The lipid coatings consist of natural waxes (beeswax and paraffin), acetylated monoglycerides, and surfactants. The coatings from lipids are hydrophobic in nature turning them the ideal moisture barriers in addition to lowering the respiration and providing the shine on the fruit surface. However, the low polarity makes the lipid coatings more brittle, greasy, and thicker, thus destroying the gloss and appearance of fruits (Perez-Gago et ah, 2002). They result in the development of rancid flavor on storage. Therefore, the combination of lipids along with either polysaccharides or proteins results in the coatings having better mechanical and barrier residences and moisture permeability than the lipid only (Bravin et al., 2004). The frequently applied lipid coatings are included for discussion here.
Since 1930, a number of waxes, including the paraffin, camauba, and beeswax, were being commercially employed to protect the apples, pears, melons, and citrus on the basis of improvement in the appearance and moisture barrier properties (Singh et al., 2016). The crude petroleum obtained paraffin wax is comprised of hydrocarbon mixtures and employed to coat the uncooked fruit. Camauba wax is extracted from palm tree leaves, beeswax from honeybees and candelilla wax from candelilla plant (Rhim and Shell- hammer, 2005). The coatings based on these waxes also lower the surface abrasion and skin browning of fruits by the alteration of interior composition and modification of mechanical integrity. They have excellent water barrier property as compared with the other lipid or nonlipid based edible coatings. The thin layer of wax coatings can be consumed, but their thicker layer must be removed before intake of fruits (Bourtoom, 2008). The coating mixture carrying 3% cassava starch (w/w), 1.5% glycerol (w/w), 0.2% carnauba wax (w/w), and 0.8% stearic acid (w/w) showed the improved water vapor and gas barrier and mechanical properties (Chiumarelli and Hubinger, 2014). In addition, the polyethylene wax is extracted from petroleum is employed to formulate the emulsion coatings. The emulsion coatings obtained from wax have been used in the food industry from a few decades. These coatings are the best moisture barrier but do not contribute any shiny appearance to the fruit surface.
188.8.131.52 FATTY ACIDS, MONOGL YCERIDES, AND ACETOCL YCERIDE
The vegetable oils derived fatty acids and monoglycerides (prepared from trans-esterification of fatty acids and a molecule of glycerol) are mainly employed as dispersing and emulsification agent in the coating formulations (Jimenez et al., 2012). The monoglycerides after acetylation turn to the flexible solid alike to waxes from the molten state. The lipids may be generally stretched up to 102% of their initial length without fracturing; however, acetoglycerides have the potential to extend equal to 800% of its length that improves higher water vapor barrier properties of these compared with the polysaccharide coatings except methylcellulose (Hassan et ah, 2017).
The formulations of the edible coatings also contain the resins mainly to coat the citrus fruits. Among resins, the insect Laccifer /ясса-derived shellac resin is recently exploited for coating purpose. Initially, the shellac coatings are used in the pharmaceutical industry and today are successfully used in some fruits as well (Peamchob et al., 2003; Chauhan et al., 2015Л The shellac resin-based coatings are restricted to the oxygen, carbon dioxide, and ethylene migration, thus initiating the anaerobic respiration easily and induce the undesirable flavor changes. Therefore, resin coatings are not recommended for climacteric fruits due to the damaged ripening (Baldwin and Baker, 2002). They are a moderate barrier to the water vapors and dry quickly producing a glossy fruit surface. Shellac coatings along with coatings from shellac camauba and candelilla wax were applied to different cultivars of apples and results shown that shellac-based coatings provide the highest glossiness and minimum firmness loss than others (Bai et al., 2002).
Proteins, the polymer of amino acids, have the great potential in the development of the edible coatings due to the easy modification of its side chains of amino acids; however, they are the lowest exploited materials among all biopolymers. A variety of sources including the wheat, com, soybeans, milk, peanut, etc., are used extensively to extract the proteins for coating formulation. The extended structure needed in coating composition can be achieved by the denaturation using the acid, base, heat, and solvent after which the protein chains may combine with each other through the ionic, hydrogen, hydrophobic, and covalent bonding. This chain-to-chain interaction in protein forms the coatings that are flexible and excellent oxygen, aroma, and oil barrier but sensitive to water, like polysaccharide-based coatings (Ramos et al., 2011). The relative humidity and temperature can significantly influence the protein-based coatings that are hydrophilic in nature with poor mechanical characteristics.
The fibrous protein-collagen, when hydrolyzed under the controlled conditions, forms the gelatin that contains a high concentration of proline, glycine, and hydroxyl-proline. The gelatin coatings are in demand because of their low cost and easy availability (Hanani et al., 2013). These coatings are clear, good oxygen barrier and contain fair mechanical properties (Hanani et al., 2012). Basically, the gelatin coatings are employed commercially in the pharmaceutical industry and its potential in food coating needs to be examined.
Zein, the prolamine protein, found mainly in corn, is water insoluble but dissolves easily in the 70-80% ethanol and glycol esters (Dickey and Parris, 2002). Zein-based coatings are glossy, tough, grease-proof, hydrophobic, fair adhesive, and binding agent and good water vapor barrier than other protein coatings. Its water barrier characteristic can be significantly modified by incorporating the fatty acids or some cross-linking reagent. But the addition of cross-linking agents makes edibility of coatings as a big concern. They have oxygen and carbon dioxide permeability less than polysaccharides coatings and plastic films like propylene, PVC, LDPE, and polystyrene, but greater as compared with gluten coatings. These coatings are frequently employed as a replacement for the shellac-based coatings in case of the fresh and dried fruits. The ripening in tomatoes is effectively suppressed without affecting the firmness with the application of zein coatings. Such coatings are preferred over commercial shellac coatings to maintain the glossiness and other aesthetic properties of apples (Bai et al., 2002, 2003a, 2003b).
184.108.40.206 WHEAT GLUTEN
The hydrophobic protein of wheat flour is called gluten that is basically globular protein and insoluble in water. Elasticity and cohesiveness offered by gluten are the two basic properties supporting the coating preparations. Gluten comprises of mainly two proteins, named as glutenin and gliadin, contributing to the elastic and extensible nature of gluten. Glutenin is insoluble in 70% ethanol, while gliadin is soluble. Interestingly, the low- strength wheat gluten is water insoluble but may dissolve in the low or high pH aqueous solutions. The disintegration of original disulfide bonds occurs at the drying stage of coating preparation and new disulfide bonds, hydro- phobic and hydrogen bonds are synthesized during drying (Chiralt et al., 2018). The coating flexibility can be unproved by the addition of glycerin and plasticizers. The sensory and mechanical properties of coatings are determined by the degree of purity of wheat gluten. More clear and stable coatings are obtained from pure gluten.
The random coil nature of major milk protein, casein, makes it easier to process and one of the best materials for formulating the edible coatings due to their various functional characteristics. They can fonnulate the coatings from aqueous solutions only without any treatment and have potential to make ample intermolecular electrostatic, hydrophobic, and hydrogen bonds so that interchain cohesion may be improved (Coltelli et al., 2016). Plasticizers when added to denatured solution at 80-100°C, the flexibility and touglmess are improved with the increase in water vapor permeability (Khwaldia et al., 2004). Casein is a favorable substance to prepare the emulsion coatings due to the property to act as a surfactant. Casein is fairly water soluble and takes nearly one day to achieve the 50% weight gain when dipped in water. Coatings from casein are odorless, transparent and flexible; however, the major limitation of these coatings is their high cost. During storage of raisins, the casein coatings did not exhibit the significant reduction of moisture transfer. But the emulsion coatings made from casein and lipids controlled the moisture loss of fruits significantly. The browning of apples was delayed effectively using the caseinate coatings (Le Tien et al., 2001).
220.127.116.11 WHEY PROTEIN
These constitute nearly 20% of the milk proteins and contain mainly /?-lactoglobulin. Whey proteins form the transparent, flavorless, and flexible coatings, like caseinate coatings, and are a promising material for edible coatings due to their high nutritional value (Cao et al., 2007). Whey protein-based coatings are good barrier against gas, aroma, grease and oil exchange and have accurate mechanical properties (Andersson, 2008; Trezza and Krochta, 2002). But they have high moisture permeability due to their hydrophilic nature. They are most suitable to formulate the emulsion or composite coatings that have better moisture barrier characteristics (Certel et al., 2004).
18.104.22.168 SOY PROTEIN
High protein content in soybeans, that is, 38-44% is desirable for the extraction of soy protein isolate (90% protein) or soy protein concentrate (65-72% protein). The two primary proteins in soy protein are 7S (conglycinin, 35%) and 11S (glycinin, 52%) resulting in the creation of disulfide bond that on heating, breaks and produces new hydrogen and hydrophobic bonds alike to the gluten (Ciannamea et al., 2014). The coatings from soy protein are prepared from the soy protein isolate in combination with a plasticizer (glycerol and sorbitol) to modify the flexibility. They are poor moisture barrier because of their inherent hydrophilic nature (Rliim et al., 2000). But such coatings strongly suppress the oxygen permeability mainly in an atmosphere of low relative humidity, thus increasing the shelf-life of fruits. Soy protein- based coatings significantly reduce the senescence in kiwifruit (Xu et al., 2001).
The edible coatings prepared from the multiple edible components are the need of time with the aim to produce a superior film with increased mechanical strength and gas bander features. They contain the amendatory functional characteristic of each constituent along with minimum downsides. Composite coatings basically include the interaction of lipids with one or two hydrocolloids either polysaccharides or proteins, thus integrating the benefits of lipid and hydrocolloid component while decreasing or screening the drawbacks of each. These coatings are used as a suspension, emulsion, or dispersion of the nonmiscible components or solution in a solvent or like a sequential layer, such as multilayer coating. Emulsion coatings are a newly emerging class in the composite coatings that are formulated using the meth- ylcellulose and fatty acids to alter the moisture bander properties. Bilayer coatings contain some necessary properties from one component (e.g., lipid) and rest from the other component (e.g., polysaccharide or protein), for example, lipid/polysaccharides, lipid/lipid, protein/protein, protein/lipid, etc. Sadly, these coatings are hardly applied to coat the whole fruits.
The quality of fresh apples was preserved when coated with the composite of tapioca starch and leaf gum obtained from decolorized Hsian Tsao (Pan et al., 2013). Perez-Gago et al. (2005) formulated a composite coating based on the carnauba wax or beeswax as the lipid component and whey protein concentrate, whey protein isolate, or hydroxypropyl methylcellulose as a hydrophilic component and found it highly successful in the suppression of enzymatic browning reaction in the apple slices.