Advances in Fresh and Minimaly Fruits

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Recent Advances in Edible Coatings for Fresh and Minimally ProcessedFruitsMaria Vargas a; Clara Pastor a; Amparo Chiralt a; D. Julian McClements b; Chelo Gonzlez-Martnez aa Department of Food Technology-Institute of Food Engineering for Development, UniversidadPolitcnica de Valencia, Valencia, Spain b Biopolymers and Colloids Research Laboratory, Departmentof Food Science, University of Massachusetts, Amherst, MA, USA

To cite this Article Vargas, Maria, Pastor, Clara, Chiralt, Amparo, McClements, D. Julian and Gonzlez-Martnez,Chelo(2008) 'Recent Advances in Edible Coatings for Fresh and Minimally Processed Fruits', Critical Reviews in FoodScience and Nutrition, 48: 6, 496 511To link to this Article: DOI: 10.1080/10408390701537344URL: http://dx.doi.org/10.1080/10408390701537344

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Critical Reviews in Food Science and Nutrition, 48:496511 (2008)Copyright C Taylor and Francis Group, LLCISSN: 1040-8398DOI: 10.1080/10408390701537344

Recent Advances in Edible Coatingsfor Fresh and MinimallyProcessed Fruits

MARIA VARGAS,1 CLARA PASTOR,1 AMPARO CHIRALT,1D. JULIAN McCLEMENTS,2 and CHELO GONZ ALEZ-MARTINEZ11Department of Food TechnologyInstitute of Food Engineering for Development, Universidad Politecnica de Valencia,Camino de Vera s/n, 46022, Valencia, Spain2Biopolymers and Colloids Research Laboratory, Department of Food Science, University of Massachusetts, Amherst, MA01003, USA

The development of new edible coatings with improved functionality and performance for fresh and minimally processed fruitsis one of the challenges of the post harvest industry. In the past few years, research efforts have focused on the design of neweco-friendly coatings based on biodegradable polymers, which not only reduce the requirements of packaging but also lead tothe conversion of by-products of the food industry into value added film-forming components. This work reviews the differentcoating formulations and applications available at present, as well as the main results of the most recent investigationscarried out on the topic. Traditionally, edible coatings have been used as a barrier to minimize water loss and delay thenatural senescence of coated fruits through selective permeability to gases. However, the new generation of edible coatingsis being especially designed to allow the incorporation and/or controlled release of antioxidants, vitamins, nutraceuticals,and natural antimicrobial agents by means of the application of promising technologies such as nanoencapsulation and thelayer-by-layer assembly.

Keywords Edible coatings, fruits, biopolymers, composites, micro- and nanoencapsulation, multilayered structures

INTRODUCTION

The application of edible coatings is one of the most inno-vative methods to extend the commercial shelf-life of fruits by,among other mechanisms, acting as a barrier against gas trans-port and having a similar effect on the storage under controlledor modified atmospheres (Park, 1999). Two of the most impor-tant advantages of this technology are the reduction of syntheticpackaging waste and the incorporation of preservatives and otherfunctional ingredients into biodegradable raw materials obtainedfrom natural sources. The latter is in response to the growing de-mand for safe, healthy foods as well as to the increasing concernsover the environment.

When developing edible coatings, the classical approach hasbeen to characterize their properties when they are cast and thenpeeled off from a plate. This approach can be very useful to

Address correspondence to Chelo Gonzalez-Martinez, Department of FoodTechnology, Institute of Food Engineering for Development, UniversidadPolitecnica de Valencia, Camino de Vera s/n, 46022, Valencia, Spain. Telephone:0034 96 387 93 62 Fax: 0034 96 387 73 69. E-mail: [email protected]

compare coatings, but an important weakness is that it does nottake into account the interaction between a coating and a fruitsurface and its subsequent influence on coating properties. Thus,it is of interest to gather some of the available methodologiesthat are used to characterize coated fruits and to discuss theirlimitations and significance.

This paper reviews some of the most recent advances in theapplication and development of edible coatings for fresh andminimally processed fruits, focusing on the main methodologiesavailable to characterize coated commodities, in order to set thebasis to obtain new and highly functional edible coatings byintroducing some of the techniques that have not yet been usedin food science.

EDIBLE COATINGS: DEFINITIONS ANDREGULATORY STATUS

Edible coatings may be defined as a thin layer of materialthat covers the surface of the food and can be eaten as part ofthe whole product. The composition of edible coatings must

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RECENT ADVANCES IN EDIBLE COATINGS FOR FRESH AND MINIMALLY PROCESSED FRUITS 497

therefore conform to the regulations that apply to the food prod-uct concerned (Guilbert et al., 1995).

According to the European Directive (ED, 1995; 1998) andthe USA Code of Federal Regulations (FDA, 2006) edible coat-ings are those coatings that are formulated with food-grade addi-tives. According to what the USA Code of Federal Regulationsstates about their application to fresh citrus fruits, the amountof edible coating ingredients used must be only that which isnecessary to accomplish the intended effect, and the ingredientshave to be GRAS and be listed in the above-mentioned Code.

Among the ingredients that can be incorporated into theformulation of edible coatings, the European Directive (1995;1998) includes the following: arabic and karaya gum, pectins,shellac, beeswax, candelilla wax, and carnauba wax. This Direc-tive was modified in 1998 by introducing new ingredients suchas lecithin, polysorbates, fatty acids, and fatty acid salts. On theother hand, the Food and Drug Administration mentions otheradditives used as components of protective coatings applied tofresh fruits and vegetables like morpholine, polydextrose, sor-bitan monostearate, sucrose fatty acid esters, cocoa butter, andcastor oil.

As stated by Kester and Fennema (1986), edible coatingshave to follow some functional requirements, which depend onthe kind of coated product and its metabolic pathways, such as:

Sensory properties: Edible coatings must be transparent, taste-less and odourless

Barrier properties: Coatings must have an adequate water va-por and solutes permeability and selective permeability togases and volatile compounds.

Moreover, the edible coating formulations have to containsafe, food-grade substances and the cost of the technology andraw materials from which coatings are produced has to be rela-tively low.

COMPOSITION OF EDIBLE COATINGS FOR FRESHAND MINIMALLY PROCESSED FRUITS

There is a very wide range of compounds that can be usedin the formulation of edible coatings and their choice dependsmainly on the target application. The major components arepolysaccharides, proteins and lipids and their properties are dis-cussed below. The minor components usually include polyolsacting as plasticizers (such as glycerol) or acid/base compoundsused to regulate pH (such as acetic or lactic acid).

Polysaccharides

Polysaccharides are the most widely used components foundin edible coatings for fruits (Kester and Fernema, 1986; Krochtaand de-Mulder-Johnston, 1997), as they are present in most com-

mercially available formulations. Polysaccharides show effec-tive gas barrier properties although they are highly hydrophilicand show high water vapor permeability in comparison withcommercial plastic films.

The main polysaccharides that can be included in edi-ble coating formulations are starch and starch derivates, cel-lulose derivates, alginate, carrageenan, chitosan, pectin, andseveral gums. Table 1 shows the most relevant properties ofpolysaccharide-based films, together with the permeability val-ues of common synthetic materials used for food packaging.

On the molecular level, polysaccharides vary according totheir molecular weight, degree of branching, conformation, elec-trical charge, and hydrophobicity. Variations in these molecularcharacteristics will lead to variations in the ability of differentpolysaccharides to form coatings, as well as to variations inthe physicochemical properties and performance of the coatingsformed.

Almost all of them are highly water soluble, so they cannotbe used for coating samples that will remain immersed in asolution (for example dipped in juices) or in a high relativehumidity environment. In some cases, cross-linking treatmentsin the presence of monovalent and divalent ions can be used tomake the coatings insoluble.

Starch is the natural polysaccharide most commonly usedin the formulation of edible coatings because it is inexpensive,abundant, biodegradable, and easy to use. Native granular starchis converted into a thermoplastic material by conventional meth-ods in the presence of plasticizers, such as water and glycerol(Thire et al., 2003). Coatings made from starch become brittlein dry atmospheres and lose strength and barrier properties inhigh humidity (Peterson and Stading, 2005). The addition ofplasticizers overcomes their flexibility and extensibility (Maliet al., 2002). In addition, during storage at high relative humid-ity or high plasticizer content starch-based materials are in arubbery state, which allows the development of crystallinity byincreasing macromolecular mobility (Delville et al., 2003). Inthese conditions, retrogradation, which involves amylose andamylopectin recrystallization, does occur (Rindlav et al., 1997).Crystallites may be acting as physical crosslinking points, whichgenerate internal stresses or cracks which in turn lead to the dam-age of the coatings (Delville et al., 2003), thus modifying thephysicochemical properties of the starch-based material (Famaet al., 2007).

Another polysaccharide that is of high interest is chitosan, ob-tained from the deacetylation of chitin (poly--(14)-N-acetyl-D-glucosamine), which is mainly obtained from crab and shrimpshells (Hirano, 1999). Films and coatings based on chitosan haveselective permeability to gases (CO2 and O2) and good mechani-cal properties. However, their uses are limited mainly because oftheir high water vapor permeability (Butler et al., 1996; Caner etal., 1998). Moreover, chitosan shows antifungal and antibacterialproperties, which are believed to be originated from its poly-cationic nature (Cuero, 1999; Tharanathan and Kittur, 2003),although the precise mechanism of its antimicrobial activity isstill unknown (Srinivasa and Tharanathan, 2007). In addition,

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the cationic nature of chitosan offers the possibility to take ad-vantage of the electrostatic interactions with anionic polyelec-trolytes, such as pectin to develop laminate coatings (Hoaglandand Parris, 1996) and multilayered structures (Marudova et al.,2005; Krzemisk, 2006).

Cellulose (poly--(14)-D-glucopyranose) derivativessuch as methylcellulose (MC), hydroxypropylmethylcelluloseand the ionic carboxymethylcellulose are also commonlyfound in the formulation of edible coatings, especially incommercial products. MC is formed by the alkali treatmentof cellulose, followed by a reaction with methylchloride. MCis stable at a wide range of pH (211), compatible with otherwater-soluble polysaccharides, and being the least hydrophilicof the cellulose ethers, it would be expected to be moreresistant to water transmission (Kester and Fennaema, 1986;Nisperos-Carriedo, 1994). MC is a non-ionic ether, exhibitsthermal gelation, high solubility, and efficient oxygen and lipidbarrier properties (Turhan and Sahboz, 2004; Bravin et al.,2004).

Alginates and carrageenans can also be used to prepare ediblecoatings. Alginates are the salts of alginic acid, which is a linearcopolymer of D-mannuronic and L-guluronic acid monomers.Alginate coating formation is based on the ability of alginates toreact with di-valent and tri-valent cations such as calcium, fer-rum or magnesium, which are added as gelling agents (Cha andChinnan, 2004). Carrageenan is a complex mixture of at least fivedifferent water-soluble galactose polymers designated as , ,, and -carrageenan. Gelation of and -carrageenan occursin the presence of monovalents or divalent cations. Carrageenanfilm formation includes this gelation mechanism during moder-ate drying, leading to a three-dimensional network formed bypolysaccharide double-helices and to a solid film after solventevaporation (Karbowiak et al., 2007).

Table 2 Proteins used in the formulation of edible coatings for fruits and properties of protein-based films.

Water vapor permeability (WVP)a O2/CO2 permeability (P)b

WVP RH gradient T RH TProtein Source Others 1011 (%/%) (C) PO2 PCO2 (%) (C) ReferencesZein Corn GRAS 8.913.2 0/85 21 0.25 1.13 60 20

Park, 1999; Gennadios and Weller, 1990;McHugh and Krochta, 1994; Bai et al., 2003

Gluten Wheat Fragile 4.3 0/50 23 1.88 46.88 91 25Gontard et al., 1996; Gontard et al., 1992;

Guilbert et al., 1996; Hernandez-Munoz etal., 2004

Soy Soybean Flexible 354 100/50 25 0.067 - 50 25Gennadios and Weller, 1991; Cho and Rhee,

2004; Cho et al., 2007Whey proteins Milk Flexible 417 100/55 25 0.001- 0.01 - 50 25

Krochta, and De-Mulder-Johnston, 1997;McHugh and Krochta, 1994; McHugh andKrochta, 1994; McHugh and Krochta, 1994,Mei and Zhao, 2003; Mate et al., 1996

Sodium caseinate Milk Brittle 42.5 0/81 25 0.76 4.56 77 25Guilbert et al., 1996; Dangaran et al., 2006;

Khwaldia et al., 2004

a(g m1s1Pa1);b(mL.m/(m2.d.Pa); *with plasticizer

Proteins

Proteins that can be used in the formulation of edible coat-ings for fruits include those derived from animal sources, suchas casein and whey protein, or obtained from plant sources likecorn-zein, wheat gluten, soy protein, peanut protein, and cotton-seed protein (Gennadios, 2002). Table 2 shows examples of theseproteins used for coating purposes together with their main func-tional characteristics. Proteins exhibit a wide variety of differentmolecular characteristics depending on their biological originand function. For example, proteins may vary in their molecularweights, conformations (globular, random coil, helix), electri-cal characteristics (charge versus pH), flexibilities (rigid versusflexible), and thermal stabilities. Differences in these molecularcharacteristics will again ultimately determine the ability of par-ticular proteins to form coatings and the characteristics of thecoatings formed.

Casein based edible coatings are attractive for food appli-cations due to their high nutritional quality, excellent sensoryproperties, and good potential for providing food products withadequate protection against their surrounding environment.

Whey proteins have been the subject of intense investiga-tion over the past decade or so. With the addition of plasticizer,heat-denatured whey proteins produce transparent and flexiblewater-based edible coatings with excellent oxygen, aroma, andoil barrier properties at low relative humidity. However, the hy-drophilic nature of whey protein coatings causes them to be lesseffective as moisture barriers.

Proteins that are insoluble in water, such as corn zein andwheat gluten, produce insoluble coatings, whereas proteins thatare soluble in water produce coatings of varying solubility, de-pending on the protein and the conditions of coating forma-tion and treatment (Krochta, 2002). For example, whey protein

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500 M. VARGAS ET AL.

Table 3 Lipids used in the formulation of edible coatings for fruits and properties of lipid-based films.

Water vapor permeability (WVP)a O2/CO2 permeability (P)b

WVP RH gradient T RH TLipid Source Others 1011 (%/%) (C) PO2 PCO2 (%) (C) ReferencesShellac (E904) Insects GRAS 0.4620.66 0/100 30 0.083 0.29 60 20 Bai et al., 2003; Hagenmaier and

Baker, 1995; Hagenmaier, 2000Beeswax (E901) Beeswax GRAS 0.058 0/100 25 0.092 0 25

Hagenmaier and Baker, 1997Candelilla wax (E902) Plant exudates GRAS 0.017 0/100 25 0.537 2.04 60 30

Guilbert, 2000; Hagenmaier andBaker, 1997; Bai et al., 2003

Carnauba wax (E903) Plant exudates GRAS 0.033 0/100 25 0.016 - 0 25Guilbert, 2000; Martin-Polo et al.,

1992Fatty acids (E471) Plant or animal material GRAS 0.223.47 12/56 23

116; 151; Martin-Polo et al., 1992;Martin-Polo et al., 1992

a(g m1s1Pa1); b(mL.m/(m2.d.Pa).

isolate produces totally water-soluble coatings but heat-denatured solutions of whey protein isolate produce coatingsin which the protein is insoluble (Perez- Gago et al., 1999).

Moreover, protein solubility is considered to be dependent onthe pH, so this parameter should be taken into account duringthe formulation and application of coatings. Only if the proteinshave been denatured, does solubility becomes unimportant.

Lipids

Edible lipids used to develop edible coatings are shown inTable 3 and include beeswax, candelilla wax, carnauba wax,triglycerides, acetylated monoglycerides, fatty acids, fatty alco-hols, and sucrose fatty acid esters. Edible resins include shellacand terpene resin.

Lipid-based edible coatings have a low affinity for water,which explains why they have low water vapor permeability.The latter is extremely important, as a great number of studiesdeal with the use of coatings on fresh fruits and vegetables tocontrol their desiccation (Morillon et al., 2002).

Due to the fact that each hydrophobic substance has its ownphysicochemical properties, each lipid-based edible coating be-haves in a different way as regards moisture transfer. The po-larity of lipid constituents has to be considered, that is to say,the distribution of electrostatic potentials on the molecules thatdepends on the chemical group, aliphatic chain length, and onthe presence of unsaturation. As the carbon number of fatty al-cohols and fatty acids increases (from 14 to 18), so does theireffectiveness to act as moisture barriers, because the non-polarpart of the molecule increases and therefore favors neither watersolubility in the film nor, as a consequence, moisture transferacross the film (Morillon et al., 2002).

Composites

The main disadvantage of lipid based coatings is their poormechanical properties, and thus, at present, research efforts are

focused on the design of composite coatings that are basedon both lipids and hydrocolloids (proteins or polysaccharides)to take advantage of the special functional characteristics ofeach group, thereby diminishing their drawbacks (Greener andFennema, 1994). Generally, lipids contribute to the improve-ment of the water vapor resistance whereas hydrocolloids con-fer selective permeability to O2 and CO2, as well as durability,structural cohesion, and integrity (Krochta, 1997).

Composite coatings can be created by the subsequent depo-sition of different layers (multilayered coatings) or can be madeby the deposition of a single layer of material. Bilayer coat-ings are formed in two stages: In the first stage the layer ofpolysaccharide or protein is cast and dried and in the secondone, the lipid layer is applied (Krochta, 1997). As an example,Debeaufort et al. (2000), developed bilayers by adding a mixtureof lipids (paraffin oil, paraffin wax, or a mixture of hydrogenatedpalm oil and triolein) onto a methylcellulose layer. Wong et al.(1994), coated apple cubes with double layers of polysaccha-rides (cellulose, carrageenan, pectin, or alginate) and acetylatedmonoglyceride.

Nevertheless, in monolayer composite edible coatings, thelipid is dispersed in the hydrophilic phase of an emulsion(Shellhammer and Krochta, 1997). Bertan et al. (2005), usedstearic and palmitic acid to allow the incorporation of a hy-drophobic exudate into a gelatine-based coating. This mixturewas emulsified using triacetin as plasticizer. Bosquez-Molina etal. (2003), obtained emulsified coatings by mixing mesquite gum(structural matrix) and a combination of some lipids (candelillawax, mineral oil, oleic acid, or beeswax).

Emulsified coatings are less efficient than bilayers due to thenon-homogeneous distribution of lipids but they have receivedmore interest because they need only one drying step instead ofthe two necessary for the bilayer films, and they can be appliedon food at room temperature. Moreover, being both, hydrophilicand lipophilic, allows them to adhere to any support, whateverits polarity, and to exhibit good mechanical resistance (Quezada-Gallo et al., 2000).

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CHARACTERIZATION OF COATED FRUITS

The use of edible coatings to extend the shelf-life and improvethe quality of fresh and minimally processed fruits has comeunder examination during the past few years due to their eco-friendly and biodegradable nature. Moreover, these outer layerscan provide a supplementary and sometimes essential means ofcontrolling physiological, morphological, and physicochemicalchanges in fruits.

The effectiveness and functionality of each coating dependson their physicochemical and barrier properties, which are veryoften closely related with the molecular arrangement of the dif-ferent components of the coating, that is, its microstructure. Thecoating is usually characterized and afterwards, the propertiesare correlated with those observed in the coated fruit, however,sometimes, these coating properties are affected by the fruit sur-face during its application.

On the other hand, after the coating has been applied, the fruitresponse has to be analyzed and therefore, the characterization ofthe coated fruit becomes an important task. The most importantproperties to be measured when characterizing coated fruits arethe following.

Thickness and Microstructure

The thickness of the edible coatings is directly related withother important properties such as permeability to gases. Thecoatings that are obtained by casting and drying on levelledplates can be pealed off and their thickness can be easily mea-sured by using a hand-held micrometer. However, it is difficultto measure the thickness of coatings accurately once they havebeen applied to the fruit surface. In these cases, an estimation ofthe coating thickness can be obtained by means of the quantifi-cation of the surface solid density, SSD (Villalobos et al., 2004;Vargas et al., 2006; 2006b) (Eq. 1).

SSD = MFa XsAs

(1)

The mass fraction of solids (Xs) of each film-forming solu-tion has to be considered to calculate SSD as well as themass of the coating solution adhered to the fruit surface (MFa),which can be calculated by weighing the fruits before and af-ter coating application. The estimation of the average samplearea (As) sometimes requires the use of image analysis tech-niques or volumetric and surface area measurements; the moreirregular the shape of the fruit, the more complicated thesemeasurements become. Films thickness can be also defined interms of coating solution properties such as viscosity, density,draining time, and the solid concentration of the original solu-tion. In addition, as stated by Cisneros-Zevallos and Krochta(2003), surface tension effects can play a major role in the fi-nal thickness, especially when considering a high or mediumhydrophobic surface (low or medium surface energy), like the

skin of most fruits covered with a natural wax layer (Choi et al.,2002).

One of the most promising, recent techniques that could bepotentially used to determine the thickness of coatings is spec-troscopic ellipsometry. This technique has been used for thecharacterization of films and multilayered structures (Schram etal., 2000), although, to the best of our knowledge, it has not yetbeen used in edible coatings. An ellipsometer shines linearlypolarized light on the sample surface and then the ellipticityof the light reflected off the sample surface is analyzed. Theprinciple of the method is based on the determination of thecomplex reflectance ratio of the reflection coefficient of light,polarized both parallel and perpendicular to the plane of inci-dence. The spectroscopic ellipsometry data are interpreted byfitting the calculated ellipsometric response of an optical modelof the presumed surface structure to the experimental data bymeans of a least-squares regression analysis (Zuber et al., 1995;Jellison, 1996). The data are measured over the entire wave-length range and compared to the mathematically generatedmodel to obtain the film thickness and the refractive index of thecoating.

On the other hand, different kinds of microscopy techniques(e.g. confocal, Scanning Electron Microscopy, Atomic ForceMicroscopy, etc.) can be used not only to measure the thicknessof the coating but also to characterize the surface roughness andtopography of coated fruits (Hershko and Nussinovitch, 1998).Moreover, the analysis of coating microstructure plays an im-portant role in reaching a better understanding of the importantcoating properties like permeability to gases and resistance towater vapor transmission.

Water vapor resistance

The direct measurement of the products permeability to wa-ter vapor exchange under controlled environmental conditionswould make it possible to determine in situ if the coating showsthe required properties for a specific combination of storageconditions (Amarante and Banks, 2001). Thus, the water vaporresistance (WVR) of coated fruits can be obtained by monitoringthe weight loss of samples both at a controlled temperature andunder controlled relative humidity conditions and expressed byusing Equation 2 (Wong et al., 1994; Vargas et al., 2006; 2006b;Avena- Bustillos et al., 1994; 1997; Pastor et al., 2005).

W V R = aw %RH100 Pwv

R T AsJ

(2)

where J , is the slope of the weight loss curve in stationary con-ditions, As , is the sample area, aw, the water activity of samples,Pwv , the saturated vapor pressure, T , the absolute temperature,and R, the universal constant of gases.

One of the problems that arises when using this methodologyis how to measure the sample area accurately. This area canbe tailored and easily determined in fresh-cut fruits but can beparticularly difficult to measure in irregular-shaped whole fruits.

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An indirect method of determining the water vapor perme-ability of coated fruits is to form a coating by casting and dryingon a plate and measure its water vapor permeability by stan-dard methods based on gravimetric techniques (ASTM, 1995).Numerous studies have used this technique to characterize thepermeability properties of a wide variety of edible coatings(Gennadios et al., 1994; McHugh and Krochta, 1994; 1994b;1994c; Avena-Bustillos et al., 2006; Clasen et al., 1996; Vil-lalobos et al., 2006). However, it must be pointed out that thesevalues are useful for comparing different formulations, but thatthe permeability properties of coatings could be altered whenthey are applied to a real fruit surface. For instance, they canbe modified because of a partial absorption of the barrier layerby the fruit surface or, in the case of emulsified coatings, bya heterogeneous lipid distribution due to surface irregularities(Morillon et al., 2002). Thus, it is always recommended thatmeasurements be taken on the coated fruit.

Gas Permeability

The gas permeability of coatings can be evaluated by measur-ing the internal composition of coated fruits, generally in termsof O2 and CO2 concentration, as well as some important volatilecompounds such as ethanol and acetaldehyde, which play an im-portant role in the metabolism of coated fruits. The internal atmo-sphere of the coated fruits is typically measured by withdrawingsamples from the core of the fruit with a syringe and injectingthem in a gas chromatograph (Chen and Nussinovitch, 2001).Salveit (1982) used a partial vacuum procedure for extractinginternal gas samples from fruits and vegetables, previously sub-merged in a saturated salt solution. As the pressure is reduced, in-ternal gases expand, and there is a mass flow of gas out of the fruitthrough the lenticels, stomata, pores, and other regions of lowresistance. Gasses dissolved within the tissue will come out ofsolution as the vacuum is applied. Appropriated gas samples canbe removed after the pressure is returned to normal and injectedinto the gas chromatograph. This methodology has been appliedto coated fruits and vegetables by several authors (Cisneros-Zeballos and Krochta, 2003; Avena Bustillos et al., 1994).

Alternatively, changes in the internal composition of coatedfruits can be determined by measuring their respiration rate.To this end, fruits are stored in a tightly-sealed glass container,and the headspace is sampled at different time intervals in orderto analyze CO2 and O2 content by using gas chromatography(Wong et al., 1994; Vargas et al., 2006; 2006b; Pastor et al.,2005; Jiang and Li, 2001; Maftoonazad and Ramaswamy, 2005;Lee et al., 2003).

Appearance: Color and Gloss

Many studies have focused on the study of color, opacity andgloss of edible coatings (Nussinovitch et al., 1996; Ward andNussinovitch, 1996; Trezza and Krochta, 2000; 2000b; 2001).

The changes in fruit color caused by coating application canbe measured by means of colorimeters, which calculate chro-

matic parameters such as luminosity, chroma, and hue, fromthe reflection spectra of samples, considering a standard ob-server/illuminant system (Hutchings, 1999). The differences inluminosity differences can be related with changes in the re-flectance properties of samples after coating is applied.

On the other hand, the gloss of the coated fruits can be mea-sured using a flat surface glossmeter. The results are reported ingloss units (from 0 to 100) relative to a highly polished planarsurface of black glass, which serves as a standard and is usu-ally arbitrarily assigned with a gloss value of 100. The use ofthis technique is limited to planar samples obtained from thecoated fruit peel. Following this procedure, Chen and Nussi-novitch (2001) measured gloss in strips of mandarin peel thatwere coated with hydrocolloids (xanthan or locust bean gum)and wax.

Other Properties

In terms of coating application, it is essential to know prop-erties related with wettability, such as surface free energy, theinterfacial tension between the coating solution and the sur-face of the coated fruit, as well as the contact angle (Choi etal., 2002; Hershko and Nussinovitch, 1998; 1998; Wong et al.,1992). These parameters are mainly determining the amount ofliquid that adheres to the surface and so, the final thickness of thefilm on the fruit. In this sense, Cisneros-Zevallos and Krochta(2003) found that the average liquid film thickness on coatedapples was a function of viscosity, draining time, density of thebiopolymer solutions, surface tension of fruit, surface tension ofliquid, and the surface roughness.

In order to perform contact angle measurements, specimensmust be planar in nature (e.g. fresh-cut fruit slices). Thus, whenperforming the measurements on whole fruits, the coated-peelhas to be cut into small pieces and if possible, fixed on a planarsurface.

On the other hand, it is important to ensure that the coatingshave as little impact as possible on the sensory quality of coatedfruits in terms of color, gloss, basic tastes (bitterness, sourness,and sweetness), aroma, and firmness. The evaluation of the sen-sory attributes of coated fruits is usually performed by means ofdescriptive analysis (Eswaranandam et al., 2006) or consumerand free-choice profiling panels (Han et al., 2005). In some cases,especially when incorporating lipids into coatings, consumersreject the samples because of their artificial color and waxy ap-pearance (Han et al., 2005; Tanada-Palmu and Grosso, 2005).

Finally, it should be mentioned that the changes in the inter-nal atmosphere of coated fruits and the subsequent delay in theirmetabolism are also reflected in their mechanical properties. Thelatter are generally evaluated via compression assays carried outusing an Instron Universal Testing Machine or a Texture Ana-lyzer. Firmness or resistance to fracture are the most commonlyreported parameters when evaluating the quality of stored coatedfruits (Vargas et al., 2006; Tanada- Palmu and Grosso, 2005;Del-Valle et al., 2005).

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APPLICATION OF EDIBLE COATINGS TO FRESH ANDMINIMALLY PROCESSED FRUITS

Edible coating technology is a promising method to preservethe quality of fresh and minimally processed fruits. Researchand development efforts are leading to an improvement of thefunctional characteristics of the coatings, which depends on theproperties of the fruit to be preserved or enhanced. These canbe achieved by a precise optimum control of gas permeability,texture, and color changes by means of quantitative or qualitativechanges in coating formulation.

Table 4 shows some of the formulations that have alreadybeen applied to fresh fruits. It should be mentioned that cellu-lose derivatives are incorporated in most commercial productssuch as Semperfresh (AgriCoat Industries Ltd., Berkshire, UK),Pro-long (Courtaulds Group, London), Nature-Seal (EcoscienceProduct System Divison, Orlando, FL), and Natural Shine 9000(Pace Internacional, Seattle, USA) among others.

Edible coatings can affect the quality of coated fruits in sev-eral different ways, since there are many mechanisms involved.These mechanisms include controlled moisture transfer betweenthe fruit and the surrounding environment, the controlled re-lease of chemical agents like antimicrobial substances, flavorcompounds, and antioxidants; the reduction of the internal oxy-gen partial pressure with a decrease in fruit metabolism, as wellas some kind of structural reinforcement (Shaidi et al., 1999).Therefore, some of the effects that can be observed in coatedfruits during storage are a reduction in respiration rate (Wonget al., 1994; El Gaouth et al., 1991), a decrease in weight loss(Baldwin et al., 1999), a delay in the occurrence of enzymaticbrowning (Baldwin et al., 1999; McHugh and Senesi, 2000; LaTien et al., 2001) and, in general, a significant extension of fruitshelf-life.

Nowadays, there is a new generation of edible coatings thatis being applied to minimally processed (MP) fruits, which arethose fruits that have been cut, peeled and/or slightly processedto be ready to eat but show a quality and freshness similar tothe fresh product, still having living tissues (Perez, 2003). Someexamples of the application of these coatings to MP fruits areshown in Table 5.

The main problem when applying the coatings to MP fruitsis the low adherence presented by the highly hydrophilic cutsurface of the fruit, as it remains wet for a long time and alsobecause of the possible presence of liquid exudates. As a con-sequence, the drying process of the coating on the fruit surfaceslows down (if drying is possible) and a partial loss of coatingintegrity could occur.

As regards the commercial coatings tested on MP fruits, theuse of Nutrisave (Nova Chem, Halifax, NS, Canada) should bementioned. This is a chitosan derivative that has been appliedto MP pears and apples, subsequently reducing the respirationrate and weight loss significantly, and delaying microbial decay(Baldwin et al., 1995).

It is also interesting to point out the application of ediblecoatings as a pre-treatment in osmotic processes carried out to

obtain minimally processed fruits. In osmotic dehydration a cel-lular tissue is immersed in a concentrated solution of sugars orsalts to promote water loss in the cells due to differences inwater chemical potential established between the external solu-tion and the internal liquid phase of the cells. Nevertheless, adiffusion of external solutes and hydrodynamic gain of the ex-ternal solution also occur (Chiralt and Fito, 2003; Chiralt andTalens, 2005). Edible coatings can be used in order to controlextensive solute uptake while having no serious negative effectin water removal (Lenart and Piotrowski, 2001). In this sense,Lenart and Dabrowska (1999) coated apple slices with pectinbased coatings prior to osmotic dehydration, thus obtaining abetter dehydration efficiency with the lowest solute gain by us-ing low methoxyl pectin coatings. Similar results were obtainedby Matuska et al. (2006) in strawberries coated with differentconcentrations of sodium alginate, carrageenan, or guar gumsolutions before osmotic dehydration.

IMPROVEMENT OF FUNCTIONAL PROPERTIES OFEDIBLE COATINGS

The functionality of edible coatings can be improved by in-corporating antimicrobial agents (chemical preservatives or an-timicrobial compounds obtained from a natural source), antioxi-dants, and functional ingredients such as minerals and vitamins.

Antioxidants are added to edible coatings to protect fruitsagainst oxidative rancidity, degradation, and discoloration(Baldwin et al., 1995). For example, the antioxidants citricand ascorbic acid were incorporated into methylcellulose-basededible coatings in order to control oxygen permeability and re-duce Vitamin C losses in apricots during storage (Ayranci andTunc, 2004).

On the other hand, the addition of chemical preservatives isof great interest for MP fruits, which have an extremely shortshelf-life because of microbiological limits as well as sensoryand nutritional losses that occur during their distribution andstorage. Thus, the most recent investigations in the edible coat-ing technology deal with the addition of antimicrobial agentsto coating formulations. Eswaranandam et al. (2006), incor-porated malic and lactic acid into soy protein coatings to ex-tend the shelf life of fresh-cut cantaloupe melon. In the sameway, edible coatings for MP fruits can contain antibrowningagents (Lee et al., 2003; McHugh and Senesi, 2000; Baldwinet al., 1996; Perez- Gago et al., 2006), and texture enhancerslike CaCl2 (Wong et al., 1994; Le Tien et al., 2001).

With reference to the use of natural antimicrobials, thedevelopment of coatings which use inherently antimicrobialpolymers as a support matrix is very promising. For example,chitosan, which is mainly obtained from the deacetylation ofcrustacean chitin, is one of the most effective antimicrobial film-forming biopolymers. Chitosan is a cationic polysaccharide,which, among other antimicrobial mechanisms, promotes celladhesion by the interaction of the positive-charged amines with

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Table 4 Application of edible coatings to fresh fruits

Coating Composition Application References

Polysaccharide-lipid Maltodextrin, CMC,1 propylene glycol, fattyacid esters, sodium benzoate

MangoDaz-Sobac et al., 1996

MC, 2PEG, stearic acid, citric acid, ascorbicacid

ApricotAyranci and Tunc, 2004

Paraffin wax, beeswax, soybean oil; CMCfrom sugar beet pulp; emulgin PE, oleicacid and sodium oleate.

Peach, pear mandarinTogrul and Arslan, 2004; Togrul

and Arslan, 2004bHPMC, beeswax, shellac, estearic acid and

glycerolPlum

Chitosan Chitosan and Tween 80 Strawberry, grape, cherry, litchi,peach, Japanese Pear, kiwi Perez-Gago et al., 2003; Vargas

et al., 2006; Vargas et al.,2006b; El Gaouth et al., 1991;Zhang and Quantick, 1998;Devlieghere et al., 2004; Parket al., 2005; Han et al., 2004;Park and Zhou, 2004; Ribeiroet al., 2007; Du, 1997; Zhangand Quantick, 1997; Li and Yu,2000; Romanazzi et al., 2002;Romanazzi et al., 2003

Carrageenan Carrageenan, glycerol and Tween 80 StrawberryRibeiro et al., 2007

Protein-Polysaccharide CMC, 3WPI, caseinates and glycerol StrawberryVachon et al., 2003

Cellulose MC and glycerol Strawberry, avocado Maftoonazad and Ramaswamy,2005

Starch Starch and glycerol StrawberryGarca et al., 1998; Garca et al.,

1998Soy protein Soy protein, glycerol, malic acid, lactic acid Apple

Eswaranandam et al., 2006Zein Zein and propylene glycol Apple Bai et al., 2003SemperfreshTM Sucrose esters of fatty acids and sodium salts

of CMCMost fruits

Krochta et al., 1996;Nisperos-Carriedo andBaldwin, 1994

Nu-Coat Flo, Ban-seel, Sucrose esters of fatty acids and sodium saltsof CMC

Apple, banana, cherry, cucumber,guava, mango, melon, pear, plum Baldwin, 1994

Brilloshine Sucrose esters and wax (shine) Most fruitsBaldwin, 1994

TAL Pro-Long Blend of sucrose esters of fatty acids andsodium salts of CMC

PearFaber et al., 2003;

Nisperos-Carriedo andBaldwin, 1993;Nisperos-Carriedo andBaldwin, 1994

Pro-Long Sucrose polyesters of fatty acids and sodiumsalts of CMC

MangoDhalla and Hanson, 1988

Nutri-Save N,O-carboxymethyl chitosan Apple, breadfruit, cherry, pearFaber et al., 2003; Worrell et al.,

2002; Lau and Meheriuk,1990; Lau and Yastremski,1991; Meheriuk et al., 1991

Crisp Coat 868 Starch StrawberryRibeiro et al., 2007

FreshSealTM Polyvinyl alcohol, starch and surfactant FruitsFaber et al., 2003; Posey et al.,

2005Nature-SealTM Composite polysaccharides Pome fruit

Faber et al., 2003AgriCoat Composite polysaccharides Avocado, pear, pome fruit

Faber et al., 2003Food Coat Fatty acids and polysaccharides Sweet cherry

Alonso and Alique, 2004

1Carboxymethylcellulose; 2Polyethilene glycol;3Whey protein isolate.

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the negative charges in the cell membranes, causing leakage ofintracellular constituents (Helander et al., 2001). Chitosan basedcoatings were shown to protect highly perishable fruits likestrawberries, raspberries, and grapes from fungal decay (Var-gas et al., 2006; El Gaouth et al., 1991; Zhang and Quantick,1998; Romanazzi et al., 2002; Devligehere et al., 2004; Parket al., 2005). Moreover, chitosan-based edible coatings can bealso used to carry other antimicrobials compounds such as or-ganic acids (Outtara et al., 2000), essential oils (Zivanovich et al.,2005), spice extracts (Pranoto et al., 2005), lysozyme (Park et al.,2004) and nisin (Pranoto et al., 2005; Cha et al., 2003).

On the other hand, some authors have incorporated naturalantimicrobial compounds into protein or polysaccharide-basedmatrices, thereby obtaining a great variety of multicomponentantimicrobial coatings. In this sense, Seydim and Sarykus (2006)developed whey protein based antimicrobial coatings by addingoregano, rosemary, and garlic essential oils; Rojas-Grau et al.(2006), used apple puree and high methoxyl pectin combinedwith oregano, lemon grass, or cinnamon oil at different concen-trations. However, these coatings showed their efficacy in vitroagainst a wide spectrum of microorganisms but they were nottested in real food systems and, as a result, there is a lack ofavailable information about their possible impact on the aromaand flavor of the coated products, which becomes more sig-nificant when plant and herb essential oils/extracts or phenolicflavors are added (Han, 2002). Thus, it is recommended to studythe influence of the incorporation of antimicrobial compoundsinto edible films and coatings on sensory properties of coatedcommodities (Min and Krochta, 2005). Moreover, the most com-monly chosen model systems when testing these kinds of coat-ings are fish (Min et al., 2005) or meat-based products (Janeset al., 2002; Langu and Johnson, 2005; Theivendran et al., 2006)and limited examples of fresh or MP coated fruits are available.Therefore, the direct application of natural antimicrobial coat-ings to whole and MP fruits is still an interesting and innovativeresearch field.

Table 5 Application of edible coatings to minimally processed (MP) fruits

Coating Composition Application References

Double layer:Polysaccharide/ Lipid

Carregeenan, pectin, cellulose, alginate,monoglycerides

Fresh-cut apple cylinders Wong et al., 1994

Microemulsion Fatty acids and wax Whole peeled grapefruitHagenmaier and Baker, 1997

Emulsion 1WPC, WPI or HPMC, stearic acid, beeswax orcarnauba wax

Fresh-cut apple piecesPerez-Gago et al., 2005

Carrageenan Carrageenan glycerol, PEG 200 Fresh-cut apple cubes Lee et al., 2003Protein/ Polysaccharide WPC, glycerol, CMC, CaCl2Apple puree Apple puree, ascorbic acid, citric acid, soy oil Fresh-cut apple Pieces. McHugh and Senesi, 2000Soy protein Soy protein, glycerol, malic acid, lactic acid Cantaloupe melon cubes

Eswaranandam et al., 2006Chitosan Chitosan Sliced mango fuit

Chien et al., 2007Nature-SealTM Composite polysaccharide Sliced apple Sliced banana

Faber et al., 2003AgriCoat Composite polysaccharide Sliced apple Sliced banana

Faber et al., 2003

1Whey protein concentrate.

Antimicrobial and antioxidant coatings have advantages overdirect applications of the antimicrobial or antioxidant agentsbecause they can be designed to slow down the diffusion of theactive compounds from the surface of the coated commodity.By slowing their diffusion into coated foods, the preservativeactivity at the surface of the food is maintained. Thus, a smalleramount of antimicrobials/antioxidants would come into contactwith the food to achieve a target shelf-life, compared to dipping,dusting or spraying the preservatives onto the surface of the food(Min and Krochta, 2005).

Finally, nutraceuticals can be incorporated into the formu-lation of edible coatings, providing an alternative way to for-tify unprocessed foods, such as fresh fruits, and encourag-ing their consumption. Following this approach, Han et al.(2004), improved the nutritional and physicochemical qualityof strawberries and raspberries by means of chitosan-basedcoatings enriched with calcium and Vitamin E. In the sameway, Park and Zhao (2004) incorporated high concentrationsof minerals (calcium and zinc) and Vitamin E into a chitosanmatrix.

FUTURE TRENDS IN THE EDIBLE COATINGSTECHNOLOGY

Recent studies in this field have focused on the developmentof new technologies that allow for a more efficient control ofcoating properties and functionality. To this end, new method-ologies have been developed, most of them based on compos-ite or multilayered systems. Nevertheless, applications to foodproducts are still scarce.

One of these new methodologies consists of the develop-ment of multilayered coatings by means of the layer-by-layer(LbL) electrodeposition (Weiss et al., 2006). LbL assembly,which is performed by alternating the immersion of substratesDo

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Figure 1 (a) Components that could be used to develop multilayered ediblecoatings for fruits and (b) example of a possible laminated coating assembledover a fruit surface.

in solutions of oppositely charged polyelectrolytes with rinsingsteps, produces ultrathin polyelectrolyte multilayers on chargedsurfaces. A requirement for multilayer formation is that the addi-tion of an opposite charged polyelectrolyte to a charged surfaceresults in a charge reversal, which permits the successive de-position of oppositely charge polyelectrolytes (Krzemiski et al.,2006). Chitosan, poly-L-lysine, pectin, and alginate are the mostcommon biopolymers that can be used in the formation of thesemultilayered structures (Marudova et al., 2005; Krzemiski et al.,2006; Bernabe et al., 2005). It is also possible to utilize othercharged species to assemble the multilayered structures, includ-ing charged lipid droplets, solid particles, micelles, or surfac-tants. Examples of adsorbing substances that can be used inLbL assembly, together with a possible multilayered structure,are shown in Fig. 1.

As mentioned above, the application of edible coatings toMP fruits has to face some technical problems related to thedifficult adhesion of materials to the hydrophilic surface of thecut fruit. The LbL electrodeposition technique could eventu-ally solve these problems, because even if LbL assembly hasnormally been applied to solid substrates, it has the potential tobe applied on hydrogel surfaces (Serizawa et al., 2005). Thus,the LbL technique could be used to coat highly hydrophilic foodsystems such as fresh-cut fruits and vegetables. Figure 2 showsan example of how the LbL electrodeposition technique could

Figure 2 Schematic representation of coating a MP fruit with a multilayered edible coating by using the LbL assembly with three dipping and washing steps.

be used to coat a MP fruit (e.g. a fresh-cut apple slice) with amultilayered edible coating.

In the near future, multilayered edible coatings will receivemore attention (than single layer coatings) as they could be spe-cially engineered to incorporate and allow the controlled releaseof vitamins and other functional or antimicrobial agents. A pos-sible multilayered structure could include three layers: a matrixlayer (e.g. biopolymer based) that contains the functional sub-stance; an inner control layer to govern the rate of diffusion ofthe functional substance by allowing its controlled release; anda barrier layer that prevents the migration of the active agentfrom the coated food as well as controlling the permeability togases. This mass transfer control can be employed, for exam-ple, to incorporate antimicrobials into edible coatings, whichrequire a high concentration together with a very slow diffusionrate to be preserved to maintain the efficiency of the antimicro-bial functions against spoilage and pathogenic microorganisms(Han et al., 2002).

Another promising technique that can be potentially used toincorporate functional ingredients and antimicrobials into edi-ble coatings for fruits is micro- and nanoencapsulation. Micro-and nanoencapsulation is defined as a technology for packagingsolids, liquids, or gaseous substances in miniature (micro andnanoscale), sealed capsules that can release their contents at con-trolled rates under specific conditions. Release can be solventactivated or signalled by changes in pH, temperature, irradia-tion, or osmotic shock. This technique is being applied more andmore in the food industry since encapsulated materials can beprotected from moisture, heat, or other extreme conditions, thusenhancing their stability and maintaining viability. This tech-nique is especially suitable for incorporating ingredients thatadd value to the food product like enzymes and pro- and prebi-otics, as well as functional ingredients that are very susceptibleto lipid oxidation such as omega-3-fatty acids or to mask odorsor tastes (Lopez- Rubio, 2006). Moreover, micro and nanoscaleencapsulation systems seem to allow for better encapsulationand release efficiency than traditional encapsulation systems.

Finally, the most recent approach to improve coating prop-erties is to make nanocomposites by incorporating nanosizedclay materials such as layered silicates into biopolymer based

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matrices. Rhim et al. (2006), incorporated different types ofnanoparticles (montmorillonites, nano-silver, and silver-zeolite)into a chitosan matrix, obtaining composites with better me-chanical, water vapor barrier, and antimicrobial properties thanthe traditional chitosan coating. Cellulose nanofibers have alsoshown good possibilities as reinforcements in composite coat-ings for food packaging. However, even if these studies seemto be promising, the major concern of the scientific communitywhen incorporating these nanomaterials into edible coatingsor food is still unsolved: the lack of studies into their possibletoxicity.

CONCLUDING REMARKS

Both, fresh and minimally processed fruits are highly per-ishable products and new technologies designed to extend theirshelf life and ensure their safety are in demand both, in the foodindustry and by consumers. Edible coating technology seems tobe very promising as long as consumers accept this techniqueas safe and friendly. Limitations mainly arise from the relativelylow capability to control film properties (since good water bar-rier properties are mainly provided by lipids and waxes) and thepoor sensory acceptance of these lipid compounds (especiallyregarding their impact on color, taste, and flavor rejection).

New trends have focused on highly functional micro- ornanostructured, multilayered composite coatings, which are de-veloped by using techniques that, at present, have hardly everbeen applied in food systems.

Future studies are expected into the development of tailor-made coatings. These coatings would be designed to providehighly specific functional performances based on the selectionof the most appropriate film forming and active ingredients andassembling them in the most effective arrangement.

ACKNOWLEDGEMENTS

The authors acknowledge financial support from the SpanishMinisterio de Educacion y Ciencia by the Project AGL2004-01009. Author M. Vargas is grateful to the Research, Develop-ment, and Innovation Office of the Universidad Politecnica deValencia (Spain) for the concession of a grant (PAID-00-06).

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