Keywords

21.1 Introduction

Fruits and vegetables are a basket of nutrients required to keep ones’s body healthy. However, they are very perishable and are victim of many physical, physiological, biochemical and pathological deteriorations. These factors affect the commercialization of the products resulting in huge economic losses (Gutiérrez et al. 2016). Since the purchase of any product depends on its aesthetic appeal, it must be attractive in terms of colour, firmness and the overall appearance. However, perishable horticultural products are prone to shrinkage, softening, colour change etc. during handling and storage. Of the several techniques to extend the shelf-life of fruits and vegetables, edible coatings have gained importance due to ease of use and fabrication according to the functional attribute desired. The application of these coatings on the product surface can be achieved by dipping, spraying or brushing. The coatings adhere to the food surface and create a modified atmosphere that provides a barrier to oxygen, moisture and the movement of solutes (Gutiérrez and Álvarez 2017; Tapia-Blácido et al. 2018). The eco-friendly polymer based edible coatings not only reduce the requirements of synthetic plastic packaging, but also uses the by-products of the food industry. The traditional use of edible coating was mainly focussed on minimizing water loss and delaying senescence through selective permeability to gases. However, the new generation of edible coatings is being specially designed to allow the incorporation and controlled release of nutraceuticals, antioxidants and natural antimicrobial agents through diverse fabrication techniques (Gutiérrez 2017, 2018a). These coatings are thin layers of edible material applied to the product surface and are used as an alternative to natural protective waxy coatings (Ansorena et al. 2018).

21.2 Historical Background of Edible Coatings

Edible coatings have been used for centuries to prevent the moisture migration, improve the food appearance and increase product shelf life. The use of wax coating on fruits by dipping is one of the oldest methods that was in vogue at the beginning of the twelfth century (Krochta and Mulder-Johnston 1997). The coating of citrus fruits with wax to retard desiccation was practiced in China in the twelfth and thirteenth centuries (Hardenburg 1967). In the 1930’s, hot-melt paraffin waxes were commercially used as edible coatings for apples and pears (Park 1999). Since 1980’s sucrose fatty acid esters (SFAEs) , Semperfresh™, Tal Pro-long™, zein proteins are commercially used to coat fruits and vegetables, thus, the firmness, colour and weight during storage are retained (Dhall 2008). During fifteenth century, a protein-based edible film called Yuba was traditionally used in Asia to improve the appearance and preservation of some foods (Gennadios et al. 1993). In the nineteenth century, to prevent oxidation and rancidity of dried nuts during storage, sucrose was applied as an edible protective coating. The more important application of edible coatings so far concerns the use of an emulsion made of oil and waxes in water that is spread on fruits and vegetables to retard the moisture loss, improve their appearance, in terms of colour and sheen, and slow down the softening and start of mealiness (Debeaufort et al. 1998).

21.3 Functionality of Edible Coatings

As previously mentioned, edible coatings are thin layer of edible material that is applied to the product surface in addition to or as a replacement for natural protective waxy coatings, which provide a barrier against the movement of gas, moisture and solutes, and can be eaten along with the commodity (Dhall 2008). It is thus important that the composition of edible coatings must comply with food regulations (Guilbert et al. 1995). A good quality of edible coatings should meet some important criteria such as moisture barrier, solute or gas barrier, water/lipid solubility, good mechanical characteristics, nontoxicity, etc. The effective use of coatings depends greatly on temperature, thickness, alkalinity, type of coating and condition of fruits or vegetables (Park et al. 1994).

In addition to improving shelf life of fruits and vegetables, edible coatings also impart functionality to the product. The improvement in sensory properties of food can also be achieved by using an edible coating containing either colourants, flavours, anti-browning agents, antimicrobials and other functional substances, thereby making the products more attractive. Novel edible materials have been derived from many natural sources that are safe to use. Among all these applications, the use of edible coatings as active packaging stands out as a promising application for food packaging (Bracone et al. 2016). These are promising new approaches to extend the quality and shelf life of fruits and vegetables. Active packaging refers to the incorporation of additives into the package in order to maintain or extend the product quality and shelf life (Gutiérrez and Alvarez 2017a, b).

21.4 Properties of Edible Coating

The properties of edible coating depend mainly on molecular structure rather than chemical constitution and molecular size. Material of any kind cannot be used for coating purposes. Arvanitoyannis and Gorris (1999) detail certain specific requirements, such as:

  1. 1.

    It should not interfere with the quality of fruits and vegetables and should not impart an undesirable odour.

  2. 2.

    The coating material should be water resistant.

  3. 3.

    It should not reduce oxygen or build up excessive carbon dioxide to avoid anaerobic respiration.

  4. 4.

    It should have reduced water vapour permeability.

  5. 5.

    It should improve mechanical handling properties and appearance, maintain structural integrity, carry active agents such as antioxidants, vitamins, etc. and retain volatile flavour compounds.

  6. 6.

    It should be easily emulsifiable.

  7. 7.

    Materials must be dissolved in all types of solvent such as water, alcohol or a mixture of other solvents.

  8. 8.

    It must be economical.

21.5 Composition of Edible Coatings

It is known that edible coatings improve the food quality, since they are selective barriers to oxygen uptake, moisture transfer and loss of volatile aromas and flavours. The moisture barrier properties of edible coatings have been extensively studied by measuring their water vapour properties, because water plays a key role in spoilage reactions in foods. However, the barrier properties of edible coatings mainly depend on the material from which they are derived. Most of the edible coatings are derived from natural polymers having film-forming ability, such as polysaccharides, proteins and lipid (Álvarez et al. 2017). Edible coatings are produced from materials with film-forming ability. It includes polysaccharides such as cellulose, starch, chitin, chitosan (Cs), alginates, pectins etc., and proteins namely casein, whey protein, keratin, collagen, gelatin, soy protein, wheat gluten, corn-zein, peanut protein, cottonseed protein etc., lipids such as wax and oil-based coatings, mono-glycerides, fatty acid resins and emulsions etc. (Gennadios et al. 1994; Gutiérrez and Alvarez 2017c; Gutiérrez 2018b; Merino et al. 2019). Their presence and abundance determine the barrier properties of material regarding oxygen, water vapour and carbon dioxide transfer in food systems.

21.6 Current Status and Recent Advances

Edible coatings are currently used in many food applications to improve handling, extend shelf life and modify appearance and flavour. It can be considered as a novel technique for carrying antimicrobials to the food surfaces of high moisture content, maintaining the flavour profile of foods with high aroma and retarding oxidation in intermediate and low moisture foods. Edible coatings also have the potential to maintain the quality of foods even after opening the package. There are multiple applications of edible coatings in the food industry. These include extending the shelf life of oxygen-sensitive foods, reducing packaging of whole and pre-cut fresh fruits and vegetables, extending product shelf life of frozen foods by reducing moisture loss, respiration and colour change, avoiding oxidation, as well as preventing moisture, aroma or colour migration (Marín et al. 2016). It is expected that the active packaging systems market has a promising future due to its integration in packaging materials or systems.

21.7 Applications of Active Edible Coatings on Fruits and Vegetables

The uses of edible coatings made from polysaccharides, proteins, lipids and composites containing several food additives, probiotics, antioxidants and oxidizing agents are discussed in this section. For some time, the main polysaccharides of interest for the coating fruits and vegetables have been cellulose, gums, starch, pectin and Cs (Saberi and Golding 2018). Polysaccharide coatings have a poor moisture barrier but have a good gas barrier and mechanical properties. Proteins from soy, milk, corn and wheat gluten are also used to prepare edible coatings (Garrido et al. 2018). Amongst them, zein is the only commercially produced protein used for such applications. Lipids such as bee wax, rice bran oil, paraffin wax and acetylated monoglycerides are also used in coating formulations. Lipids are hydrophobic and therefore, act as good moisture barriers. However, its mechanical properties are inferior to protein and polysaccharide-based films.

21.7.1 Edible Coatings as Carriers of Flavour, Colourant and Texturizer

As previously outlined, edible coatings can maintain and deliver desirable concentrations of functional ingredients to improve the product quality. Edible coatings can be incorporated with flavours, seasonings, vitamins, colourants and other beneficial plant-derived compounds. Table 21.1 briefly describes the use of edible coatings as carriers of flavours, colourants and texturizers.

Table 21.1 Application of active edible coatings for imparting desirable flavour, colour and textural attributes

21.7.2 Active Edible Coatings Carrying Nutraceuticals

Edible coatings also have the capacity to contain many active ingredients that could be used to enhance the nutritional and functional quality of food products (Gutiérrez and Álvarez 2016; Gutiérrez 2017). However, the integration of nutritional or nutraceutical ingredients into coatings of foods (Table 21.2) has been reported only by few workers, though there is growing interest in this area.

Table 21.2 Edible coatings as carriers of nutraceuticals

21.7.3 Edible Coatings as Carrier for Probiotic Organisms

Probiotics are live microorganisms, which at a given concentration can confer health benefits to the host (Sanders 2008). Probiotics help to support the growth of beneficial microorganisms, reduce potentially harmful bacteria and reinforce the body’s natural defence mechanisms. Table 21.3 describes the beneficial use of edible coatings as carriers of probiotics in various fruits and vegetables. Such active packaging systems help to deliver novel health-enhanced products for the consumers.

Table 21.3 Application of probiotic-containing edible coatings on fruits and vegetables

21.7.4 Edible Coatings as Carrier of Antimicrobial Agents

Active compounds such as antimicrobials are incorporated into edible coatings to inhibit spoilage/pathogenic bacteria by maintaining effective concentrations of active compounds on food surfaces. There are several categories of antimicrobials that can potentially be incorporated into edible coatings: (1) organic acids, namely lactic, acetic, propionic, benzoic, sorbic, (2) fatty acid esters such as glyceryl monolaurate, (3) polypeptides such as peroxidase, lysozyme, lactoferrin, nisin, and (4) plant essential oils such as cinnamon, oregano, lemongrass and nitrites and sulphites, as listed in Table 21.4 (Álvarez et al. 2018). The incorporation of these antimicrobials can help in delivering safe food.

Table 21.4 Examples of antimicrobials added into edible coatings applied on fruits and vegetables

21.7.5 Active Edible Coatings Carrying Antioxidants

Antioxidants are free radical acceptors that delay, inhibit or interrupt the stage of propagation of autooxidation. These are used to protect against oxidative rancidity and enzymatic browning in fruits and vegetables. The incorporation of natural antioxidants into edible coatings can improve their functionality and applicability in foods. A wide range of natural antioxidants such as essential oils and plant extracts, as well as pure compounds, such as α-tocopherol and ascorbic acid have been incorporated into edible coatings to improve their bioactive properties. Table 21.5 shows the beneficial effect of antioxidant-containing edible coatings. In addition, antioxidants prevent the loss of vitamin C and the browning reactions, especially in the fresh-cut fruits and vegetables, thereby maintaining colour and improving the shelf-life and acceptability of the commodity.

Table 21.5 Coatings incorporating antioxidants

21.7.6 Commercially Available Active Edible Coatings

Since the research on active edible coatings began, many products have been launched in the commercial market, which may or may not be specific to each product. Table 21.6 gives some of these formulations of active edible coatings that are available in the market.

Table 21.6 Commercially available active edible coatings

21.8 Limitations for Use

Edible coatings are being developed with incorporation of active compounds such as antioxidants, antimicrobials, flavourings and nutraceuticals. Research is being done to maintain the mechanical properties, since they can be affected dramatically with the incorporation of food grade additives. Therefore, the development of coatings with improved mechanical, functional and barrier properties is an important task. One of the main obstacles is the cost, which restricts its application in high value products. In addition to the cost, other limiting factors for the commercial use of edible coatings are the lack of materials with the desired functionalities, the investment cost for the installation of coating equipment, the strictness of the regulations and the difficulty of the production process. Despite these limitations, the food industry is looking for edible coatings that can be used in a broad spectrum of foods to add value to their products, increase product shelf life and/or reduce packaging. There is also a trend to develop new generation edible coatings which release active compounds using nanotechnological solutions such as nanoencapsulation and multilayer systems.

21.9 Regulatory Status for the Use of Edible Coatings

Since edible coatings are an integral part of the edible portion of food products, they should follow all required regulations pertinent to food ingredients. To maintain product safety and quality, all components, as well as functional additives should be food-grade and non-toxic materials. In addition, all process facilities should be acceptable for food processing and should strictly comply with current Good Manufacturing Practice (GMP). Acceptable ingredients for use in edible coatings should be Generally Recognized As Safe (GRAS), and used within any limitations specified by the Food and Drug Administration (FDA). According to the European Directive (1998) the ingredients that can be incorporated into the formulation of edible coatings are arabic and karaya gum, pectins, shellac, beeswax, candelilla wax, carnauba wax, lecithin, polysorbates, fatty acids and fatty acid salts. On the other hand, the FDA mentions other additives used as components of protective coatings applied to fresh fruits and vegetables such as morpholine, polydextrose, sorbitan monostearate, SFAEs, cocoa butter and castor oil (Vargas et al. 2008).

21.10 Conclusion

The trend of packaging research worldwide should be directed to produce active edible coatings that are affordable and can maintain high quality, wholesome, fresh-like products. These coatings are promising in postharvest management and will receive more attention in the near future. Natural ingredients have great potential for use as additives in edible coatings without the adverse effects associated with chemical preservatives. Future research should encourage additional studies to overcome the above limitations.