Improved Postharvest Techniques for Fruit Coatings

Abstract

Fruits, particularly tropical fruits, have a high moisture content, distinct morphological characteristics, and physiological changes, all of which contribute to their high rate of perishability. Nonetheless, their organoleptic and nutritional qualities make them one of the most important horticultural products. Fruit coating, which imitates natural packaging, is a postharvest solution that is practical and cost-effective for a variety of applications, including on-shelf display, transportation, and storage in support of the supply chain of fruits and vegetables. Gas and moisture permeability, microbiological resistance, and esthetic enhancement are the coating functions. Using modified materials and procedures, edible coatings for fresh and freshly cut fruits are currently being developed. Edible coatings infused with essential oils or volatiles may help to prevent disease resistance while also providing consumers with a fragrant preference. When considering how to advance fruit coating technology when agricultural wastes are the primary source of new coating materials, composite coatings, nanoparticles, encapsulation, and multiple-layer coatings all hold a great deal of promise. Future research may center on the optimal material for particular fruits during the logistics phase.

1. Introduction

Fruits, especially tropical fruits, are highly perishable and quickly lose their quality after being picked. Fruit postharvest loss in tropical areas could be up to 50%. The high level of physiological changes that occur in fruit is mostly attributable to the high level of moisture content [1]. The amount of water that evaporates from fruit is a significant factor in determining both the quality of the fruit and the economic quantity losses [2]. As a result, packaging that includes coating has the potential to successfully limit the loss of water from the fruit that has been stored. On the other hand, standard packaging might not be appropriate for certain fruits or environments. For instance, the sharp spines of a durian can easily puncture the plastic film, or the headspace in the package can accumulate condensed water and cause the fruit to rot. Edible coatings are a simple kind of fruit packaging that is applied to individual fruits in order to facilitate easier postharvest handling [3]. However, for fruit coating to be successful, it is necessary to consider not just different coating processes but also the characteristics of the fruit itself.

2. Background related to fruit-based coatings

The extracellular barrier of cuticles protects the plant’s surface and interior from environmental stresses. The cuticle, composed of polysaccharides and a lipid matrix termed “cutin” [4], can dramatically transform the epidermis’s outer cell wall. Many factors are essential determinants of shelf life and storage capacity, including the cuticle’s moderating effect on water transpiration, fruit dehydration, and vulnerability to rots, pests, and diseases. Fruit cuticles have been proven to be highly responsive to environmental factors, and their quality changes even after harvest. When a fruit matures, the cuticle grows thicker [5]. Cutin polymers can be formed from long-chain fatty acids (C₂₀-C₃₄) but are typically made up of esterified and oxygenated C₁₆ and C₁₈ fatty acids [6]. It may also contain trace amounts of glycerol, phenylpropanoids, primary and secondary alkanes, alcohols, aldehydes, ketones, etc. [7]. Fatty acids are essential for the biosynthesis of cutin and wax, which occurs primarily in chloroplasts [8]. The components of wax extracted from the fruit surface of guavas comprise fatty acids and primary alcohols as the major components, followed by sterols, n-alkanes, and aldehydes. Interestingly, the wax also contains triterpenoids, a natural pesticide against insects and pathogenic fungi [9]. Thus, understanding the components and function of the cuticle should be useful in improving the coating materials used in fresh fruit markets.

Fresh fruits are perishable due to their quick deterioration due to dehydration, softening, discoloration, microbiological decay, disorder, and loss of nutrients [1]. These properties result from the metabolic processes occurring within the fruit, which are accelerated mainly by an improper relative humidity and gas composition in the storage atmosphere. High water loss and rapid metabolic changes associated with ripening and senescence are the primary causes of fresh fruit deterioration after harvest [10, 11, 12, 13]. Artificial coatings mimic the protective properties of the fruit’s natural cuticle and can postpone ripening and senescence by acting as a barrier to water and gases. Currently, edible coatings that are safe for food or handling provide a sustainable and effective solution for preserving the high quality of fruit throughout the postharvest value chain [3]. It is generally accepted that edible coatings as a type of MAP (modified atmosphere packaging) add value to fruits by shielding them from contamination, improving their esthetics, and preventing the loss of flavorful volatiles during preparation and storage (Figure 1). For the success of a fruit coating business, a thorough understanding of fruit characteristics, nature, respiration, and transpiration behavior is essential.

Figure 1.

Fruit coating on mature green “Nam Dok Mai” mango during air drying.

2.1 Fruit types

Three distinct fruit categories can be identified based on their floral structure: simple fruit, aggregation fruit, and multiple fruit. Some fruits, like the papaya, the rose apple, and the chili, have an internal cavity despite having a variety of morphologies. In addition, some fruits produce an inner supplementary tissue called aril (mangosteen, durian). Fruits that are either an aggregate (sugar apple, strawberry) or multiple fruit (pineapple, jackfruit) are made up of many fruitlets that have fused together. Variations in fruitlet maturation might cause uncertain ripening of the entire fruit [14]. Fruit coating could improve the ripening quality of the uncertain ripening by maintaining moisture and gas diffusion in the whole fruit.

2.2 Fruit structure/fruit parts

Dermal tissues (peel), cortical tissues (pulp), vascular bundles (veins), seeds, and intercellular spaces make up the anatomy of most fruits. The peel consists primarily of parenchyma, of which some are transformed into guard cells (Figure 2A) and the lenticular opening channel (Figure 2B). Fruit stomata and lenticels are responsible for gas and water vapor exchanges, which could be an issue for fruit coating. In rambutan (Nephilium lappaceum L.), the epidermal tissues, spinterns, include up to 100–200 apertures/mm² of stomata [10] that are constantly open [15]. Thus, stomata on spinterns that connect up to 20 groups of vascular bundles in the mesocarp are primarily responsible for moisture loss from rambutan fruit (Figure 2A). Replacement of water lost by spinterns with water from the skin [16]. The aril is still edible, despite water loss from the pericarp, producing skin withering, spintern drying, and pericarp browning (Figure 3).

Figure 2.

Anatomical structure of cross-sectioned rambutan fruit showing spintern stomata and vascular bundle networks (A) and cross-sectioned papaya fruit showing lenticel and pericarp tissues (B).

Figure 3.

Fruit pericarps and arils of ordinary fruit (left) and dehydrated fruit (right).

Vascular bundles (phloem and xylem tissues) are gathered at the fruit’s stylar end (peduncle/pedicel) that connects the fruit to the mother plant. In tangerine fruit, the vascular bundles start from the stylar end through the albedo mesocarp flavedo under the peel (Figure 4). The maturing fruit’s peel is typically covered with the cuticle. Consequently, the cut peduncle is where the fruit quickly loses its moisture to the air after being harvested. Fruit edible flesh could be derived from the ovary wall, some accessory organs (receptacle and petal), or the unique tissue of “aril” covering the seed, which comprises a majority of parenchymal cells carrying high water content, stored chemicals, and biological metabolisms. Furthermore, in some fruits, there is a fruit cavity (papaya, rose apple) or seed cavity (mango, apple) that is a gas container in the fruit. The fruit’s intercellular spaces and cavity (Figure 2B) play crucial roles in the fruit’s ability to transpire and respire. The gaseous atmosphere in the intercellular spaces and cavity can be modified depending on the respiration rate and metabolism of the cells. The portions of intercellular spaces in several tropical fruits are shown in Table 1. Moreover, in fruit cavities, such as papaya, the different varieties exhibit varying volumes of cavities [17]. Intercellular spaces are important for cellular modification under hypoxic conditions [18]. Thus, the higher portion of intercellular air spaces is more tolerant to hypoxic conditions caused by fruit coatings. For example, apples and rose apples are tolerant to hypoxia from fruit coatings, in contrast to tangerines and guavas, which contain tiny intercellular spaces. Excessive CO₂ from respiration could accumulate in the room to reduce cellular toxicity, or O₂ demand can be taken from the portions to delay hypoxic cellular conditions. Coating material types and concentrations must be carefully considered. An off-flavor may result if a wax coating effectively prevents water loss from fruits with few intercellular spaces, such as tangerines. Thus, the modulation of the proper coating formula for each fruit depends on the fruit characteristics of each cultivar.

Figure 4.

Fruit structure of tangerine fruit when cross-sectioned (left) and longitudinally sectioned (right).

Commodity	Total intercellular air spaces (%)
‘Gala’ apple	1.25
‘Klom Sali’ guava	0.24
‘Phet’ rose apple	3.32
‘Khaew Wan’ tangerine	0.42

Table 1.

Percentages of intercellular spaces in some tropical fruits (own data).

2.3 Maturity, respiration, and ethylene production characters

Fruits are classified as climacteric or non-climacteric based on their respiratory and ethylene production patterns during maturation. The success of fruit coatings depends on the fruit’s maturation, transpiration, and respiration rates. After fruit setting and during fruit growth due to cell division, the respiration rate is at its highest; it then gradually decreases to its lowest point during the early stage of fruit maturation. During ripening, respiration and ethylene production rates in climacteric fruits rise sharply, peak, and drop off, while both rates are not apparent in non-climacteric fruits. Storage temperatures play important roles in the respiration (Table 2) and ethylene production rates of fruits (Table 3) [19]. When fruit is coated with exogenous waxes, natural gases, and humidity, exchanges between the fruit and its respective microclimates are disrupted. The respiration, ethylene production, and gas permeability of the coated fruit will alter the gas concentrations within the cellular fruit, decreasing O₂ and increasing CO₂ and C₂H₄. By measuring the concentration of gases in the fruit’s intercellular spaces and/or cavity, the fruit gets into an equilibrium of internal gases (Figure 5). Ethylene and CO₂ accumulation at different levels in “Solo” papaya during on-tree ripening [20] are shown in Table 4.

Commodity	Respiration rates (mL CO2⋅kg^−1⋅h^−1)
	13°C		25°C
	Max	Min	Max	Min
“Rongrien” rambutan	20.40	—	48.00	—
“Monthong” durian	58.39	6.72	106.05	28.70
Mangosteen	—	—	20.50	—
“Nam Dok Mai” mango	26.00	11.00	72.50	17.05
Longkong	36.37	—	81.66	—
“Khao Pan” pummelo	4.00	—	9.25	—

Table 2.

Respiration rates of some tropical fruits at 13°C and 25°C (adopted from Kosiyachinda and Tansiriyakul [19]).

Commodity	Ethylene production rates (μL⋅kg^−1⋅h^−1)
	13°C	25°C
“Rongrien” rambutan	0.78	0.66
“Monthong” durian	2.40	9.52
Mangosteen	0.22	3.60
“Nam Dok Mai” mango	0.67	—
Longkong	2.19	1.39
“Khao Pan” pummelo	0.026	0.026

Table 3.

Ethylene production rates of some tropical fruits at 13°C and 25°C (adopted from Kosiyachinda and Tansiriyakul [19]).

Figure 5.

Dynamic and metabolism changes in coated fruits and the surroundings.

Ripening stage	Concentrations of gases in the fruit cavity
	Carbon dioxide (%)	Ethylene (μL⋅L^−1)
Mature green	1.8	0
Fully ripe	5.5	2.8
Overripe	5.0	2.3

Table 4.

Internal CO₂ and C₂H₄ concentrations in the fruit cavity of “solo” papaya during fruit ripening (adopted from Akamine and Goo [20]).

Many tropical fruits, such as mangosteen, longan, longkong, rambutan, durian, and salah, contain two parts: the pericarp (peel) and aril (flesh), which individually develop during fruit maturation. The aril of some tropical fruits is derived either from the funiculus (durian and lychee) or integument (mangosteen and rambutan) of the seed [14]. During maturation and ripening, the pericarp and the aril mature independently at different levels. Fruits like the mango, whose flesh develops from the ovary wall, soften, change color, and release their natural aromas as they ripen regularly and consistently. However, the pericarp (dusk) of the durian fruit releases most of the climacteric ethylene during whole fruit ripening, which in turn causes the aril to ripen. Double climacteric blooming occurs in several cultivars (fruit ripening and dehiscence). The respiration and ethylene production rates of durian pulp during ripening are much lower than those from the pericarp [21]. The endogenous ethylene produced in the husk is required for whole fruit and pulp ripening [22]. Thus, a coating of fresh-cut durian must undergo proper ripening before husk removal because unripe pulp often fails to ripen regularly [23]. Interestingly, stages of maturity are crucial for fruit coating. In “Nam Dok Mai” mango, for example, the mature green fruit was induced to get anaerobic conditions and produce off flavor at a high concentration of composite coating, but the treated fruit was typical when the ripe fruit was coated at the same concentration [24].

3. Basic materials for fruit coatings

After harvesting, coating fruits with shellac or fat-related substances was a frequent procedure in the past. Later, fat-based plant extracts from bananas, pineapple leaves, and carnauba trees were utilized. In contrast, coating materials often consist of a single complex molecule that is effective at preventing water loss but has poor gas exchange. Therefore, it is vital to understand the composition before searching for an acceptable covering. There are three major chemical properties of edible coatings.

3.1 Polysaccharides

The polysaccharide-based coating comes from modified or extracted polysaccharides from natural products, typically including many starches from plants (corn, wheat, rice, cassava, and potato), glucomannan, galactomannan, inulin, plant cellulose and pectin, plant gum, alginate from brown seaweeds, pullulan from the fungus Aureobasidium pullulans, and chitosan from shrimp. The edible film made of polysaccharides is transparent, strong mechanically, and impervious to lipids. From those materials, chitosan, a plant disease elicitor, has been studied for coating many fruits. With stored longans (Dimocarpus longan Lour. cv. “Daw”), chitosan coating at 1.0% and 1.5% revealed a delay in pericarp browning and an effective retardant of disease growth with less than 4% disease incidence at 4°C [25]. Gac fruit (Momordica cochinchinensis Spreng) at the yellow stage coated with 0.5%, and 1.0% chitosan delayed fungal infection and enhanced fruit appearance [26]. Chitosan coatings have been successfully used to maintain the quality of many tropical fruits such as pineapple [27], banana [28], and papaya [29].

The polysaccharide-based coating is useful for coating freshly cut fruits because it can prevent fat remnants from leaking out. However, the constraint of the polysaccharide coatings is a lack of control over water loss and O₂ and CO₂ exchanges [30]. Moreover, if the logistics of handling fresh produce involve a high-temperature fluctuation, or if the fresh produce is immediately removed from the cold storage and placed at room temperature, the coating may be in a reversible phase, resulting in the peeling of the coating from condensed water droplets on the surface. Researchers have attempted to create an edible polysaccharide coating or film using antioxidants from plants. Typically, this is done to enhance the physical and chemical properties of the film and coating for safe consumption.

3.2 Proteins/oligo peptides

Proteins from plants, animals, and microorganisms are biopolymers that can be utilized to make films with tunable physical and functional properties when mixed with plasticizers or other components. The proteins such as gelatin, casein, collagen, whey proteins, egg white, etc. have been studied. Covalent unions (side chain cross-linking) and electrostatic or ionic interactions between protein chains contribute to coating formation [31]. Protein films’ mechanical and hydrophobic barrier properties, and hence their suitability for food packaging applications, are strongly influenced by the final chain contacts and bonds. The major bonding processes are controlled by production conditions such as pH, salt addition, heating, enzyme action, drying, and reactions to food-grade chemicals [32]. Protein films have been enhanced with antimicrobials and antioxidants, among other active compounds. Some protein extracts have been studied for fruit coatings. Park et al. [33] used a corn-zein coating to delay fruit ripening in tomato fruit, while Avena-Bustillos et al. [34] used a casein (milk) protein coating to reduce weight loss in zucchini. Furthermore, zein and gelatin coatings could delay ripening in mangoes stored at 32°C [35].

Although the protein-based coating belongs to the hydrophilic group, the covalent cross-links, and electrostatic networks boost the coating’s structural stability, limiting the coating’s reversible phase when droplets condense on the surface during logistics.

3.3 Lipids

Fatty acid derivatives, as a significant component of the natural cuticle, play an important role in preventing water loss and gas exchanges in fruits. The properties of flexibility, hydrophobicity, and cohesion that edible films require are provided by lipids [36]. The quality of fruits can be maintained by edible coatings made of lipids, which are effective barriers against moisture, O₂, and CO₂, but not C₂H₄. Edible coatings such as carnauba wax, shellac, bee wax, and some plant oils based on lipids can cover fruits and vegetables. Films and edible coatings made from fat have gained appeal due to their functional and nutritional benefits. Shellac and carnauba have long been applied to postharvest fruits. Ten percent shellac coating prevented fresh weight loss and disease infection, and reduced the respiration and ethylene production rates of gac fruit at 25°C storage, while 15% shellac coating led to a high accumulation of acetaldehyde since day nine [37]. Shellac and carnauba emulsions were coated on “Nova” mandarins (Citrus reticulata) at 20°C storage. The carnauba waxes resulted in minor weight loss compared to the uncoated control and shellac coating, but shellac-coated fruit showed the highest fruit shine. The highest levels of CO₂ and the lowest level of O₂ were found in shellac-coated fruit, resulting in the highest ethanol content in the juice due to induced anaerobic respiration [38].

Many lipid-based coatings can provide the best water transpiration prevention due mainly to their strong hydrophobic properties, but for fresh produce, the switch from aerobic to anaerobic respiration caused by too low O₂ and/or high CO₂ in the coated fruit must be considered. Although long fatty acid derivatives are the most abundant in natural cuticles, attachment to sterols, terpenoids, polysaccharides, and phenolics may modify the complex structure, increasing gas exchange permeability.

4. Recent approaches to enhance the efficiency of coatings for fruit preservation

Fruit coatings have been improved and developed, mostly relying on complex cuticle structures, fruit types, proposed uses, and raw material availability. Modified materials and techniques are elucidated in this chapter.

4.1 Modified materials

4.1.1 Plant residues

Plants are a massive source of fruit-coating materials, particularly from agricultural waste. New materials from plant cell structures, including cell wall components and stored chemical substances such as gum, polysaccharides, proteins, and lipids, have been extracted and researched for food packaging [39].

Recently, agricultural wastes have been used to produce some coating components in carboxymethyl cellulose (CMC). Ketrodsakul [40] developed a process to extract crude cellulose from corn stems, an agricultural waste left after corn harvesting in central Thailand, and turn them into CMC. To improve the quality of CMC from corn stems (Figure 6A), Clorox was used to remove green chlorophylls from the samples (Figure 6B). The chemical spectrum analysis using Fourier transform infrared spectroscopy (FT-IR) shows that the CMC from corn stem has characteristics of sub-residues similar to a commercial CMC (Figure 6C). The modified CMC can be used as a material coated on mango fruit surfaces, demonstrating complete covering over the stomata of mango peel (Figure 7B), compared to the uncoated control (Figure 7A).

Figure 6.

Visual appearances of CMC from corn stem cellulose, extracted with 1.0 M NaOH (A) and then with Clorox treatment for 12 h (B), and the FT-IR spectrums of a commercial CMC compared with the CMC from corn stem cellulose extracted with 1.0 M NaOH and bleached with Clorox (C).

Figure 7.

Scanning electron microscopes (1500 x) of stomata on uncoated mango peel (A) and 2% corn-CMC coated mango peel (B).

Supapvanich et al. [41] used Aloe vera gel coatings on fresh-cut “Taaptimjaan”’ rose apples stored at 4°C. A. vera coating preserved the white index and slowed the browning and chilling injury symptoms, particularly at 75% (v/v). Modified atmosphere coatings by A. vera dips delayed the increases in phenolic concentration and polyphenol oxidase (PPO) activity.

4.1.2 Animal residues

New coating materials derived from animal production waste have been gradually discovered. Sericin, for example, a natural protein from silk industry wastewater, is hydrolyzed and used for food coatings. The FDA approves sericin and its derivatives as “GRAS” substances [42]. Sericin coatings on fresh-cut mango [43] and apples [44] had a lower water loss and browning index than the control at low-temperature storage by decreasing the activities of browning-related enzymes, mainly PPO. Because sericin hydrolysates contain serine (30–33%), glycine (19%), and aspartic acid (17.8%) [45], holding high hydroxyl (–OH) groups that could absorb water, leading to a reduction in water loss and inhibiting the browning-related enzymes in the fresh-cut produce.

4.1.3 Microorganism products

Some bacteria cultivated under specific conditions can produce edible polymers as by-products. For instance, Acetobacter can oxidize sugars, sugar alcohols, and ethanol and produce acetic acid as the primary end product that generally contains bacterial exopolysaccharides. Acetobacter species such as Acetobacter xylinus [46] are capable of synthesizing cellulose and have many uses in some fermented food products, which produce soluble polystyrene and contain rhamnose, glucose, mannose, and glucuronic acid as their acetane-related structures. Bacterial cellulose (BC) is one of the promising biomaterials that can be developed as a food packaging plastic material and is produced through the fermentation of high carbohydrate-containing substrates such as agricultural and industrial waste [46, 47]. Yanti et al. [47] produced CMC from a modified BC film and used glycerol as a plasticizer.

There is currently very little information about studying fruit coated with BC-based materials. However, an antimicrobial composite edible film from fermented cheese whey with Candida tropicalis was found to inhibit Pseudomonas aeruginosa, in vitro [48]. Thus, edible BC films can potentially be used for fresh-cut and intact fruit coating.

4.1.4 Composite coating materials

Composite coatings consist of two or more biopolymers, which can minimize the disadvantages of each component. As a result, composite coatings can contain active components such as antibiotics, metal nanoparticles, essential oils, and antioxidants to improve the function of coatings in maintaining the quality of fruits. Because of differences in the polarities of materials, suitable emulsifiers or plasticizers may require a better mixture of the solutions. When compared to chitosan coating alone, the layer of chitosan integrated with tiny Tween-20 provides smooth and complete coverage on mangoes (Figure 8).

Figure 8.

Mango fruit coated with chitosan and chitosan+Tween-20.

4.1.4.1 Polysaccharide and polysaccharide composite coating

Because of their hydrophilic properties, polysaccharide-based materials for fruit coatings, including homopolysaccharides and heteropolysaccharides, have low barrier properties to water vapor, CO₂, and O₂. It has been demonstrated that combining different polysaccharides improves coating functions.

Chitosan coatings, cross-linked to hydroxypropyl methylcellulose (HPMC) by incorporating neem oil, reduced the number of hydroxyl moieties. Thus, decreasing hydrophilic characteristics could improve the moisture barrier of coatings. The emulsion of neem oil in chitosan cross-linked to HPMC presented a higher contact angle than the chitosan solution on the surface of pitaya, which showed a slight hydrophobic characteristic. Composite chitosan coating with HMC reduced weight loss and delayed the senescence of pitaya fruit compared to chitosan coating [49]. Chitosan combined with A. vera retarded the weight loss of blueberry fruit compared to chitosan coating alone due to the hydrophobic character of the A. vera liquid fraction. Furthermore, the composite coating based on chitosan and A. vera delayed microbial spoilage via the antifungal activity of both chitosan and A. vera. [50]. Furthermore, κ-carrageenan has been introduced to improve chitosan-based composite coating based on the interaction between oppositely charged polysaccharides. Because of the formation of a hydrogen-bond network between chitosan and κ-carrageenan, the composite coating has better water vapor barrier properties [51]. The composite coating reduced the physiological weight loss, improved the accumulation of phenolics, and suppressed the activities of the major chlorophyll-degrading enzymes, resulting in the retention of chlorophyll content in the bracts of dragon fruit and their green color (Figure 9) [52].

Figure 9.

Visual appearance of dragon fruit coated with chitosan- and ĸ carrageenan-based composite coating compared to the uncoated control dragon fruit during storage at 10°C.

On the other hand, sodium alginate combined with hydroxyethyl cellulose (HEC) generated a continuous and smooth coating layer on the strawberry fruit surface. Meanwhile, sodium alginate alone could not form a continuous film on the fruit, and hydroxyethyl cellulose alone displayed a coating layer with wrinkles and was multi-porous [53]. The single coatings could not reduce the weight loss caused, but the sodium alginate-HEC composite coating significantly decreased the weight loss of strawberry fruit. Furthermore, this composite coating was a good gas barrier that retarded the loss of phenolics and flavonoids due to the degradation of strawberries.

4.1.4.2 Polysaccharide and lipid composite coatings

Composite coatings based on polysaccharides and lipids are probably used to enhance the moisture and gas barrier properties due to the hydrophobic characteristic of lipids. Fagundes et al. [54] discovered that a composite coating of HPMC and beeswax containing an antifungal compound had 2.5 times lower water vapor permeability than chitosan coating, resulting in effective weight loss and respiration rate reductions in cherry tomatoes. The composite coating on strawberry fruit reduced weight loss by 15–20%, while the chitosan coating reduced weight loss by 11% compared to uncoated fruit [55]. At 22°C, a composite coating based on wheat straw arabinoxylan and oat bran-glucan stearic acid ester applied to apples reduced weight loss by 1.2 times, compared to uncoated fruit. Aside from that, arabinoxylan and β-glucan in the stearic acid ester composite coating inhibited microbial contamination while preserving fruit sensory quality [56].

Plant oils have recently been used to combine with polysaccharides to form complex composite coatings. Olive oil, containing high levels of monounsaturated fatty acids and antioxidants, was emulsified with chitosan and alginate [57]. Both composite coatings of chitosan and alginate emulsions with olive oil reduced the fig fruit’s respiration rate and fungal decay. Furthermore, chitosan-olive oil coating and alginate-olive oil coating reduced the weight loss of figs by 15.69% and 22.66%. In addition, 1.0% sucrose fatty acid ester (SFE) slightly reduced the fruit softening and respiration rates of gac fruit during 16 days of storage at 25°C [58]. The sucrose moieties esterified to the fatty acid materials could provide more gas permeability to the SFE coating.

4.1.4.3 Protein and lipid composite coatings

Shellac has been used as a fruit coating for protecting fruits from water transpiration, gas, and microbial spoilage. However, shellac is unstable due to the polymerization of the structure’s hydroxyl and carboxyl groups. The electrostatic interaction between the negative charge of shellac and the positive charge of gelatin could protect the shellac’s active site, thus reducing the esterification process and enhancing the shellac’s stability. The composite film based on 60% shellac and 40% gelatin acted as an effective barrier to prevent moisture and gas movement, resulting in slight decreases in weight loss, firmness, and maintaining the quality of bananas at low temperatures for more than 30 days, compared with uncoated fruits [59]. Furthermore, the hydrophilic nature of whey protein from flaxseed was used to improve the hydrophobic property of bee wax by blending both to form a composite coating. The composite coating significantly reduced the water vapor permeability but increased the oxygen permeability of the whey protein isolate coating. The composite coating reduced the shriveling of the plum due to weight loss and delayed the softening of the fruit [60]. The stability of composite coatings depends on the amount of lipid added. A whey protein isolate-based composite coating containing 10% lipid showed fewer defects compared to the coating containing 5% lipid during the 15 days of storage at 5°C.

4.1.4.4 Composite coating containing growth regulators and plant extracts

Recently, the incorporation of plant residues and extracts as growth regulators, natural antioxidants, and antimicrobials into biopolymers to develop composite coatings has been increased to enhance the quality and shelf life of many fruits [61]. A pomegranate pericarp extract (PPE), containing enriched phenolics, was incorporated with chitosan-pullulan to formulate a composite coating for mangoes in cold storage. The chitosan-pullulan composite coating enriched with PPE effectively retained the fruit’s phenolic content, flavonoid content, and antioxidant activity due to the barrier property of the coating against moisture and gas transference [62]. Nguyen et al. [63] developed the composite film of passion fruit peel pectin combined with chitosan and then incorporated Piper betle L. leaf extract (PLE) for the preservation of purple eggplants. Despite the fact that the addition of PLE increased the water vapor permeability of the pectin/chitosan composite film due to the increased concentration of polar groups, the composite coating outperformed the control film against bacteria. Moreover, alginate combined with 0.45 mg.L⁻¹ longkong peel extract (LPE)-silver particles coating prevented severe browning, weight loss, and decay incidence during storage of longkong fruit during storage at 13°C and 90–95% RH by limiting the growth of fruit browning mostly via decreasing peroxidase (POD) and PPO activity [64].

The polyphenols in Cleistocalyx operculatus (Roxb.) fruit (CFE) were successfully extracted and added to chitosan and gum Arabic edible coatings for banana fruit. The chitosan-gum-CFE-based composite coating showed high effectiveness in improving the freshness of bananas stored at ambient conditions. The surface structure of the banana showed a wrinkle and crack structure (observed by scanning electron microscopy) for uncoated bananas and a smooth surface for bananas coated with the composite coating [65]. “Nam Dok Mai” mango (Mangifera indica L.) is usually encountered with postharvest decay due to anthracnose’s invasion during 25°C storage. Double layers of chitosan, which contains positive charges, and sodium alginate, which contains negative charges coated on “Nam Dok Mai” mango fruit delayed the peel color changes, and retarded the decay. The untreated control developed disease black spots on day six, and the symptoms worsened throughout storage. Interestingly, mangoes coated with both materials revealed 12.5% of the disease symptoms on day eight and then were steady until day 12 [24]. The chitosan- and κ-carrageenan-based composite coating was more effective in retaining the chlorophyll content and nutritional quality of dragon fruit when combined with GA₃ or MeJA pretreatment. This composite coating, combined with hot water treatment, controlled the diseases by regulating H₂O₂ accumulation and antioxidant enzyme activities and maintained the overall quality of dragon fruit [66]. Moreover, fruit dehiscence during ripening is a crucial postharvest problem in “Chanthaburi II” durian, a new hybrid cultivar of Thailand. Fruit coating with 1% chitosan+100 mg⋅L⁻¹ GA₃ effectively reduced the dusk dehiscence [67] (Figure 10). Gibberellic acid induces vegetative development in the plant parts that perform against ethylene responses.

Figure 10.

Fruit dehiscence in “Chanthaburi II” coated with 1% chitosan (B), 100 mg⋅L−1 GA3 (C), or 1% chitosan+100 mg⋅L−1 GA3, compared to the uncoated control (A) on day eight at 25°C.

4.2 Modified techniques

4.2.1 Nanotechnology

Nowadays, nanotechnology is considered the most promising innovative technique in food packaging due to its high safety and quality impact. Nanomaterials can be prepared using modified techniques. The materials show a higher effect than ordinary materials because of their smaller size and adhesive forces [68]. In food packaging, nanomaterials can be mixed in the polymer matrix to the increase gas barrier properties of films and coatings, or designed to be an active component in coatings.

4.2.1.1 Biopolymer nanocomposite coatings

Candeuba wax solid lipid nanoparticles (267–344 nm) were used as coatings on guava fruit, compared with xanthan gum coating at a low temperature. The coating based on 65 g⋅L⁻¹ solid lipid nanoparticles had lower permeabilities to O₂ and CO₂, responsible for reducing the respiration rate of guava fruit and maintaining the nutritional quality of the fruit for five weeks. Furthermore, 65 g⋅L⁻¹ of solid lipid nanoparticle-based coating retained total phenolic and ascorbic acid content by delaying the oxidative reaction in guava fruit. However, the coating based on 75 g⋅L⁻¹ of solid lipid nanoparticles resulted in anaerobic respiration, which caused physiological damage to the fruit [69].

Chitosan nanoparticles with a low molecular weight were successfully created and used as a coating for banana fruit. Cavendish bananas coated with chitosan nanoparticles showed uniform and smooth skin. A chitosan nanoparticle-based coating delayed the ripening of banana fruit by two to three days, compared to the uncoated control [70]. Chitosan nanoparticles added to a Moringa oleifera plant extract or aloe vera gel also had a significant impact on the firmness, ethylene rate, respiration, and total phenolic content of Cavendish bananas during storage. Banana fruit coated with this composite coating showed a lower weight loss and a higher score for consumer evaluation compared to the fruit coated with aloe vera or M. oleifera plant extract alone [71].

4.2.1.2 Nanoparticles incorporated in coatings

Antibacterial ZnO nanoparticles were combined in a κ-carrageenan solution for coating bananas (Musa sp., AAA group) during storage at ambient temperature. The nano-ZnO treatment significantly reduced the weight loss of banana fruit, while the κ-carrageenan-based coating reduced the fruit’s respiration. Furthermore, κ-carrageenan-based coating combined with nano-ZnO delayed peel color changes by maintaining chlorophyll content, reduced weight loss, retained firmness, and reduced fruit disease incidence [72] (Figure 11). In addition, 500 mg⋅L⁻¹ nano-ZnO was mixed in a 10% shellac solution to improve the postharvest quality of gac fruit (M. cochinchinensis Spreng) at 25°C. Throughout the 12 days, the nano-ZnO coating effectively inhibited disease infection and severity on fruit [37].

Figure 11.

Appearance of banana fruit treated with various coatings on day 10 at ambient temperature.

Hmmam et al. [73] developed carboxymethyl cellulose (CMC) and guar gum-based silver nanoparticles (AgNPs) coatings for “Seddik” mango fruit. Nanoparticles were formed at an average size of 84.8 to 213 nm for CMC-AgNPs and 61.7 to 132 nm for guar gum-AgNPs. The guar gum-AgNP coating significantly reduced the weight loss and respiration rate of mango fruit during storage, compared to the CMC-AgNP coating and uncoated fruit. The application of CMC- or guar gum-based AgNP coatings retarded the ripening and prolonged the postharvest life of mango fruit. Vieira et al. [74] fabricated an active coating based on hydroxypropyl methylcellulose (HPMC) and silver nanoparticles to extend the papaya’s shelf life. HPMC, glycerol, and silver nanoparticles were well dispersed into the nanocomposite film due to the chemical bonds between HPMC chains and AgNPs. AgNPs did not affect the water vapor, oxygen, or carbon dioxide permeabilities. The coating based on HPMC and 0.25% AgNPs retained color and firmness, reduced weight loss, and delayed the change to soluble solids of papaya fruit during storage.

4.2.2 Encapsulation

Incorporating bioactive chemicals into food items confers numerous advantages for food preservation and the development of functional foods. However, bioactive compounds may cause a quick loss of function or be evaporated through the air. Encapsulation with edible coatings is a possibly advanced technology that can mitigate the disadvantages of employing bioactive chemicals by storing the compounds and managing the release control [75]. Depending on the qualities and objectives of the bioactive chemical, various encapsulation methods may be utilized. Most of the target compounds are volatiles or essential oils. These methods are more successful than a direct application on the food surface because edible coatings prevent the agents from migrating away from the surface, retaining a high concentration of bioactive compounds where needed. The encapsulators should contain electrostatic charges as well as emulsifiers such as dextran, oligosaccharides, oligopeptides, glycerol, phospholipids, etc.

Many reports have studied the encapsulation of active compounds integrated into some coatings for fresh produce storage. Cinnamaldehyde exhibits antifungal functions but is easily evaporated into the atmosphere. Thus, using an inclusion complex method, cinnamaldehyde was encapsulated in β-cyclodextrin to produce a complex that can be used to preserve fresh-cut produce. The 25:70 cinnamaldehyde/β-cyclodextrin ratio demonstrated the highest encapsulation efficiency and capacity, whereas, in the first three hours, the 25:75 ratio had faster control release. Antimicrobial activity was tested against two strains of gram-positive (Staphylococcus aureus and Bacillus cereus) and two strains of gram-negative (Escherichia coli and P. aeruginosa) bacteria. β-cyclodextrin with encapsulated cinnamaldehyde inhibited all tested bacterial strains [76]. Much work has been done on encapsulating fruit aroma/flavor compounds through interactions with some polysaccharides such as starch. For example, flavor molecules (aldehydes, alcohols, terpenes, ketones, and fatty acids) can be wrapped in a left-handed single helical structure of starch (with high linear amylose). Alternatively, the interaction between starch and flavor compounds included polar interactions. The hydrogen bonds are formed between the hydroxyl groups of starch and flavor compounds [77].

4.2.3 Multilayer coating

Typical coating techniques may not cover the whole fruit correctly, causing improper permeability of gases and water vapor between the coated fruit and the atmosphere. Thus, efforts are being made to find multi-component edible coatings that are rationally developed to boost the overall performance of edible coatings. The technique of multilayer coating or layer-by-layer (LBL) electrostatic deposition is one method that uses thin multilayers to improve the performance of edible coatings. With the LBL method, coating characteristics and functionality may be efficiently controlled by alternating the deposition of polyelectrolytes with opposite charges onto fruit surfaces [78]. LBL coating forms integrated thin films by alternating layers of various materials carrying different charges or functional groups. The first layer often holds polyelectrolytes with positive charges; thus, the second polyelectrolyte layer should have a negative charge opposite the first layer. Each additional layer flips the polarity of the charge on the surface. Repeating these steps multiple times creates a multilayered LBL coating (Figure 12).

Figure 12.

The coating process with layer-by-layer electrostatic of opposite coating materials using a dipping and washing procedure (adopted from Costa et al. [78]).

The method of constructing LBL edible coatings allows for the combination of the best characteristics of various coating materials. For example, antibacterial polysaccharides can be combined with well-adhesive proteins, or active polysaccharides can be combined with polysaccharides that improve adhesion and texture. The feasibility of using the LBL edible coating method for complete surface coverage is elucidated by the contact angles of a coating droplet on the surface (Figure 13). Chitosan holding positive charges is in orange, whereas polystyrene sulfonate (PSS) having negative charges is in a clear drop. Each coating layer began with chitosan and ended with PSS. The contact angle of a chitosan droplet is less than 90° (flat shape) on all surface coatings, indicating high adhesive force between the layers. On the other hand, a droplet’s contact angle is over 90° (round shape), showing high cohesion between PSS molecules.

Figure 13.

Contact angle of a drop of chitosan (orange) and PSS (clear) on different chitosan/PSS coating layers.

Prior to storage at 25°C, a multilayer coating of oppositely charged chitosan (CTS: +) and polystyrene sulfonate (PSS: -) was treated on mature green “Nam Dok Mai” mangoes. Fruit coated at 3½ layers delayed ripening and reduced disease infection without off flavors, whereas fruit coated at 5½ layers had fermentation disorders at the end of storage [79]. Subsequently, allyl isothiocyanate (AIT), a natural antifungal compound, was integrated into the first layer of a multilayer coating of 0.5% CTS and 0.5% PSS. The concentrations above 0.15% AIT effectively inhibited, in vitro, the mycelial growth of Colletotrichum gloeosporioides. The multicoating delayed changes in weight loss, firmness, and antioxidant capacities of mango. Furthermore, mangoes coated with 0.5% CTS and 0.5% PSS + 0.15% AIT (Figure 14C) significantly reduced anthracnose disease severity in C. gloeosporioides-inoculated fruit (Figure 14A and B) [80].

Figure 14.

Disease growth of Colletotrichum gloeosporioides inoculated “Nam Dok Mai” mangoes multilayer-coated with 0.5% chitosan/ 0.5% PSS (B), and 0.5% chitosan/0.5% PSS + 0.15% AIT (C), compared to the uncoated control (A) on day 10 at 25°C storage.

Figure 15A shows an in vitro culture of Colletotrichum gloeosporioides on different PDA media with 1% chitosan, 500 mg⋅L⁻¹ prochloraz (a commercial fungicide), and some plant extracts. The fungal growth is wholly inhibited by 5000 μL⋅L⁻¹ galangal extract or 1000 μL⋅L⁻¹ sweet-flag extract. To reduce anthracnose during 25°C incubation, “Nam Dok Mai” mango fruit was treated with a double coating plus a sweet flag extract [24]. Mango fruit coated with 1% chitosan+3500 μL⋅L⁻¹ sweet flag extract for the first layer and 0.1% sodium alginate for the second layer effectively inhibited disease infection for nine days at 25°C (Figure 15B) [24].

Figure 15.

In vitro cultures of Colletotrichum gloeosporioides on different chemicals and plant extract PDA media on day 13 at 25°C (A), and “Nam Dok Mai” mangoes coated with a double coating of 0.5% chitosan+3500 μL⋅L⁻¹ sweet-flag extract and 0.1% sodium alginate (lower row), compared to the uncoated control (upper row) during 25°C storage (B).

5. Modulation of fruit coatings: Fields for future research

5.1 Advantages and limitations of fruit coatings

Fruits have a natural cuticle that varies depending on the type and stage of development. The lack of protective fruit cuticles is caused by improper maturity [81] or postharvest handling [82]. Coating as a part of packaging can add a polymer layer to the fruit surface. Aside from adding visual luster (Figure 16), fruit coating can extend shelf life and days to decay, reduce chilling injury and browning, and delay ripening by preventing water loss and creating modified environments inside the coated fruits.

Figure 16.

Visual appearances of uncoated gac fruit (left) and fruit coated with 8% shellac+nano-silver particles (right).

Some concerns have been raised, however, about fruit coating. There is a possibility that the concentrations and components of coatings cause the fruit to undergo anaerobic respiration due to the inefficiency of its respiratory and transpiratory systems [24]. It is possible that the fruit’s failure to ripen was due to unsuitable coatings. Some coating materials may cause a “plastic-like” film to form on the fruit’s surface (Figure 17A), or they may cause a toxic response in the surface tissues of the fruit peel (Figure 17B). In some cases, a combination of postharvest treatments is required for effective fruit quality preservation. Coating alone may not be sufficient for preventing water loss in the long-term storage of some fresh commodities, such as rambutans [83] that contain a high number of spinterns. The sucrose fatty acid ester coatings, for example, cannot maintain the buying quality of rambutans (Figure 18), so those additional postharvest handlings, such as MAP at low-temperature storage, would be intensively required. To effectively reduce postharvest disease during low-temperature storage, Nguyen et al. [84] used hot water treatment prior to the chitosan- and κ-carrageenan-based composite coating of dragon fruit.

Figure 17.

A “plastic-like” appearance of chili fruit coating (A) and peel toxicity tests of coating materials at different concentrations (B).

Figure 18.

Rambutan fruit coated with different coating materials of sucrose fatty acid esters (sucrose myristate (M-1695), sucrose palmitate (P-1670), and sucrose stearate (S-1670)) for 12 days at 13°C.

Some coating materials, particularly polysaccharide-based coatings, can become reversible during temperature fluctuations. Furthermore, in some cases in which the internal atmosphere is modified, fruit coating can reduce or inhibit the production of natural fragrances such as ester volatile compounds [85].

5.2 Developments in fruit coating research

Fruit coting is a practical method of handling fresh produce after harvest. The coating material chosen is determined by the intended use, coating types and techniques, and fruit types (high or low respiration rates). Nowadays, most fruit coating materials are edible and can be directly consumed safely. A trend in the industry is to find new coating materials, especially from agricultural waste [39, 86], that are flexible at various temperatures (up and down). As a result, advanced polymers should be used to improve coating efficiency, particularly in disease prevention. Another interesting issue is adding additional volatiles to coating materials, as fruit coating may reduce the release of the fruit’s aroma or volatiles. MA or hypoxic conditions could inhibit some volatile production, particularly ethylene, depending on volatiles such as ester compounds [85]. Encapsulating some natural volatiles into the coating could prevent microorganisms and entice customers’ aroma preferences as indicated in aroma sensory research [87].

Furthermore, in the final step, the product’s feasibility should be tested prior to retailing. Figure 19 from our collaborative research with the Food and Agribusiness, Trade, and Investment Queensland shows the cold chain logistics of “Murcott” mandarins treated with different coatings and MA storage shipped from Queensland, New Zealand, to Bangkok, Thailand. The alterations of temperature and relative humidity of the atmosphere during cold chain shipping and transport were recorded. The fruit quality was monitored in Thailand after arrival and subsequent storage in Bangkok in 2015, and the merchandise has since been sold on shelves in many modern trade stores in Thailand.

Figure 19.

The study of cold chain logistics of coated “Murcott” mandarins shipped from Queensland, New Zealand, to Bangkok, Thailand, in 2015.

6. Conclusions

The effectiveness of fruit coating is correlated with the intended purposes of use, which may include improving glossy, disease resistance, gas and moisture permeability, or some combination of these. Furthermore, the fruit’s nature, such as structure, types, maturity stages, and physiological metabolism, has different behaviors. The information is crucial for formulating the coating for each product. Some fruits are not conducive to standard MAP but lend themselves well to fruit coating. For instance, durian fruit has many tiny, sharp spines that can pierce plastic wrapping. Even with coating, the shelf life of some fruits, such as rambutan, is limited by the presence of many stomata on the peel. Coating materials and processes are now being researched to enhance them, including composite combinations, encapsulation of nanoparticles, and multilayer coating. Eventually, fruit coating is both environmentally beneficial and economically effective.

Acknowledgments

We appreciate the Postharvest Technology Innovation Center, Ministry of Higher Education, Science, Research, and Innovation, Bangkok, Thailand for some research data.