An edible film is a thin layer of edible material, which can be placed on or between food products while an edible coating is a layer which is coated on a surface of food product. The major difference between edible coating and edible film is that the edible coating is applied in liquid form on the surface of product, usually by dipping the product in a solution of coating material formed by the different types of substance (carbohydrate, protein, lipid or composite mixture), and edible film is first moulded as solid sheets, which are used to wrap the food product.
Fresh and minimally processed fruits and vegetables is one of the major growth segments in the food industry. To increase the shelf life and storability of fresh produce and to prevent from microbial contamination, the food industry is seeking new strategies. Edible coating is a technique which may be used to increase shelf life of fresh and minimally processed fruits and vegetables and provide a safe atmosphere free from microbial contamination, also maintain the quality of food products by regulating the transfer of gases (O2, CO2) and moisture. It also maintains the taste and aroma in a food system. Edible coatings which are prepared by suitable method can be utilised to maintain nutritional characteristics, quality, marketability and economic value.
Functional Attributes
1. It regulates the transfer of moisture from surface of food product to environment and eliminates moisture loss. During post-harvest storage of fresh produce moisture loss leads to changes in colour, texture, taste and weight loss.
2. Edible coating prevents gas exchange between the fresh produce and environment, which results to slow down the rate of respiration, delay the ripening and reduces the decay percentage. Due to gas-barrier property of edible coating enzymatic oxidation, browning discolouration and texture softening is reduced during storage.
3. By restricting the exchange of volatile components, edible coating prevents the loss of flavour and colour compounds and natural volatile components from fresh produce.
4. It protects the fresh produce from any type of physical injury which is caused either by mechanical impact or other mechanical factors like pressure and vibrations.
5. It reduces the microbial population on the food surface, delay the ripening of fruits/vegetables also delaying browning and discolouration of fruit surface and improving quality. As it carries many antioxidant and antimicrobial agents and colour and flavour characteristics to the food produce.
Mechanism of Action of Edible Coating
Permeability Property
Permeability is an important characteristic of edible coating to think about when selecting coating materials. Permeability of any coating solution is calculated from a combination of Henry's law of solubility and Fick’s first law of diffusion. Due to the hydrophilic nature of most coating material and lack of homogeneous structure, permeability of coating material cannot be predicted. Addition of plasticisers or components of low-molecular-weight influences flexibility and permeability of coating material.
Effect on Water Loss
Water vapour permeability is defined as movement of water vapour through a coating material per unit area and thickness. Water loss generally occurs due to evaporation. It describes the vapour pressure difference at a definite temperature and RH. Movement of water vapour is dependent on temperature and relative humidity.
Effect on Gas Exchange
A novel edible coating should have good gas-barrier properties. It slows down the rate of respiration of fruits/vegetables after harvest by preventing the transfer of flavour and aroma components with the surrounding. Inside the food produce edible coating creates a modified or controlled atmosphere that will enhance the shelf life of fruits/vegetables after harvesting.
Edible Coating Materials for Fruits and Vegetables
There are different types of coating material are used like polysaccharides, lipids, proteins, and resins which can be used alone or in different combinations. Coating materials are selection on the basis of their hydrophilic and hydrophobic nature, easy development of coatings, water solubility, and sensory properties.
Polysaccharides-based Coating
Polysaccharides-based coatings were prepared by various coating materials such as alginates, carrageenan, cellulose derivatives, starch and starch derivatives, chitosan, various plant and microbial gums, chitosan, aloe vera and pectinates. Polysaccharides-based coatings create a modified atmosphere to maintain the quality of food produce. Polysaccharides are hydrophilic in nature, and can be used as gas barrier while cannot retard water loss effectively whereas, some polysaccharides, can be used to retard moisture loss of fruits/vegetables.
Lipid-based Coatings
Various fatty acids, waxes and resins are generally used for coating material for fruits/vegetables as lipid-based coating. It provides barrier mainly for moisture and improves surface texture. As compared to polysaccharides and protein-based coatings lipid-based coating has good compatibility with other coating materials and having high water vapour and gas-barrier properties. However, lipid-based coatings show unwanted waxy taste and lipid rancidity.
Protein-based Edible Coating
To increase the shelf life and maintain the edible quality of food produce, various edible coatings are prepared which are made of protein, polysaccharide and oil-containing materials which help to prolong the shelf life and maintain the quality of fruits and vegetables. Protein and polysaccharide films are generally good barrier against intermediate relative humidity (RH) and good mechanical properties. Much attention has been lately paid to whey protein concentrate (WPC) films because they are transparent with excellent barrier properties to oxygen and flavours. Generally, meat and other foods are covered with dry particles (breaded) or dipped in liquid solutions of these particles (battering).Owing to their structural characteristics, flours, gluten, protein-based materials such as whey proteins and supportive polysaccharides structured by gum and starch are commonly used.
Herbal Coating
Edible coatings can be securely eaten with the food product which is applied on surface of food produce to maintain their quality & enhance their shelf-life after harvesting. Edible coating material can be prepared from various herbal extract such as Aloe Vera, tulsi, mint, lemongrass, cinnamon, oregano, neem and clove for food produce. Herbs-based edible coatings can be used in various fruits and vegetables such as papaya, apple, orange, grapes, tomato, cucumber and mango in order to prevent loss of moisture, delay ripening of fruits & reduce microbial growth. Herbal edible coatings can increase the storage time of fruits/vegetables of about 15-35 days when kept at low temperature while untreated fruits are deteriorated within 8-10 days. Use of herbal extracts as an edible coating for fresh produce would be an innovative & interesting thing that can be used as an alternative for chemical treatment that can be commercially applied.
Challenges in Developing Edible Coatings for Food Produce
The success of an edible coating depends on its permeability to moisture and gases (O2 and Co2,) and chemical attributes of the coating material, types of food produce, and environmental factors in which produce is stored.
There are some points which should be considered when developing an edible coatings formulation:
Edible coating materials which have hydrophilic in nature show inadequate moisture-barrier properties;
Due to inappropriate storage condition, unsuitable transportation and marketing of food produce o2 and co2 barrier properties were varied;
Improper technique of coating and poor coating adhesion between coating layer and the surface of fruits and vegetables; and
Unwanted flavour is added to the food produce as applied some of the coating material which results unnecessary sensory attribute.
