Most fruits and vegetables possess a natural waxy layer on the surface, called cuticle. This waxy layer generally has a low permeability to water vapor. Applying an external coating will enhance this natural barrier or replace it in cases where this layer has been partially removed or altered during postharvest handling or processing. Coatings provide a partial barrier to moisture and gas exchange, improve the mechanical handling property through helping maintain structural integrity, retain volatile flavor compounds, and carry other functional food ingredients.
Biopolymers such as proteins, polysaccharides, lipids, and resins are the common coating-forming materials that can be used alone or in combinations. The physical and chemical characteristics of the biopolymers greatly influence the functionality of resulting coatings (Sothornvit and Krochta 2000). Selection of coating materials is generally based on their water solubility, hydrophilic and hydrophobic nature, easy formation of coatings, and sensory properties. This section discusses the coating materials feasible for fruit and vegetable applications, and the innovations in this application. Table 4 summarizes the recent reports on using edible coatings for fresh and minimally processed fruits and vegetables.
Lipid compounds include neutral lipids of glycerides which are esters of glycerol and fatty acids and the waxes which are esters of long-chain monohydric alcohols and fatty acids, while resins are a group of acidic substances that are usually secreted by special plant cells into long resin ducts or canals in response to injury or infection in many trees and shrubs (Hernandez 1994). Edible lipids including neutral lipids, fatty acids, waxes, and resins are the traditional coating materials for fresh produce, showing the effectiveness in providing moisture barrier and improving surface appearance (Kester and Fennema 1986; Hernandez 1994; Hagenmaier and Baker 1994, 1995; Morillon and others 2002). Very comprehensive reviews on the applications of different types of lipid-based coatings for fruits and vegetables have been done by Hernandez (1994), Baldwin (1994), Baldwin and others (1997), and Min and Krochta (2005).
Waxes (carnauba wax, beeswax, paraffin wax, and others) have been commercially applied as protective coatings for fresh whole fruits and vegetables since the 1930s with the purpose of blocking moisture transport, reducing surface abrasion during fruit handling (Lawrence and Iyengar 1983; Warth 1986), and controlling soft scald formation (browning of the skin) in fruits such as apples by improving mechanical integrity and controlling internal gas composition of the fruits (Kester and Fennema 1986). In general, wax coatings are substantially more resistant to moisture transport than other lipid or nonlipid coatings (Schultz and others 1949; Landmann and others 1960; Watters and Brekke 1961; Kaplan 1986). Commercial applications of wax coatings is rather extensive on citrus, apples, mature green tomatoes, rutabagas, cucumbers, and other vegetables such as asparagus, beans, beets, carrots, celery, eggplant, kohlrabi, okra, parsnips, peppers, potatoes, radishes, squash, sweet potatoes, and turnips (Hardenburg 1967), where high glossing and shine surface are desired. Waxes-based coatings are continuously evaluated for their applications in citric fruits, melons, and some tree fruits such as apples and pears (Mannheim and Soffer 1996; Hagenmaier and Baker 1997; Petracek and others 1998; Alleyne and Hagenmaier 2000; Hagenmaier 2000; Bai and others 2002, 2003b; Da-Mota and others 2003; Fallik and others 2005; Porat and others 2005).
Shellac and other resin-based coatings generally have lower permeability to O2, CO2, and ethylene gas, and shellac coatings also dry fast and produce a shiny surface on coated produce (Baldwin 1994). Resin coatings are fairly effective at reducing water loss, but are least permeable to gases among the available coating film-formers, meaning that fruit can easily undergo anaerobic respiration and flavor changes that are usually undesirable. Some climatic fruit do not tolerate resin coatings at all due to impaired ripening from the MA created by these materials (Baldwin and Baker 2002; Porat and others 2005). The gas permeability of shellac and several experimental coating formulations, including candelilla wax and shellac carnauba, was measured by Bai and others (2003b) on different varieties of apples. It was found that the shellac coating resulted in maximum fruit gloss, lowest internal O2, highest CO2, and least loss of flesh firmness for all of the apple varieties. However, the shellac coating gave an unusual accumulation of ethanol in freshly harvested and 5-mo-stored ‘Fuji,’ candelilla and carnauba-shellac coatings maintained more optimal internal O2 and CO2 and better quality for ‘Fuji,’‘Braeburn,’ and ‘Granny Smith’ apples, although even these coatings may present too much of a gas barrier for ‘Granny Smith.’ It was recommended the best coatings as shellac for ‘Delicious,’ and carnauba-shellac for ‘Braeburn’ or ‘Fuji’ (Bai and others 2003b).
Triglycerides or neutral lipids can form a continuous stable layer on the food surface based on their high polarity relative to waxes. Most fatty acids derived from vegetable oils are considered GRAS (generally recognized as safety) substances and have been suggested as substitutes for the petroleum-based mineral oils used in the preparation of edible coatings (Hernandez 1994; Baldwin and others 1997). However, these coatings may suffer from flavor instability, while partially hydrogenated vegetable oil that is resistant to rancidity sometimes gives better results (Kochhar and Rossell 1982).
The beneficial properties of lipid-based coating, including waxes-, resins-, neutral lipids-, and fatty acid-based coatings, include good compatibility with other coating-forming agents and high water vapor and gas-barrier properties in comparison with polysaccharides- and protein-based coatings (Greener and Fennema 1992). However, lipid-based coatings present a greasy surface and undesirable organoleptic properties such as waxy taste and lipid rancidity (Guilbert 1986). Waxes and shellac tend to restrict the gas exchange of O2 and CO2 between atmosphere and fruit to the extent that the internal O2 level becomes too low to support aerobic respiration, resulting in high levels of internal ethanol, acetaldehyde, and internal CO2 (Petracek and others 1998; Alleyne and Hagenmaier 2000). This leads to accumulation of off-flavors in the fruit (Mannheim and Soffer 1996; Baldwin and others 1997; Hagenmaier 2002). In addition, some lipid materials, such as shellac, are unstable when subjected to temperature changes, where a white waxy layer usually appears when moving fruits from cold storage to the grocery display shelves due to temperature fluctuation. Currently, lipid-based coating materials are usually studied in combination with polysaccharide- or protein-based coating materials for forming composite coatings, taking advantages of the desirable properties of different materials. A more detailed discussion on the composite coatings appears in a later section.
Polysaccharides that have been evaluated or used for forming films and coatings include starch and starch derivatives, cellulose derivatives, alginates, carrageenan, various plant and microbial gums, chitosan, and pectinates; they were reviewed by Nisperos-Carriedo (1994), Krochta and Mulder-Johnston (1997), and Debeaufort and others (1998). These coatings can be utilized to modify the internal atmosphere, thereby reducing respiration of fruits and vegetables (Banks 1984; Drake and others 1987; Motlagh and Quantick 1988; Nisperos-Carriedo and Baldwin 1990). Due to the hydrophilic nature of polysaccharides, the advantages of using these materials are more apparent as a gas barrier rather than retarding water loss. However, certain polysaccharides, applied in the form of high-moisture gelatinous coatings, can effectively retard moisture loss of food by functioning as sacrificing agents rather than moisture barriers (Kester and Fennema 1986).
Starch and derivatives Starch, the reserve polysaccharide of most plants, is one of the most abundant natural polysaccharides used as food hydrocolloid (Whistler and Paschall 1965; Narayan 1994) because of its wide range of functionality and relative low cost. Starch films are often transparent (Lourdin and others 1997; Myllärinen and others 2002) or translucent (Rindlav and others 1997), odorless, tasteless, and colorless, and have low permeability to oxygen at low-to-intermediate RH (Mark and others 1966; Roth and Mehltretter 1970). Starch films have low oxygen permeability comparable to ethylene vinyl alcohol copolymer (EVOH), a commercial synthetic oxygen-barrier film, at ambient environment (such as 20 °C, 50% to 60% RH) (Forssell and others 2002), but the oxygen permeability is greatly affected by the water content of the films (Gaudin and others 2000; Forssell and others 2002).
Dextrins, derived from starch with smaller molecular size, are often used as film-formers and edible adhesives (Smith 1984). Coatings from dextrins provided a better water vapor resistance than starch coatings (Allen and others 1963). Pullulan is an extracellular microbial polysaccharide from starch that is edible and biodegradable. Pullulan films cast from aqueous solution are clear, odorless, and tasteless, and have good oxygen-barrier properties too (Yuen 1974; Conca and Yang 1993). Pullulan-based coatings have shown potential for preserving fresh strawberries and kiwifruits because of their barriers to moisture, O2, and CO2 (Diab and others 2001). In general, due to its good oxygen barrier, starch is a good candidate for coating fruits and vegetables having high respiration rates, thus suppressing respiration and retarding oxidation of coated products.
Cellulose and derivatives Cellulose is the structural material of plant cell walls (Nisperos-Carriedo 1994). For producing films, cellulose is first dissolved in an aggressively toxic mixture of sodium hydroxide and carbon disulfide and then recast into sulfuric acid to produce cellophane (Petersen and others 1999). Cellulose ethers are polymer substances obtained by partial substitution of hydroxyl groups in cellulose by ether functions (Felcht 1985). In general, cellulose derivatives possess excellent film-forming property, but are too expensive for large-scale commercial usage. The most common commercially produced cellulose derivatives are carboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxypropyl cellulose (HPC), and hydroxypropylmethyl cellulose (HPMC). These materials are nonionic and compatible with surfactants, other water-soluble polysaccharides, and salt (Nisperos-Carriedo 1994), and can be dissolved in aqueous or aqueous-ethanol solutions, producing films that are water-soluble and resistant to fats and oils (Krumel and Lindsay 1976; Nelson and Fennema 1991; Gennadios and others 1997).
CMC is by far the most important cellulose derivative for food applications (Sanderson 1981). Edible coatings made of CMC, MC, HPC, and HPMC have been applied to some fruits and vegetables for providing barriers to oxygen, oil, or moisture transfer (Morgan 1971; Sacharow 1972; Krumel and Lindsay 1976; Maftoonazad and Ramaswamy 2005), and for improving batter adhesion (Meyers 1990; Dziezak 1991). CMC coatings have shown the capabilities for helping retain the original firmness and crispness of apples, berries, peaches, celery, lettuce, and carrots when used in a dry coating process (Mason 1969), preserving important flavor components of some fresh fruits and vegetables (Nisperos-Carriedo and Baldwin 1990), and reducing oxygen uptake without increasing carbon dioxide level in the internal environment of coated apples and pears by simulating a controlled atmosphere environment (Lowings and Cutts 1982; Banks 1985; Meheriuk and Lau 1988; Santerre and others 1989).
Seaweed extracts Alginates are the major structural polysaccharides of brown seaweed known as Phaeophyceae (Whistler and BeMiller 1973; Sanderson 1981). Alginates possess good film-forming property, producing uniform, transparent, and water-soluble films. Alginate-based films are impervious to oils and fats but, as other hydrophilic polysaccharides, have high water vapor permeability (Cottrell and Kovacks 1980; King 1983). However, alginate gel coating can act as a sacrificing agent, where moisture is lost from the coating before the food significantly dehydrates (Kester and Fennema 1986). The coating can also improve the adhesion of batter to the surface of fruits and vegetables (Fisher and Wong 1972). Alginate coatings are good oxygen barriers (Conca and Yang 1993) that can retard lipid oxidation in various fruits and vegetables (Kester and Fennema 1986), and have been found to reduce weight loss and natural microflora counts in minimally processed carrots (Amanatidou and others 2000). Calcium-alginate coatings were found to improve the quality of fruits and vegetables, such as reducing shrinkage, oxidative rancidity, moisture migration, oil absorption, and sealing-in volatile flavors, improving appearance and color, and reducing weight loss of fresh mushrooms in comparison with uncoated ones (Hershko and Nussinovitch 1998).
Carrageenan, extracted from several red seaweeds, mainly Chondrus crispus (Whistler and Daniel 1985) and a complex mixture of several polysaccharides, is another potential coating material for fruits and vegetables. Carrageenan-based coatings have been applied to fresh fruits and vegetables such as fresh apples for reducing moisture loss, oxidation, or disintegration of the apples (Bryan 1972; Lee and others 2003). In combination with antibrowning agents such as ascorbic acid, carrageenan-based coatings resulted in positive sensory results and reduction of microbial levels on minimally processed apple slices (Lee and others 2003). By acting as a sacrificial moisture layer, carrageenan coating was able to protect moisture loss of grapefruits (Bryan 1972). In addition, κ-carrageenan films can effectively carry food-grade antimicrobials such as lysozyme, nisin, grape fruit seed extract, and EDTA for a wide range of applications as a food package material (Choi and others 2001).
Other gums, including exudate gums (gum arabic or acacia gum and gum karaya) and microbial fermentation gums (xanthan gum), have also been studied as coating materials for fruits and vegetables. Xanthan gum provides uniform coatings with good cling and improved adhesion in wet batters. Gum arabic has been used for coating pecan nut halves to eliminate moist and oily appearance (Arnold 1968). Spraygum and Sealgum (Colloides Naturels Inc., Bridgewater, N.J., U.S.A.), water-soluble adhesive film-forming polymers based on acacia gum, showed considerable promise as an inhibitor of after-cooking darkening of potatoes (Mazza and Qi 1991).
Emulsion and bilayer coatings
Recent emphasis and interest in the development of edible coatings have been focused on composite or bilayer coatings, such as integrating proteins, polysaccharides, and/or lipids together for improving functionality of the coatings. This is based on the fact that each individual coating material has some unique, but limited, functions and that together their functionality can be enhanced. Polysaccharides and proteins are polymeric and hydrophilic in nature, thus good film-formers with excellent oxygen, aroma, and lipid barriers at low relative humidity. However, they are poor moisture barriers compared to synthetic moisture-barrier films such as low-density polyethylene (LDPE). On the other hand, lipids are hydrophobic with better moisture-barrier properties than those of polysaccharides and proteins. However, the nonpolymeric nature limits their cohesive film-forming capacity (Krochta 1997). In composite films and coatings, the polysaccharide or protein provides the film integrity and entraps the lipid component, and the lipid component imparts the moisture-barrier property (Krochta 1997).
Composite film/coating can be categorized as a bilayer or a stable emulsion. For bilayer composite films/coatings, lipid generally forms an additional layer over the polysaccharide or protein layer, while the lipid in the emulsion composite films/coatings is dispersed and entrapped in the matrix of protein or polysaccharide. The amphiphilic character of proteins enables proteins to stabilize the protein–lipid emulsions through the balance between forces, primarily electrostatic and hydrophobic. Polysaccharides stabilize emulsions by strongly attaching to the surface of the lipid and significantly protruding into the continuous phase to form a polymeric layer or a network of appreciable thickness (Callegarin and others 1997). In many cases, addition of emulsifier is required to improve emulsion stability.
The barrier and mechanical properties of the composite films/coatings are affected by the composition and distribution of the hydrophobic substances in the film/coating matrix (Kamper and Fennema 1985; Debeaufort and others 1993). In general, bilayer films/coatings are more effective water vapor barrier than emulsion films/coatings due to the existence of a continuous hydrophobic phase in the matrix, and their moisture-barrier property can be improved by increasing the degree of lipid saturation and chain length of fatty acids (Kamper and Fennema 1984a, 1984b; Hagenmaier and Shaw 1990). For emulsion composite films/coatings, the type of lipid, location, volume fraction, polymorphic phase, and drying conditions significantly impact moisture-barriers property (Gontard and others 1994). Moisture barriers of whey protein–lipid emulsion films/coatings are improved when the hydrocarbon chain length of fatty acid alcohols and monoglycerides increased from 14 to 18 carbon atoms (McHugh and Krochta 1994a). Beeswax and fatty acids are more effective in reducing water vapor permeability of WPI-based emulsion films/coatings than fatty acid alcohols due to the lipid polarity.
The improved moisture-barrier properties of composite coatings have made them promising candidates for coating fresh and minimally processed fruits and vegetables. Cole (1969) reported that a bilayer coating formed with amylose ester of fatty acids and protein prevents dehydration and oxidative degradation of fruits and vegetables. Composite coatings with soy or zein protein with amylose ester or fatty acids provided an effective moisture barrier on carrot and fresh-cut apple slices (Williams 1968). Wheat gluten with lipid (beeswax, stearic acid, and palmitic acid) based bilayer coatings significantly retained firmness and reduced weight loss of fresh strawberries (Tanada-Palmu and Grosso 2005). Chitosan–lauric acid composite coatings prevented fresh-cut apple slices from browning and water loss (Pennisi 1992). A casein–lipid emulsion coating formed a tight matrix that binds to the cut apple surfaces and protects apple slices from moisture loss and oxidative browning (Krochta and others 1988, 1990). A sodium caseinate/stearic acid emulsion coating reduced white blush and respiration rate of peeled carrots, and a calcium caseinate acetylated monoglyceride emulsion coating reduced water loss of apples, celery sticks, and zucchini as a result of increased water vapor resistance of the emulsion coatings (Avena-Bustillos and others 1994a, 1994b, 1997). Caseinate–lipid emulsion coatings offer advantages over commercial wax coatings in that they can be applied to fresh produce at room temperature. The protein matrix also improves adhesion of the coatings to food surfaces. HPMC–lipid composite coatings consisting of beeswax or shellac significantly reduced texture loss and internal breakdown of plums (Perez-Gago and others 2003a). Composite coatings prepared from WPI or WPC as the hydrophilic phase and beeswax or carnauba wax as the lipid phase exerted an antibrowning effect on fresh-cut apples (Perez-Gago and others 2003b, 2005, 2006). Locust bean gum, shellac and beeswax coatings prolonged the storability of the cherries by reducing moisture loss (Rojas-Argudo and others 2005). An emulsion coating with CMC as the hydrophilic phase and paraffin wax, beeswax, or soybean oil as the hydrophobic phase also extended shelf life and reduced weight loss of apples, peaches, and pears (Toğrul and Arslan 2004, 2005).