Edible coatings started to be developed as solution against fruit and vegetables rapid dehydration, enzymatic browning and texture decay, which had as major follow-up the enhancing of these goods shelf-life. Along the years, besides being a partial barrier to moisture, gases (oxygen, carbon dioxide, volatile substances) and microorganisms, and improving mechanical resistance of fruits and vegetables, edible coatings managed to be good carriers for a diversity of food additives and contributors to the production of aroma volatiles (Olivas and Barbosa-Cánovas, 2005). In other words, what started as an alternative to plastic bag or casserole packaging could become the packaging of the future (Fundazioa, 2006).
The term, edible film, has been related to food applications only in the past 50 years. One semi-sarcastic tale was that spies' instructions were written on edible films, so that in the off-chance they were captured, they could easily destroy their secrets by eating them. In most cases, the terms film and coating are used interchangeably to indicate that the surface of a food is covered by relatively thin layer of material of certain composition (Pavlath and Orts, 2009). Application of edible coatings to preserve quality of fruits and vegetables was first mentioned by Hardenburg (1967) for oranges and lemons coated with wax in China to preserve the fruits longer. In the U.S., the first patent on use of edible coatings dates back to 1916 belonging to Hoffman for a method of preserving whole fruits with molten wax. In 1972, Bryan patented a method to preserve grapefruit halves with edible coating composed of low methoxyl pectin and locust bean gum dispersed in grapefruit juice (Olivas and Barbosa-Canovas, 2009). Currently, edible films and coatings are used in various applications, mainly to decrease water vapor transmission rate preventing texture decay (Chien et al., 2007; Garcia and Barret, 2002; Olivas et al., 2003; Tapia et al., 2007), to preserve texture of fruits and vegetables (Kader, 1986; Knee, 1980; Maftoonazad et al., 2007; Olivas et al., 2007), to reduce respiration and, hence increase shelf life of a commodity(Kays and Paull, 2004) , to control browning (Bai et al., 2003; Baldwin et al., 1996; Olivas et al., 2007; Perez-Gago and Krochta, 2001), to preserved flavour (Baldwin et al., 1999; Fellman et al., 2003; Ke et al., 1994; Olivas et al., 2007; Olivas et al., 2003)
X.2. Types of edible coatings
The main components of edible films and coatings are presented in figure 1. The main categories are: carbohydrates, proteins and lipids and the edible film characteristics formed by these components are described in Table 1 - 3.
Figure - Main components of edible films and coatings for intact and minimally processed fruits and vegetables
(CHS carbohydrates; FA fatty acids; MC methylcellulose; HPMC hydroxypropylmethylcellulose; HPC hydroxypropylcellulose, CMC carboxymethylcellulose) (Olivas and Barbosa-Canovas, 2009)
In addition to the edible coatings presented in figure 1 there are also composite films, a blend of polysaccharides, protein, and/or lipids combined in order to improve the permeability or mechanical properties as dictated by the need of a specific application. These heterogeneous films are applied either in the form of an emulsion, suspension, or dispersion of the non-miscible constituents, or in successive layers (multilayer coating or films), or in the form of a solution in a common solvent. The method of application affects the barrier properties of the films obtained (Khin et al., 2005).
X.3. Antimicrobial properties of edible films
As fruits and vegetable have been related with many foodborne outbreaks (EFSA, 2012) new methods of packing used today consider also the usage of antimicrobial compounds that may provide inhibitory effects against spoilage and/or pathogenic bacteria reducing the risk of foodborne diseases related with this food category. Antimicrobial coatings with acid-chitosan solutions of the surface of tomato stem scars inoculated with a four-strain cocktail of Salmonella (serovars Montevideo, Newport, Saintpaul, and Typhimurium) provide an alternative antimicrobial intervention for decontamination of tomatoes (Jin and Gurtler, 2012). After 8 days at 10 °C, the population of L. monocytogenes was 10-fold lower on coated sweet corn than on non-coated sweet corn indicating a barrier effect of zein coating (Carlin et al., 2001). An antimicrobial edible film developed from defatted mustard meal demonstrated a potential to be applied to foods as wraps or coatings to control the growth of L. monocytogenes (Lee et al., 2012). Also, some edible coatings or different antifungal compounds that may be incorporated in the edible films proved to be effective for different fruits and vegetables during refrigerated storage (Mehyar et al., 2011; Valencia-Chamorro et al., 2009, 2010).
The antimicrobial substances that can be incorporated into edible coatings may be divided in different categories like: organic acids (acetic, benzoic, lactic, propionic, sorbic), fatty acid esters (glyceryl monolaurate), polypeptides (lysozyme, peroxidase, lactoferrin, nisin), plant essential oils (cinnamon, oregano, lemongrass), nitrites and sulphites (Franssen and Krochta, 2003).
Although several types of antimicrobials incorporated into edible coatings have been used for extending shelf-life of fresh commodities, their use in fresh-cut fruits is yet limited. Currently, organic acids and plant essential oils are the main antimicrobial agents incorporated into edible coatings for fresh-cut fruits. Despite the good results achieved so far with the incorporation of essential oils into edible coatings, the major drawback is their strong flavour which could change the original taste of foods (Rojas-Grau et al., 2011). It has been shown that some volatile essential oils can be used to prepare apple-based antimicrobial edible films with good physical properties, and that the films are effective against major foodborne pathogens (Du et al., 2009).
X.4. Challenges for ecology
Increased consumer demand for higher quality food in combination with the environmental need to reduce disposable packaging waste have led to increased interest in research into edible films and coatings (Del-Valle et al., 2005). Food packaging specialists are more and more concerned about the preservation and protection of all types of foods and their raw materials, particularly from oxidative and microbial spoilage in order to extend their shelf-life. Continuous awareness by one and all towards environmental pollution by the latter and as a result the need for a safe, eco-friendly atmosphere has led to a paradigm shift on the use of biodegradable materials, especially from renewable agriculture feedstock and marine food processing industry wastes. Such an approach amounts to natural resource conservation and recyclability as well as generation of new, innovative design and use (Tharanathan, 2003).
Edibility, biodegradability and increased food safety are the main benefits of active edible films. Their environmental friendly aspects make them alternatives in packaging systems, without the ecological costs of the synthetic non-biodegradable materials (Geraldine et al., 2008). The main problems of biopolymers are their higher costs and less optimal physical and processing properties when compared with synthetic plastics. In addition, there have not been sufficient incentives for downstream processors to incorporate the biodegradable materials into their products (Siracusa et al., 2008). Polymer cross-linking and graft copolymerization of natural polymers with synthetic monomers are other alternatives of value in biodegradable packaging films. Although their complete replacement for synthetic plastics is just impossible to achieve and perhaps may be even unnecessary, at least for a few specific applications our attention and needful are required in the days to come (Tharanathan, 2003).
X.5. General consumer perceptions
In order to maximize the potential of a new technology, it is important to consider consumer concerns and acceptance. Nowadays an increasing interest is shown by consumers concerning what goes on or into their food as they become increasingly educated and informed about health and nutrition. Consumers usually judge the quality of fresh-cut on the basis of appearance and freshness at the time of purchase (Barry-Ryan et al., 2007). Some edible coatings are invisible some are so much a part of the food that we are not aware of them even though they may be visible but the consumers need to be informed. The Institute of Food Technologists' Expert Panel on Food Safety and Nutrition wrote in 1997 that " (…) use of such films as coatings on foods must be declared appropriately to the consumer, no matter how small the amount used (…) " (www.rense.com).
Consumers are concerned about the types of products that are coated, safety of the coating materials, sensory qualities of the resulting products, end benefits of edible films coating and the cost of the food products packaged with edible films coating (Wan et al., 2007). A study by Sonti showed that compared to caucasians, asians were more and hispanic/spanish were less aware of edible coating in 2003 and a 7% increase in purchase intent was observed after advantages of edible coating had been described to consumers (Sonti, 2003).
The research in the field of sensory analysis of fruits and vegetables coated with edible films is yet at the beginning. One example of consumer's opinion regarding edible coating suggested that chitosan coatings of strawberries did not change consumer acceptability of flavor, sweetness, or firmness of the samples that have similar sensory descriptors as those of fresh berries after 1 week of storage at 2°C (Han et al., 2005).
X.6. Regulatory aspects
In order to be used in the food industry, as an integral part of the edible portion of food products, all edible coatings have to be in agreement with the European regulations for food ingredients. The main documents which are regulating the use of edible coatings are European Directives for food additives other than colors and sweeteners 1995/2/EC and 1998/72/EC that classify edible films and coatings as food products, food ingredients, food additives, food contact substances, or food packaging materials.
In Europe, the ingredients that can be incorporated into edible coating formulations are majorly regarded as food additives and are listed within the list of additives for general purposes, although pectins, Acacia and karaya gums, beeswax, polysorbates, fatty acids, and lecithin are mentioned apart for coating applications. In any case, the use of these coating forming substances is permitted, provided that the „quantum satis‟ principle is observed (Rojas-Grau et al., 2011).
In 2008 the Commission of the European Communities introduced a new Directive 2008/84/EC which lays down specific purity criteria on food additives other than colours and sweeteners taking into account the specifications and analytical techniques for food if the primary purpose of the substances is shelf-life extension.
In the U.S. the FDA regulations established that edible coatings must be Generally Recognized as Safe (GRAS) under conditions of its intended use with amount applied in accordance with good manufacturing practices. Furthermore, the U.S. FDA requires that all film coating substances products are determined safe for their intended use before they are permitted for sale in the United States. It is the responsibility of the Film Coating Substances manufacturer to ensure that food contact materials comply with the specifications and limitations in all applicable authorizations.
A very important aspect that must be regulated for the use of edible coatings refers to the presence of allergens. Pre-packaged fruits and vegetables that have a protective edible coating or wax must include the source of any allergen or gluten on their labels as several edible films and coatings are formed from milk protein (whey, casein), wheat protein (gluten), soy protein and peanut protein that could cause allergic reactions.
X.7. Economic aspects
Despite advantages claimed for edible coatings, they are not yet used at industrial level at a large scale. Costs are considered to be the main obstacle on their way to large scale application. Even so the edible films industry has know an important growth in the past years. As recently as 1967, edible films had very little commercial use, and were limited mostly to wax layers on fruits. During intervening years, a significant business grew out of this concept (i.e., in 1986, there were little more than ten companies offering such products, while by 1996, numbers grew to 600 companies). Today, edible film use has expanded rapidly for retaining quality of a wide variety of foods, with total annual revenue exceeding $100 million (Pavlath and Orts, 2009).
Some coating formulations are already on the market having commercial names, such as Pace International, LLC line of products: RainGardâ„¢, SemperFreshâ„¢ (composed of sucrose esters of fatty acids, sodium salt of CMC, and mono- and diglycerides), Pac-Rite®, Natural Shine Penbotecâ„¢, EpiShield â„¢ (www.paceint.com), Nature Sealâ„¢ composed of cellulose derivatives, but without sucrose fatty acid esters and Tal Pro-longâ„¢ composed of sucrose polyesters of fatty acids and the sodium salt of CMC, owned by RPM International, Inc. (www.mantrose.com).
X.8. Production and market issues
The development of new edible coatings with improved functionality and performance for fresh and minimally processed fruits is one of the challenges of the post harvest industry. In the past few years, research efforts have focused on the design of new eco-friendly coatings based on biodegradable polymers, which not only reduce the requirements of packaging but also lead to the conversion of by-products of the food industry into value added film-forming components (Vargas et al., 2008). Nevertheless the success of edible coatings for fruits and/or vegetables depends mainly on selecting films or coatings appropriate for a specific product. Together with their advantages edible coatings used in different purposes for fruits and/or vegetables have their limitations. For example the efficacy of films and coatings is dramatically affected by the water availability so the moisture content of the foodstuff and the relative humidity in the ambient should be taken into account in order to develop effective films and coatings with antioxidant activity (Bonilla et al., 2012). Furthermore wax and SFAE (Sucrose Fatty Acid Esters) mixtures, the most widely used edible coatings for fruits and vegetables, are not equally effective on all produce but more than that consumers tend to be wary of waxy coatings considering that a waxy taste is not desirable (Park, 1999). The production of edible films from fruit purees is an interesting way of reducing the huge post-harvest losses of tropical fruits but their main limitation is related to the relatively poor mechanical and barrier properties of biopolymers when compared to synthetic polymers (www.worldfoodscience.org).
X.9. Further developments
Nowadays, there are many technologies that can be used to improve quality and increase safety of fresh products. However, the use of innovative edible coatings stands out among other techniques in the struggle for maintaining freshness of fresh and cut fruits and vegetables. Many studies have dealt with the obtaining and characterization of low-cost coating materials, the optimization of methods for coating application, and the effects and implications of applications on different food matrices. However, in most cases the commercial implementation of edible coatings still requires further research and consideration of some critical issues such as consumer acceptance and regulatory aspects (Rojas-Graü et al., 2010 ).
One major advantage of using edible coatings is that several active compounds can be incorporated into the polymer matrix and consumed with the food. In this way, incorporation of active ingredients into edible coatings for fresh and minimally processed fruits and vegetables is feasible through nanotechnological solutions. However, current applications of nanosystems to fresh products are still scarce. More research about the effect of nanotechnologies on quality and nutritional aspects of fresh and minimally processed fruits and vegetables should be conducted.
Further studies on edible films coating should put the light to the important steps to consider when commercializing edible films coating, which are to evaluate sensory attributes of integrated coated products, to properly label ingredients of edible films coating with a focus on marketing the natural ingredients added and to target marketing strategies to advertise direct consumer benefits of the resulting edible film coating products (Wan et al., 2007).
X.10. Research and training needs
X.11. Questions for learning
What
X.12. Acknowledgements
X.13. References
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X.14. Sources of further information and advice
