Initially, edible coatings were developed to replace and decrease the usage of other kinds of chemicals and synthetic compounds that may be harmful to customers’ health. An edible coating is a covering sheet composed of biological or chemical ingredients and utilized as a monolayer film or multilayer films on the surface of the product ^[1][29]. Edible coatings help maintaining the phytonutrients (antioxidants, phenolics, pigments) and controlling physicochemical qualities (inhalation and exhalation rate, weight loss, total dissolved matters, pH) of fruits for longer time ^[2][30]. As a result, fruit deterioration is delayed, fruit quality is improved, and fruit shelf life is extended ^[3][31]. To be effective, edible coatings must meet several functional requirements (Figure 1), including (i) being free of toxic materials and harmless for human beings; (ii) having superb boundary capabilities regarding water, humidity, O₂, CO₂, and C₂H₄; and (iii) improving the visual as well as textural properties of the coated products ^[4][32]. The coating should not alter the sensory properties of the fruit ^[5][33]. Therefore, edible coating formula needs to be carefully considered during development. Furthermore, the coating should control the gas exchange to avoid fruit fermentation and undesirable off-flavor ^[6][12]. In the case of fruit coating, when the oxygen level falls below 3%, anaerobic respiration takes over, which generates undesirable flavors and induces other issues such as colorant and structure alterations. Therefore, high concentration of O₂ (>8%) and low concentration of CO₂ (<5%) are recommended to prevent or delay deterioration, hence preserving food quality ^[7][34].

The coating-forming solutions can comprise a single main component from proteins, polysaccharides, lipids, or a mixture of them to achieve desired properties ^[8][35]. New edible coating candidates should be inexpensive and available in large quantity. The coating should provide easy application, have good adhesive characteristics, and dry quickly with uniform thickness. Moreover, the coating performance and structural stability must be maintained during long-term storage. The coating must be flexible enough to adapt to specific morphological changes such as fruit shrinkage or mechanical damage ^[9][36].

The characteristics of coating polymers determine their use and function ^[10][37]. Proteins and polysaccharides establish strong molecular interactions in polymers. They have superb mechanical and gas isolation (oxygen and carbon dioxide) qualities that inhibit the common ripening process in many fruits ^[11][12][38,39]. The most universally used polysaccharides in food production can be attributed to cellulose and its derivatives, namely pectin, chitosan, and gums ^[13][40]. However, studies showed that the drawback of their application is the poor performance in preventing fruit moisture loss ^[14][41]. In addition, usage of proteins in edible coatings may be limited because they are potentially allergenic or refused due to religious belief ^[15][42]. Coatings made from lipids (fatty acids, acylglycerol or waxes) are great moisture barriers due to their hydrophobic property ^[9][36]. Unfortunately, coatings based on lipids were found to be poor in mechanical attributes and brittleness due to their lack of cohesiveness and structural integrity ^[16][43]. Lipid molecules are frequently added to matrices to reduce the water sensitivity of a hydrocolloid-based coating ^[16][43]. Therefore, mixtures of different components can be used for production of edible coatings that enhance the physicochemical characteristics and solve the drawbacks of the individual components ^[17][44].

Coatings that are based on polysaccharides and proteins achieved excellent barrier characteristics. However, they are also less flexible due to strong intermolecular forces along the polymer chain. As a result, blisters, flakes, or cracks may appear in the coating as the fruit shrinks during storage ^[5][33]. The primary role of plasticizers is to strengthen coating flexibility and decrease brittleness. Glycerol, mannitol and sorbitol are the food grade plasticizers typically used in edible coatings ^[18][45]. In addition, sugars with small molecules (such as fructose-glucose syrups and honey), other polyols (such as glycerol derivatives and propylene glycols), lipids and by-products (such as phospholipids, fatty acids, lecithin, oils, and waxes), and water are also popular examples of food grade plasticizers that can be added to coatings ^[19][46]. The plasticizer absorbs more water into the coating matrix and decreases the intermolecular interactions along the polymer chain, thereby preventing the coating from blistering, flaking and cracking ^[20][21][47,48]. Furthermore, the amount of plasticizer used in the coating formulation must be examined according to its influence on the permeability of the wrapping. Plasticizers increase the moisture and gas permeability through the coating by reducing polymer interactions and increasing intermolecular space ^[20][47]. Even though plasticizers are supposed to give the coatings elastic structure, overusing plasticizers results in decreased moisture resistance and weaker mechanical strength. Table 1 summarizes the advantages and challenges of plasticizers reported in edible coatings of fruits.

When using hydrophilic polyols (such as glycerol or sorbitol) as plasticizers, the water solubility, elongation, and the moisture permeability of coatings were substantially improved following the plasticizer concentration. The observed total color difference and the puncture strength, on the other hand, decreased with higher plasticizer content. The mechanical properties of puncture strength and elongation of coatings are more affected by glycerol than sorbitol. In addition, sorbitol-plasticized coatings had decreased water vapor permeability, which can be increased with higher plasticizer concentration. However, this rise was less than that found with glycerol-plasticized coatings ^[25][52]. Additionally, Yang and Paulson ^[26][53] found that sorbitol did not show a plasticizing effect on gellan films, although it had been widely used in protein-based films. That study showed that polyethylene glycol and glycerol were effective in plasticizing gellan films at 60% concentration. It has been proved that at lower concentration, the films turned to be more fragile, and their manipulation became challenging, while glycerol concentration beyond 75% results in sticky behavior. Additionally, the presence of plasticizers promoted the homogenous coating structure, preventing phase separation. By limiting the development of pores or cracks, the integrity of the coating can be maintained ^[27][54]. The coatings with low protein content in the formulation showed smoother surfaces when treated with plasticizers ^[28][55].

Following the selection of the wrapping composition, the application of the solution to the surface of the fruit is the next important step. There are different approaches about how to cover the food surface with edible coating (Figure 2). Dipping is the simplest method consisting of three steps: (i) immersion and dwelling, (ii) deposition and (iii) evaporation of solvents ^[29][56]. After the excess solution has been drained away, the food is typically dried at ambient condition or treated with a dryer ^[30][11]. Previous studies showed that the density and morphology of coatings precipitated by dipping are significantly affected by several factors including time for immersion, speed of withdrawal, number of cycles for dip-coating, coating solution parameters such as density, viscosity, surface tension, substrate surface characteristics, and drying conditions ^[31][57]. However, there are numerous disadvantages of the dipping method. Dipping commonly results in a layer with heavy thickness leading to substantially reduced fruit respiration, damage food surfaces and degraded function. In addition, microorganisms and dirt from the fruit surface may contaminate the coating solution, hence challenging the industrial up-scaling. Another drawback of the dipping approach is the large quantity of solution needed for coating per unit mass of product to guarantee optimum dipping conditions ^[5][33].

The spreading technique is effective for coating solutions with high viscosity. In general, the wetting level and spreading rate are the key factors used to describe how the coating solution is spread across the food surface. Several parameters influence the efficiency of coating deposition by spreading, including substrate quality, particularly drying conditions, liquid characteristics, and surface geometry ^[32][58]. Specialized operators and technicians typically perform brushing. Thus, the human factor has a significant impact on the quality of coating and thickness homogeneity.

Spraying is the process of using a set of nozzles to distribute small droplets on the fruit surface. Spraying methods are used in three different ways, including air spray atomization, pressure atomization and air assisted airless atomization ^[30][11]. The spraying technology allows multi-layer applications such as interlayer solutions, as well as consistent coating with homogeneous thickness ^[33][59]. Additionally, the coating thickness is greater than that of dipping method due to the low viscosity of the solution ^[30][11].

The electrospraying method uses a strong electric field to produce charged droplets, with a very narrow size distribution, that are micrometric and sub-micrometric in size ^[34][60]. The electrospraying process can adjust droplet specifications such as size, or produced layer thickness by controlling the flow rate as well as the viscosity of the solution ^[35][61].

The layer-by-layer deposition method is based on the electrostatic interactions of the food surface with charged polyelectrolytes. These electrostatic interactions improve adhesion of the coating to the food surface and may be used to create coatings with two or more thin layers that are chemically or physically linked to each other. Such linked multilayer coating improves the effectiveness compared to conventional edible coatings ^[36][62]. The use of the multilayer coating approach to increase the compactness of the coating layers during postharvest storage of fruits were documented for polysaccharides and charged polyelectrolytes capable of hydrogen and covalent bonding. Polysaccharides and charged polyelectrolyte demonstrated efficiency in fruit preservation when applying the multi-layer coating method to improve the tightness of the coatings ^[37][63].

The final technique in the list is cross-linking, which is defined as the process of combining polymer chains using covalent and non-covalent linkages. Cross-linked coatings are typically created by spraying, dipping, or spreading the coating solution onto the food surface. A cross-linking agent is then added to provide a more compact and stable coating. Cross-linked coatings have substantial benefits, including better mechanical properties, chemical and thermal stability as well as better molecular migration ^[38][64]. Cross-linking is particularly effective for biopolymer materials formed from proteins or polysaccharides. Proteins are used more frequently than polysaccharides with this technique due to the greater number of functional groups in proteins ^[10][37].

The results of different application techniques for edible coatings are listed in Table 2. The findings in the list confirm the idea that coating technology in postharvest preservation of fruits can be considered a sustainable solution.