The vegetables partake in our daily diet as an indispensable constituent, being the reservoir of essential vitamins, minerals, antioxidants, bioflavonoids, dietary fiber and flavor compounds. The harvested vegetable produce hits our tables after undergoing the phase of post-harvest wherein they begin to dehydrate, deteriorate and lose their appearance, taste and nutritional value. As a result of inadequate handling, storage and microbial contamination, the post-harvest losses of vegetables experienced in recent decades are appalling with a notable loss of 25-30 per cent
(Puttalingamma, 2015). Since the harvested vegetables consist of living tissues, they remain 'alive' even after harvesting and consequently undergo physiological and biochemical changes that cause detrimental quality and shelf life changes. The main postharvest phenomena
viz., respiration, transpiration and ethylene production are reported to contribute to the deterioration of the postharvest life of the vegetables
(Olivas and Barbosa-Canovas, 2009) while the other factors of spoilage are loss of moisture, nutrients and minerals and nutritionally important pigments as well as spoilage due to microbial pathogens.
The quality of the vegetable produce is a product of organoleptic, nutritional and hygienic traits and is dynamic. Neither the producer nor the consumer could accept spoilage during post-harvest storage and can never compromise to produce quality be entertained. Besides, the consumer of today expects higher than ever before, demanding their food to be more nutritive, safer to eat with diverse variety and prolonged shelf life. In this context, the technique of edible coating emerges as a strategy to meet out the objectives of preventing spoilage during storage, prolonging produce shelf life colligating with the appealing visual aspect. Edible coatings are the cheaper alternative that is focused to avert the postharvest losses and being used for extending the postharvest life and lowering production cost.
The use of edible films has grown intensely since the mid-1980s but by no means it is a 21
st century innovation, they were used at least as early as the 1100’s, when merchants in citrus-growing regions of southern China used wax to preserve oranges shipped by caravan to the Emperor’s table in the North. People in Europe for centuries preserved fresh fruit with “larding,” a coating of the melted fat from hogs. Those coatings sealed off the fruit, preventing the exchange of gases with the air, essential for sustaining good quality.
The Ready-to-eat fruits and vegetables market now account for about 10 per cent of all produce sales, with sales exceeding $10 billion annually especially due to increase in the number of health-conscious consumers who look for more foods that require minimal preparation like cut fruit and premixed salads.
Fruits and vegetables skins provide natural protection against drying out, discoloration and other forms of spoilage. edible films, however, by regulating the transfer of moisture, oxygen, carbon dioxide, lipids, aroma and flavour compounds in food systems, edible films and coatings can increase shelf life and improve food quality. They generally provide the same protection against bacteria as the natural skin if the foods are handled under sterile conditions. The success of an edible film or coating in extending the shelf life and enhancing the quality of food strongly depends on its barrier properties to moisture, oxygen and carbon dioxide, which in turn depends on the chemical composition and structure of the film-forming polymer and the conditions of storage.
Ongoing researches on edible films are consistently being made. In the last three decades, considerable progress has been made in developing these materials driven by the increasing consumer demand for safe, high-quality, convenient food with good shelf lives, along with an ecological awareness of the limited natural resources and the environmental impact of packaging waste. Ahead are the various aspects of edible coatings having the shelf life of vegetables is its focus.
Materials for edible coatings used in vegetables
The basic components of our everyday foods (carbohydrates, proteins and lipids) are the ingredients of the edible coating. As a general rule, polysaccharides are used to control gas transmission from product to outer environment; fat to reduce water transmission; protein for mechanical stability
(Palvath and Orts, 2009). The important prerequisite is that the material used should be a GRAS (Generally Recognized As Safe) compound.
Polysaccharide based edible coatings
The polysaccharide coatings are either gum-based or starch-based. The polysaccharide gums easily form films of good tensile strength. The formation of micelles enables the gums with their film-forming ability and the structural differences of gums are attributed to the presence or absence of branching, electrical charge and molecular weight. The commonly used gums are Agar (from red seaweed), Alginate (
Laminaria sp.,
Macrocystis pyrifera,
Ascophyllum nodosum), Carrageenan (
Eucheuma spp.), Gaur gum (
Cyamopsis tetragonoloba), Gum Arabic (
Acacia spp.), Konjac gum (
Amorphophallus spp.), Pullalan (
Aureobasidium pullulans) and Xanthan gum (
Xanthomonas campestris).
The starches commonly occur in seeds, roots and tubers and best-used starches for edible coating are arrowroot, barley, cassava, corn, pea, potato, rice, sago, sorghum and wheat. Based on the amylose content, they are classified as high-amylose, regular and waxy starches.
Chitosan
Among polysaccharides of plant and animal origin, chitin-derivative chitosan is becoming a leader in edible films and coatings due to its unique properties. Chitin, the second most ubiquitous natural polysaccharide after cellulose, obtained from the crustacean shells (crabs and lobsters). This close resembles of cellulose is sought after the following traits in edible coatings
(No et al., 2007).
• Antimicrobial activity.
• Barrier against moisture and gases.
• Antioxidant property.
• Protection of nutrients, texture, aroma and quality of the coated item.
Protein-based edible coatings
The protein-based coatings are prepared from solutions comprising three main components
viz., protein, plasticizer and solvent. The properties of the protein coating are influenced by the intrinsic properties of the coating or the extrinsic processing factors. The intrinsic properties of the protein include amino acid composition, crystallinity, hydrophobicity/ hydrophilicity, surface charge, pI, molecular size and three-dimensional shape. Extrinsic factors include processing temperature, drying conditions, pH, ionic strength, salt-type, relative humidity during processing and storage, shear and pressure.
The commonly used animal-origin proteins for coating purposes are the casein and whey protein. The common plant proteins are wheat protein, soy protein and corn zein. The water-insoluble nature, glossy and grease-proof coating with low water vapor permeability makes the corn zein protein unique from any other agricultural protein and thus have potential usage in biodegradable packing.
Lipid-based edible coatings
The basic need for lipid coating is barricading the moisture transport to or from the produce and its atmosphere, due to its relatively low polarity. In general, the lipid-based coatings are often used supported on polysaccharide (polymer) structure matrix, for mechanical strength. Paraffin wax, carnauba wax (Copernica cerifera), beeswax (white wax) and Candelilla wax are more employed. Resin (wood resin) and rosin (coumarone indene) are allowed in the vegetable coating.
Incorporation of active ingredients into edible coatings
One of the typical distinctive functions of the edible coatings is their ability to blend with active ingredients into their matrix for improving coating functionality. These ingredients are classified into three classes
(Dhall, 2013).
Antimicrobial agents
The antimicrobial edible coatings offer inhibitory effects on spoilage and pathogenic bacterial and fungi. They include organic acids (acetate, benzoate, lactate, sorbate), fatty acid esters (glyceryl monolaurate), polypeptides (lysozyme, peroxidase, lactoferrin, nisin), plant essential oils (cinnamon, oregano, lemongrass), nitrites and sulfites.
Texture enhancers and stabilizers
Calcium salts
The loss of firmness in fruit tissues is mainly due to the action of pectinases. One of the best ways to control tissue softening is treatment with calcium salts. The calcium salts form a cross-linked network, increasing the mechanical strength, thus delaying the tissue softening and subsequent senescence.
Plasticizers
The edible coating formulated with polysaccharides and proteins are stiff and brittle due to their extensive interaction between the molecules. Therefore, a plasticizer is essential for such materials to enable them with plasticity and more functionality. Glycerol, acetylated monoglyceride, polyethylene glycol and sucrose are common plasticizers used.
Emulsifiers
Being surface-active agents of amphiphilic nature, they not only reduce the surface tension of water-lipid or water-air interfaces but also modify the surface energy to control adhesion and wettability of the coating surface
(Krochta, 2002).
Neutraceuticals
Many efforts and experiments have been made in the process of incorporation of minerals and minerals into edible coatings to enhance the nutritional value of some vegetables. Many studies have validated the successful application of Vitamin C, E and Calcium incorporation into fresh produce through the edible coating. Antioxidants such as BHA, BHT, ascorbic and citric acid prevent the enzymatic browning and in vegetables and mushrooms
(Dhall, 2013).
Methods of application of edible coatings
Below are the methods of application of the edible coating to the whole/fresh-cut/minimally processed fruits and vegetables.
Dipping
It is the most common method employed for edible coating. The coating is given to produce as it gets dipped into the edible film-forming formulation. The formulation should necessarily have appropriate density, viscosity and surface tension to give a thick coating layer over the produce. This technique is done through three-stage
viz., immersion and dwelling, deposition and evaporation.
Brushing
Brushing is the controlled spreading of a suspension onto the material surface and further dried. This method is also called as spreading and considered as an effective alternative against the large dimension films. The wetting degree, spreading rate and the contact angle measurement are to be carefully evaluated for efficient spreading over the surface by the particular liquid formulation.
Spraying
When the coat forming solution doesn’t have good viscosity, then the spraying technique is deployed. A typical spraying system produces fine spray with drop size distribution up to 20 µm.
Solvent casting
It is the most used application method of hydrocolloid edible films. Water or water-ethanol dispersions of the coating formulation is spread on a suitable substrate and dried.
Although coating application methods are diverse, their selection relies on the desired product, coating thickness, solution rheology and drying technique employed.
Application of edible coatings to fresh-cut and minimally processed vegetables
The highly engaged life of today has buildup the demand for fresh-cut and minimally processed vegetables, as they provide a ready-to-cook product. The fresh-cut as well as the minimally processed vegetables are subjected to primary processing operations such as cutting, dicing, slicing,
etc. leaving the portions of cut pieces naked. Though these cut pieces are packed, the loss due to respiration and gaseous exchange between internal and external atmosphere is unavoidable. To overcome the problems originating from the cut surface, the edible coating can be a potential candidate in the way of extending shelf-life of fresh-cut and minimally processed vegetables. The application of the edible coating to fresh-cut (Table 1) and minimally processed vegetables (Table 2) and their effects are given in the tables.
Future outlook
The hydrophilicity of some edible coating materials does not offer a good moisture barrier. Researches are needed to be focused on formulating formulations with high-moisture barrier property and improved functionality. There is also a necessity to develop novel coating application techniques to increase the coating efficiency, adhesion and durability. For antimicrobial packaging, the nanomaterials, particularly Ag nanoparticles can be incorporated into the coating formulation to improve its antimicrobial activity. After all, the sensory qualities are to be taken prime most importance and not a compromise should be made between the quality and shelf-life of the cut vegetables.
