Progress in Fruit and Vegetable Preservation: Plant-Based Nanoemulsion Coatings and Their Evolving Trends

Abstract

Innovative technologies in the food industry are focused on integrated approaches to improve the sustainability of the food system that cover the whole supply chain. Huge post-harvest losses of fruits and vegetables and the use of synthetic chemical preservatives for this purpose are a matter of grave concern for any country. High demands for safe and healthy food have contributed to maximizing efforts to investigate post-harvest technology. Since fruits and vegetables are extremely perishable foods, they require the best post-harvest methods to maintain their storage stability and increase shelf-life. A solution for this emerging problem was found in the application of nanoemulsion edible coatings, described as thin-layered edible coatings or films with the possibility to provide additional benefits such as antioxidant and antimicrobial properties. These coatings provide protection against moisture loss, respiration, gaseous exchange, microbial spoilage, etc., offering promising results to safeguard the physicochemical during the time of storage and transportation of fruits and vegetables. This review summarizes the newest studies of nanoemulsion coatings on fresh products, providing valuable information regarding preparation and application methods and applied polymers and bioactives. Moreover, it gives a detailed description of the influence of nanoemulsion coating application (shelf-life, weight loss, colour, etc.) on fresh fruits and vegetables during storage.

1. Introduction

Current trends in people’s lifestyles, fast urbanization, and improvements in technology lead to higher demands for safe and healthy food, with requirements to contain additional health benefits [1]. The global change in the consumption of fruits and vegetables has been followed by an increase in the knowledge and awareness of healthy compounds in these products, such as antioxidants and other micronutrients. Hence, it is inevitable that the expansion of fresh and minimally processed fruits and vegetables in the food industry will be followed by the consumers’ high expectations regarding their freshness and quality. Unfortunately, changes in colour, browning, sweating, or other undesirable variations could turn down consumers from buying it [2,3]. In addition to the evident variations in organoleptic characteristics, ensuring the safety of food is of great importance. This concern becomes particularly pronounced when considering fruits and vegetables, as they are typically consumed directly after harvest and are frequently devoid of any sterilization treatments. The fundamental susceptibility of these commodities to microbial contamination and spoilage underscores the critical need to prioritize food safety measures in their production and distribution [4].

Fruits and vegetables are renowned for their sensory appeal, including taste, aroma, texture, and visual appeal [5,6]. Any deviation from their natural qualities due to contamination or spoilage not only diminishes their overall quality but can also pose significant health risks to consumers. Unlike many other food products that undergo various processing steps or cooking procedures before consumption, fruits and vegetables are often consumed raw, which heightens the importance of their initial safety [7,8,9]. Moreover, fruits and vegetables are frequently handled by multiple actors within the supply chain, from growers and harvesters to distributors and retailers, increasing the potential for contamination at various points along the journey from farm to fork [10,11]. Fruits and vegetables constitute a main part of a healthy diet, but very high amounts can be damaged by microbial spoilage, leading to food safety concerns and economic losses. Moreover, the Food and Agriculture Organization (FAO) estimates that more than 1.3 billion tonnes of food are wasted each year, with fruits and vegetables accounting for 40%–50% of the total waste produced. In the past decades, numerous conventional methods have been developed for food preservation, but there are only a few strategies for extending the shelf life of fruits and vegetables. One of the innovative and globally accepted methods for achieving this goal is the use of thin-layered edible coatings or films, which, throughout the extension of shelf life, provide good quality for selling, distribution, and storage. Moreover, the implementation of nanoemulsion technology empowers edible coatings and films due to their small size and high surface area, which can lead to encapsulation and target delivery of different compounds, providing additional benefits such as antioxidant and antimicrobial properties [12,13]. Furthermore, a significant feature of edible coatings is that their utilization is not limited to fruits and vegetables since there are other highly perishable types of food such as meat and poultry, dairy products, bakery products, and fish and seafood [14].

Since nanoemulsion-based edible coatings represent a promising strategy in food technology, many research and review articles on this topic have been published, such as [1,6,9,10,15]. These review articles offer insight into recent developments in edible films and coatings [10], advances in coating materials [6], potential applications [1], and the formulation of coatings for fruits and vegetables [9]. However, there are not many commercially available formulations of edible coatings, as well as just 16 patents regarding nanoemulsion edible coatings and films with either antifungal, antibacterial, or antioxidant properties for application on fruits and vegetables [12,16]. Therefore, there is a need for further advancements in composition, formulation, application methods, and practical applications of these coatings across diverse produce items. This review offers insight into the core concepts, techniques, and components of nanoemulsions. The special significance is reflected regarding the effects on shelf life and various physicochemical and microbial qualities of treated fruits and vegetables during the postharvest period. Moreover, this review article stands out as it covers applicability to real food and its effects in the real-time preservation of fruit and vegetable commodities.

2. Edible Coatings

Edible packaging, which includes edible films and edible coatings, is described as a thin layer for primary packaging made from edible components without any potential health risk for consumers [17,18]. These coatings represent a replacement for natural protective waxy coatings, which can be completely made from renewable materials like natural polymers [19]. Moreover, they can be produced as single- or multi-layer coatings based on desirable characteristics [20]. Edible coatings can be applied in liquid form directly to food, and they are considered an integral unit with food products, while edible films are carved into a solid sheet and commonly used as wraps, pouches, bags, capsules, and casings [21]. Besides application procedures, another difference between edible coatings and films is their thickness, where coatings are thinner than 0.025 mm, while films are usually thicker than 0.050 mm [22].

Awareness of important features of edible coatings, e.g., decreasing oxygen permeability, reducing respiration rates, etc., which represent a barrier to chemical and biological changes, can provide necessary compatibility between packaging and food products [23]. Furthermore, edible coatings can contain many active compounds, e.g., antimicrobials, antioxidants, nutrient dyes, and spices, resulting in improvements in exceed softening and surface browning, preventing microbial spoilage, and increasing nutritional and antioxidant properties [5,20]. Edible coatings and films demonstrated promising results in food preservation due to the controlled diffusion and gradual release of incorporated antimicrobial compounds to the surface of food. The choice of antimicrobial agents should be correlated with the type of food, chosen polymer, and target microorganism(s) [24].

When it is used for fresh fruits and vegetables, it is desirable to have low water vapour permeability in order to decrease desiccation, while oxygen permeability should be minimal to provide retard respiration, but anaerobic conditions must not be reached to avoid ethanol production and off-flavour formation [20]. Organoleptic and functional properties of edible coatings must be suitable for each type of food product, so their formulation depends on their expected purpose [23].

Considering all the above-mentioned characteristics of edible coatings, they can evidently play a crucial role in maintaining and improving food safety for several reasons; their potential roles, advantages, and limitations are summarized in Figure 1 [25,26,27].

Future research should focus on improving coating application methods, developing sustainable and cost-effective formulations, and expanding the range of food products that can benefit from these coatings [28]. Edible coatings offer a viable and sustainable solution for improving food safety by providing microbial barriers, reducing cross-contamination, controlling moisture levels, and extending the shelf life of various food products. While they come with certain limitations, their advantages in terms of naturalness, customization, improved quality, and waste reduction make them a valuable asset in the search for safer and longer-lasting food options [8,29].

3. Nanoemulsions

The use of nanotechnology has increased and gained an indispensable role in the areas of food and food packaging, along with promising results in many scientific fields in recent decades. Nanotechnology is engaged in the production, processing, and application of materials whose thickness is less than 1 nm [30]. The characteristics of nanomaterials (small size and high surface area) are the reasons for their widespread use in medicine and industrial applications [31]. Possible applications of nanotechnology are encapsulation and target delivery of particles (pharmaceutical, antimicrobial, and antioxidant), increasing the shelf life of food and food storage, detecting contamination, as well as upgrading the flavour and sensory properties. The aforementioned characteristics of nanotechnology explain why one of the most promising applications of nanoemulsions is the utilization of edible coatings and films [32].

These specific structures can change the nutritional content of food and have the ability to magnify the bioaccessibility of hydrophobic bioactive compounds [33]. Bioactive compounds show low stability and bioavailability, as well as degradation and deterioration because of oxidation [34]. Furthermore, many bioactive compounds break free easily from the matrix due to their small size, resulting in a reduction in efficiency and the shelf life of the food; therefore, it is important to design biodegradable packaging in a way that controls the release of bioactive compounds [35]. It can be concluded that it is difficult to design functional products, but nanoemulsions through the process of encapsulation of bioactive compounds can protect them from inadequate processing conditions (oxidation, light, pH, temperature variations, and other influences) [34].

The mentioned nano-scale structures contain two immiscible liquids whose size of droplets can be between 20 and 200 nm, depending on the composition and fabrication method. The formation of nanoemulsions begins with a dispersion of small droplets of one immiscible liquid in another immiscible liquid; therefore, the usual phases are oily and aquatic. Hereof, nanoemulsions can be the oil-in-water (O/W) and the water-in-oil (W/O) types [15,36]. The small size provides better stability to gravitational separation and droplet aggregation, as well as optical clarity and higher bioavailability of encapsulated bioactive chemicals while enhancing phytochemical properties [32,37]. The content of the oil phase includes essential oils (EOs), flavour oils, triglyceride oils, preservatives, flavours, colours, oil-soluble vitamins, nutraceuticals, and pharmaceuticals. Hence, oil phases demonstrate various physicochemical properties, where crystallization, melting behaviour, and thermal history are important parameters due to their influence on viscosity, melting point, polarity, refractive index, and interfacial tension of the nanoemulsion system. The water phase commonly consists of water, but it can possess other important components, such as co-solvents, buffers, salts, preservatives, carbohydrates, and proteins [31].

Even though nanoemulsions have a promising quality for edible packaging, the main issue is long-term stability because of thermodynamically unfavourable systems, in which the positive free energy wants to be released through flocculation, coalescence, Ostwald ripening, and gravitational separation [15]. To ensure the stability of nanoemulsions for commercial applications, it is necessary to control their composition and structure. Furthermore, the oil and water phases should be chosen carefully, along with the selection of the most suitable additives. The unavoidable particle of nanoemulsions is a surfactant that has a key role in maintaining the functional abilities of nanoemulsions, and it should be chosen carefully for each application and type of nanoemulsion. These added additives will protect nanoemulsions from the breakdown [15,31]. Specifically, a hydrophilic emulsifier is used for O/W nanoemulsions, while a lipophilic emulsifier is used for W/O nanoemulsions. Based on the location of the oil and water phases, nanoemulsions can have different structures and form simple or multiple nanoemulsions [31]. The most common surfactants are lecithin; sodium deoxycholate; tween 20, 40, 60, and 80; as well as span 20, 40, 60, and 80. Additionally used surfactants are casein, β-lactoglobulin, polyethylene glycol polysaccharides (e.g., gums and starch), sodium dodecyl sulfate, etc. [36].

3.1. The Main Features of Nanoemulsion-Based Coatings

Even though edible coatings have been used for centuries, their full potential was seen when the food processing sector adopted nanotechnology as a cutting-edge approach and opened fresh ideas for post-harvest storage methods. The use of nanoemulsion edible coatings is a biodegradable alternative approach to address problems during storage for agricultural commodities by creating barrier-enhancing protective nanoemulsions and nanoparticle coatings for use on fruits and vegetables that can help to reduce physiological disturbances on the organoleptic properties of fruits and vegetables (Figure 2). The nanoemulsion technology ensures the use of lower amounts of materials with improved efficiency due to an increase in surface area [38]. Furthermore, edible coatings should not hamper the minimum rate of respiration and impact the produce’s colour and flavour. However, it should have high adhesive qualities and be thin enough to apply evenly to fruits. Moreover, for the commercial application of edible coatings, it is valuable that the main components are inexpensive and conveniently accessible [9].

Recent advancements in nanoemulsion-based edible coatings have focused on tailoring formulations to meet the specific needs of different fruits and vegetables. These innovations include the incorporation of natural antimicrobial agents, antioxidants, and flavour and texture enhancers to improve product quality, safety, and consumer appeal of different foods [25]. Formulation adjustments are made to ensure compatibility with the unique characteristics of each type of produce, such as fresh fruits and vegetables [28].

Nanoemulsion coatings can be applied using various methods, such as dipping, spraying, or brushing, depending on the produce type and desired outcome. These methods facilitate the uniform deposition of the coating, which serves as a protective barrier against moisture loss, microbial contamination, and oxidation [10]. The versatility of nanoemulsion coatings is showcased by their successful application on a wide array of fruits and vegetables, including but not limited to berries, citrus fruits, leafy greens, and root vegetables [29]. These coatings have been shown to extend shelf life, maintain freshness, and preserve nutritional quality across different produce categories. Recent advances have also focused on improving the effectiveness of edible coatings by optimizing factors such as coating thickness, drying methods, and post-harvest storage conditions. Additionally, considerations such as temperature, humidity, and the specific needs of different produce varieties have been considered to enhance the performance of these coatings [6].

3.2. Nanoemulsion-Based Edible Coatings’ Polymers

3.2.1. Coatings Polymers

The flexibility of edible coatings’ composition and performance creates new opportunities for the creation of innovative packaging techniques that satisfy the two important objectives of food preservation and quality improvement. Furthermore, edible coatings are a desirable option since they lessen the need for packaging made of plastic, which reduces waste and has a positive influence on the environment [27]. Edible packaging is typically made from edible polymers that can be divided into three groups: polysaccharides, proteins, and lipids [39], which are present in Figure 3. All compounds employed in coatings must be food-grade and safe for human consumption, as well as other additives. Furthermore, the choice of coating material is of greatest importance to ensure desirable characteristics for each type of fresh fruit and vegetable. However, one type of edible polymer can possess excellent properties in certain areas, but it can be inadequate in others. Hence, the utilization of a combination of edible polymers has most of the advantages of a single polymer, resulting in the improvement in technological functionality and biological characteristics of edible films and coatings [40,41,42].

Coating polymers should be reasonably priced and widely accessible. The coatings should be simple to apply, have good adhesive properties, dry rapidly, and have a consistent thickness. Furthermore, during long storage, the coating’s functionality and structural stability must be preserved. The covering must be adaptable to certain morphological changes, e.g., mechanical damage or fruit shrinkage. Since the properties of coating polymers dictate their use and function (

Table 1. Examples of nanoemulsions polymer(s) for creating coatings for fruits and vegetables.

Nanoemulsion Polymer(s)	Active Compound	Functional Role	Fruits/Vegetable	Ref.
Polysaccharide-based
Pectin	Ascorbic acid, citric acid, and sodium chlorite	Decreased microbial deterioration; little effect on the nutritional or sensory value.	Apple	[43]
Chitosan	Carvacrol	Prevented microbial growth for 13 days at 5 °C.	Cut carrot	[44]
Starch	Basil leaf extract	Extended the shelf life of eggplant up to 16 days; maintained firmness; reduced moisture loss; prevented the growth of total soluble solids, and prevented colour changes.	Eggplant	[45]
Alginate	Lemon EO * or orange EO	Inhibited bacterial and fungal growth for 15 days of storage.	Red raspberry	[46]
Xanthan	Cinnamic acid	Slowed down the process of browning; increased their shelf-life by up to 4 days and 8 days of storage.	Fresh-cut pear	[47]
Protein-based
Collagen	/	Reduced the rate of gas transport, which made them useful instruments for extending shelf life.	Mangoes and apples	[48]
Whey protein	/	Maintained various fruit qualities, such as the total phenolic content, browning, and product weight loss.	Apples	[49]
Soy	Cinnamaldehyde	Proved to be an effective antioxidant and moisture barrier.	Banana	[50]
Zein	Carvacrol, thymol, and eugenol EO	Reduced growth of some major pathogens on the peel surfaces of melons.	Melon	[51]
Lipid-based
Carnauba wax	Lemongrass oil	Prevented foodborne pathogens (Salmonella typhimurium and Escherichia coli) and prolonged shelf life.	Grape berries	[7]
Beeswax	/	Preserved different fruits’ quality metrics and taste characteristics.	Mandarin	[52]
Soy lecithin	Lysophosphatidylethanolamine	Improved the shelf life of banana fruits; provided normal colour development and low ethylene production.	Banana fruits	[53]

* Essential oil.

), it is important to know their main features [28].

3.2.2. Polysaccharide-Based Coatings

Polysaccharide-based coatings are characterized to be a poor moisture barrier due to their hydrophilic nature and have a selective permeability to O₂ and CO₂ and resistance to oils while expressing nontoxicity. They are a source of hardness, crispness, compactness, viscosity, adhesiveness, and gel-forming properties. Also, they can be easily found at low prices [54,55]. The presence of hydroxyl groups in the polysaccharides, which results in internal hydrogen bonding, is primarily responsible for their capacity to form films. Furthermore, polysaccharides are especially suitable for short-term food packaging applications. The most-used contributories for edible films and coatings are alginate, chitosan, cellulose, starch, pectin, alginate, and carrageenan [56]. As presented in Table 1, pectin-based nanoemulsion coatings show promise in reducing microbial deterioration in apples. While these coatings have little impact on the nutritional or sensory qualities of the fruits, they contribute to increased shelf life and reduced spoilage [43]. On the other side, chitosan-based nanoemulsion coatings are effective in preventing microbial growth in cut carrots, thus extending their shelf life during storage at lower temperatures [44]. One more polysaccharide is used for nanoemulsion coatings in starch in combination with basil leaves. A study conducted by Kumar et al. [45] showed benefits in extending the shelf life of eggplants, maintaining firmness, reducing moisture loss, and delaying undesirable changes in texture and colour. Alginate-based nanoemulsion coatings incorporating EOs of citrus exhibit antibacterial and antifungal properties that contribute to the preservation of red raspberries during storage. Finally, xanthan can also be considered one of the polysaccharides used for nanoemulsion systems since Sharma and Rao [47] published that coatings based on xanthan and cinnamic acid can slow down the browning process in fresh-cut pears.

3.2.3. Protein-Based Coatings

Proteins are macromolecules formed by amino acids in different molecular conformation structures that can form fibrous or globular forms. They are hydrophobic molecules incorporated with hydrophilic amino acid residues that influence permeability. The source of proteins for edible packaging can originate from either animals (casein, whey gelatin, etc.) or plants (soy protein, corn zein, etc.) [55]. Proteins-based films ensure mechanical stability, but plasticizers must be added to improve flexibility. They are excellent barriers, except for water, which can be overcome through the addition of hydrophobic components. Moreover, they provide additional nutritional value [54,57]. According to the overview in Table 1, some proteins have been used in nanoemulsion coating systems. For example, collagen-based nanoemulsion coatings are effective in reducing the rate of gas transport. This property makes them valuable for extending the shelf life of fruits like mangoes and apples [48]. In the work of Li et al. [50], soy coatings enriched with cinnamaldehyde were applied to bananas. The outcome of their research was confirmation of an effective moisture barrier and significant antioxidant features of the formulated coating. The utilization of whey protein has been widely presented and investigated. Feng et al. [49] observed significant preservation of apples as they protect total phenolic content, browning, and weight loss. Further, Boyaci et al. [51] formulated coatings assembled from zein protein and different EO, such as carvacrol, thymol, and eugenol EO. This combination of ingredients reduced the growth of some major pathogens on the peel surfaces of melons, providing effective protection against microbial spoilage.

3.2.4. Lipid-Based Coatings

Lipids are a heterogeneous group that can be divided into classes: simple lipids, derived lipids, and lipid components. In the food industry, simple lipids, including oils and fats, are mostly used. Lipids are used in edible packaging due to various antibacterial, antioxidant, mechanical, barrier, and functional properties, but they are not polymers. Therefore, they do not have a self-supporting structure and cannot function without other compounds. The homogenous distribution of the small size of lipid fragments can improve stability and decrease the water vapour permeability of edible films and coatings. The main advantage of lipid components is their possible compatibility with other polymers, resulting in an improved barrier and mechanical properties, but films and coatings loaded with lipid components can be sensitive to oxidation and retention of off-flavours and have a bitter aftertaste [55,58,59]. Table 1 shows a few examples of lipid-based coatings. Regarding the study of Kim et al. [7], the incorporation of lemongrass oil in carnauba wax showed promising results in the prevention of foodborne pathogens, such as Salmonella typhimurium and Escherichia coli, and improved the shelf life of grape berries. The application of beeswax as a coating solution on mandarins, due to the many health benefits of beeswax, could preserve many fruits’ quality and taste characteristics. However, the use of beeswax is a bit limited due to potential consumer allergic reactions [52]. Ahmet and Palta [53] applied soy lecithin coatings on bananas. Their study revealed that the incorporation of lysophosphatidylethanolamine, as an active compound, led to improvements in the shelf life of tested fruits, as they succeeded in maintaining normal colour development and low ethylene production.

Overall, the nature of these materials can be mostly hydrophilic (polysaccharides), hydrophobic (lipids), or both (proteins), and the use of solvents is limited to water or ethanol to maintain edible properties as they are considered to be safe for human consumption [54]. The mechanism of linkage of biopolymers involves multiple intermolecular forces such as electrostatic, hydrophobic, ionic interactions, and covalent bonds. Therefore, the properties of edible films and coatings depend on the physical and chemical features of the chosen biopolymer or a combination of them. Additionally, plasticizers and other additives (Figure 3) can be added to the biopolymers to achieve desirable properties in the edible films and coatings. Plasticizers are small molecules that are added to films and coatings to reduce brittleness and flaking or enhance flexibility or toughness [60]. Functional additives are essential participants of films and coatings because they enhance, for example, nutritional, health, and sensory properties, as well as antimicrobial and antioxidant activity, which will be discussed in the next chapter. Other additives are used to solve technical problems that may occur during the process [40].

3.2.5. Composite Coatings

The utilization of a combination of edible polymers in the development of edible films and coatings offers several advantages over using a single polymer. This approach can lead to improvements in both technological functionality and biological characteristics, enhancing the overall effectiveness of these food-grade coatings [29]. For example, different polymers have varying abilities to create barriers against moisture, oxygen, and other gases. Combining polymers with complementary barrier properties can result in improved overall barrier performance. For instance, one polymer may excel at moisture resistance, while another is better at blocking oxygen. Together, they provide a more robust protective barrier for food products. Furthermore, edible films and coatings may need to exhibit specific mechanical properties, such as flexibility, tensile strength, or elasticity, depending on the application [61]. Combining polymers allows for the fine-tuning of these properties to meet the specific requirements of the food product. For example, a blend of polymers can result in a coating that is both flexible and strong. Some edible coatings are designed to release bioactive compounds, such as antimicrobials, antioxidants, or flavour compounds, over time [62,63]. By using a combination of polymers, it is possible to control the release kinetics more precisely, ensuring that the bioactive components are released when and where they are needed. One more important characteristic is adhesion to the food surface, and this parameter can be critical for the effectiveness of edible coatings. Combining polymers with different adhesive properties can enhance the coating’s ability to stick to the food substrate, reducing the risk of delamination or cracking [25].

Certain combinations of polymers can exhibit synergistic effects, where the properties of the blend are greater than the sum of the individual components. This can result in coatings with unique characteristics that cannot be achieved with a single polymer. Combining polymers can lead to improved biological characteristics, such as increased antimicrobial activity or better compatibility with specific food products. Utilizing a combination of polymers can also lead to more sustainable coatings by reducing the need for single-use plastics or synthetic materials. Natural and biodegradable polymers can be blended to create coatings that are eco-friendly and have minimal environmental impact. Different food products have diverse requirements and challenges when it comes to coatings. Using a combination of polymers provides flexibility, allowing food scientists to tailor coatings to suit a wide range of applications [26,63].

3.3. Nanoemulsion-Based Coatings’ Active Compounds

Searching sustainable and health-conscious solutions for the food industry, many researchers have made significant strides in identifying environmentally friendly active compounds with minimal health risks. This exploration is driven by the growing consumer demand for food products that are not only free from harmful pesticide residues but also aligned with a broader commitment to environmental well-being. In this context, plant-based extracts have emerged as a promising category of natural preservatives, offering a safer and more sustainable alternative to synthetic chemical pesticides and preservatives [64]. Particularly noteworthy is their role in preventing postharvest illnesses and preserving the quality of food products after harvesting, marking an essential alteration towards eco-friendly and health-conscious food preservation practices [65]. Raw vegetables, fruits, and herbs/spices contain active compounds that can express antimicrobial and antioxidant activity. Natural phytochemicals can be found in a variety of foods, including fruits and vegetables (such as guava, garlic, pepper, onion, and cabbage), seeds and leaves (such as olive leaves, nutmeg, parsley, caraway, fennel, and grape seeds), and herbs and spices (such as lemongrass, basil, oregano, ginger, rosemary, thyme, and cinnamon) [45,66]. As previously mentioned, nanoemulsion active compounds have emerged as essential ingredients in modern food industry approaches. Among these active compounds, EOs, derived from plant sources, have garnered significant attention due to their multifaceted applications and potential health benefits. Recognized for their aromatic and therapeutic properties, EOs serve as a valuable category of natural extracts with wide-ranging capabilities [22]. EOs are natural phytochemicals that have been widely used as a valuable source of bioactive components that have antioxidant and antibacterial properties, which pique the interest of the food industry as they have been recognized as safe (GRAS) for the environment and human health. As a result, there is a growing interest in using these oils for sustainable agriculture, and a lot of research has been performed that shows that plant EOs and extracts may often be used as medicines and food preservatives, as can be seen in Table 1, wherein many active ingredients are EOs. Studies on the use of EOs and their components against microbial diseases found in horticulture products have produced a collection of findings that can guide the development of practical solutions for food safety. Also, these EOs differ in terms of their biological activity, physical and chemical characteristics, and scent. Therefore, it is critical to choose the best option or combination for each individual application [67,68].

3.4. Nanoemulsion Coating Production Methods

Nanoemulsion coatings have gained significant prominence in various industries due to their ability to encapsulate and deliver active compounds, enhance product stability, and improve functionality. The production of nanoemulsion coatings involves several methods, each with its unique advantages and applications. These methods are fundamental to harnessing the potential of nanoemulsions for diverse industrial needs [1]. Methods for nanoemulsion production are separated into two groups: low-intensity and high-intensity methods (Figure 4). Low-intensity methods are based on the spontaneous formation of small droplets in surfactant–oil–water mixtures when change occurs in the system composition or environmental conditions, and methods within this group are the phase-inversion temperature, emulsion-inversion point method, and spontaneous and membrane emulsification [69]. On the other hand, high-intensity methods have found their use in industrial applications. These methods use mechanical devices, such as valve homogenization, micro fluidization, and sonication, to disrupt and merge the oil and water phases, creating intense turbulence, shear, and cavitation flow profiles [35,70]. The implementation of certain fabrication methods depends on the nature of the components and the required characteristics of nanoemulsions, but most approaches are high-energy ones. However, because nanoemulsions might experience destabilizing processes, a good formulation is essential for ensuring their long-term stability. The ability to withstand physical change is a definition of stability [71]. Coalescence and Ostwald ripening are the two methods by which emulsions are destabilized. According to Rao and McClements [72], Ostwald ripening occurs when the size of oil droplets increases as oil molecules diffuse from small to big droplets through the continuous phase in relatively water-soluble oils (such as EOs), while coalescence occurs due to weak stearic repulsion when two oil droplets merge into a single, bigger droplet.

The production of nanoemulsion films and coatings starts with dissolving polymers into an adequate solvent. Afterwards, in the solution of the chosen polymer or combination of polymers, active compounds are added (as part of nanoemulsions or unattended), along with plasticizers and additives, to make the final formulation of the coating. There are two process methods for the production of nanoemulsion coatings: the dry-process and wet-process methods. The first method relies on the thermoplastic characteristics of polymers and includes moulding or extrusion. This method does not use solvents that make it environmentally friendly, and thus the obtained films are unable to cover uneven surfaces. On the other hand, the most common method used on the laboratory scale is the casting method (Figure 5). In this wet-process method, a solution of coating is dispersed on the surface and then air dried for a certain amount of time in order to achieve evaporation of the solvent. The edible films are obtained when the solution is dispersed on a flat surface and afterwards peeled off. However, coatings are formed on the product surface if the solution is directly applied to food products via most common application methods (dipping or spraying) [73,74].

3.5. Nanoemulsion Coating Application Method

The application of nanoemulsion coatings and edible films in the food industry plays a pivotal role in enhancing the quality, safety, and shelf life of a wide range of products. While both coatings and films share the common goal of providing protection and functionality, they can be applied through various methods, each with its own advantages, disadvantages, and limitations. In this context, it is essential to understand these application methods in more detail [75]. The application of edible films is to be as sheets that surround the food, unlike edible coatings, where there are a few application methods, such as dipping, fluidized bed, panning and spraying. However, they may be created from the same raw materials, have the same goal, and serve the same function, with the only difference being how they cover the food [14]. Moreover, applications of edible coatings have demonstrated a considerable impact as a postharvest treatment on the physiological effect and physicochemical qualities. Their application is presented in Table 2, which summarizes the most recent main component of the edible coating’s active component and polymers that have been used on fruits and vegetables, as well as producing techniques and application methods. Apparently, as can be seen in Table 2, the two most common methods for coating application are dipping and spraying.

One of the methods used for applying nanoemulsion coatings and edible films is the immersion or dipping method. This method is adequate for distributing an even coating from all sides on the target fruit/vegetable, even when it has a rough surface. Firstly, the fruit/vegetable is dipped in a coating solution for a specific amount of time to create a semipermeable covering on the surface. The typical dipping duration is between 30 s and 5 min. The following step can be submerging in a crosslinking solution for a certain amount of time to increase the coating’s stability and toughness. Secondly, the fruit/vegetable is removed from coating solutions to allow the draining of surplus solution in favour of obtaining a uniform and controlled coating thickness [6,76]. Another common method for applying nanoemulsion coatings and edible films is spraying, which provides an equal distribution of the coating solution across the fruits/vegetables, resulting in an average thickness of coating. Furthermore, a coating solution is applied via a nozzle and can be applied as a layer-by-layer coating. Due to high pressure (about 60–80 psi), this method does not require a significant amount of coating solution, and the solution can be less viscous [75].

Table 2. Recent applications of edible coatings on fruits/vegetables based on different polymers with the addition of active compounds.

Target Fruits/Vegetable	Nanoemulsion Active Compound	Nanoemulsion Polymer	Producing Technique	Application Method	Ref.
Papaya	Ginger oil	Carnauba wax and hydroxypropyl methylcellulose	Ultra-Turrax homogenizer	Hand-coating procedure	[77]
	Oregano EO *	Sodium alginate	Ultrasonication	Dipping	[78]
	Eucalyptus staigeriana, Lippia sidoides, and Pimenta pseudocaryophyllus leaf extracts	Carboxymethylcellulose	/		[79]
	Citral oil	/	Phase-inversion method		[80]
Okra	Basil oil and aqueous extract of Sapindus mukorossi	Alginate	Ultrasonication	Dipping	[81]
Apple	Rosemary EO	Water chestnut starch	Ultra-Turrax homogenizer, followed by ultrasonication	Dipping	[82]
	Lemongrass oil	Carnauba–shellac wax	Ultra-Turrax homogenizer, then high-pressure homogenizer		[83]
	Cinnamon EO *	Pectine from apple pomace	Ultra-Turrax homogenizer		[84]
	Lemon EO	Aloe vera gel and hydroxypropyl methylcellulose		Spraying	[85]
Apricot fruits	Pomegranate peel extract	Chitosan	Ultrasonication	Dipping	[86]
Tomato	Sweet orange EO	Sodium alginate	Ultrasonication	Dipping	[87]
	Thymol	Quinoa-chitosan	Spontaneous emulsification, ultrasound, and a combination of both methods		[88]
	Cardamom EO	Carboxymethyl cellulose	Ultrasonication		[89]
Strawberry	Lime, fennel, and lavender	Yam starch	Ultra-Turrax homogenizer	Spraying	[90]
	Cymbopogon martinii and Mentha spicata EO	Arrowroot starch, cellulose nanocrystals, carnauba wax		Dipping	[91]
Fresh-cut red bell pepper	Tea tree oil	Chitosan	Magnetic stirrer	Dipping	[92]
Cherry	Eryngium campestre L. EO	Chitosan	Sonication with an ultrasonic bath, then Ultra-Turrax homogenizer	Dipping	[93]
Pineapple	Eugenol (clove EO) and Aloe vera gel	Chitosan	Spontaneous emulsification and ultrasonication	Spraying	[94]
	Citral and sesame oil	Sodium alginate	Ultrasonication	Dipping	[95]
Citrus fruits	Eugenol, carvacrol and cinnamaldehyde	Sodium alginate with citric acid, sucrose ester, Vitamin C, and potassium sorbate	Ultra-Turrax homogenizer, then passed through a microfludizer	Applied on the surface of the peel with a soft brush	[96]
Salacca fruits	Orange oil	Carnauba wax	Homogenizer	Spraying	[97]
Blackberries	Lactic and acetic acid	Chitosan	Ultrasonication	Spraying	[98]
Grape berries	Lemongrass oil	Carnauba	Ultra-Turrax homogenizer, then high-pressure homogenizer	Dipping	[7]
Garambullo fruits	Tomato oily extract	Gelatin	Ultra-Turrax homogenizer	Dipping	[99]
Peach	Aloe vera gel extract	Chitosan	/	Dipping	[100]
Fresh-cut kiwi fruits	Opuntia ficus-indica Mucilage	Aloe arborescens gel	Ultra-Turrax homogenizer	Dipping	[101]
	Ascorbic acid and vanillin	Alginate and carboxymethylcellulose	Ultra-Turrax homogenizer, followed by ultrasonication		[102]
Pear	Lemon peel EO	Guar gum and chitosan	/	Dipping	[103]
Jujube fruits	/	Aloe vera gel, carboxymethyl cellulose, and pectin	/	Dipping	[104]
Guava	Cyclea barbata leaves extract	Alginate	/	Dipping	[105]
Capsicum fruits	Mulberry leaf extract	Pectin	/	Dipping	[106]
Guava fruits	Ginger extract and garlic extract	Gum arabic and aloe vera gel coating	/	Dipping	[13]
Melon	Citral oil	/	Phase-inversion method	Dipping	[80]
	Citral	Chitosan and carboxymethyl cellulose	Ultrasonication		[107]
Lettuce	Oregano oil	/	Ultrasonication	Dipping	[108]

* Essential oil.

Table 2. Recent applications of edible coatings on fruits/vegetables based on different polymers with the addition of active compounds.

Target Fruits/Vegetable	Nanoemulsion Active Compound	Nanoemulsion Polymer	Producing Technique	Application Method	Ref.
Papaya	Ginger oil	Carnauba wax and hydroxypropyl methylcellulose	Ultra-Turrax homogenizer	Hand-coating procedure	[77]
	Oregano EO *	Sodium alginate	Ultrasonication	Dipping	[78]
	Eucalyptus staigeriana, Lippia sidoides, and Pimenta pseudocaryophyllus leaf extracts	Carboxymethylcellulose	/		[79]
	Citral oil	/	Phase-inversion method		[80]
Okra	Basil oil and aqueous extract of Sapindus mukorossi	Alginate	Ultrasonication	Dipping	[81]
Apple	Rosemary EO	Water chestnut starch	Ultra-Turrax homogenizer, followed by ultrasonication	Dipping	[82]
	Lemongrass oil	Carnauba–shellac wax	Ultra-Turrax homogenizer, then high-pressure homogenizer		[83]
	Cinnamon EO *	Pectine from apple pomace	Ultra-Turrax homogenizer		[84]
	Lemon EO	Aloe vera gel and hydroxypropyl methylcellulose		Spraying	[85]
Apricot fruits	Pomegranate peel extract	Chitosan	Ultrasonication	Dipping	[86]
Tomato	Sweet orange EO	Sodium alginate	Ultrasonication	Dipping	[87]
	Thymol	Quinoa-chitosan	Spontaneous emulsification, ultrasound, and a combination of both methods		[88]
	Cardamom EO	Carboxymethyl cellulose	Ultrasonication		[89]
Strawberry	Lime, fennel, and lavender	Yam starch	Ultra-Turrax homogenizer	Spraying	[90]
	Cymbopogon martinii and Mentha spicata EO	Arrowroot starch, cellulose nanocrystals, carnauba wax		Dipping	[91]
Fresh-cut red bell pepper	Tea tree oil	Chitosan	Magnetic stirrer	Dipping	[92]
Cherry	Eryngium campestre L. EO	Chitosan	Sonication with an ultrasonic bath, then Ultra-Turrax homogenizer	Dipping	[93]
Pineapple	Eugenol (clove EO) and Aloe vera gel	Chitosan	Spontaneous emulsification and ultrasonication	Spraying	[94]
	Citral and sesame oil	Sodium alginate	Ultrasonication	Dipping	[95]
Citrus fruits	Eugenol, carvacrol and cinnamaldehyde	Sodium alginate with citric acid, sucrose ester, Vitamin C, and potassium sorbate	Ultra-Turrax homogenizer, then passed through a microfludizer	Applied on the surface of the peel with a soft brush	[96]
Salacca fruits	Orange oil	Carnauba wax	Homogenizer	Spraying	[97]
Blackberries	Lactic and acetic acid	Chitosan	Ultrasonication	Spraying	[98]
Grape berries	Lemongrass oil	Carnauba	Ultra-Turrax homogenizer, then high-pressure homogenizer	Dipping	[7]
Garambullo fruits	Tomato oily extract	Gelatin	Ultra-Turrax homogenizer	Dipping	[99]
Peach	Aloe vera gel extract	Chitosan	/	Dipping	[100]
Fresh-cut kiwi fruits	Opuntia ficus-indica Mucilage	Aloe arborescens gel	Ultra-Turrax homogenizer	Dipping	[101]
	Ascorbic acid and vanillin	Alginate and carboxymethylcellulose	Ultra-Turrax homogenizer, followed by ultrasonication		[102]
Pear	Lemon peel EO	Guar gum and chitosan	/	Dipping	[103]
Jujube fruits	/	Aloe vera gel, carboxymethyl cellulose, and pectin	/	Dipping	[104]
Guava	Cyclea barbata leaves extract	Alginate	/	Dipping	[105]
Capsicum fruits	Mulberry leaf extract	Pectin	/	Dipping	[106]
Guava fruits	Ginger extract and garlic extract	Gum arabic and aloe vera gel coating	/	Dipping	[13]
Melon	Citral oil	/	Phase-inversion method	Dipping	[80]
	Citral	Chitosan and carboxymethyl cellulose	Ultrasonication		[107]
Lettuce	Oregano oil	/	Ultrasonication	Dipping	[108]

* Essential oil.

The application process should be performed in hygienic surroundings to avoid contamination and guarantee the efficiency of the coating. Additionally, it must be applied evenly and successfully adhered to the produce’s surface. Furthermore, it is important to be aware of the features of each application method. Therefore, in Figure 6, the main benefits and disadvantages of the two most common application methods are summarized [27]. The dipping method provides even application of coatings on fruits or vegetables’ surfaces that are uneven or rough; therefore, it does not have limitations on the type of produce. However, the application is rather time-consuming due to the requirement to carefully perform immersion and drying steps. On the other hand, the spraying method allows uniform and consistent cover with explicitly precise distribution control but has difficulties when applied on severely uneven surfaces. However, it is efficient for large-scale applications and uses lower amounts of the coating solution, even though sometimes products can be over-sprayed, resulting in material wastes [109,110].

As mentioned earlier, examples of the application of spraying and dipping methods are presented in Table 2. In the case of papayas, several nanoemulsion coating systems have been developed. In the study of [77], ginger oil is employed as the active compound to coat papayas. The coating formulation includes carnauba wax and hydroxypropyl methylcellulose as the nanoemulsion polymer. The producing technique involves the use of an Ultra-Turrax homogenizer, a high-powered device for emulsification. The application method chosen is a hand-coating procedure. This approach likely provides precise control over the coating process and ensures even coverage on the papaya surface. Ginger oil, known for its aromatic and potentially bioactive properties, could contribute to the preservation and flavour enhancement of the papayas. In the case of a recent study by Tabassum et al. [78], oregano EO is the selected active compound for coating papayas. The coating formulation incorporates sodium alginate-based edible coating, which is known for its ability to provide a protective and barrier layer. Ultrasonication is used as the producing technique, suggesting the creation of a stable and fine nanoemulsion. The chosen application method is dipping, which ensures comprehensive coverage and uniformity. Oregano EO is a natural ingredient with antimicrobial and antioxidant properties, making it a valuable choice for extending the shelf life and quality of papayas. Papayas with Eucalyptus, Lippia sidoides, and Pimenta pseudocaryophyllus leaf extracts developed by Zillo et al. [79] include a carboxymethylcellulose coating, which is likely selected for its film-forming and protective characteristics. Interestingly, the producing technique is not specified, suggesting that the method may vary or be proprietary. This study highlights the potential of utilizing a combination of natural extracts to create innovative nanoemulsion coatings for papayas, although specific details about the production process are not provided. In this study, citral oil is employed as the active compound, and the application method is dipping. Citral oil is known for its citrusy aroma and potential antimicrobial properties, which can contribute to both flavour enhancement and food safety in papayas. The producing technique involves a “phase inversion method”, which is a technique that induces a change in the phase of the emulsion, often resulting in the formation of nano-sized droplets [80].

For instance, in the coating of pineapples, chitosan-based coatings incorporated with clove EO and aloe vera gel were formulated by Basumatary et al. [94]. Aloe vera gel is known for its gel-forming ability, while together with clove EO, provides many health benefits. The coatings were prepared via two methods, spontaneous emulsification and ultrasonication, which provided stable nanoemulsion coatings, which were applied via spraying. In the work of Prakash et al. [95], pineapples were dipped in a coating solution prepared from sodium alginate enriched with citral and sesame oil. Citral was added due to its antimicrobial activity, while sesame oil ensures the stability of nanoemulsion. Utilization of ultrasonication provided the nano distribution of droplets, resulting in even distribution of active components.

In Table 2, there are many examples of apple coatings; all reviewed research articles used different EOs, such as rosemary, lemongrass, cinnamon, and lemon EO. However, contrary to other scientists who chose the dipping method, Farina et al. [85] decided to spray their apple samples with aloe vera gel and hydroxypropyl methylcellulose coating, prepared using an Ultra-Turrax homogenizer, which provided an attractive natural shine to fruits. The same production method for coatings was involved in the work of Naqash et al. [84], who used pectin, as a main polymer, obtained from pomace, which is an environmentally and economically responsible practice. Additionally, the expressed antimicrobial activity of the formulated coating is due to the presence of cinnamon EO and its benefits. For the preparation of carnauba–shellac wax coating, firstly, an Ultra-Turrax homogenizer and then a high-pressure homogenizer were used, resulting in a stable nanoemulsion for even 5 months of storage [83]. In a recent study by Bashir et al. [82], for dipping apples, water chestnut starch coatings were prepared via ultrasonication, which provided an adequate solution for post-harvest protection against decay, weight loss, maintaining firmness and high concentrations of anthocyanins and ascorbic acid in corps.

Besides seeking improvements in obtaining optimal formulation of edible coatings (polymers, active compounds, additives), storage conditions, and producing methods, further research is necessary concerning the application method. In conclusion, a variety of parameters, including the type of produce, desired coating qualities, and manufacturing scale, determine whether to dip or spray edible films and nanoemulsion coatings. The best strategy must be chosen depending on the application needs because each method has a unique mix of benefits and drawbacks [79,81].

4. Studies of Nanoemulsion Coating Applications on Fruit/Vegetable Samples

Even though edible coatings have been used to preserve food since the 12th century in East Asia, nowadays, they have gained huge popularity due to the numerous benefits of edible packaging, such as the impact on colour, shelf life, firmness, and weight loss, as graphically presented in Figure 7 [111,112]. As presented in Table 1, the application of edible coatings on fruits/vegetables is in the expanding interest of scientists, as can be seen in a vast number of publications that give insights into different aspects of the effects of the implementation of different formulations of coatings.

4.1. pH

The pH is viewed as an important parameter for the evaluation of the quality of fruits and vegetables. Manzoor et al. [99] kept track of changes in the pH value of kiwi fruit slices and the influence of alginate and carboxymethylcellulose coatings incorporated with ascorbic acid and vanillin. They observed significant differences on the 7th day of storage between control and treated samples. Moreover, alginate coating containing 1% vanillin exhibited the highest preservation of pH values of kiwi slices compared to carboxymethylcellulose coatings. In the work of Prakash et al. [95], the pH of the pineapple samples was unaffected by the various coating treatments and stayed between 3.9 and 4.5 throughout storage. On the other hand, throughout the storage period, the pH values of all sweet cherries increased significantly regardless of their treatment. The pH differences were found to be less pronounced in coated cherries than in untreated cherries [93]. In the study of Anjum et al. [13], a significant change in pH was observed for guava fruits that increased up to day 9, then decreased from day 12 to day 15. The explanation for the rise in pH is due to the decrease in organic acids and potential sugar conversion.

4.2. Total Phenolic Content

The hidden worry is that fresh fruit and vegetable storage causes nutritional loss because of water retention, oxidation, microbial breakdown, and enzymatic breakdown. Therefore, another important element in assessing the organoleptic and nutritional quality of fruits and vegetables is the levels of total phenolic content (TPC) [92]. In the study by Sathiyaseelan et al. [92], the TPC considerably changed between preservation methods and days of preservation. Nanoemulsion coating succeeded in maintaining the TPC in red bell pepper, at a high value of 92.37 mg/g of GAE, while the TPC value of control fruits was 64.59 mg/g of GAE. In the work of Oliveira Filho et al., 2022 [91], during storage, the total phenolic compound concentration declined in all samples. Strawberries coated with Cymbopogon martinii and Mentha spicata EOs showed the lowest TPC decrease among the treatments tested in this study (1.43–1.52 to 1.03–1.11 mg GAE/g of fruit and 1.57–1.68 to 1.07–1.13 mg GAE/g, respectively) as a comparison to treatments without using EOs, where TPC was 1.7–0.78 mg GAE/g of fruit. For example, the TPCs of cherries significantly dropped during the 21-day storage period at 4 °C, but the coating treatment had a notable inhibitory effect on this decreasing tendency. The highest TPC value (50.21 g/mL gallic acid) was measured in coated sweet cherries after 14 days of storage, while the lowest TPC value (10.36 g/mL gallic acid) was found in the control group after 21 days of storage. The outcomes also showed that the most efficient method for reducing TPC loss in the coated cherries was to encapsulate Eryngium campestre L. EO in chitosan coating, which kept the TPC value above 1.13 μg/mL gallic acid for the first seven days of storage [93].

4.3. Colour

The key factor encouraging a consumer’s decision to consume fruits or vegetables is their appealing colour, flavour, and texture. As a result, keeping the colour of fruits and freshly cut items is essential for the food industry [92]. Changes in visual colour indicate fruit ripening. In the case of papaya, colour changes as the fruit ripens from green to yellow/orange due to the degradation of green chlorophyll and the production of yellow/orange carotenoids. Their study revealed that control fruits had lower hue values, which indicates papaya peel colour shift from green to yellow colour, than coated fruits, where the highest hue values were found for the carnauba-based coating with an 18% solid phase [77]. Contrarily, in the work of Sathiyaseelan et al. [92], significant colour changes in fresh-cut red bell peppers between the control and treatment samples were not detected. For example, in the study of Phothisuwan et al. [97], carnauba wax coatings with different orange oil concentrations were applied on fresh salacca fruits that initially had yellowish-white flesh that later turned yellowish-brown. The findings of this study disclosed that the salacca fruits had the maximum lightness and yellowness values when coated with 2.5% carnauba wax that contained 0.08% orange oil. Increased blackness in the salacca was associated with a higher content of orange oil (0.16%), which may be related to orange oil’s influence on colour change. Additionally, coating with 0.16% orange oil resulted in the salacca flesh turning an unacceptably light shade of brown at the end of the test. The finding from this study revealed how important the concentration of EO in the formulation of the coating is.

4.4. Firmness

The texture profile is one of the most noticeable physiological qualities that is attributed to consumer acceptance. Firmness, one of many textural variables, more accurately depicts the texture of the tissue and, as a result, can better indicate fruit freshness. It is directly correlated with water loss rates as well as internal metabolic activities such as cellulose hydrolysis, membrane integrity loss, and depolymerization of starch and pectin [82,94]. Basumatary et al. [94] prepared chitosan-based edible coating with eugenol oil and aloe vera gel, and its characterization revealed that the firmness of the pineapple samples declined throughout the course of the 20-day storage period, but it was at its lowest within the first 4 days. After 12 days of storage, the coated pineapples maintained a firmness of around 43 N, which is roughly twice as firm as the control samples, whereas the hardness of the control samples decreased from about 60 N to 23 N. According to Sathiyaseelan et al. [92], fresh-cut red bell pepper maintained its hardness for 12 days when it was coated with a combination of calcium chloride, low-molecular-weight chitosan, and tea tree oil. When compared to the control, the findings revealed that the combined treatment also preserved the springiness, gumminess, chewiness, and resilience of fresh-cut red bell pepper. However, in the work of Phothisuwan et al. [97], the control revealed the same trend as the salacca hardness preserved with carnauba wax with and without orange oil, but lower values were discovered. The study of Arabpoor et al. [93] showed that uncoated cherries lost stiffness more quickly than the coated ones, suggesting that chitosan nanoparticles with Eryngium campestre essential oil effectively delayed tissue softening. Compared to uncoated samples, fruit softening was likewise delayed in cherries treated with only chitosan nanoparticles. However, on day 14 of storage, the tissues of cherries coated with chitosan nanoparticles were significantly softer than those coated with chitosan nanoparticles with EO. The semi-permeable surface of coated fruits may have contributed to fruit firmness persistence by limiting metabolic gaseous exchange (oxygen and carbon dioxide) across the coating barrier, followed by a decrease in metabolic activity and oxidizing enzyme effectiveness [82].

4.5. Weight Loss

Weight loss rate is an important indicator for measuring the degree of preservation of fruits and vegetables. Therefore, Arabpoor et al. [93] studied the effects of chitosan coating loaded with Eryngium campestre L. EO on the preservation of cherries during a 21-day storage period. Their study revealed that coatings served as an efficient barrier to prevent water loss, extending the fruits’ shelf life. Their results showed that the control fruits had the maximum weight loss during storage, while coated cherry fruits had a weight loss of only 16%. For example, Bashir et al. [82] observed minimum weight loss in apples of 22% coated with starch-based nanoemulsion coatings with rosemary EO, while weight loss in control samples was 35%. According to Miranda et al. [74], the weight loss of papaya fruits was minor when maintained in cold storage, regardless of the type of coating. Contrarily, fruits lost more weight when it was placed in a warmer environment (22 °C or room temperature). They found out that carnauba-based coatings were more effective than hydroxypropyl methylcellulose due to their hydrophobic nature. In the work of Tabassum et al. [78], all the samples showed a notable increase in weight loss during storage, but the uncoated sample showed the greatest increase. In the study of Yang et al. [96], when stored until day 60, the weight loss rates of citrus fruits for the nanoemulsion treatment (3.14%) and control (4.12%) were significantly different, indicating that nanoemulsion coating had a better preservation effect on cold storage. According to Vilaplana et al. [98], compared to untreated and chemically fungicide-treated blackberries, fruits coated with chitosan made with lactic acid showed lower weight loss after 14 days of storage at 4 °C.

4.6. Shelf Life

Since fruits and vegetables are living tissues and extremely perishable foods, they require the best post-harvest methods to maintain their storage stability and increase their shelf life. After harvest, physiological processes limit the shelf life of fruits and vegetables [113]. The study on citrus fruits disclosed a statistically significant difference, in detail, during cold storage; rotten fruits were present in the control and nanoemulsion-treated samples from 20 to 30 days, respectively. When the fruits were preserved for 60 days, the decay rate of the nanoemulsion-treated fruits was 4.1%, compared to 7.4% for the control group [96]. With regard to the shelf life of fresh-cut or minimally processed fruits, it is important not to exceed 10⁶ CFU/g of aerobic plate count, which has been reported as the limit value. In the study of Prakash et al. [95], pineapple shelf life was prolonged for 3 days, as uncoated samples reached the limit on the 9th day, while coated samples reached it on the 12th day. According to Anjum et al. [13], the shelf life of guava fruit extracts was a substantial effect of combined edible coatings and plant extracts. The average shelf life of control guava fruits was 6 days, while those of ginger extract + gum arabic, aloe vera gel + gum arabic, and garlic extract + gum arabic coatings were 9, 10, and 13.1 days, respectively. In the case of garlic extract + gum arabic coatings, the shelf life of guava was doubled.

4.7. Acidity

The organic acid content of fruits is directly related to titratable acidity (TA), which usually declines due to the conversion of organic acids into sugar during the reverse-glycolysis process of fruit ripening [86,94]. Also, acidity is a key element that significantly affects fruit flavour [102]. Based on the work of Basumatary et al. [94], the TA values of the control sample rose throughout the first 12 days of storage from roughly 0.58 to almost 0.98 but then sharply dropped to roughly 0.35 on day 20, indicating accelerated ripening and senescence. All coated samples had their TA values maintained for the first 12 days of storage, but the aloe vera gel coating had them maintained for the entire 20-day storage duration in this study. Gull et al. [83] reported that at the end of the storage period, control fruit samples showed the largest decline, with a value of 0.22%. But coating applications considerably prevented this decline. Coated samples of apricots were treated with chitosan-based coatings containing pomegranate peel extract in concentrations of 0.75% and 0.50%, which displayed TA values of 0.33 and 0.25%, respectively; fruits coated with 1% of pomegranate peel extract efficiently maintained high TA values (0.34%). This might be explained by the coatings’ barrier properties, which limited the oxygen supply and slowed down the ripening process. Manzoor et al. [102] found that alginate coating more successfully maintained TA than carboxymethylcellulose coatings, but all coated kiwis showed higher TA than untreated samples during 7 days of storage. In the study of Yang et al. [96], the TA values decreased steadily during storage, and nanoemulsion treatment revealed higher TA content (0.58%) than control TA content (0.53%), which was expected given the use of organic acids in the respiratory process. After 10 days of storage, there was a significant difference between the control group, where the TA value fell abruptly from 62.30 to 35.52, while on the 13th day in the nanoemulsion treatment group, it decreased from 62.30 to 40.13. Based on the work of Sortino et al. [101], during 9 days of storage of kiwi fruit slices, it is notable that control samples showed a significant decrease in TA values, from 1.8 to 0.9 g/L, while TA values for samples coated by aloe gel with Opuntia ficus-indica Mucilage extract were between 1.8 and 1.4 g/L. According to Prakash et al. [95], the TA of the sliced pineapples, which varied between treatments during the storage period and ranged from 0.51 to 0.62, was not significantly affected by the various citral nanoemulsions used in the edible coatings.

4.8. Total Soluble Solids

It is suggested that the rise in soluble solids during ripening involves a mechanism of cell wall breakdown that provides a source of carbon for sugar production. Investigating the impact of coarse emulsion and nanoemulsion alginate-based coating containing oregano EO on the total soluble solids (TSS), scientists found out that the TSS of all the samples progressively grew in the first days of storage. All emulsion-coated samples displayed a rapid drop in the TSS after 12 days. Meanwhile, the nanoemulsion-coated samples displayed a rise in the TSS by the end of the storage period, preventing sugar degradation for a longer time and extending the shelf life of fresh-cut papaya as a result, while the uncoated sample had a sudden increase in the TSS until day 8 and then a quick reduction, indicating a limited shelf life [78]. Similarly, Sortino et al. [101] investigated the TSS in kiwi fruit slices and found out it did not substantially differ across treatments during storage; as for control and coated samples, the TSS increased from 12.9 Brix to 14.1 and 13.6 Brix, respectively. In the work of Yang et al. [96], the TSS content of untreated and nanoemulsion-treated fruits gradually increased throughout the first 30 to 40 days of storage and considerably decreased as storage time increased. On the 60th day, the TSS concentrations in the nanoemulsion treatment and control were 14.41% and 15.14%, respectively, indicating that the conversion rate of reducing sugars was significantly slowed down by the nanoemulsion covering. Regarding the TSS values of pineapple, coated samples were maintained at about 17 °Brix, while uncoated samples increased slightly from 15.96 to 18.93 °Brix during storage [95].

4.9. Vitamin C

Ascorbic acid or vitamin C (VC) is a natural antioxidant that is important in illness prevention; immune system stimulation; and the promotion of healthy skin, gums, tendons, and ligaments. However, due to its high degree of oxidation, it is unstable to a variety of conditions, including temperature, pH, oxygen, metal ions, and enzymes (such as ascorbate oxidase or peroxidase). Therefore, it frequently serves as a fruit quality indicator [102]. Yang et al. [96] analyzed the VC content of citrus fruits covered by alginate-based enriched with a cinnamaldehyde, eugenol, and carvacrol nanoemulsion. Their study revealed that VC contents of untreated and nanoemulsion-treated fruits, on the 60th day of storage, were 21.28 mg/100 g and 24.38 mg/100 g of VC, respectively. In the study of Prakash et al. [95], in pineapples, initial ascorbic acid content was determined after 12 days of storage. The initial values were between 18.60 and 19.38 mg/100 g, which, it was discovered, fell throughout the storage time. However, it disclosed that the VC content was notably lower (2.59 mg/100 g) in the uncoated samples at the end of the storage period, while in coated samples was between 10.6 and 12.57 mg/100 g. According to Anjum et al. [13], differences between coated and control samples in VC content of guava fruits were not observed, but there was an evident trend of decrease throughout storage time.

4.10. Microbial Analyses

Microbial analyses are important to evaluate fruit and vegetable safety. The cut surface of a fruit is exposed to environmental deterioration and more vulnerable to decomposition by numerous bacteria in the absence of a physical or chemical barrier in the form of a protective epidermis, which directly affects the health of consumers [78]. A study by Vilapana et al. [98] compared chemical antifungal products and chitosan coatings enriched with lactic and acetic acid in the post-harvest life of berries, which were impacted by the presence of fungi like Mucor sp. Their study revealed that blackberries treated with lactic acid coating had a greater disease reduction after 7 days at 4 °C than fruits treated with an ascorbic acid coating (40.29%), and both coatings considerably outperformed fruits sprayed with synthetic fungicide (34.32% disease reduction) in terms of soft mould control. Even though chitosan coatings with lactic and ascorbic acid were less efficient than synthetic fungicide treatment, Vilaplana et al. [98] pointed out that it still may be an alternative approach for the reduction of soft mould in blackberries. On the other hand, microbial analyses for total bacterial, yeast, and mould count on fresh-cut papaya were revealed to be greatly lower between the coated and untreated samples. On day 8 of storage, the bacterial count of the uncoated sample was 6.83 log CFU/g, which exceeded the critical limit. On the other hand, up until day 16 of storage, practically all of the samples containing alginate coating with oregano EO had a bacterial count that did not surpass 3 log CFU/g. The difference in the restriction of the yeast and mould growth for the course and nanoemulsion coating was in a 4-day prolongation for nanoemulsion coatings, which implicates the superiority of nanoemulsions compared to emulsions [78]. In the study by Robledo et al. [88], the effect of quinoa protein/chitosan coating with thymol EO against the growth of mould (Botrytis cinerea) on cherry tomatoes was evaluated. After 7 days at 5 °C, tomatoes coated with thymol nanoemulsion quinoa protein/chitosan displayed a significant reduction in fungal growth when compared to control samples. In control samples (uncoated and coated only with quinoa protein/chitosan), yeast and mould counts taken on days 0 and 2 revealed a population between 2.6 and 3.6 log CFU/g that grew to approximately 6.28–6.45 log CFU/g at day 7 of storage, while in coated samples reached 4.7 log CFU/g. In this regard, no inhibition of fungal proliferation was observed when quinoa protein/chitosan alone was used; however, on day 7, the thymol nanoemulsion of quinoa protein/chitosan displayed the inhibition of fungal development. In the study of Sortino et al. [101], microbiological analyses of aerobic mesophilic bacteria, pseudomonads, and yeasts were performed. It was found that at the end of storage, minimal concentrations of aerobic mesophilic bacteria in untreated and treated samples were 3.3 and 1 log CFU/g, respectively. The pseudomonads were discovered after 7 days, and their concentrations after 9 days were 4.5 and 1.5 log CFU/g in the control and coated samples, respectively. A similar pattern was seen with yeasts, with concentrations of 2 and 1.5 log CFU/g. It is evident that the amount of aerobic mesophilic bacteria, pseudomonads, and yeast on the kiwi fruit slices was dramatically reduced during storage time due to an aloe gel mucilage coating solution. In the study by Prakash et al. [95], a rise in the total plate count and yeast and mould count of all the samples was observed during the storage period. Regardless, alginate-based coating with citral nanoemulsion significantly reduced the microbial growth compared to control, where the most effective coating contained a higher concentration of citral nanoemulsion. The results showed that coating with 1% citral nanoemulsion reduced the growth of yeast and mould counts by 2.77 log CFU/g and the total plate count by 4.68 log CFU/g compared to the uncoated control during 12 days of storage. In the study of Abdalla et al. [114], after 12 days of storage, in the control sample, we found substantial contamination that prevented us from counting colonies. Along with the white and yellow colonies that looked like yeast, there were also colonies of black filamentous fungi that had a velvety aspect. The strawberry treated with the pectin coating was the second sample with the highest contamination (4.1 log CFU/g), which displayed three distinct forms of apparent contamination. Finally, there was no obvious contamination on the fruits coated with film chitosan and film pectin–chitosan with EO.

5. Is industrialization and Wider Application of Plant-Based Nanoemulsion Coatings for Fruits and Vegetables Possible?

Based on this review and all presented results in the scientific field, the industrialization and wider application of plant-based nanoemulsion coatings for fruits and vegetables are indeed possible and promising. However, several factors contribute to the feasibility of their industrial adoption.

5.1. Optimization of Nanoemulsion Coating Formulations

Current research and development efforts have led to important advancements in the formulation of plant-based nanoemulsion coatings. Researchers are continually refining the composition and properties of these coatings to meet industry-specific needs, as shown in Table 2. However, this includes optimizing the choice of plant-based polymers, active compounds, and stabilizers to ensure both effectiveness and economic feasibility. As the primary goal of these coating is to extend the shelf life of fruits and vegetables, reduce postharvest losses, and preserve product quality, the food industry is interested in this technology since minimizing waste and ensuring food safety is the priority for the current decade. This makes nanoemulsion coatings an attractive option, and the needs of modern consumers can be satisfied through this sustainable and eco-friendly food-related technology. By creating a protective barrier on the surface of the produce, these coatings can reduce water loss, inhibit oxygen penetration, and inhibit microbial growth. This can increase marketable timeframes, which are highly desirable outcomes for the food industry. Also, using natural antimicrobial and antioxidant agents makes this solution acceptable on many levels for modern society [1]. Many of these coatings incorporate natural antimicrobial agents, such as EOs, which can effectively inhibit the growth of spoilage and foodborne pathogenic microorganisms. It is very important to emphasize that many regulatory bodies have recognized the potential of plant-based nanoemulsion coatings, and some guidelines and safety assessments have been established in the recent past [24]. This step can lead to the adoption and standardization of this type of coating system in the food industry.

5.2. Application of Coatings

As innovation is necessary in the food industry, but also in coating application techniques and equipment, much research needs to be performed on easier and more cost-effective ways to apply nanoemulsion coatings on an industrial scale. This primarily refers to advancements in spraying and dipping methods [27]. Furthermore, as the food industry seeks to reduce its high environmental impact, the development of eco-friendly coatings using biodegradable materials is gaining special traction. This addresses concerns about the ecological footprint of food packaging and preservation, which can be improved with plant-based nanoemulsions. Despite these promising aspects, several challenges and considerations need to be addressed for the widespread industrialization of plant-based nanoemulsion coatings. One of the obstacles for producers, especially for small- and medium-sized enterprises, is the high cost of coating formulations, as well as specific application techniques. Therefore, achieving the cost-effectiveness of coatings while delivering the desired benefits is crucial for industrial adoption. One of the challenges during the industrialization of these types of coating systems can also be uniformity during application. Ensuring consistent coating coverage on a large scale, especially for irregularly shaped or textured fruits and vegetables, requires efficient application methods and equipment. Scaling up production from laboratory experiments to industrial levels can pose technical challenges that also need to be addressed [115]. However, social barriers can similarly affect the wide application of this product. Therefore, consumer education and understanding all the advantages of plant-based nanoemulsion coatings is necessary.

5.3. Research Needs

Summarizing these facts, it can be said that plant-based nanoemulsion coatings have the potential to improve food technology by addressing critical challenges related to the shelf life, food safety, and sustainability of targeted products. Their industrial and worldwide adoption depends on overcoming cost barriers, refining application methods, and navigating regulatory documents. As consumer preferences continue to evolve in the future, the food industry can make a solid base for this challenge using eco-friendly plant-based nanoemulsion coatings. Therefore, continued research, investment in technology, and regulatory support are going to play crucial roles in realizing the full potential of these coatings in the food industry. Profound studies of nanoemulsion coatings systems are crucial to understanding their properties to ensure the ability to manipulate and predict their behavior. Further research should focus on better understanding physicochemical processes occurring in fresh products, as well as the effects of nanoemulsion coatings, with the objective of securing mutual compatibility. Furthermore, it would be significant to broaden the spectrum of food products that can be coated and to enhance coating application techniques. Another important aspect is searching for new sources of bioactive compounds that would be more efficient and provide a satisfactory effect on the prolongation of shelf life. Further, performing more scale-up studies will provide beneficial information on the production of nanoemulsion coatings on an industrial scale, which could lead to wider implementation of edible coatings in the food industry.

6. Future Perspectives

The implementation of post-harvest techniques is necessary due to the high rate of food losses, especially for fresh-cut and minimally processed fruits and vegetables that require additional protection to maintain the highest quality and safety. The application of a nanoemulsion edible coating system on fruits and vegetables provides a substantial impact as a postharvest treatment and offers promising results to safeguard the physicochemical during the time of storage and transportation, as well as to provide antibacterial and antifungal properties, retain nutrition, and guarantee the prolongation of shelf-life of products. These coatings act as effective barriers, shielding the produce from external factors such as oxygen, moisture, and microbial contaminants, ensuring the freshness and quality of fruits and vegetables for extended periods and contributing to both consumer satisfaction and a reduction in food waste. However, to make plant-based nanoemulsion coatings technologically feasible and economically available, advanced and thorough research is required regarding functionality, composition, sensory and mechanical properties, and the application aspect of coatings on real food. Future perspectives should include a comprehensive study of different sources of active compounds and their effects, a profound understanding of impacts regarding specific application to fruits and vegetables, and an adequate combination of components in coating formulations to fulfil the necessary qualifications for widespread application.