Bio-Coatings for Preservation of Fresh Fruits and Vegetables

: In response to increasing concerns over food waste and safety, and the environmental impacts of traditional conservation methods, this review aims to explore the potential of bio-coatings in preserving the freshness of fruits and vegetables. Our primary objective is to provide a comprehensive analysis of recent advancements in bio-coating technologies, detailing their beneﬁts in terms of enhancing food safety, prolonging shelf life, and reducing waste. This paper delves into various forms of bio-coatings, their applications, and their effectiveness in maintaining post-harvest quality. We further elucidate the underlying mechanisms that govern their preservation efﬁcacy. This review is intended for researchers, industry professionals, and policy makers who are interested in sustainable preservation alternatives and their implications for food security and environmental sustainability. By the end of this review, the audience will gain a thorough understanding of the current state of bio-coating technology and its prospects in the food preservation industry.


Introduction
Consuming fresh food is undoubtedly the best way to enjoy various flavors and nutrients, but their preservation helps to enjoy all these even out of season [1]. Food production and supply are not always in balance with the needs of the population. In the case of surplus production of fresh fruits or vegetables, which are perishable or semiperishable, it is important to store and preserve them to ensure a continuous food supply. Some fruits and vegetables cannot be grown in every type of soil and climate, so preserving food will aid importing them abroad. After harvest, the ripening and aging process can be delayed by different preservation methods [2], maintaining the taste and quality, and extending the sale/consumption ratio of food out of season. Preservation makes the product available on the market in a wide variety, and when there is no discrepancy between supply and demand, stabilization of food prices can also be observed. In broad terms, the preservation of fresh fruits and vegetables consists of handling and treating them to stop or slow down their decay or spoilage (contamination by microorganisms, loss of nutritive value, loss of flavor, change in texture, microbial and enzymatic decomposition) while ensuring a longer shelf life for the food. Fruits are susceptible to a variety of postharvest diseases caused by bacteria and fungi. These diseases can lead to visible decay and a loss of quality [3]. The loss of fruit quality during ripening and post-harvest is also influenced by several physiological changes. These changes can be categorized under factors such as maturity, respiration, ethylene production, and enzymatic reactions [4,5]. The stage of maturity at which fruits are harvested affects their quality and shelf life. When fruits are harvested too soon or too late, they may not have the flavor or texture that they should, and their shelf life may be shortened [6]. The process by which the fruit's stored organic elements are transformed into energy is called respiration. After the fruit is harvested, this process continues, causing it to lose weight and nutritional content. In  Figure 2 displayed the publication trend of scientific articles that were published on the topic of "Bio Coatings for Preservation of Fresh Fruits and Vegetables" (data were taken from the ISI Web of Science database, which can be found at www.webofscience.com). The scientific community's heightened interest in the field is reflected in the significantly increased number of publications written on the subject that have been published for the past decade.  Figure 2 displayed the publication trend of scientific articles that were published on the topic of "Bio Coatings for Preservation of Fresh Fruits and Vegetables" (data were taken from the ISI Web of Science database, which can be found at www.webofscience.com, accessed on 16 July 2023). The scientific community's heightened interest in the field is reflected in the significantly increased number of publications written on the subject that have been published for the past decade.
Given the contemporary challenges in food preservation and the emergent promise of bio-coating technology, this review seeks to provide a comprehensive update on the research and applications of bio-coatings in preserving fresh fruits and vegetables. Our primary objectives are as follows: (a) We aim to collate and analyse the most recent studies in this field to understand the current landscape of bio-coating technology, detailing the specific types of bio-coatings used and the types of fruits and vegetables they are applied to. (b) We plan to elucidate the underlying mechanisms that make these bio-coatings effective in preserving the freshness of fruits and vegetables, including their impact on moisture control, ripening processes, and microbial growth. (c) Considering the increasing importance of sustainability, we aim to assess the environmental impacts of these bio-coatings, including their production, usage, and disposal, as compared to traditional preservation methods. (d) Beyond the laboratory, we seek to evaluate the practical implications of bio-coating technology, such as its commercial viability, regulatory aspects, and consumer acceptance. Given the contemporary challenges in food preservation and the emergent promise of bio-coating technology, this review seeks to provide a comprehensive update on the research and applications of bio-coatings in preserving fresh fruits and vegetables. Ou primary objectives are as follows: a) We aim to collate and analyse the most recent studies in this field to understand the current landscape of bio-coating technology, detailing the specific types of bio-coat ings used and the types of fruits and vegetables they are applied to. b) We plan to elucidate the underlying mechanisms that make these bio-coatings effec tive in preserving the freshness of fruits and vegetables, including their impact on moisture control, ripening processes, and microbial growth. c) Considering the increasing importance of sustainability, we aim to assess the envi ronmental impacts of these bio-coatings, including their production, usage, and dis posal, as compared to traditional preservation methods. d) Beyond the laboratory, we seek to evaluate the practical implications of bio-coating technology, such as its commercial viability, regulatory aspects, and consumer ac ceptance. e) Finally, we aim to identify gaps in the current research and suggest potential oppor tunities for future studies in the field.
In achieving these aims, we hope to offer researchers, industry professionals, and policy makers a robust understanding of the present and potential future of bio-coatings in preserving fresh fruits and vegetables.

Materials
Bio-coatings can be enriched with essential nutrients, vitamins, or bioactive com pounds that contribute to the nutritional value of fresh fruits and vegetables. These nutri ents can be derived from natural sources or encapsulated within the bio-coating material ensuring their controlled release and preservation over time [22][23][24]. To protect spoilage causing microorganisms the coating composition may incorporate natural antimicrobia compounds such as plant extracts or essential oils (EO). This minimises the possibility o In achieving these aims, we hope to offer researchers, industry professionals, and policy makers a robust understanding of the present and potential future of bio-coatings in preserving fresh fruits and vegetables.

Materials
Bio-coatings can be enriched with essential nutrients, vitamins, or bioactive compounds that contribute to the nutritional value of fresh fruits and vegetables. These nutrients can be derived from natural sources or encapsulated within the bio-coating material, ensuring their controlled release and preservation over time [22][23][24]. To protect spoilagecausing microorganisms the coating composition may incorporate natural antimicrobial compounds such as plant extracts or essential oils (EO). This minimises the possibility of microbial contamination and deterioration, maintaining the freshness and quality of the packaged fruits and vegetables for a longer amount of time [23]. Due to volatile molecules that can cover up the original flavor of the treated fresh fruit or vegetable, bio-coatings based on essential oils can significantly impact sensory characteristics. Alternatives to conventional techniques could include combining numerous preservation systems or using an EO compatible with specific kinds of food [25][26][27][28][29][30].
The ability of bio-coatings to tailor their composition and functionality opens new possibilities for developing innovative packaging solutions that address the dual goals of food preservation and quality enhancement [31]. Figure 3 illustrates advantages of bio-coatings on shelf life, quality, and nutritional content of fresh fruits and vegetables: (1) Bio-coatings can help reduce the rate of moisture loss reduce the moisture loss rate and microbial growth rate, thereby extending its shelf life [31]. (2) Bio-coatings can help to preserve the appearance, texture, and flavor of produce during storage and transport. This can be particularly beneficial for high-value produce, such as berries, tomatoes, and citrus fruits [32]. (3) Bio-coatings can help to preserve the nutritional content of produce, such as vitamins and antioxidants, by reducing the rate of oxidation and degradation [33].
Coatings 2023, 13, 1420 5 of 34 (4) By increasing the shelf life and preserving the quality of food products, bio-coatings can reduce waste and increase the availability of fresh products for consumption [34]. and microbial growth rate, thereby extending its shelf life [31]. 2) Bio-coatings can help to preserve the appearance, texture, and flavor of produce ing storage and transport. This can be particularly beneficial for high-value prod such as berries, tomatoes, and citrus fruits [32]. 3) Bio-coatings can help to preserve the nutritional content of produce, such as vita and antioxidants, by reducing the rate of oxidation and degradation [33]. 4) By increasing the shelf life and preserving the quality of food products, bio-coa can reduce waste and increase the availability of fresh products for consumption By addressing these advantages and safety considerations, we can maximize th tential benefits of using bio-coatings on fresh fruits and vegetables while ensurin safety and satisfaction of consumers. All materials used in coatings must be food-g and safe for human consumption. Any additives, such as antimicrobials or antioxid must also be deemed safe. The potential allergenicity of the coating materials shou considered. If allergenic substances are used, these should be clearly labelled to pre allergic reactions. Coating materials and processes should comply with all relevant l national, and international food safety and labelling regulations. Despite their pote to carry antimicrobial agents, incorrect formulation or application of coatings could c an environment conducive to microbial growth. Ensuring that coatings are correctl plied under sanitary conditions is essential. Coatings should not adversely affect the texture, or overall sensory quality of the produce. Any potential impact on sensory a utes should be thoroughly tested [35][36][37].
Typically, biomolecules such as polysaccharides, proteins [38], and lipids are us create edible bio-coatings (Figure 4), which are then applied as a thin film to the su of the product to regulate moisture transfer, gas exchange, or the oxidation pro thereby increasing the product's shelf life [39,40]. By addressing these advantages and safety considerations, we can maximize the potential benefits of using bio-coatings on fresh fruits and vegetables while ensuring the safety and satisfaction of consumers. All materials used in coatings must be food-grade and safe for human consumption. Any additives, such as antimicrobials or antioxidants, must also be deemed safe. The potential allergenicity of the coating materials should be considered. If allergenic substances are used, these should be clearly labelled to prevent allergic reactions. Coating materials and processes should comply with all relevant local, national, and international food safety and labelling regulations. Despite their potential to carry antimicrobial agents, incorrect formulation or application of coatings could create an environment conducive to microbial growth. Ensuring that coatings are correctly applied under sanitary conditions is essential. Coatings should not adversely affect the taste, texture, or overall sensory quality of the produce. Any potential impact on sensory attributes should be thoroughly tested [35][36][37].
Typically, biomolecules such as polysaccharides, proteins [38], and lipids are used to create edible bio-coatings ( Figure 4), which are then applied as a thin film to the surface of the product to regulate moisture transfer, gas exchange, or the oxidation process, thereby increasing the product's shelf life [39,40].

Arabic gum African baobab pulp extract Blueberries Dipping
Arabic gum (10%, w/v), Better preservation of total phenols and total anthocyanins delayed the increase in total soluble solids better [74] Tragacanth gum with Eremurus extract Sweet cherries Dipping Tragacanth gum (7.5 or 12.5 g/L) Eremurus extract (10 or 12.5 g/L) Reducing post-harvest losses and increasing the shelf life [75] Guar gum with Ethanolic extract of Spirulina platensis Xanthan gum (XG) Apple 3D food printing 0.4% in proportion 2:1, 1:1, and 1:2 addition of xanthan gum (XG) and basil seed gum (BSG) Higher hardness, gumminess, stiffness and self-supporting ability [85] Carboxymethyl cellulose (CMC) combined with chitosan Citrus fruit Layer-by-layer 1.5% CMC and 1.0% chitosan solutions Enhanced fruit glossiness and appearance [86] Protein/guar gum and mango puree/calcium chloride Fresh-cut mango Cross-linked coating for coating solution-A, 400 mL of denatured protein solutions and 100 mL of guar gum solutions were mixed. As a plasticizer, 10% of glycerol (on a protein basis) was added to all the solutions. For coating solution-B, 500 mL of 2% calcium chloride (w/v) solution, which contains 30% of fresh mango puree (v/v) Decreasing deterioration of fresh-cut mangoes, enhancing shelf life, and keeping quality during low-temperature storage [87] Water-in-oil (w/o) emulsions maltodextrin and whey protein isolate (WPI)

Properties
Fresh vegetables and fruits are frequently coated with polysaccharide-based materials to extend their shelf lives. Their organized structure, which contains a hydrogen network, is why they are regarded as an efficient oxygen blocker. Starch, a polysaccharide that is abundant in nature and inexpensive, is made up of the linear polymer amylose and the highly branched polymer amylopectin. Aiming to achieve three essential objectives, namely: preserving physico-chemical and sensory properties, improving microbial stability, and increasing the post-harvest shelf life of fresh strawberries during storage, Aryou Emamifar and Sudabeh Bavaisi considered obtaining edible coating based on polysaccharide and nano-zinc oxide (nano-ZnO). They combined sodium alginate (SA) with nano-ZnO as antimicrobial agents and used low temperature storage. Similar to alginate in terms of applications, gellan gum is an anionic polysaccharide that is easily processed into transparent gels and has low water vapour permeability [89].
According to Petriccione et al. [90], strawberries stored at 2 • C that had been coated with a 1% or 2% chitosan solution had a much lower degradation rate than untreated strawberries of the same cultivar. Due to research by Jongsri et al. [91], high molecular weight Chitosan (HM-CTS) applied during post-harvest storage of 'Nam Dok Mai' mango can slow down deteriorative processes, keep the fruit's quality intact, and lengthen its storage life. Ma et al. (2013) studied the effects of CHI and oligoCHI on the resistance of peach fruit against brown rot brought on by Monilinia fructicola (M. fructicola) [92]. They demonstrated the effectiveness of these natural substances in controlling disease and maintaining peach fruit quality.
Several reports indicated a suitable way to improve the effectiveness of CHI-based coatings by adding plant extracts [93][94][95]. In 2003, Bautista-Banos et al. [96] investigated the in vitro fungicidal effect of CHI and plant extracts, alone and combined, on Colletotrichum gloeosporioides (C. gloeosporioides), which causes anthracnose on papaya fruits. They considered CHI an important natural product with good capacity in control of anthracnose on papaya fruit. It acts as a barrier that limits the penetration of C. gloeosporioides without maintaining a great firmness of fruit. They also observed a trend towards better firmness in the case of fruits treated only with papaya seed extract or with papaya seed extract combined with CHI. The same action was observed at the grapefruit seed extract (GSE). GSE is one of several bioactive chemicals generally recognized as safe for use in food. GSE is widely utilized as an antibacterial in the food industry, and its effectiveness as a coating material or in edible packaging films has been discovered very recently [97].
Van Thi Tran et al. [98], obtained in 2021 CHI-based coatings with different concentrations of tea seed oil (TSO) and monitored their antifungal capacity on Japanese pears (Pyrus pyrifolia Nakai) inoculated with a spore suspension of B. cinerea. The results demonstrated that incorporating different concentrations of TSO improved the in vitro and in vivo antifungal capacity of chitosan coatings.
Treatments with CHI particles combined with Zataria multiflora and Cinnamomum zeylanicum essential oils revealed a strong capacity to inhibit gray mold proliferation on strawberries, especially that of B. cinerea [99]. In 2022, Chun Yang et al. [100] studied CHI-based coatings with high phenolic extracts such as turmeric (TU) and green tea (GT) for strawberry preservation. Their study demonstrated that the post-harvest treatment of strawberries by coating with CHI and TU inhibited the proliferation of B. cinerea during 7 days of storage at 20 • C, and the one based on CHI and GT prolonged the antioxidant properties of strawberries post-harvest to 8 days at 20 • C, without significantly affecting the aroma and taste.
Natália Ferrão Castelo Branco Melo et al., in 2018 [101], prepared and evaluated coatings with edible CHI nanoparticles as possible treatment to improve the post-harvest quality of grapes. As a result, grapes were cleaned with sodium hypochlorite (1%), washed in drinking water, dried for two hours, and then submerged for three minutes in coating solutions containing chitosan nanoparticles the day after they were harvested. Grapes with and without CHI nanoparticle coating were kept either at ambient temperature (25 • C/12 days) or in the refrigerator (12 • C/24 days). Coatings with edible CHI nanoparticles delayed the ripening process of grapes, and implicitly decreased weight loss.
Recent studies showed that Aloe vera gel (AVG) having film-forming properties is easy to apply, has antifungal and antimicrobial actions and acts as a natural barrier to humidity and air [102][103][104][105][106]. AVG delays browning and ripening, delays firmness and weight loss and preserves phenolic content.
The world's most popular and delicious fruit, strawberries, are susceptible to microbial and fungal infection during storage. Coatings of banana starch-CHI with different concentrations of AVG show that AVG presence significantly reduces fungal decay, increasing the shelf life of strawberries up to 15 days of storage, a 5% lower weight loss compared to uncovered fruits, maintaining at the same time their color and firmness. The positive results are attributed to the cross-linking process between AVG molecules and starch [103]. Additionally, the effect CHI coatings with beeswax, such as CHI-beeswax-CHI coatings on the quality of Fragaria ananassa cv Camarosa strawberries, was studied in 2013 by Velickova et al. The authors showed a significant decrease in fruit senescence and weight loss while preserving the produce color and texture [107].
Cellulose derivatives are successfully used in obtaining coatings [108]. For example, the hydroxypropyl methylcellulose (HPMC) coating exhibits excellent properties, acting as effective lipid, oxygen, and carbon dioxide barriers, while having high water vapour permeability. Roberta Passafiume et al. [109], in 2020, analysed the effect of three types of edible coatings based on AVG, AVG with HPMC, and AVG with lemon essential oil (LEO) on the quality of fresh-cut kiwifruit. In the case of coatings based on AVG, respectively AVG with LEO, the quality of the Hayward kiwis was very well maintained, because the coatings functioned as a barrier to gas and slowed the growth of bacteria. However, the coatings based on AVG with LEO give the fruit a taste characteristic of LEO, and AVG used alone give the fruit an herbaceous taste, so that the coatings based on AVG with HPMC come with the advantage of not altering the natural taste of kiwi slices. Essentially, AVG combined with HPMC or LEO has demonstrated the ability to preserve the quality of freshly cut kiwi. For the preservation of cherry tomatoes and grapes, edible coating based on soy protein isolate (SPI), HPMC, and glycerine (GLY) shown much lower weight loss rates [110]. Additionally, for tomatoes preservation, in 2020, Stefania Frassinetti et al. [111] prepared bio-coatings based on gelatine (G) or G enriched with blueberry juice (GB) showing that GB preserved the nutritional quality of tomatoes. Poveronov et al. [112], in 2014, combined G and CHI in edible films or coatings and demonstrated inhibition of microbial proliferation on fresh-cut melon. Strawberries coated with CHI had a longer shelf life and preserved its qualitative qualities for a longer period than uncoated strawberries, according to a study published in the "Foods" journal [113]. In this investigation, different concentrations of CHI were applied to strawberries, and they were kept at 2 • C for nine days. The findings demonstrated that CHI coatings extended fruit shelf life by slowing weight loss, microbial development, and degradation. The study also discovered that the CHI coatings had no detrimental impact on the sensory characteristics of the strawberries, proving that the coating had no impact on the fruit's flavor or texture. The tomatoes were coated with alginate that had been enhanced with oregano essential oil (OEO) and stored at room temperature for two weeks, according to Pirozzi et al. [114]. They demonstrated how alginate coatings may extend the shelf life of tomatoes and improve their quality while being transported and stored. Moreover, to inhibit foodborne pathogens' growth and to prolong the shelf life of tomatoes, Das et al., prepared coatings based on carboxymethyl cellulose and cardamom oil [115]. The findings revealed the antibacterial effect of these coatings against Escherichia coli and Listeria monocytogenes and demonstrated the shelf life extension of tomatoes during storage. These coatings are being designed as an ecologically friendly and sustainable alternative for synthetic coatings and traditional preservation methods [116,117]. According to the authors, using alginate coatings in combination with natural additives can be a successful strategy for maintaining the quality and increasing the shelf life of apples.
Dordevici et al. [118] produced κ-carrageenan films by incorporating spent coffee grounds oil. κ-Carrageenan is a naturally derived biopolymer extracted from certain species of red seaweeds. It is widely used in the food industry for its gelling, thickening, and stabilizing properties. In the context of bio-coatings for preserving fresh fruits and vegetables, κ-carrageenan has shown promising results. Being derived from natural sources and biodegradable, using κ-carrageenan as a bio-coating aligns with the current shift towards more sustainable and environmentally friendly food packaging and preservation solutions. Despite these benefits, challenges remain. For instance, the functionality of κcarrageenan coatings can be influenced by the type of fruit or vegetable, storage conditions, and the specific formulation of the coating [119].
These examples illustrate the potential and the properties of bio-coatings to lengthen the shelf life of produce and maintain its quality in commercial and research settings. The continuous conduct of research and development in this field has the potential to assist in the discovery of novel coating materials and formulas that are superior, leading to an even higher level of success in the use of bio-coatings for the preservation of fresh fruits and vegetables.
Below, based on what we discussed above, we present a suggestion for classifying and integrating the mechanisms of different bio-coatings in inhibiting the deterioration of fruits and vegetables: (a) Bio-coatings can act as an effective barrier against moisture loss, a common cause of deterioration in fruits and vegetables. This can help maintain the firmness and prolong the produce's shelf life.
(b) Bio-coatings can also control the exchange of gases, such as oxygen and carbon dioxide, slowing down the respiration rate and ripening process in fruits and vegetables. (c) Certain bio-coatings can incorporate antimicrobial agents that inhibit the growth of spoilage and pathogenic microorganisms, further enhancing the shelf life and safety of the produce. (d) Some bio-coatings can also contain antioxidants, preventing oxidative browning and off-flavor development in fruits and vegetables. (e) Bio-coatings can slow down ethylene production and perception, effectively delaying the ripening and senescence of fruits. (f) By providing a protective barrier, bio-coatings can help in preserving the nutritional quality of fruits and vegetables, preventing the loss of vitamins and other essential nutrients during storage. (g) By maintaining freshness and reducing blemishes or discoloration, bio-coatings can also help in preserving the visual appeal of fruits and vegetables, which is an essential factor in consumer acceptance.
By classifying the mechanisms in this way, we can better understand how different bio-coating's function, and it allows us to design and formulate coatings that are best suited to specific types of fruits and vegetables, considering their unique post-harvest handling and storage requirements. This integrated approach will ensure the most effective use of bio-coatings in preserving the freshness, quality, and safety of fruits and vegetables.

Bio-Coatings Methods for Fruits and Vegetables Preservation
The choice of a coating method may depend on the type of fresh fruits and vegetables, the coating material, and the desired coating thickness. The application method should be carried out under hygienic conditions to prevent contamination and ensure the effectiveness of the coating. It is also essential to apply the coating evenly and that it adheres properly to the surface of the produce, maximizing its effectiveness. The coating material can be applied in its pure form or mixed with other ingredients such as antioxidants, preservatives, or antimicrobial agents, thus enhancing its effectiveness [120].

Nanoencapsulation
As mentioned before, for the food industry, as an alternative to synthetic preservatives, researchers have paid attention to the use of natural preservatives such as EO [121,122]. However, there are also some limitations, such as its volatility or its rapid oxidation. In this regard, for bio-efficacy in terms of antifungal, antimycotoxigenic and antioxidant capacity, new studies have applied nanotechnology for incorporating the EOs into the polymer matrix. By nanoencapsulation, the polymer matrix will capture the EO and act as a carrier matrix improving thus the efficacy of the EO [123].
Nanoemulsions can be used as edible coatings, creating a barrier on the surface of fruits and vegetables, reducing water loss, controlling respiration rate, and inhibiting microbial growth. Additionally, the small droplet size of the nanoemulsion allows for a more complete coverage of the surface, including small crevices and pores, and enhances the adhesion of the coating [124]. The composition of the nanoemulsion can be optimized to include bioactive compounds such as antioxidants, antimicrobials, or enzymes, which can enhance the preservation of fruits and vegetables. For example, nanoemulsions containing plant extracts, essential oils, or CHI have effectively inhibited the growth of pathogens and spoilage microorganisms, and delay senescence and ripening of fresh produce [125,126]. Nanoemulsions as edible coatings can be applied through various methods, such as spray coating or dip coating. The choice of the method will depend on factors such as the type of fruit or vegetable being coated, the desired shelf life, and the properties of the nanoemulsion [12]. Nanoemulsions represent a promising strategy for the preservation of fresh fruits and vegetables, and they have the potential to reduce food waste, enhance food quality, and provide added nutritional value. However, further research is needed to optimize the composition and application of nanoemulsions as edible coatings, and to evaluate their safety and consumer acceptance.
In general terms, this nanotechnology offers protection over a long period, ensuring maintenance of firmness and nutritional and organoleptic characteristics. In 2015, M. Sessa et al. [127] concluded in their research that the modified CHI with nanoencapsulated LEO prolong the shelf life of rucola leaf with no significant effect of the organoleptic characteristics of the vegetable in comparison to CHI coating or EO alone.
In-Hah Kim et al., in 2013 [128], developed highly stable nanoemulsions-solutions based on Carnauba wax and LEO for coating plums. The results revealed their ability to inhibit Salmonella typhimurium (S. typhimurium) and Escherichia coli (E. coli) O157:H7 contamination, maintaining the firmness of coated fruits and reducing respiration rates during storage.

Microemulsion Formulation
Formulation of the microemulsion involves mixing the coating materials in a suitable solvent system to form a microemulsion. This typically involves using an emulsifier (a surfactant or co-surfactant) to ensure stability of the microemulsion. The properties of the microemulsion and the final coating will depend on the emulsifier used and the proportion of the oil-to-water phase. To ensure the stability of the microemulsion, an emulsifier (a surfactant or co-surfactant) is often used [129]. Microemulsion is applied to the surface of the fresh fruits or vegetables. Several techniques, such as dipping, spraying, or brushing, can be used to accomplish this. Following application, the fruit or vegetable is typically let too dry at a specified temperature and relative humidity [130].

Microspinning (Electrospinning)
Is a flexible method for producing ultra-thin fibers from a variety of materials. Electrospinning is used to create nanofibers, which are then used to coat fruit through dipping, spraying, painting, etc. Electrospun nanofibers have a very high surface area-to-volume ratio, making them perfect for thinly and uniformly coating fruits and legumes [131]. Because the fibers are so fine, they can go inside the fruit or legume's surface pores and provide a more thorough coating than other techniques. One benefit of electrospinning is that depending on the number of layers used, coatings can be made with various thicknesses, from a few nanometres to several microns [132,133]. This method requires specialized equipment, making it more expensive and difficult to scale up for commercial production. Although electrospinning has significant potential for use in food applications, it also comes with several difficulties. Electrospun mats are sensitive and may not fully respond to unusual shapes, making applying them directly on fruits or vegetables difficult. Some edible polymers might also need particular electrospinning settings or solvents that are not always food-grade or may present other difficulties [134].
Less common but still used are the following briefly mentioned methods:

Melt Extrusion
In this method, the biopolymer is heated until it melts and then extruded or pressed into a thin film. This method is often used with lipid-based coatings, such as waxes or resins [135].

Coacervation
This method is often used to encapsulate active ingredients within a bio-polymer coating. In coacervation, the polymer is dissolved in a liquid, and then a second, immiscible liquid is added. The polymer separates from the solution and forms tiny droplets, which can be collected and dried to form a coating [136].

Phase Inversion Method
The phase Inversion method is used to produce porous coatings. The biopolymer is first dissolved in a solvent, and then a non-solvent is added. This causes the polymer to precipitate out of the solution and form a porous structure [137].

Methods of Coating Application in Fresh Fruits and Vegetables
Some standard methods for applying bio-coatings ( Figure 5) to fresh fruits and vegetables are presented below: Coatings 2023, 13, x FOR PEER REVIEW 18 of 33 Figure 5. Examples of some bio-coating methods for fresh fruits and vegetables (the graphical representation was made using the BioRender program by the Toronto-based BioRender Corporation).

Spraying Method
The bio-coating material it can also be applied to the surface of the fruits and/or vegetables using a spray nozzle by Spraying method [140]. Spraying (using spray nozzles) is the standard method used by the industry in packing lines to coat fruit in many different types of fruit (citrus, apples, tomatoes, pears, etc.). The biopolymer is dissolved into a suitable solvent to create a solution. The solution is then atomized into a fine mist using a spray nozzle, which is directed onto the surface of the fruit or vegetable [141]. One of the advantages of spraying is that it is a fast and efficient method that can be easily scaled up for commercial production. Adding antimicrobial agents or antioxidants to the biopolymer solution can help extend the coated produce's shelf life. However, spraying method also has some limitations [142].

Spreading Method
Another method for applying bio-coatings is the Spreading method. This method is used for producing with a small surface area, such as mushrooms or grapes [143]. The coating material is applied to the surface of the fruits and/or vegetables using a brush.

Dipping (Dip Coating) Method
In the Dipping (Dip coating) method the fruit and/or vegetable is immersed in a solution of the coating material for a predetermined amount of time, then removed and allowed to dry. The fruit or legume is then removed from the solution and excess solution drains off. Dip coating is commonly used for fruits and vegetables that have a relatively smooth surface [31]. It may not be suitable for fruits or legumes with delicate skins or membranes, as they may be damaged during immersion. Dip coating it is a simple and inexpensive method that can be easily scaled up for commercial production [138]. Adding antimicrobial and/or antioxidants extract to the biopolymer solution can help extend the shelf life of the coated fruit and/or vegetable [139].

Spraying Method
The bio-coating material it can also be applied to the surface of the fruits and/or vegetables using a spray nozzle by Spraying method [140]. Spraying (using spray nozzles) is the standard method used by the industry in packing lines to coat fruit in many different types of fruit (citrus, apples, tomatoes, pears, etc.). The biopolymer is dissolved into a suitable solvent to create a solution. The solution is then atomized into a fine mist using a spray nozzle, which is directed onto the surface of the fruit or vegetable [141]. One of the advantages of spraying is that it is a fast and efficient method that can be easily scaled up for commercial production. Adding antimicrobial agents or antioxidants to the biopolymer solution can help extend the coated produce's shelf life. However, spraying method also has some limitations [142].

Spreading Method
Another method for applying bio-coatings is the Spreading method. This method is used for producing with a small surface area, such as mushrooms or grapes [143]. The coating material is applied to the surface of the fruits and/or vegetables using a brush.

Vacuum Infusion
Vacuum infusion is used for the fresh fruits and vegetables with a porous structure, such as melons or cucumbers [144]. The biopolymer solution is introduced into the vacuum chamber, where it is absorbed into the pores of the fruits and vegetables [145] but applying this method to food such as carrots or eggplants, could improve the organoleptic characteristics and nutritional values, increasing the storage time and preserving their tissue hardness. This can be particularly useful for preserving fruits and legumes with thick skins or membranes that require a deeper coating [146]. The vacuum infusion is not suitable for fruits or vegetables with delicate structures, as they may be damaged during the vacuum process [147].

Solution Casting Process
Solution casting for applying bio-coatings involves dissolving the biopolymer in a suitable solvent and then casting it onto the surface of the fruit or vegetable [148]. Solution casting is used to obtain stand-alone films in a Petri dish or similar. Dipping and spraying are the most common application methods in fruits, and the solvent (mostly water) evaporates as the fruit dries. Care should be taken when choosing the solvent to dissolve the biopolymer since it must be compatible with both the biopolymer and the fruit or vegetable that will be coated. The solvent evaporates once the solution is thrown over the fruit or legume's surface, leaving behind a thin layer. By varying the biopolymer solution's concentration or the number of coating layers used, the coating's thickness can be managed [141,148]. Solution casting has the benefit of being an easy, inexpensive process that is simple to scale up for commercial production [149]. Additionally, depending on the application technique, the coating's thickness and uniformity could vary, which may impact the coating's effectiveness [150]. In conclusion, solution casting is a promising method for obtaining bio-coatings for the preservation of fruits and vegetables, and it has the potential to improve food quality and reduce food waste [151].

Multilayer Coating (Layer by Layer)
Another process for applying bio-coatings is Multilayer coating (Layer by layer) method. The procedure consists of applying a thin layer of edible bio-based coating to the fruits' and vegetables' surfaces [152]. The fruits and vegetables are coated with multiple layers of coating using the multilayer coating method. Each layer has a specific purpose, such as preventing infectious agents from penetrating the fruit and vegetables or limiting the amount of oxygen that reaches [153]. The coatings are typically made from natural materials, such as CHI, cellulose, and pectin that are safe for human consumption. The multilayer coating method has been shown to be effective in extending the shelf life of a variety of fresh fruits and vegetables, including apples, strawberries, tomatoes, and cucumbers. The method is also environmentally friendly, as it reduces the need for chemical preservatives and packaging materials [154].

Cross-Linked Coating Method
Cross-linking is a process in which polymer chains are linked together via chemical bonds, creating a three-dimensional network of interconnected chains. This technique is used in the formulation of coatings to improve their performance, as the cross-linked structure typically provides improved mechanical strength, water resistance, and stability [83]. In the context of preservation of fresh fruits and vegetables, the Cross-linked coating method can offer several advantages. Cross-linked coatings have tighter polymer networks which can reduce the permeability of gases (such as oxygen and carbon dioxide) and water vapor, slowing down the ripening process and moisture loss in fruits and vegetables. These coatings typically have greater mechanical strength and resistance to abrasion or damage, ensuring that the protective layer remains intact during handling and transportation. Cross-linked coatings are less likely to dissolve or degrade, making them more stable and durable for longer storage periods. Cross-linked coatings can also be used as a matrix for encapsulating and releasing active agents, such as antimicrobial and antioxidant compounds. The cross-linked structure can provide controlled release of these compounds, enhancing the shelf life and safety of the produce. For instance, chitosan, a naturally derived biopolymer commonly used in edible coatings, can be cross-linked using agents such as genipin [155].
However, it's important to note that the safety of the cross-linking agents and the potential migration of substances from the coating to the food should be thoroughly assessed to ensure food safety. The coatings should always comply with relevant food safety regulations and standards.

D Food Printing Method
3D food printing is a burgeoning technology that has the potential to revolutionize the food industry. It involves the use of a 3D printer to deposit materials layer by layer to create a food product with a specific structure, texture, and potentially, nutritional profile [156]. 3D printing technology could be used to create precise, uniform edible coatings on fruits and vegetables, enhancing their shelf life and quality. 3D food printing allows for the incorporation of various ingredients into the food structure. This could potentially be leveraged to include natural preservatives or antimicrobials in the printed food, helping to extend the shelf life of fresh produce. 3D printing can help in reducing food waste by allowing for the creation of food products from produce that would otherwise be discarded due to aesthetic imperfections. This does not preserve fresh produce per se, but it can help in maximizing the utility of harvested fruits and vegetables. While still a relatively new concept, there is potential for 3D printing to create biodegradable packaging for fruits and vegetables. This could be designed to provide protection and potentially incorporate preservation techniques (e.g., modified atmosphere packaging) [85].

Innovations in the Development and Application of Edible Bio-Coatings for Fresh Fruits and Vegetables
The food processing industry is part of what concerns the world economy. Innovations in developing and applying edible bio-coatings for fresh fruits and vegetables are in continuous evolution (Figure 6).

Fruits and Vegetables
The food processing industry is part of what concerns the world tions in developing and applying edible bio-coatings for fresh fruits and continuous evolution ( Figure 6).

Smart Technology (Artificial Intelligence (AI))
Many organizations in the food industry recognized the need for smart technology such as Artificial Intelligence (AI) [157] to solve fruits and vegetable supply problems, maintain food quality and safety, increase profits, and reduce food waste. Research studies [158,159] demonstrated that AI-assisted drying process of fruits and vegetables can extend the freshness period as well as their nutritional values. To check for internal rot or pesticide residues, and to determine the shelf life of a product, Software can be used [160]. It will detect defects in the fresh products, even when they appear perfect to the human eye. The food industry is known to have a slow transition to Industry 4.0 [161] that aims [162] to achieve efficiency between supply chain members, to enhance the connectivity between human and physical systems, to improve traceability, quality, and food safety, following the impact on the environment. Therefore, it is expected that green technologies such as harvesting, extraction, processing, and pasteurization to be applied in the coming years in combination with Industry 4.0 technologies such as AI [163], smart sensors or robotics [164] to ensure a healthier food future [165,166]. AI technology can always be improved to detect specific causes of disease in fruits and vegetables, helping producers to increase production and profits. Moreover, Society 5.0, which the Japanese government introduced, is also in progress [167].

Digital Technology
As a digital technology, 3D printing [168,169] has applications in various industries, including food processing. Additionally, 4D [170] printing is applied. There are new challenges involving the use of materials based on plants for 3D food printing by extrusion. With the advancement of 3D printing technology, it might be possible to create custom bio-coatings with precise thickness and uniformity, or even multiple layers of different biopolymers. For example, a 3D printer could be programmed to apply a thin, even layer of chitosan coating onto fresh fruits or vegetables, potentially reducing microbial growth and prolonging shelf life. As mentioned before alginate coatings [45,46] can help to maintain the structural integrity of fruits and vegetables, reducing mechanical damage and water [171]. A 3D printer could be used to apply this coating in a controlled and uniform manner. As in the case of pectin, a complex carbohydrate found in the cell walls of fruits and vegetables [172], a 3D printer could apply a precise layer of bio-coating to the fresh fruits and vegetables. These are only some examples of the bio-based materials that could potentially be used in 3D printed coatings for fresh fruits and vegetables. The exact composition of the coating could be customized based on the specific needs of the produce, including its ripeness level, the environmental conditions it will be stored in, and how long it needs to be preserved.

Solid Natural Residues Use
Current research studies focus on finding innovative models in the circular economy, including the use of edible coatings in the food processing industry. For this purpose, it is necessary to inform consumers [173] about the advantages of eating food preserved by edible bio-coatings in reducing waste and pollution by optimizing the resources needed and reusing them [174]. There are concerns regarding the processing of solid residues remained after squeezing fruits, vegetables or plants into fibrillated cellulose that can be used to obtain edible bio-coatings for fresh fruits. Luana Amoroso et al. [175] studied the use of waste resulted from squeezing fresh or stale carrots to produce cellulose nanofibers. These nanofibers suspension films sprayed on the surface of bananas led to an important delay in enzymatic browning of the peels, so this research could be considered significant in achieving sustainable food materials.

Plant Dyes Use
Bio-coatings for preserving fresh fruits and vegetables using plant dyes can also offer potential preservation benefits. Plant dyes, derived from natural sources, can provide color and additional functional properties to bio-coatings [176].
Plant-based dyes that are safe for consumption, derived from sources such as fruits, vegetables, or herbs, can be chosen. Examples include beetroot extract (for red/pink color), turmeric extract (for yellow color), spinach extract (for green color), or purple cabbage extract (for purple color) [177]. A bio-coating formulation would be prepared using a suitable edible matrix, such as starch, pectin, or alginate. The plant dyes would be integrated into the matrix during the formulation process. These would give the produce vibrant colors, enhancing its aesthetic appeal and maybe enhancing consumer preference. Natural antioxidants are present in many plant colors, which can help lower oxidative damage and delay spoiling processes. These antioxidants may extend the shelf life and freshness of the fruits and vegetables. Together with the plant pigments, the bio-coating creates a layer of defence on the surface of the produce that serves as a physical barrier against outside contaminants, moisture loss, and light exposure. By performing so, you can preserve quality and stop microbial growth [178,179]. In addition to controlling gas exchange, the bio-coating made of plant pigments can also control respiration rates and ethylene generation. By doing so, the ripening process can be slowed and the fruits and vegetables' shelf life can be increased [180]. To achieve the expected preservation effects, maximizing both the bio-coating formulation and the amounts and combinations of plant pigments would be necessary. Additionally, it's essential to confirm that the plant dyes used comply with food safety regulations and are safe for consumption [135].

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) Technology
Another innovative method to create bio-coating for preserving fresh fruits and vegetables is the use of clustered regularly interspaced short palindromic repeats (CRISPR) technology. Using CRISPR technology, probiotic microorganisms with improved antibacterial capabilities could be created [181]. These could be incorporated into a bio-coating for fresh fruit preservation resistance in crops [182]. Researchers could use CRISPR to precisely edit the genes of a crop plant to enhance its resistance to specific diseases or pathogens. Targeting and modifying key genes associated with disease susceptibility can potentially make the plant more resistant to infections. The edited genes or CRISPR components could be incorporated into a bio-coating formulation. The bio-coating would act as a protective layer applied to the surface of the seeds or plant tissues. The bio-coating would adhere to the seeds or plant surfaces and gradually release the CRISPR components into the plant cells over time. This controlled release would allow for the plant to efficiently uptake of the CRISPR machinery. Once inside the plant cells, the CRISPR components would activate or deactivate specific genes involved in disease resistance. For example, the CRISPR system could target and suppress favourable genes for pathogen invasion or activate genes that enhance the plant's defence mechanisms. By introducing these genetic modifications using CRISPR, the crop plant would potentially exhibit enhanced resistance against specific diseases or pathogens, thereby reducing crop losses and improving overall yield and quality. The specific implementation of CRISPR technology in bio-coatings for disease resistance is still an area of ongoing research [183,184].

Macrophages Use
Additionally, the application of macrophages such as CRISPR technologies is constantly evolving. Macrophages are immune cells that are typically associated with the immune response and tissue repair in living organisms. Using macrophages for preserving fresh fruits and vegetables is not a commonly explored approach. Their application in food preservation is not well-established, but macrophages cell membrane-based nanoparticles is a new promising platform for novel bio-coatings development [185].
Due to the food industry's significant adverse effects on the environment, adopting more sustainable practices that boost both productivity and sustainability has become necessary. Bio-based edible coatings, which are essential to meeting the demands of the market today, are a prime illustration of this. PolyNatural is an outstanding example of how the food industry is now driven fundamentally differently because of the agricultural sector and new technologies. Given the rising demand for organic products every year, this scale-up is essential to the Agtech industry. In 2021, the value of organic goods sold in the United States alone exceeded USD 9 billion, an increase of 5.5% from the previous year. Additionally, 12% of the nation's overall agricultural product market comprised organic goods, reflecting an increasing trend [186].
Many scale-ups and start-ups are working on non-chemical methods of extending the shelf life of fruits and vegetables. The Global Startup Heat Map indicates that 289 companies are now leading the way in the food industry with their ground-breaking and environmentally friendly solutions. Five scale-ups from this limited group deserve special attention for their natural bio-coating advances. StartUs Insight [187] has helped to make this list of the best innovative firms in natural preservatives and coatings possible. More than 2 million startups and scale-ups worldwide are analysed by this platform using big data and artificial intelligence. We are discussing important developments that will help the fruit export business grow and adopt a sustainable approach to food production, a market where maintaining product shelf life is essential to protect investments and maintain positive bilateral relations.
Chinova Bioworks is developing a natural preservative called Chiber derived from white mushroom fibers [188] Chiber's applications are not limited to fresh fruits and vegetables; it may also be utilized in beverages and dairy products that originate from vegetables, so organically increasing the shelf life of a wider variety of food kinds.
The level of innovation of Mori [189] that has been applied to the company's flagship product, which is a coating for fresh fruits and vegetables produced from silk protein, makes it, without a doubt one of the most appealing scales-ups now available.
Because it prevents microbial development, dehydration, and the oxidation of food, this solution can double the amount of time that fruits and vegetables can be stored at room temperature before going bad.
An Israeli startup company [190] has developed a coating for fresh fruits and vegetables that is entirely comprised of natural ingredients and improve the period they may be preserved without using any chemicals. Since the solution offered by Sufresca is derived from plants and is odourless, colorless, and tasteless, it does not change the flavor of the products, nor does it affect the color of the items when it works to protect them from dehydration and fermentation.
Encapsulation of natural flavors in all natural materials is the product of a Swiss company [191] that just recently started. It is a powder that encapsulates the flavors of food and then releases them once the meal is consumed in an environmentally responsible manner. This is a significant advancement for the food industry since it raises overall product quality, reduces the amount of money spent on processing, and, as a result, enhances the profitability of businesses operating in the area.
With its Shel-Life ® solution [192], a food-grade organic coating that increases the shelf life of fruits and vegetables, it is currently one of the top scale-ups. Shel-Life ® is a 100% organic coating that stops fruits and vegetables from drying out, preserving their freshness for longer while being kind to the environment. It is built of natural polymers and vegetable lipids. In other words, food that is fresher and of greater quality is obtained, maintaining its integrity, and promoting consumer health. Since PolyNatural's Shel-Life ® was implemented in January 2022, it has helped save more than 840 tons of fruit from going to waste. This shows how valuable this solution is for reducing the carbon footprint of food production.
With these innovations in the development and application of edible bio-coatings for fresh fruits and vegetables it is now possible to extend the shelf life these without adding chemicals or petroleum derivatives. This helps the food industry advance onto a more sustainable way of accomplishing issues.

Challenges and Limitations of Bio-Coatings for Produce Preservation
Despite their numerous benefits, bio-coatings pose challenges that need to be addressed to ensure their wide adoption. One of the major factors hindering their broad-scale implementation is the cost of manufacturing and raw materials. Compared to conventional coatings, the production and formulation of bio-coatings can be more expensive, thereby limiting their cost-effectiveness. The industry must focus on the development of more cost-efficient production processes and sourcing strategies for bio-coating materials. Another aspect affecting the use of bio-coatings is their availability. The raw materials needed for these coatings may not be readily available in certain regions or countries. This lack of access can serve as a barrier to their widespread use. Strategies need to be developed to ensure that these materials can be sourced or substituted locally to facilitate their use in different parts of the world. If the bio-coatings have a short shelf life, they may not be as effective as other methods in maintaining the freshness of food over extended periods. The efficacy of bio-coatings in maintaining the quality of food may be interdependent on a few elements, including the kind of product, the features of the fruit or vegetables, the storage circumstances, as well as the formulation and surface properties of the coating [31,141,193]. The successful application of bio-coatings also relies heavily on controlling storage conditions, composition of the covering film, and the process of its deposition. When these elements are optimized, food preservation can be significantly enhanced. While commercial applications of bio-coatings are currently limited, ongoing research into integrating multiple naturally occurring components is expected to improve their functionality and suitability [194]. Bio-coatings have the potential to reduce waste, contributing to environmental protection significantly. However, certain criteria must be met to fulfil their intended purpose of enhancing food quality, functionality, and safety over extended periods. These include cost-effectiveness, stability, non-stickiness, the inclusion of functional ingredients, good adhesion, and no impact on sensory properties. They should also control moisture content without affecting the appearance of the fruits and vegetables. They must easily obtained and inexpensive; uniform and stable during production and storage; non-sticky when handling the fruit or vegetable; to contain functional ingredients such as antimicrobial and antioxidant agents; to have good adhesion to surface; to not affect the smell, aroma, taste, and texture of fruit and vegetable; to maintain a desired moisture content by controlling the water migration in the fruit and vegetable; and to not affect the appearance of the fruit and vegetable [195].
Finally, the role of surface characteristics cannot be overlooked in the development and application of bio-coatings. Understanding these characteristics is essential to enhance food preservation and film adhesion. While wetting is necessary for appropriate adhesion, it is not sufficient. The coating should be designed to maximize surface contact with the product for optimal wettability. However, achieving good wettability does not automatically translate into good adhesion, suggesting that other factors also play a role in this process. Further studies are needed to understand and optimize these factors It [196,197].

Conclusions
In conclusion, our comprehensive review of the current research shows that biocoatings present an innovative and environmentally friendly approach to preserving the freshness of fruits and vegetables. These sustainable solutions extend the shelf life of these products and reduce dependency on traditional packaging, which often contributes to environmental pollution. By enhancing food safety, bio-coatings could also play a pivotal role in reducing foodborne diseases, a significant concern worldwide. While the application of bio-coatings is promising, more research is necessary to understand their long-term impacts and their interaction with different types of food products. Specifically, further research should optimize bio-coating compositions to cater to a wide range of fruits and vegetables with different preservation needs. Additionally, studies should investigate consumer acceptance of bio-coatings, as public perception and acceptance will significantly influence the adoption rate of this technology.
Looking towards the future, we predict a growth in bio-coating technologies integrated with smart and active packaging solutions. These might include indicators for freshness or ripeness, or even release of antimicrobial agents over time. Additionally, the future might see bio-coatings that preserve the fruits and vegetables and enhance their nutritional content. Commercial scalability of these technologies, while maintaining cost-effectiveness, will be crucial for widespread adoption.
The exploration and implementation of bio-coatings are a testament to our continual strive for sustainable solutions in the food industry, reflecting our growing awareness of and responsibility towards our environment. With the ever-present challenge of feeding an increasing global population, bio-coatings stand at the frontier of innovative, sustainable, and effective food preservation methods.