more research is essential to determine this technology's potential for future application on a commercial scale. In this context, the objective of this review is to update the available information on the use of nanoemulsions as coatings for preserving fresh fruits and vegetables.

**Figure 1.** The distribution of publications related to 'nanoemulsion as edible coating for fruits and vegetables' (2005–2021): ScienceDirect databases. Data for 2021 is as of September 2021.

#### **2. Edible Coatings—An Overview**

The first reports of the use of coatings on fruits appeared in the 12th century in China, where wax was applied to citrus (lemons and oranges) to reduce mass loss and preserve the fruit [15]. However, it was only in 1922 that the commercial scale application of waxes began in order to increase postharvest conservation of fruits and vegetables, thus reducing postharvest losses [16].

Currently, edible coatings are used as a strategy to increase the shelf life and postharvest quality of many fresh fruits and vegetables during storage [17,18]. Edible coatings are defined as thin layers applied on the fruit surface, forming clear films produced from food-grade materials and adding to, or as a substitute for, the waxes naturally present on the fruit surface. As these films become part of the food and are consumed as such (for fruits where the peel is consumed), the materials used in their composition must be GRAS (Generally Recognized as Safe), that is, be non-toxic and safe for food [19].

Edible coatings are formulated from various biopolymers such as polysaccharide, lipid, and protein compounds, or by combining materials resulting in improved properties (Table 1). They act as an obstacle to water vapor, gases, and solutes [20] as shown in Figure 2.

**Table 1.** Summary of diverse structural materials frequently used for edible coating.


**Figure 2.** Main functions of edible coatings on fruits and vegetables.

The mechanism of action for coatings on fruit is similar to packaging with a modified atmosphere; the coating produces a physical barrier that modifies gas exchange between the interior of the fruit and the surrounding atmosphere, increasing the concentration of CO2 and decreasing O2 [30]. This environment can effectively decrease respiration rate, conserve stored energy, delay microbial growth, and therefore, extend the useful life of the fruit [31]. The coating efficiency depends on the coating thickness formed on the fruit surface, since there is a negative correlation between thickness and coating permeability [32]. Another important point is related to low permeability coatings, based on resins such as shellac, for example, which can restrict gas exchange almost entirely, leading to the accumulation of CO2 within the fruit, and the production of compounds

resulting from the fermentation process that can cause off-flavor, such as acetaldehyde and ethanol, thus affecting fruit quality [18,33].

In addition to maintaining quality and postharvest conservation of fruits and vegetables, the coating materials can also act as carriers of compounds such as food coloring, flavoring, antimicrobials, antioxidants, antagonistic microorganisms, among others [34,35]. In this sense, several natural bioactive compounds have been incorporated into edible coating materials such as essential oils [36–38], plant extracts [39,40], vitamins [34], antagonistic microorganisms [41,42], and antibrowning or firming agents in fresh cut fruit. [43,44].

#### **3. Methods to Apply Edible Coatings**

The effectiveness of coatings in preserving fresh fruits and vegetables is influenced by the application method, which will be chosen according to the nature of the food to be coated, the surface attributes, the rheological properties of the solution, and the main purpose of the coating [45]. The adhesion of coatings to food surfaces is essential for performance of their intended function [16,46]. Wettability is used to quantify the interfacial interaction that occurs between the food surface and the coating. This variable must be taken into account when assessing the performance of the coating solution on the food surface [31].

Dipping (Figure 3a), spraying (Figure 3b), and hand coating (Figure 3c) techniques are the most common methods for applying edible coatings to fresh fruits and vegetables. Other techniques such as fluidized bed and foaming are also available; however, these techniques are rarely used on commercial and laboratory scales [45].

**Figure 3.** Dipping (**a**), spraying (**b**), and (**c**) hand coating techniques to apply edible coatings.

On a laboratory scale, immersion is one of the main methods used for coating fruits due to its simplicity, without dependence on equipment, and uniformity of film obtained. In this method, the entire surface of the food is submerged in the film-forming solution at a constant speed, allowing full surface coverage, ensuring complete surface wetting [47]. After application, the excess solution is drained to eliminate the overload of film-forming solution on the fruit surface [48]. Finally, the food is dried with the excess solvent and liquid being evaporated to leave the film in contact with the food surface. Drying can take place at room temperature or using a heated air tunnel after draining the solution. This technique allows the application of coating solutions with a wide viscosity range [46]. A negative

point of this technique is the possibility of cross-contamination from fruit to fruit during the immersion process due to the accumulation of residues and microbial organisms [45].

To avoid this problem, products that will be coated must be properly cleaned and sanitized, and the coating solution replaced frequently [15]. According to Raghav et al. [16], in general, fruits and vegetables are immersed for 5–30 s in the coating solution.

In turn, the spraying technique, most popular in packing houses, provides a homogeneous and attractive coating. In addition, it avoids the possibility of contaminating the coating solution [49]. This process increases the liquid surface through the formation of drops and distributes them over the food surface [45]. During spray application, the fruit or vegetable is placed on a plate or rotating rollers at a coordinated speed, under dispersing nozzles activated manually or automatically. This procedure is repeated until the desirable coating thickness is achieved. A drawback of this technique is that viscous solutions cannot be sprayed as they clog the equipment [50].

Another method to apply a filmogenic solution is by gloved hands to the fruit surface. Fruits can be coated by spreading a uniform amount of coating solution by hand while wearing latex gloves. It is appropriate on a laboratory scale to avoid solution contaminations and to minimize waste of experimental coating solutions during screenings. However, a negative aspect consists of the non-homogeneous film thickness formed on the entire fruit surface [18,35].
