CURRENT TRENDS IN EDIBLE COATINGS
From the past few decades, antimicrobial components have been incorporated in the edible coatings to prevent the spoilage of fruits, thus increasing their shelf-life (Valencia-Chamorro et al., 2011). But now-a-days, nanoparticles and nanoemulsions contribute a lot to improve the barrier functions of edible coatings. Nanotechnology, today is the fastest emerging attractive and promising field in the food industry. It is also like a magical spell in the edible coatings. Nano-based structures have increased surface area, thus allowing the greater homogeneity and distribution on the fruit surface and pores (Rao and McClements, 2012). Solid lipid nanoparticles are the submicronic colloidal systems employed to trap and deliver the hydrophobic active components thus offering huge potential in the fruit preservation in the future. In addition, the microbial polysaccharides-based coatings are also promising substances to improve the functionality of coatings. They are today the most highlighted substrate for industrial applications due to their easy availability, renewable resources, less manufacturing cost, high efficiency, bio-compatibility, and eco-friendly nature. All these advantages of microbial polysaccharides accelerated their application in the edible coating formulations. A novel teclmique, called as herbal edible coating, is recently developed and creating miracles in food preservation and quality improvement. This method utilizes the combination of herbs and other edible coatings to coat the fruits. Frequently used herbs are aloe vera, basil leaves, turmeric, lemon grass, Indian lilac, rosemary, etc., that contain the antioxidants, essential vitamins, and minerals, thus offering the nutraceuticals, medicinal, and antimicrobial properties to coating formulations in addition to acting as a water vapor barrier (Martinez-Romero et al., 2006). Recently, the poultry feather fibers are also being investigated to extract the protein called keratin using different teclmiques. They contain nearly 90% keratin that is renewable, biodegradable, and promising biopolymer due to its balanced lipophilic and hydrophilic amino acid composition (Esparza et al., 2017).
MAJOR CHALLENGES IN APPLICATION OF EDIBLE COATINGS
No doubt the applications of edible coatings prolong the shelf life, increase the microbiological safety and quality of fruits, but they are also associated with some drawbacks, such as increased physiological disorders, due to low oxygen or high carbon dioxide levels, probable allergenicity (protein- based coatings), poor surface adhesion, and undesirable sensoiy traits of some coating materials. The consumer’s acceptance of coated fruits is primarily determined by their organoleptic properties. The quality is deteriorated by using the thick coatings that act as a strong barrier between the internal and external atmosphere of fruits, thus restricting the gas exchange and resulting in the anaerobic respiration (Cisneros-Zevallos and Krochta, 2003). The anaerobic conditions inside the fruit generate the carbon dioxide, ethanol, and acetaldehyde leading to fermentation and off-flavor production, thus adversely affecting the quality (Wills and Golding, 2016). The internal atmosphere modification may result in the following physiological disorders: flesh breakdown, core flush, ethanol accumulation, and off- flavors generation, as abstracted by Paul and Pandey (2014). Therefore, coating thickness must be adjusted as per the cultivar, storage conditions, and fruit composition. Further, the unacceptable alterations in sensoiy traits have been reported when antimicrobial substances particularly the essential oils are added into edible coatings (Burt, 2004). The higher amounts of sulfur-containing components may also induce unpleasant odor as noticed in case of as N-acetylcysteine and glutathione incorporation in edible coatings (Rojas-Grau et al., 2008). Likewise, the nutraceutical components can provide off-flavor and bitter taste (Drewnowski and Gomez-Cameros, 2000). However, few studies supported the incorporation of additives, like antioxidants, antibrowning agents, and antimicrobial substances, in edible coatings to improve their functionality and shelf-life extension of fruits (Lee et al., 2003; Eswaranandam et al., 2006; Raybaudi-Massilia et al., 2008). Today, there is a strong need to examine the effect of coating materials and coating additives on the sensoiy properties of coated products.
LEGISLATIVE ISSUES OF EDIBLE COATINGS
Edible coatings may be divided into following classes: food ingredients, food products, food additives, food packaging materials, and food contact substances as per the European Directive and USA regulations. They must contain safe and food-grade functional components while fulfilling the standards of hygiene during processing (Nussinovitch, 2003). Each countiy has own provisions for edible coatings and define the ED and USDA approved additives. The organic acids, like acetic, tartaric, lactic, citric, propionic, malic, and their salts, are having the GRAS status for general usage (Embus- cado and Huber, 2009) while essential oils are the allowed food additives in USA. Europe has mentioned the ingredients to be added in the edible coating formulations that include pectins, beeswax, arabic, and karaya gum, lecithin, candelilla, and carnauba wax, shellac, polysorbates, and fatty acids and their salts along with their category as an antioxidant, antimicrobial agent, etc., and E-number. In addition to these USFDA also permitted other substances, like sorbitan monostearate, polydextrose, cocoa butter, SFA esters, morpholine, and castor oil. While in India the regulations regarding the edible coatings are governed by PFA section ZZZ (23) of Rule 42 (2006) that tells that the beeswax or Carnauba wax may be used to coat the fruits and vegetables under the suitable label declaration. But these must be redefined as vegetarian consumers do not prefer to eat wax-coated fruits as they may contain the animal-based waxes or may also enclose the pesticides. In order to avoid this confusion, the packers or producers must follow the FDA regulations 1994 that need infonnation about the origin of waxes or resin, like plant, animal, petroleum, or shellac based. FDA also enables the producers to label “No wax or resin coating” in the package of fruits containing no coating substances. Despite these, another major concern is the presence of allergens in the edible protein-based coatings extracted from milk, soybeans, peanuts, wheat, fish, and nuts and this must be clearly labeled on the package (Franssen and Krochta, 2003). Recently, the addition of nanomaterials also complicates the safety of edible coatings because these nano-size materials may enter in the cells and stay in the human eventually. The data about their toxicological effects are also very limited in scientific literature. Therefore, there is a strong need for accurate information about the influence of nanocomposites on human health for which persistent exposure is indispensable before commercializing any nano-based formulation.
Based on the recent trend of reducing the chemicals consumption, the researchers are focusing toward the incorporation of natural food ingredients canying the antioxidant and antimicrobial activities that do not compromise human health. Essential oils are one such category that produces the exciting compounds to coat the fruits. Instead of their several benefits in improving the health, barrier properties, and microbiological safety, sensory traits of fruits may get compromised. But this might be improved by encapsulating the essential oils in the nanoliposom.es that release them gradually. The aloe vera gel is extensively employed recently in the edible coating due to its functional properties. It is a strong moisture barrier without using the lipid. In addition, the next generation edible coating containing the nanomaterials are the most interesting categoiy in the future that have huge potential in the preservation of quality and extension of shelf-life. The nano-scale active nutrients, like enzymes, prebiotic, omega-3-fatty acids, probiotic, and nano-sized delivery system of functional agents, such as antioxidants, antimicrobials, antibrowning agents, etc., also boost the nutritional value of coating formulation without affecting the sensoiy properties. There are some challenges in the application of edible coating such as off-flavor and physiological disorder development, hygroscopic and hydrophilic nature of some coating materials. Extensive research must be conducted in order to fulfill these gaps in the edible coating usage so that they can be more functional and advantageous in the extension of shelf-life of fruits.
The increasing consumer demand for fresh fruits reflects the need of edible coatings with distinct functionality. The shelf-life of fruits is prolonged by the use of edible coatings because they reduce the respiration, microbial load, and dehydration, improve the firmness, appearance, and maintain the aroma compounds thus preserving the fruit quality. The edible coatings can be prepared from a number of materials, like proteins, hydrocolloids, and waxes, but today much attention is given to the protein and polysaccharide coatings containing the inherent nutraceuticals, antioxidant, or antimicrobial compounds because they are eco-friendly, safe, and may replace the lipid- based traditional coatings. These coatings provide excellent gas barrier but are poor in restricting the moisture transfer. Consistent efforts are required to develop the emulsion coatings that are an effective moisture barrier. Recently the concept of herbal coating and essential oil containing the edible coating has emerged that offer the best results with minimum limitations. Despite this, the researchers must focus on the hardly-exploited interactions among the active components, edible coating materials, and fruit surface to develop novel coatings with better sensory properties and functionality.