EDIBLE COATINGS: APPROACH TO EXTEND SHELF-LIFE IN FRUITS

Edible coatings seem to be a promising technique to increase the shelf-life of fruits while maintaining their nutrition value, quality, and safety during storage. The kind, application, and dose of edible coatings are regulated by the standards of the country where either fruits are being exported or coating is being applied. The characteristics of fruit to be preserved or coated is determined by the composition or functionality of edible coatings. The gas and water vapor permeability of formulated edible coatings is the best criteria to evaluate their effectiveness in the storage life of fresh produce. The diffusion characteristics of flesh and skin of fruits must be studied along with their internal gas compositions after coating them. Also, the impact of edible coating on the fruit properties is necessary to determine while selecting the right coating material for fruits. Numerous research works revealed in the literature (Table 7.2) that the application of edible coatings not only enhance the shelf-life but also improve the sensory properties and barrier properties of fruit skin.

TABLE 7.2 Summary of Application of Different Edible Coatings on Fruits.

Fruit name

Coating substance(s)

Significant function(s)

References

Grapes

Aloe vera gel

Water barrier and sensory improvement

Chauhan et al. (2014)

Lemongrass oil, chitosan, glacial and acetic acid

Ensure microbiological safety

Ohetal. (2017)

Apple

Whey protein and casein

Oxygen barrier

Le Tien et al. (2001)

Pullulan based coatings

Retain the sensory attributes

Wu and Chen (2013)

Sodium alginate

Retain the sensory attributes and antimicrobial carrier

Rojas-Grau et al. (2007)

Fruit name

Coating substance(s)

Significant function(s)

References

Candelilla wax based coating

Preserve sensory qualities, reduce weight loss, and microbial load

Ochoa et al. (2011)

A. occidentale

L. tree gum with Sorbitol, Tween 80

Water vapor barrier and improve the opacity and mechanical properties

Carneiro-da-Cunha et al. (2009)

Carnauba wax

Water barrier

Chiumarelli and Hubinger (2012)

Avocado

Methylcellulose

Oxygen/carbon dioxide/ water barrier

Maftoonazad and Ramaswamy (2005)

Orange

HPMC and lipid coating containing potassium sorbate, sodium benzoate, sodium propionate, stearic acid and glycerol

Antifungal properties

Valencia-Chamorro et al. (2009)

Tomato

Aloe vera gel

Water barrier and sensory improvement

Athmaselvi et al. (2013)

HPMC and beeswax

Water barrier, retain the color, firmness, and control respiration rate

Fagundes et al. (2015)

Cherry

S emperfresh™( sucrose esters of fatty acids, CMC Sodium salt, mono- and diglycerides)

Oxygen/water barrier and antimicrobial carrier

Yaman and Bayo- indirli (2002)

Casein

Oxygen/carbon dioxide/ water barrier

Certel et al. (2004)

Berry

cactus

Sodium caseinate, sorbitol and glycerol

Phytochemical retention

Correa-Betanzo et al. (2011)

Kiwifruit

Pullulan

Oxygen/carbon dioxide/ water barrier

Diab et al. (2001)

Apricots

Pectins based coatings

Improve appearance, thermal and barrier properties.

Gorrasi and Bugatti (2016)

Blueberry

Semperfresh™ (SF)

Decreased weight loss

Duan et al. (2011)

calcium caseinate

Delayed fruit ripening and great firmness

Duan et al. (2011)

Peeled litclii fruit

Chitosan

Chitosan

Oxygen/water barrier oxygen barrier

Dong et al. (2004) Jiang et al. (2005)

Fruit name

Coating substance(s)

Significant function(s)

References

Guava

Candelilla wax and mineral oil

Reduce weight loss ethylene

Emission and retain glossiness, color and firmness

Thomas et al. (2005)

Mango

Wax; zein, shellac based cellulose derivative

Oxygen/carbon dioxide/ water barrier

Hoa et al. (2002)

Chitosan, aloe vera

Moisture barrier

Chauhan et al. (2014)

Citrus

Chitosan

Oxygen/carbon dioxide/ water barrier

Fornes et al. (2005)

Peach

Wax and CMC

Water barrier

Togrul and Arslan (2004)

Papaya

Chitosan, aloe vera, papaya leaf extract

Delayed ripening

Marpudi et al. (2011)

Cantaloupe

Aloe vera gel, CMC, ascorbic acid and glycerol

Water barrier, retain color and firmness

Yulianingish et al. (2013)

Plum

HPMC/lipid composite

Oxygen/carbon dioxide/ water barrier

Perez-Gago et al. (2003)

Quince

Semperfresh™

Oxygen/carbon dioxide/ water barrier

Yurdugul (2005)

Raspberry

Chitosan

Water barrier; Ca2+, vitamin E carrier

Han et al. (2004b)

Pectin and sodium alginate coatings

Maintain color, taste, antioxidant capacity and reduce the weight loss, and microbial growth.

Guerreiroa et al. (2015)

Sapota

Aloe vera gel

Retain color, firmness, sweetness, and juiciness

Padmja and Basco (2014)

Strawberry

Cactus mucilage

Oxygen barrier

Del-Valle et al. (2005)

Caseinate-whey protein

microbial barrier

Vachon et al. (2003)

Chitosan

Water barrier; Ca2+, vitamin E carrier

Han et al. (2004a, 2004b)

Sodium alginate & calcium Alginate gel

Water barrier, antimicrobial carrier

Moadenia et al. (2010)

Pullulan

Oxygen/carbon dioxide/ water barrier

Diab et al. (2001)

Candelilla wax, guar gum and glycerol

Antifungal agent

Oregel-Zamudio et al. (2017)

Fruit name

Coating substance(s)

Significant function(s)

References

Wheat gluten-based

Oxygen/water barrier

Tanada-Palmu and Grosso (2005)

Chitosan; HPMC

Water barrier; antimicrobial carrier

Park et al. (2005)

EDIBLE COATINGS: NOVEL CARRIER FOR ACTIVE COMPONENTS

Edible coatings have the distinct feature of fusing the several active ingredients so that the stability, safety shelf life, quality, and nutrition of the fresh fruits can be improved. The active components that may be added into the matrix include the following materials:

  • 1. Plasticizers
  • 2. Emulsifiers
  • 3. Antimicrobial agents
  • 4. Texture enhancers
  • 5. Nutraceuticals
  • 6. Antibrowning agents
  • 7. Antioxidants

PLASTICIZERS

Edible coatings especially based on polysaccharides and proteins often need the plasticizers due to their brittle and rigid structure. The boundless interactions among the polymers make their structure stiff and therefore, numerous plasticizers, such as acetylated monoglyceride, glycerol, sucrose, and polyethylene glycol are extensively employed in coating formulations to improve the flexibility by reducing the polymer glass transition temperature (Krochta, 2002; Schmid, 2013; Galus et al., 2015). Besides this, the coating base and water vapor resistance may also be influenced by the plasticizers’ addition (Sothornvit and Krochta, 2000); however, the water barrier of coatings can be weakened if plasticizers used are hydrophilic in nature.

EMULSIFIERS

Emulsifiers are generally the surface-active agents with amphiphilic nature. They are considered necessary in the preparation of polysaccharide or protein-based coating as they contain the lipid molecules. They have the potential to lower the surface tension of water-air or water-lipid interface. They also aid in the modification of coating wettability and adhesion by controlling the surface energy (Krochta, 2002). Their addition improves the hydrophilicity of whey protein-based coatings and enhances the oxygen barrier (Lin and Krochta, 2003).

ANTIMICROBIAL AGENTS

The microbial stability of fruits can be enhanced by dipping them into the edible-coating solutions carrying the antimicrobial substances that delay the microbial growth. The reduction of spoilage and disease-causing microorganisms is the main aim of incorporating antimicrobial agents. Their direct addition onto the fruit surface may decrease the effectiveness due to the neutralization or diffusion of antimicrobials quickly inside the fruit from the surface (Min and Krochta, 2005). Therefore, the edible coatings retain the sufficient active substances on the fruit surface thus exhibiting the increased inhibitory action against the microbial load. A variety of antimicrobial agents, including the glyceryl monolaurate, organic acids (acetic, lactic, sorbic, propionic and benzoic acid), polypeptides (nisin, peroxidase, lysozyme, and lactoferrin), nitrites, essential oils (lemongrass, cinnamon, and oregano), sulfites and many more are widely used in the coating matrices (Franssen and Krochta, 2003).

Being membrane soluble, the protonated acids penetrate into the cytoplasm by the process of diffusion (Ricke, 2003). The active component present in the essential oil inhibits the action of several pathogenic microorganisms (Delaquis et al., 2002), however, their inhibition mechanism is not yet fully understood (Lambert et al., 2001). Hydrophobicity of essential oils permits them to enter the cell membranes and mitochondria, thus interrupting the interior structure and resulting in the more permeable membranes. In addition, the incorporation of essential oil fulfills the current consumer demands to get natural products (Burt, 2004). One major disadvantage of essential oil is their interaction with the fruit constituents thus influencing their sensoiy properties especially taste and flavor (Gutierrez et al., 2008). The edible coating containing carrageenans and organic acids, like ascorbic acid, citric acid, and oxalic acid increased the shelf-life of apple slices by 14 days at 3°C (Lee et al., 2003). The destruction of pathogens like E. coli and Streptococcus faecalis has been increased by adding the lysozyme in the chitosan-based coating (Park et al., 2004).

TEXTURE ENHANCERS

The pectic enzymes found in fruits may lead to the softening due to the disruption of subcellular compartmentalization and intermingling of enzymes and substrates while storage (Toivonen and Brummell, 2008). The best method to retain the firmness of fruit is giving them calcium salt treatments due to the interaction of calcium ions with pectic substances to create a cross- linked network. This may be beneficial in maintaining the fruit firmness and delaying the process of ripening and senescence (Cheour and Souiden, 2015). To prevent the softening, texture enhancers are industrially incorporated in the edible coatings. The softening in strawberries is significantly reduced at 4°C by adding calcium gluconate (1%) in the chitosan coating (Hemandez-Munoz et al., 2008). The firmness of ffozen-thawed raspberries is enhanced by 25% when chitosan coating along with calcium ions is applied (Han et al., 2004).

NUTRACEUTICALS

Recently, there is a trend of adding nutraceutical components in food products to achieve health benefits. Keeping this in view, some studies have been conducted about the edible coatings containing the nutraceuticals, like vitamins, minerals, and fatty acids, so that the nutritional value can be boosted. But it is essential to know the impact of bioactive compounds on the mechanical and barrier properties of coatings. Mei and Zhao (2003) suggested the potential of milk protein coating containing the calcium (10% w/v) and vitamin E (0.2% w/v) in the improvement of water barrier characteristic of produce. But in contrast to this, the chitosan-based coatings with zinc lactate and vitamins E increase the water transfer resistance with the compromise in the mechanical properties, mainly tensile strength (Park and Zhao, 2004). Interestingly the alginate or gellan-based formulation was prepared successfully containing the cells of live bifidobacteria and coated to fresh-cut apple and papaya (Tapia et al., 2008). Bifidobacterium lactis Bb-12 remain greater than 106 CFU/g up to 10 days at 4°C that exhibits the potential of polysaccharide-based coatings to provide favorable conditions for the probiotics on the fruit surface. This study opens the door to conduct the investigations for developing novel coatings containing live probiotics.

ANTIBROWNING AGENTS

Fruits, when processed, may induce unacceptable color while marketing and storage due to the release of polyphenol oxidase enzyme that produces brown color pigments from the phenolic compounds in the presence of oxygen (Queiroz et ah, 2008). This enzymatic browning may be reduced using the antibrowning agents, like citric acid, ascorbic acid, cysteine, N-acetylcysteine, oxalic acid, and reduced glutathione (Gonzalez-Aguilar et ah, 2004). Antibrowning agents may be classified into the numerous groups, like complexing agents, chelators, reducing agents, and enzyme inhibitors, on the basis of action of browning inhibition (Altunkaya and Gokmen, 2008; Son et ah, 2001). The addition of ascorbic, citric, and oxalic acids in carrageenan and whey protein coating was beneficial in the color retention of fresh-cut apple slices while the storage of 2 weeks (Lee et ah, 2003). Likewise, Oms-Oliu et ah (2008) demonstrated the maintenance of color during the storage of pears for 2 weeks when coated with gellan, alginate, or pectin coating containing the N-acetylcysteine and glutathione.

7.6.7 ANTIOXIDANTS

Antioxidants are incorporated into the edible coatings matrix to guard the fruits from degradation and discoloration. Ferulic acid offers many physiological benefits including the antioxidant properties. It is added in the soy protein coating to coat the fresh-cut apples (Alves et ah, 2017). Some antibrowning agents, like ascorbic acid, N-acetylcysteine and glutathione act as an antioxidant as well. a-Tocopheryl acetate, when added in the chitosan coating, minimizes the alteration in color of frozen and fresh strawberries (Han et ah, 2004b).

 
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