1. Introduction
High decay rates pose a significant challenge to the storage of fruits and vegetables leading to nutrient loss and the spread of microorganisms responsible for degradation [
1], leading to huge fresh produce losses due to inappropriate storage conditions all over the world every year. Therefore, innovative technologies such as edible coatings and films (ECF) and controlled atmosphere packaging are suggested as possible solutions for maintaining the quality of agricultural products during storage and shelf time [
2,
3,
4]. The spread of harmful microorganisms poses a severe threat to fruit quality and human health [
5], encouraging extensive research into the combined application of nano-biocomposites and edible coatings [
6,
7].
Many studies suggested possible environmentally-friendly alternatives as ECF matrixes, including but not limited to carbohydrate-based polymers such as chitosan (CS), starch and β-cyclo-dextrin (β-CD) [
4,
6,
7,
8]. For examples, chitosan can form a thin film on the surfaces of pulps, which might control the decay and keep the quality of fruits by inhibiting the growth of bacteria and fungi, and reducing their respiration rates [
4]. Moreover, starch has been considered as another suitable coating material due to its properties of wide availability, renewable and biodegradable biopolymer with low cost [
6,
8]. However, various challenges were proposed, such as weak barrier qualities, low mechanical characteristics, together with low antimicrobial activity [
9], which prompted the development and application of edible coatings containing inorganic nanofillers [
10].
The preparation and application of bio-nanocomposite packaging with polymeric materials and inorganic nanoparticles (NPs) for food preservation are expected to be developed in the future [
11,
12,
13]. Of the various types of inorganic NPs, titanium dioxide (TiO
2) are always used as an antimicrobial agent to coat different materials for further application in many food products [
13]. The photocatalytic behavior of TiO
2 is hugely relied on visible light irradiation, as well as UV light, which is responsible for activating its antimicrobial properties [
14,
15,
16]. Meanwhile, the electrocatalytic activity of silver nanoparticles (AgNPs) could provide the excellent antimicrobial property for its extensive application by incorporating into edible polymers as an active food packaging [
16,
17]. ZnO, in conjunction with antimicrobial activity can act as a permeation barrier for further application [
10,
11]. Furthermore, it was found by other researchers that various sizes of NPs had influence on the mechanical and physical properties, filtering ultraviolet (UV) light, as well as their antimicrobial activity [
11,
12].
Some interesting works on preparation and property characterization of ECF with NPs have been conducted by researchers [
3,
14]. As reported by Cano et al. [
14], TiO
2-based NPs were incorporated into the CS-based matrix to obtain different nano-biocomposites. Moreover, Xu et al. [
5] found that the composite coating with graphene oxide (GO), CS and TiO
2 NPs at the ratio 1:20:4 exhibited excellent antibacterial activities against
Aspergillus niger and
Bacillus subtilis, which might induce the cell membrane rupture. Andrade et al. [
18] also showed that the β-CD-coated-Ag NPs reduced more than 99%
E. coli and
Pseudomonas aeruginosa CFU compared to the control samples without silver addition. Shankar et al. [
19] indicated the incorporation of sulfur nanoparticles (SNP) could enhance the hydrophobicity, mechanical strength, and water vapor barrier property as well as antimicrobial activity of chitosan film. Furthermore, SNP capped with chitosan film exhibited the highest antimicrobial activity against
E. coli and
Listeria monocytogenes with complete sterilization within 6 h and 12 h, respectively.
Since not many studies are currently available involving the influence of ECF with NPs on the quality of fresh produce in storage. Yu et al. [
20] examined harvested jujube to determine the combined effect of CS and nano-silicon on its quality characteristics. The quality indexes including decay incidence, weight loss, the red index, and respiration rate, exceeded those of the control samples after storage for 32 d at an ambient temperature. As reported by Shi et al. [
21], by reducing the browning index, as well as obstructing polyphenol activity and the increase of malondialdehyde (MDA), the film of CS/nano-silica significantly extended the longevity of fresh longan. They indicated that the CS/nano-silica coating showed promise for the quality preservation of fresh longan during prolonged storage.
In recent years, the preparation and application of ECF and nanostructured materials are increasing for extending the shelf life of fresh agricultural products. These research works have also been summarized by researchers. The different types of bio-based materials, their applications as packaging materials, and future trends were reviewed by Sorrentino et al. [
22]. Moreover, Dutta et al. [
23] and ** fresh of fruits and vegetables [
1,
4]. The acid-based equilibrium features of CS is illustrated in
Figure 1, which denote a linear polysaccharide consisting of (1,4)-linked 2-amino-deoxy-β-
d-glucan with one primary amino group, as well as two free hydroxyl groups [
4,
28,
29,
30,
31]. While dissolving in diluted acidic solutions [
1,
4,
28], positively charged amino groups can successfully react with numerous negatively charged surfaces of polymer materials and cells [
23]. Due to its exceptional biocompatibility and film-forming properties, CS is widely applied as an edible coating film during the storage of fresh produce, and was deemed as generally recognized as safe (GRAS) in 2005 by the food and drug administration (FDA) according to the scientific procedures for use in food (FDA/CFSAN) [
1,
4].
Starch is a natural polymer with the properties of cheap, biodegradable, renewable and plentiful, and shows promise as a coating material [
32,
33]. It consists of amylose, amylopectin, as well as two macromolecules [
34]. As a linear polymer, amylose consists of glucose units connected with α-1, 4-bonds, while short linear chains branched onto longer chains with α-1, 6 linkages denote the extensively branched polymer amylopectin [
34,
35]. Despite starch films being devoid of color, taste, and flavor, as well as being translucent or transparent, they are defined by two primary challenges namely their high sensitivity to moisture and low mechanical characteristics [
32,
34,
35,
36,
37].
Shellac is a natural resin generated from insects, which is frequently employed by the food industry to glaze and treat the surfaces of citrus fruits and confectionary products to avoid damage during storage [
15,
38]. It exhibits a chemical structure with a large number of carbonyl and carboxyl groups [
38,
39]. Although shellac exhibits no antimicrobial effect, it is always used in various food applications as the non-toxic binder and the coating material [
15]. More importantly, shellac being approved by the FDA as GRAS can be utilized for indirect food contact. Furthermore, shellac films display exceptional adherence capacity to various surfaces with excellent rigidity, strength and high gloss [
15,
38].
Polycyclic glucose oligosaccharides known as cyclodextrins (CDs) are composed of 6, 7 or 8 glucopyranose units connected with β-1,4-glucosidic bonds referred to as α-, β-, or γ- cyclodextrin, respectively [
18]. The structure in
Figure 2 illustrates β-CD molecules consisting of a hydrophilic exterior with hydroxyl groups, as well as a hydrophobic central cavity. By forming inclusion complexes that are water soluble, low polarity and non-polar molecules displaying suitable shape and size are allowed to solubilize [
18,
40]. However, the cavity of β-CD is too small for encapsulating metal NPs. The simulated enveloped process of β-CD with NPs is shown in
Figure 2 and β-CD can be used as the nanoparticle stabilizer and can simply bind to the NPs via chemisorption through hydroxyl groups [
41].
Composed of a linear series of sulfated galactans, carrageenan is a hydrocolloid extracted from red algae. It is a water-soluble, natural product with the potential to be employed as a film-forming substance [
42]. The number and location of a sulfated ester on 3,6-anhydro-d-galactose residues determine the specific classification of the galactans. Similar to other hydrocolloid-based films, they display inadequate water-vapor-barrier properties [
42,
43].
Kefiran is a naturally occurring polysaccharide with characteristics that include gelatinization, film-formation and texturization. These properties are adequate for its utilization in food packaging [
44,
45,
46,
47]. Consisting of equal parts of
d-galactose and
d-glucose, kefiran is a water-soluble substance derived from glucogalactone and is responsible for enhancing milk gel viscosity, as well as its viscoelastic qualities [
44,
45]. However, insufficient mechanical strength and high moisture permeability denote some of the unique challenges of films produced from kefiran [
44].
Consisting of β-
d-glucose, linear carboxymethyl cellulose (CMC) is an essential, water-soluble cellulose derivative [
47,
48], which is cost-efficient, non-allergenic, transparent, non-toxic, easy to process and possesses preferable film-forming qualities [
48,
49,
50]. CMC is exceptionally susceptible to water vapor even though it displays sufficient capability to form adequate transparent films and is an effective inhibitor of oxygen and lipids [
50].
A galacturonan backbone with homogalacturonans (-1,4-galacturonan) and rhamno-galacturonans branched by -1,4-galactan and -1,3- or -1,5-arabinan chains form the pectin-like framework of the soluble soybean polysaccharide (SSPS), which is derived from the cell-wall substances of soybean cotyledons [
51,
52]. While SPSS is commonly used for producing edible films, which are water-soluble and colorless, some pertinent challenges include mechanical vulnerability, high gas patency, as well as elevated absorption capacity for water moisture [
53,
54,
55].
With cost efficiency and versatility among its attributes, polylactic acid (PLA) presents a new kind of biodegradable substance, which is manufactured from sustainable plant materials such as corn starch and sugar beets [
56,
57], and obtained FDA certification approving food-contact utilization [
58]. However, no antimicrobial effect is evident in the pure PLA film [
59]. As a natural, linear and aliphatic polyester, PLA films exhibit a high level of transparency and extremely water resistant [
59,
60,
61]. They are frequently employed to enhance food preservation and prolong the storage life of fresh produce [
57,
59,
60,
61].
As a popular substance obtained from the byproducts of corn-refining, zein is produced from corn gluten meal, and the films derived from it exhibit useful qualities such as biodegradable, transparent, as well as acting as an oxygen barrier [
62,
63,
64]. Zein protein (ZP) forms part of the protein bodies and is located in corn endosperm, and three specific fractions namely α, β and γ zeins were determined and isolated according to disparate levels of solubility in aqueous alcohol mixtures [
65].
Whey protein as a by-product of cheese is a yellow-green liquid and well-known material to form biodegradable films [
44,
66]. The protein content of whey protein concentrates (WPC) ranged between 35-90%, while that of whey protein isolates (WPI) exceeds 90%. Both of them can be used to produce biodegradable films that are transparent, flexible and devoid of flavor and color [
66,
67]. Whey protein film exhibits a fairly high moisture permeability, as well as unique mechanical characteristics while inhibiting oxygen penetration [
44,
66].
6. Future Trends of ECF with NPs
Many materials have been developed to prepare coating films for the storage of fruits and vegetables recently. The evaluated works on the properties of coating films prepared by various edible materials are important for their further application in food storage. Therefore, the main benefits and drawbacks of ECF with different materials, especially complex materials should be demonstrated, which might provide some important information for further using these NPs in the coating films.
In recent years, nanosized particles as antimicrobial agents incorporated into edible coatings were the subject of many studies. However, additional research involving the interactions between nanosized particles and coating materials are necessary. Moreover, further works about the effect of these NP addition on the properties of coating materials properties should be understood. These investigation results might provide insights regarding further improvements to their physical and antimicrobial properties for practical applications.
The antimicrobial activity of edible coatings containing nanosized particles is a critical factor for its evaluation and application, while its antibacterial activity and mechanism are popular research topics. Although the antifungal activity of ECF incorporated with NPs has also been investigated, few articles about its antifungal mechanism are published. Therefore, the antifungal mechanism, especially under the induction of UV or visible light requires further investigation.
The toxicity of edible coatings containing nanosized metal particles should be examined extensively since it possesses the potential to induce allergic reactions, especially in freshly cut products due to the release and migration of metal ions. On the other hand, the main quality factors of fruits and vegetables that might be improved or reduced by the application of various ECF with NPs should also be indicated. Therefore, further research on these subjects are essential.
It is vital for further research to focus on enhancing the consistency of composite coating properties, and to investigate its effects on the storage quality of fruits and vegetables to establish the standards for its preparation and application [
4]. Further studies involving cost-effective ways to prepare and apply ECF are necessary.
Following application, the NC can attach uniformly to the pulp pericarps, achieving stability even when cracks are present. Consequently, bacteria are possibly obstructed from gaining entry to these spaces. Further research should investigate the toxicity and safety of these coatings, the possibility of NPs migration from the coating to fruit, as well as the risks involved in human consumption [
87].