Biopolymer Coatings on Paper Packaging Materials

BiopolymerCoatings on

Paper PackagingMaterials

Khaoula Khwaldia, Elmira Arab-Tehrany,and Stephane Desobry

ABSTRACT: Increased environmental concerns over the use of certain synthetic packaging and coatings in com-bination with consumer demands for both higher quality and longer shelf life have led to increased interest in al-ternative packaging materials research. Naturally renewable biopolymers can be used as barrier coatings on paperpackaging materials. These biopolymer coatings may retard unwanted moisture transfer in food products, are goodoxygen and oil barriers, are biodegradable, and have potential to replace current synthetic paper and paperboardcoatings. Incorporation of antimicrobial agents in coatings to produce active paper packaging materials providesan attractive option for protecting food from microorganism development and spread. The barrier, mechanical, andother properties of biopolymer-coated paper are reviewed. Existing and potential applications for bioactive coatingson paper packaging materials are discussed with examples.

IntroductionPaper is widely used in packaging applications and is

biodegradable and therefore perfectly safe for the environment.Paper consists of a porous cellulose structure made up of mi-crofibrils, which are composed of long-chain cellulose moleculesin a crystalline state with amorphous regions regularly disrupt-ing the crystalline structure. The hydrophilic nature of cellulose,due to the OH sites in the basic unit of cellulose (C6H10O5) andfiber network porosity, limits the water-vapor-barrier properties ofpaper. Paper packaging also easily absorbs water from the envi-ronment or from the food and looses its physical and mechanicalstrengths. Moisture migration can occur in paper by diffusion ofwater vapor through the void spaces as well as in condensedform through the fiber cell walls (Bandyopadthay and others2002).

Paper is often associated with other materials, such as plasticmaterials and aluminum, for their good barrier properties thatcould be advantageously combined with paper stiffness. Paper iscoated with ethyl vinyl alcohol (EVOH), a polymer with excel-lent oxygen-barrier properties, when gas barrier properties are re-quested (Zhang and others 1999, 2001). However, because of thepolar groups of EVOH at the origin of the hydrophilic character ofthe polymer at high relative humidity, an additional polymer layerbased on polyolefins is used to prevent water sorption (Despondand others 2005). Polyolefins are generally chosen as papercoating materials to overcome porosity and hygroscopicity of

MS 20090740 Submitted 8/1/2009, Accepted 9/10/2009. Author Khwaldia iswith Inst. Natl. de Recherche et d’Analyse Physico-chimique, INRAP, PoleTechnologique de Sidi Thabet 2020 Sidi Thabet, Tunisia. Authors Arab-Tehranyand Desobry are with Laboratoire d’Ingenierie des Biomolecules, ENSAIA, 2av. de la foret de Haye-BP 172 54505, Vandoeuvre-les-Nancy cedex, France.Direct inquiries to author Khwaldia (E-mail: [email protected]).

paper. Unfortunately, the obtained material loses its biodegra-dation and recyclability characteristics due to the addition ofsynthetic polymer layers.

Natural polymers can be used as barrier coatings on pa-per packaging materials. Such biodegradable coatings havethe potential to replace current synthetic paper coatings, suchas polyethylene, polyvinyl alcohol, rubber latex, and fluo-rocarbon in food packaging applications (Chan and Krochta2001a, 2001b). Agriculturally derived alternatives to syntheticpaper coatings provide an opportunity to strengthen the agri-cultural economy and reduce importation of petroleum and itsderivatives.

Naturally renewable biopolymers have been the focus of muchresearch in recent years because of interest in their potential useas edible and biodegradable films and coatings for food pack-aging. The properties, technology, functionalities, and potentialuses of biopolymer films and coatings have been extensivelyreviewed by Kester and Fennema (1986), Gennadios and oth-ers (1994), Gontard and Guilbert (1994), Krochta and others(1994), Anker (1996), Guilbert and others (1997), Krochta andDe Mulder-Johnston (1997), Krochta (2002), and Khwaldia andothers (2004a).

Biopolymer-based packaging materials originated from natu-rally renewable resources such as polysaccharides, proteins, andlipids or combinations of those components offer favorable envi-ronmental advantages of recyclability and reutilization comparedto conventional petroleum-based synthetic polymers. Biopoly-mer films and coatings may also serve as gas and solute bar-riers and complement other types of packaging by minimizingfood quality deterioration and extending the shelf life of foods(Guilbert and others 1996; Krochta and De Mulder-Johnston1997; Debeaufort and others 1998). Moreover, biopolymer-basedfilms and coatings can act as efficient vehicles for incorporatingvarious additives including antimicrobials, antioxidants, coloring

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Biopolymer-coated paper . . .

agents, and nutrients (Baldwin 1994; Petersen and others 1999;Ozdemir and Floros 2001; Han and Gennadios 2005).

The association of biopolymers to paper provides interestingfunctionalities while maintaining environment-friendly character-istic of the material. Renewable biopolymers, such as caseinates(Khwaldia and others 2005; Gastaldi and others 2007; Khwaldia2009), whey protein isolate (WPI; Han and Krochta 1999, 2001;Lin and Krochta 2003, Gallstedt and others 2005), isolatedsoy protein (Park and others 2000; Rhim and others 2006),wheat gluten (Gallstedt and others 2005), corn zein (Trezzaand Vergano 1994; Parris and others 1998; Trezza and oth-ers 1998), chitosan (Despond and others 2005; Ham-Pichavantand others 2005; Kjellgren and others 2006), carrageenan (Rhimand others 1998); alginate (Rhim and others 2006), and starch(Matsui and others 2004) have been investigated as paper-coatingmaterials.

Han and Krochta (1999, 2001) showed that whey-protein-coated paper improves packaging material performance ofpaper by increasing oil resistance and reducing water-vaporpermeability. Despond and others (2005), as well as Kjellgrenand others (2006), used paper coated with chitosan or chi-tosan/carnauba wax to obtain a packaging material with goodbarrier properties towards oxygen, nitrogen, carbon dioxide, andair. Rhim and others (2006) indicated that water resistance ofpaper is improved by coating with soy protein isolate (SPI) oralginate.

The main objectives of this study were to review the differ-ent types of renewable biopolymers investigated as paper coat-ing materials, to summarize the barrier, mechanical, and otherproperties possessed by biopolymer-coated paper, and finally todiscuss existing and potential applications for bioactive coatingson paper coating materials.

Materials for CoatingsRenewable biopolymers available for forming coatings on

paper packaging materials are generally made from proteins,polysaccharides, and lipids used alone or together. The choiceof materials for a coating is largely dependent on its desiredfunction. Plasticizers, regular paper pigments, antioxidants, orantimicrobial agents can be added to paper coating solutions toimprove its performance properties.

Biobased polymers can be applied to paper or paperboard withdifferent coating techniques, such as surface sizing, solution coat-ing, compression molding, and curtain coating depending on theappropriate coating material and type of paper used. Surface siz-ing is one of the most frequently used processes for applying anaqueous coating to a paper substrate. In surface sizing, the solidcontent of the coating is limited and is typically lower than 10%to 15% (Vartiainen and others 2004). A low solid content doesnot yield a fully continuous coating and increases the amountof drying needed. A higher coating weight and better gas-barrierproperties can be obtained using curtain-coating technique inwhich the paper industry has begun to show a considerable in-terest (Kjellgren and others 2006). A thick and continuous coat-ing, necessary in several cases to obtain coverage of the paper, isnot possible to obtain by solution coating. However, this coatingtechnique results in interesting mechanical properties (Gallstedtand others 2005). The compression-molding technique is suit-able for applications where complete coverage and thick coat-ings were necessary, and which, therefore, involved significantlymore coating material compared to solution coating.

Protein-based coatingsProteins cover a broad range of polymeric compounds that

provide structure or biological activity in plants or animals.

Proteins have successfully been formed into films and/or coat-ings and their film properties have been quantified (Krochta 2002;Sobral and others 2005; Gounga and others 2007). Proteins aresuitable for coating fruits and vegetables, meats, eggs, nuts, otherdry foods, and paper packaging. Protein coatings on paper in-clude milk proteins, wheat gluten, gelatin, corn zein, and SPIs.Protein-derived coatings show excellent oxygen barrier propertyat low to intermediate relative humidity as well as fairly goodmechanical properties. However, their barrier against water va-por is poor due to their hydrophilic nature (Avena-Bustillos andKrochta 1993).

Caseins and caseinates. Milk proteins, such as casein, haveseveral key physical characteristics for effective performance inedible films and coatings, such as their solubility in water andability to act as emulsifiers (Southward 1985). Sodium caseinate(NaCAS) can easily form films from aqueous solutions becauseof its random coil nature and its ability to form extensive inter-molecular hydrogen, electrostatic, and hydrophobic bonds, re-sulting in an increase of the interchain cohesion (Avena-Bustillosand Krochta 1993; McHugh and Krochta 1994; Brault and others1997). NaCAS films appear to have lower oxygen permeabil-ities than nonionic polysaccharide films (Khwaldia and others2004a). This may be related to their more polar nature and morelinear (nonring) structure, leading to higher cohesive energy den-sity and lower free volume (Miller and Krochta 1997). Moreover,they possess good mechanical properties, as has been shown inour previous research (Khwaldia and others 2004b). These prop-erties make caseinate an attractive polymer for the coating ofcellulose-based materials for food packaging purposes.

Khwaldia (2009) showed that the thickness of papers coatedwith NaCAS was affected by the coating weight. By increasingcoating weight from 3 to 18 g/m2, the dried thickness of NaCAS-paper bilayers increased. Therefore, most of the NaCAS contentformed a continuous layer on the surface of the paper, which isa porous, with rough surface, material leading to an increase ofmeasurable paper thickness. Likewise, Gastaldi and others (2007)demonstrated through microscopic observations that a calciumcaseinate coating produced a homogeneous and rather denselayer on the paper with a regular and smooth surface. Impregna-tion percentage of the calcium caseinate coating solutions waslow (4.8%) in contrast to wheat gluten solutions that presented ahigher impregnation percentage (above 50%). The performanceof coated paper as packaging material is closely dependent onthe integrity of the coating layer and its interface with paper.Impregnation measurements constituted an original approach tocharacterize the structure of interface created between paper andcoating. Impregnation rates were not only related to the viscosityof coating solution applied on paper, but also to the increase ofconcentration of coating agent during the drying stage.

On the other hand, Khwaldia (2009) showed that NaCAS coat-ing improved both paper strength and ductility and reduced wa-ter vapor transmission. They also found that increasing paraffinwax concentration in the coating led to an increase in tearingresistance of the resulting coated papers. Conversely, Khwaldia(2004) reported that the tearing resistance of coated paper wasnot affected by carnauba wax concentration.

Whey protein. Whey protein, a by-product of the cheese indus-try, is already known as an excellent barrier to oxygen, aroma, andoil and can be used as a coating material for improving the oxy-gen barrier property of food packaging (Miller and Krochta 1997).The oxygen permeability of whey protein films has been reportedto be very low and comparable to that of EVOH polymer at low orintermediate relative humidity conditions (McHugh and Krochta1994). Compared to currently used sizing agents and pigmentadhesives, whey protein may have some advantages. It forms anintact water-insoluble film out of aqueous solution, due to the

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formation of intermolecular disulfide bonds after heat denatura-tion (McHugh and Krochta 1994). Thus, such a whey protein filmhas a cross-linked structure.

Some studies have considered whey proteins as coatings on pa-per. Han and Krochta (1999) showed that whey-protein-coatedpaper improves packaging material performance of paper by in-creasing oil resistance and reducing water vapor permeability(WVP). Chan and Krochta (2001a) reported a significant reduc-tion in oxygen permeability for paperboard coated with dena-tured and undenatured WPI. Han and Krochta (2001) studiedthe increase in gloss and the increase in oil resistance of pa-per coated with WPI. The increased gloss after WPI coating maybe caused by the paper surface being more homogeneous andsmoother. The increase of surface smoothness and homogeneitywere also suggested by the previous research of Han and Krochta(1999). Gallstedt and others (2005) showed that WPI and wheyprotein concentrate (WPC) enhanced the strength and toughnessof the paper. Conversely, Han and Krochta (2001) reported thatwhey protein coating decreased the tensile strength of the pa-per, because the coated paper structure has smaller interactionforce between fibers because of coating interference. Chan andKrochta (2001b) pointed out that WPI coatings produce high andstable gloss values. WPI might replace commercial paperboardcoatings such as polyvinyl alcohol and fluorocarbon as greaseand oxygen barriers while maintaining desirable color and gloss.Although the replacement of existing polymer coating formula-tions for paper with biodegradable coating formulations mightbe possible, a reliable solution has not yet been found. Manyquestions and problems still have to be solved before biopoly-mers can be commercially used as a replacement for syntheticpolymers. These questions concern the demands on the biopoly-mer coating and the cellulose substrate as well as on the coatingprocess.

Soy protein. Generally, soy protein films have inadequate me-chanical properties and are poor moisture barriers because of thehydrophilic nature of soy protein. Researchers have attempted toimprove the properties of soy protein films that have major poten-tial applications in the food and packaging industry (Stuchell andKrochta 1994; Rangavajhyala and others 1997; Rhim and oth-ers 1999). It is estimated that in the United States, about 25000to 50000 metric tons of soy proteins are used in paper coat-ings (Myers 1993). SPI-coated paper was found to impart gasand oil barrier as well as adequate mechanical properties, forextending the shelf life of food products (Park and others 2000).Rhim and others (2006) reported that the water resistance of SPI-coated paperboards is higher than that of alginate-coated paper-boards. The contact angle of water on the alginate-coatedpaperboards decreased more than that of the SPI-coated ones.However, water resistance of the alginate-coated paperboardsposttreated with the CaCl2 solution was comparable to the SPI-coated ones. These same researchers indicated that SPI coat-ings cross-linked by formaldehyde posttreatment or compositedwith organically modified montmorillonite were more effectivein decreasing the WVP of coated paperboards. The cross-linkingtechnique is an interesting approach to enhance mechanical andwater vapor barrier properties of biodegradable films and coat-ings for food packaging applications. The more commonly usedcovalent cross-linking agents are glutaraldehyde, glyceraldehyde,formaldehyde, gossypol, and tannic and lactic acids. However,food use of films treated with such cross-linking agents is highlyquestionable. Due to the possible toxicity of these modifyingagents, further research should be done to analyze chemicalresidues remaining in the film and their migration in the event ofthese materials being used in direct contact with foods.

Wheat gluten. The functional properties of wheat gluten (WG),such as selective gas barrier properties, insolubility in water,

adhesive/cohesive properties, viscoelastic behavior, and film-forming properties, have been exploited in the development ofedible coatings based on WG (Gontard and others 1992, 1994;Gennadios and others 1993, 1994).

Techniques have been developed for the production of papercoating dispersions in which gluten is used as a binder. Thesesuspensions show good film-forming properties and the resultingcoating has a strong adhesion to various substrates (Derksen andothers 1995). According to Gastaldi and others (2007), increas-ing WG concentrations from 10% to 20% in coating solutionsdecreased adsorption percentage of dry matter from 63.3% to53.6%, respectively. This effect could be related to the increasedsolution viscosity with WG solution concentration, consideringthat a viscous solution would be less prone to penetrate insidefibrous paper.

Corn zein. Corn zein protein coatings are used as oxygen,moisture, and grease barriers for nuts, candies, and other foods(Andres 1984). Corn zein films and coatings have relative insol-ubility in water, and they form strong, glossy films resistant togrease and oxygen permeation.

Corn zein coatings do not interfere with paper recycling, donot require separating protein and paper layers, and have beensuggested as an alternative to polyolefin materials Trezza andVergano (1994). Little data are available on the biodegradabil-ity, the recyclability, and the reutilization of biopolymer-coatedpaper or paperboard. Research is needed to evaluate their recy-cling potential and their rate of biodegradation under compostingconditions.

Trezza and Vergano (1994) measured the grease resistance ofcorn zein-coated paper with respect to coating level, plasticizeraddition, and time exposure. As coating level increased, uni-formity of the coating also increased. Coating uniformity andquality are necessary for good grease resistance. Zein-coatedpapers were as effective grease barriers as were polyethylenelaminates used for quick-service restaurant sandwich packaging.These results showed the potential for fully compostable paper-based wraps and boxes for the food service industry.

Polysaccharide-based coatingsPolysaccharides are nontoxic and widely available. They also

are excellent gas, aroma, and lipid barriers. They form strongfilms, but because of their hydrophilic nature exhibit poor wa-ter vapor barrier properties (Kester and Fennema 1986; Guilbert1986). Many researchers have studied the film formation andthe properties of several polysaccharide materials (Kamper andFennema 1985; Martin-Polo and others 1992a, 1992b; Nisperos-Carriedo 1994). Polysaccharides most used for paper coating in-clude starch, alginates, carrageenan, and chitosan.

Chitosan. Chitosan, a natural polysaccharide, is derived bydeacetylation of chitin, the 2nd most abundant naturally occur-ring biopolymer after cellulose (No and Meyers 1995). Chitosan isan edible and biodegradable material that has attracted notableinterest in the food packaging area (Buttler and others 1996;Shahidi and others 1999; Tual and others 2000; Despond andothers 2001). Chitosan has been documented to possess film-forming properties for use as edible films or coatings and alsobioactive properties either in its polymeric or oligomeric form(Fang and others 1994; Begin and van Calsteren 1996; Tsai andothers 2000; Coma and others 2003).

Chitosan films are tough, long-lasting, flexible, and very dif-ficult to tear. Most of their mechanical properties are compa-rable to many medium-strength commercial polymers. It hasbeen reported that chitosan films have moderate WVP valuesand could be used to increase the storage life of fresh produceand foodstuffs with higher water activity values. Chitosan exhibitsexcellent oxygen-barrier properties due its high cristallinity and

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the hydrogen bonds between the molecular chains (Kittur andothers 1998; Gallstedt 2001). Moreover, chitosan is a good bar-rier against grease (Kittur and others 1998). Due to its positivecharge on the amino group under acidic conditions, chitosanbinds to negatively charged molecules such as fats and lipids(Jumaa and Muller 1999; Shu and others 2001). These propertiesmake chitosan an attractive polymer for the barrier coating ofcellulose-based materials for food packaging purposes.

Chitosan has been used as a papermaking additive and for thesurface treatment of paper for decades. Laleg and Pikulik (1991)tested the use of chitosan as a wet-end additive in paperboard.They reported that the mechanical properties of paperboard in-cluding chitosan as a wet-end additive were improved. The chi-tosan retention was also reported to be good, due to the differentcharges of the chitosan (cationic) and cellulose (anionic). Water-insoluble sheets of chitosan and pulp fiber have been developedto enhance the gas-barrier properties of paper (Hosokawa andothers 1991; Gallstedt and Hedenqvist 2006). The printability ofpaper increases with the addition of chitosan due to the fact thatthe paper surface becomes smoother (Thomson 1985).

Studies of chitosan coatings on paper, paperboard, andcellophane have been reported (Domszy and others 1985; Dobband others 1998; Krasavtsev and others 2002; Ho and others2003; Vartiainen and others 2004; Kjellgren and others 2006;Bordenave and others 2007). Chitosan is readily compatible withpaper and is one of the most interesting polysaccharide coatingmaterials for paper. Bordenave and others (2007) have character-ized the morphology and the microstructure of chitosan-coatedpapers by infrared spectroscopy and scanning electron mi-croscopy. Their observations suggested that the chitosanpenetrated deeply into the paper, embedding the cellulose fibers,instead of forming a layer on paper. The chitosan-coated materi-als exhibited good moisture barrier properties, but not sufficientfor food applications, and their surface hydrophilicity was toohigh.

Alginates. Alginates, which are extracted from brown sea-weeds of the Phaephyceae class, are the salts of alginic acid.Alginates are resistant to solvents, oil, and grease and exhibit in-teresting film-forming properties. Moreover, alginates could alsoact as a penetration controller when associated with pure starch.Alginates are generally used in sizing and/or coating paper toproduce surface uniformity.

Rhim and others (2006) found a decrease in contact angleof water by coating paper with alginate. Reduction in the con-tact angle of water by alginate coatings indicates an increasein hydrophilicity of the surface of paperboards, which becamesmoother and more homogeneous resulting in an increased affin-ity of the paperboards to water. According to Ham-Pichavant andothers (2005), incorporation of sodium alginate in chitosan for-mulations considerably increased the fat barrier of coated papersand, at the same time, reduced the treatment cost. Indeed, thechitosan/alginate mixture, after coating on paper, allowed fat re-sistance with a synergistic effect, taking into account the possiblelimitation of chitosan penetration into the paper and the con-tribution to a smoother surface due to film-forming capacitiesof gums at low concentration. Rhim and others (2006) reportedthat the tensile strength of paperboards was decreased with algi-nate coatings. The decrease in tensile strength of alginate-coatedpaperboards is mainly due to the swelling of cellulose fiber bysolvent penetration during coating and may be partially due tothe fact that alginate impregnated into the cellulose structure ofpaper and interfered with fiber-to-fiber interaction.

Starch. Starch is the most commonly used agricultural rawmaterial since it is inexpensive and widely available. Starchfilms have poor physical properties, but these can be improvedby blending the starch with cellulose derivates and proteins

(Arvanitoyannis and others 1996, 1998; Psomiadou and others1996; Peressini and others 2004).

Dispersion of starch granules is commonly used to function as apaper-coating agent with the main objective to smooth the surfaceof the paper without changing its barrier properties (Matsui andothers 2004). The surface sizing treatment uses native starch andmodified starches to improve paper properties, including physicalstrength, oil/grease resistance, and optical properties.

The acetylation reaction is one of the most interesting waysto decrease starch hygroscopicity. This chemical reaction allowsthe attainment of thermoplastic and hygroscopic materials (Graafand others 1995; Fringant and others 1996). Larotonda and others(2005) as well as Fringant and others (1998) used papers treatedwith starch acetate to decrease paper hygroscopicity. Larotondaand others (2005) demonstrated that significant reductions in wa-ter adsorptivity and WVP of Kraft paper might well be achievedthrough starch acetate impregnation, mainly in low relative hu-midity conditions. Indeed, the starch acetate impregnated thepaper structure and partially filled superficial and internal pores,thus decreasing the paper permeability. Furthermore, as starchacetate is much less hygroscopic than paper, its adsorptivity isreduced significantly by impregnation. Indeed, the impregnationof papers with nonhygroscopic and biodegradable materials isan interesting treatment used to reduce the hygrocopicity and theWVP of papers. This treatment could not only improve the waterbarrier property of paper packaging, but also its barrier proper-ties against gas and aroma compounds maintaining the foodstuffquality during storage (Dury-Brun and others 2008). However,the impregnation did not improve the mechanical properties ofthe paper materials (Matsui and others 2004). The properties ofimpregnated papers depend on the time of immersion, the con-centration of the material used for impregnation, and the impreg-nation procedure (with or without vacuum application).

Lipid and composite coatingsLipid compounds, such as long-chain fatty acids and waxes,

can be incorporated in the film or coating matrix because oftheir hydrorepellency. Waxes are the most efficient substancesto reduce moisture permeability. Their high hydrophobicity isa consequence of a high content in esters of long-chain fattyalcohols and acids, as well as long-chain alkanes (Kester andFennema 1986; Donhowe 1992; Hagenmeier and Shaw 1992).

Paper and paperboard, which are the most widely used mate-rials in food and drink packaging, are frequently wax-coated toimprove their water-resistance and increase the shelf life of thepackaged products (Rodriguez and others 2007). Paraffin wax ap-plied in a molten form was commonly used to produce a watervapor barrier. Recyclable packaging paper materials using au-todispersible waxes have been reported (Back 1995).

Lipid coatings provide good moisture barrier, but they havecertain disadvantages such as brittleness, lack of homogeneity,and presence of pinholes and cracks in the surface of the coat-ing. Composite coatings or multilayer coatings, applied eitherin the form of an emulsion or in successive layers (multilayercoating), have been prepared to combine the good structuraland gas-barrier properties of hydrocolloid coatings with the goodmoisture-barrier characteristics of lipids. The method of applica-tion affects the barrier properties of the coatings obtained.

Parris and others (1998) measured the water barrier and greasepermeation properties of Kraft paper coated with a combinationof zein and paraffin wax. Their data demonstrated that the zeinlayer of the bilayer coating contributes grease-proofing and thewax layer water resistance. In a previous study, WVP decreaseshave been documented for NaCAS-coated paper due to the ad-dition of carnauba wax (Khwaldia and others 2005). The WVPof coated papers decreased as the amount of wax in the coating



Table 1 --- Functions of biopolymers used in paper coating.

Biopolymers Functions Reference

WPI Increase printability of water-based ink Han and Krochta (1999)Grease barrier Han and Krochta (2001)

Chan and Krochta (2001a)NaCAS Oxygen barrier Khwaldia (2004)NaCAS/paraffin wax bilayer Water vapor barrier Khwaldia (2009)Corn zein Grease barrier Trezza and Vergano (1994)

Minimize effects of drying and brittlenessCorn zein/paraffin wax bilayer Water vapor barrier Parris and others (1998)

Grease barrierSPI Gas and lipid barrier Park and others (2000)SPI with CaCl2 posttreatment Water vapor barrier Rhim and others (2006)WG Oxygen barrier Gallstedt and others (2005)Carrageenan Grease barrier Rhim and others (1998)HPMC/beeswax Water vapor barrier Sothornvit (2009)Chitosan Fat barrier Ham-Pichavant and others (2005)

Gas barrier Kjellgren and others (2006)Chitosan/sodium alginate bilayer Fat barrier Ham-Pichavant and others (2005)Chitosan/carnauba wax bilayer Gas barrier Despond and others (2005)Chitosan/sodium alginate bilayer Fat barrier Ham-Pichavant and others (2005)Paraffin wax Water vapor barrier Parris and others (1998)

increased. The addition of hydrophobic substances (carnaubawax) to this hydrophilic matrix provides the moisture-barrierproperties.

Khwaldia (2009) found that the greatest reduction in paperWVP is achieved by addition of a wax layer to the paper alreadycoated with NaCAS, due to the high resistance to moisture transferof the paraffin wax. Indeed, the barrier ability of bilayer coatingagainst water vapor transfer is higher than that of an emulsioncoating. Emulsion coatings have the advantage that they are easierto apply on paper materials than bilayer coatings and they needonly one drying step. Moreover, no problem separation of the twolayers occurs, and their both hydrophilic and lipophilic natureallows their good adhesion onto any support.

Despond and others (2005) processed a gas-barrier multilayermaterial with paper, chitosan, and carnauba wax. Because ofthe hydrophobic character of the external wax layer, the watersorption in the multilayer decreased greatly, and gas permeabilityvalues lower than 0.5 barrer were obtained in the hydrated state.

Functional Properties

Barrier propertiesRegarding the barrier properties of packaging materials, the

critical compounds that can penetrate the packaging materialsand degrade food quality are water vapor and oxygen of thesurrounding atmosphere. To avoid the moisture transfer that canaffect food quality, WVP control is important to assure stabilityand safety during distribution and storage. The ingress of oxygen,which is strongly and irreversibly reacted with food componentssuch as lipids, vitamins, flavors, and colors, leads to permanentchange in the nature of food products (rancidity, vitamin loss, andmicrobial contamination). Good oxygen barrier properties arecritical for achieving a long shelf life for the packaged product.Other important gases to which food packaging should be lesspermeable are carbon dioxide and nitrogen.

To meet this demand, expensive synthetic barrier polymers, in-cluding EVOH copolymers and polyvinylidene chloride are com-monly used in the form of laminates as oxygen-barrier layers infood packaging materials. Such composite synthetic laminatesare not biodegradable and cannot be recycled. Therefore, there

is an increasing interest in the development of biodegradablepolymers for packaging materials that have suitable applicationproperties and can be disposed of after use in an economicallyand ecologically acceptable way. The various potential func-tions of biopolymers used in paper coating are summarized inTable 1.

Gas permeability. Greaseproof paper was coated with chitosanto obtain a packaging material with good barrier properties to-wards oxygen, nitrogen, and carbon dioxide (Kjellgren and oth-ers 2006). The oxygen permeability in the same range as thepolyethylene terephthalate was obtained at coat weights exceed-ing 5 g/m2. The oxygen permeability was not substantially af-fected by temperature changes, provided that the air permeanceof the base paper was low. A barrier against nitrogen and carbondioxide required a coat weight exceeding 5 g/m2. Trezza andothers (1998) reported a reduction in the oxygen permeability ofpaper coated with corn zein. Furthermore, Gallstedt and others(2005) studied the effects of coating procedures on oxygen barrierproperties of paper and paperboard coated with chitosan, WPI,WPC, and WG. Paper sheets were solution-coated using a handapplicator, WG was compression-molded onto paper and paper-board, and chitosan solution was also applied on paperboard us-ing curtain-coating. The coatings on the applicator-coated sheetswere too thin and discontinuous to improve the oxygen bar-rier properties. Because of the higher amount of WG material inthe compression-molding process, coatings were thick and con-tinuous, resulting in low oxygen permeability. Chitosan-curtain-coated paperboard showed the highest oxygen-barrier properties,which are comparable to those of commonly used packagingoxygen-barrier polymers. On the other hand, Khwaldia (2004)studied the combined effects of mica, carnauba wax, glycerol,and NaCAS concentrations on oxygen-barrier properties. Coat-ing significantly increased oxygen-barrier property. The oxygenpermeability of the coated paper was 13 to 90 times lower thanthat of the uncoated paper.

Water vapor permeability. A water barrier can be formed bychanging the wettability of the paper surface with sizing agentsor through coating with hydrophobic materials. Paper is oftencoated with paraffin wax, applied in a molten form, to pro-duce a water vapor barrier. Han and Krochta (1999) studiedthe wetting properties and WVP of whey-protein-coated paper.



They reported that the whey protein coating increased the water-vapor-barrier property of pulp paper. The WVP decreased by44.8% compared to the uncoated paper after WPI coating with10 g/m2. The properties of the NaCAS-paper bilayers were investi-gated by Khwaldia (2009). The WVP of NaCAS-coated paper wasdecreased consistently by increasing coating weight from 3 to18 g/m2. NaCAS coating on paper reduced WVP by 75% for18 g/m2 coating weight compared to that of the uncoated paper.In a previous study, Khwaldia and others (2005) showed that theWVP of NaCAS-coated papers decreased as the amount of wax inthe coating increased. The addition of hydrophobic substances tothis hydrophilic matrix provides the moisture barrier properties.The substantial reduction in WVP of paper by incorporation ofwaxes was expected because waxes are most efficient substancesto reduce moisture permeability due to their high hydrophobicity.

Parris and others (1998) evaluated coating formulations com-posed of the corn protein zein and paraffin wax for their water-vapor-barrier properties. The water vapor transmission rates forpaper coated with paraffin wax were found to be significantlylower than those measured using the zein-coated paper. Coatingthe paper with a 2% solution of zein in paraffin wax reduced thewater vapor transmission rates by approximately half the valuesobtained for wax-coated paper. Water vapor transmissionvalues were strongly dependent on the amount of wax in thecoating. On the other hand, Rhim and others (2006) showedthat water barrier properties of paperboards increased by SPI oralginate coating with CaCl2 posttreatment. Biopolymer-coatedpaperboards can be used in the preparation of water-resistantcorrugated fiberboard boxes for the storage of high-moisturefoods. Larotonda and others (2005) reported that Kraft paper im-pregnation with cassava starch acetate is an interesting alterna-tive for improving the hygroscopic properties and obtaining awaterproof paper. Furthermore, hydroxypropyl methylcellulose(HPMC)-based coatings reduced WVP and further reduction wasobtained when beeswax was incorporated in the HPMC-lipidcomposite-coated paper (Sothornvit 2009). Using HPMC as acoating material for paper has a benefit in terms of lower concen-tration of coating solution, while providing desirable mechanicalproperties. Indeed, a low concentration of HPMC is adequate toprovide the appropriate viscosity for coating on paper. Further in-vestigation is still needed to verify the properties of HPMC-basedcoated paper with specific products.

Bordenave and others (2007) evaluated the barrier propertiesagainst moisture and the liquid water sensitivity of chitosan-coated papers. They showed that the chitosan coating led to asignificant decrease of the paper moisture transfer but the surfacehydrophilicity remained high.

Oil permeability. Grease resistance is an important propertyof paper packaging materials used for foods containing fats oroils. Limited research has been done on quantifying the oilpermeability of packaging materials. Coated paper or paperboardwith a good grease barrier is important for packaging used in fast-food restaurants, as well as food-packaging applications suchas cereal boxes, donut boxes, and pizza boxes. Corn zein wasshown to have excellent grease resistance, both as a film andas a coating on paper. Zein coating on paper for grease barrierwas compared to quick-service sandwich packaging, and it wasfound that zein-coated papers were as effective as polyethylenelaminates used for quick-service restaurant sandwich packaging(Trezza and Vergano 1994). In their study, the zein-coated pa-pers were not heat-sealed to a 2nd sheet of paper, as were thecommercial polyethylene-laminated samples. Further research isrequired to evaluate the effects of heat sealing of zein coatingand storage on grease properties of the coated papers.

Research results also showed that a whey protein film (DeMulder-Johnston 1999) and whey protein coating on paper (Chan

2000) provided excellent oil-barrier properties. Rhim and others(1998) showed that the grease resistance of carrageenan-coatedpapers was comparable to polyethylene-laminated papers, andPark and others (2000) reported that soy-protein-coated papersimparted gas and lipid barrier, as well as adequate mechanicalproperties.

Chan and Krochta (2001a) studied the grease barrier propertyof WPI-coated paperboard. They found that a good grease bar-rier was obtained with paperboard coated with WPI and glyc-erol as plasticizer. However, glycerol plasticizer may migrateinto the paperboard during storage. Lin and Krochta (2003) con-cluded that WPC with about 80% protein coatings on paperboardgave a grease barrier comparable to WPI coatings. Sucrose-plasticized whey-protein coatings on paperboard imparted ex-cellent grease resistance, similar to glycerol-plasticized coatings.Long-term ambient storage of WPC-coated paperboard indicatedthat the use of sucrose as plasticizer imparted good grease resis-tance and minimized plasticizer migration. On the other hand,Ham-Pichavant and others (2005) explored the ability of bilayerchitosan-coated paper as fat barrier. They also investigated the na-ture of interactions between fatty acids, chosen as model lipids,and chitosan. Their experiments showed a strong pH-dependentchitosan-lipid interaction. The chitosan layer could act as a lipidtrap coating to decrease fat transfer if the pH of the chitosanfilm-forming solution was adjusted to 5.5 to 6 prior to coating.Chitosan-coated papers can be used as fat barrier packaging witha chitosan level of 5.41%. However, treatment costs remain highcompared with fluorinated resins. In an attempt to decrease bothtreatment cost and fat transfer, chitosan was associated with var-ious polymers. Incorporation of sodium alginate considerablyincreased the fat barrier of coated papers. Kjellgren and others(2006) reported that chitosan-coated greaseproof papers exhib-ited excellent grease resistance within the coat weight range of2.4 to 5.2 g/m2. The air permeability of the coated material hada great influence on grease resistance.

Mechanical propertiesIn many packaging applications, barrier properties as well as

mechanical resistance are required. In general, mechanical prop-erties of coated/laminated films in a composite structure tend torely strongly on the substrate or base film rather than the coating(Hong and others 2004). The mechanical properties frequentlymeasured to characterize paper-based packaging materials aretensile strength (TS), elongation (E), elastic modulus (EM), andtearing resistance (TR). TS is a measure of the ability of a filmto resist breaking under tension, which is dependent on thestrength of fibers, their surface area, and length, and also thebonding strength between them. E is a quantitative representa-tion of the film’s ability to stretch. EM is the fundamental mea-sure of film stiffness. TR corresponds to the average force appliedduring the tearing operation; it is likely that it relates to the frac-ture stress and/or fracture resistance or toughness of the material(Rabinovitch 2003).

Gallstedt and others (2005) studied the mechanical proper-ties of paper and paperboard coated with chitosan, WPI, WPC,and WG protein. The mechanical tests of solution-coated papershowed that chitosan was the most effective coating on a coatweight basis. This was due to its high viscosity, which limited thedegree of penetration into the paper. The researchers reportedthat the fracture stress increased with increasing coat weight forall the solution coatings. The WPI-coated sheets showed a morerapid decrease in Young’s modulus and greater increase in frac-ture strain and tear resistance, with increasing coat weight, thanthe WG- and WPC-coated sheets.

According to Khwaldia (2009), the TS of papers coated withNaCAS-paraffin wax emulsion was not affected by coating weight



Table 2 --- Potential uses of biopolymer coatings as a carrier for bioactive compounds in active food packagingsystems.

Biopolymer coating carrier Active component Target microorganism Reference

Modified starch Cinnamaldehyde E. coli Ben Arfa and others (2007b)Carvacrol B. cinerea

SPI Carvacrol E. coli Ben Arfa and others (2007a)Cinnamaldehyde E. coli Ben Arfa and others (2007b)

B. cinereaCarboxymethyl cellulose Sorbic acid Mold spoilage Ghosh and others (1977)Chitosan Lactic acid Bacillus subtilis Vartiainen and others (2004)

Nisin L. monocytogenes Lee and others (2003)E. coli

Wax Cinnamaldehyde-enriched cinnamon oil Fungal spoilage Rodriguez and others (2007)

(3 to 18 g/m2) and paraffin wax concentration (10% to 40%). In-deed, the TS of the coated paper was controlled by the TS of thebase paper because the coating weights were low in comparisonwith the coating weight of the base paper. However, the E wasincreased by increasing coating weight. Han and Krochta (2001)reported that whey protein coating decreased the TS of the pa-per. During the coating process, WPI solution swells the cellulosefiber structure and penetrates into spaces between fibers. Afterdrying, whey protein remains in the cellulose structure and in-terferes with fiber-to-fiber interaction. Because the coated paperstructure has a smaller interaction force between fibers becauseof coating interference, the TS is decreased after coating. Con-versely, in a previous study, TS and ductility increases have beendocumented for coated paper, consisting of cellulose, NaCAS,mica, carnauba wax, and glycerol (Khwaldia and others 2005).Furthermore, chitosan coating was shown to not affect the TSof the coated paper. The fracture strain was, however, slightlyincreased (Kjellgren and others 2006). Nevertheless, SPI coatingon paperboard reduced the TS by 37.5% compared to that of theuncoated paperboard, while E increased. The ring crush strengthwas, however, not affected by soy protein coating (Rhim andothers 2006).

The TR of coated paper was shown to be affected by bothcoating weight and paraffin wax concentration. NaCAS coatingon paper increased the TR by 25.3% for 18 g/m2, comparedto that of the uncoated paper (Khwaldia 2009). These resultsare in agreement with those of Gallstedt and others (2005) whoshowed that the WPI-coated sheets showed an increase in TRwith increasing coating weight.

Bioactive coatings on paper packagingActive packaging has become one of the major areas of re-

search in food packaging. Principal active packaging systems,successfully developed and utilized in the U.S. and Japan, in-volve oxygen scavenging, moisture absorption, carbon dioxide,or ethanol generation, and antimicrobial systems. Antimicrobialpackaging is of great importance because it could be a po-tential alternative solution to extend the shelf life and assurethe innocuousness and preservation of food products. The di-rect incorporation of antimicrobial agents into food formulationsmay result in partial inactivation of the active substances bythe food constituents. Indeed, their incorporation in films andcoatings could maintain high concentrations on food surfaceswith a low migration of active substances (Coma 2008). Antimi-crobial packaging materials can be prepared by adding a sa-chet in the package, by incorporating bioactive agents directlyinto the packaging material, by coating the active compoundon the surface of the packaging or by utilizing inherently an-timicrobial polymers exhibiting film-forming properties (Cooksey2001).

Biopolymer coating on paper packaging materials may serveas potential inclusion matrices of volatile and nonvolatile an-timicrobial agents to develop biodegradable active packaging(Table 2). The antimicrobial agents may either be released throughevaporation in the headspace (volatile substances) or migrateinto the food (nonvolatile additives) through diffusion. The effi-cacy of biopolymer-based coatings as carriers for incorporatingantimicrobials is mainly related to their good film-forming prop-erties, high retention ability, and release ability. The biopreserva-tives suggested for antimicrobial packaging include organic acidssuch as sorbic, propionic, and benzoic, or their respective acidanhydrides (Vojdani and Torres 1990; Weng and Chen 1997;Cagri and others 2001), bacteriocins such as nisin, pediocin, andlactin (Appendini and Hotchkiss 1996; Ming and others 1997;Padgett and others 1998), volatiles from essential oils, enzymessuch as lysozyme, lactoperoxidase, chitinase, and glucose oxi-dase (Labuza and Breene 1989; Suppakul and others 2003), andfungicides such as benomyl (Halek and Garg 1989) and imazalil(Weng and Hotchkiss 1992).

The choice of active components is often limited by the in-compatibility of the component with the packaging material orby its heat liability. Thus it is important to choose proper coatingmatrix, active agents, and plasticizers.

Essential oils and their components, which are naturally oc-curring antimicrobial agents, are well known for their potencyagainst pathogenic microorganisms and spoilage microorgan-isms (Hammer and others 1999; Cox and others 2000; Benkeblia2004). The antimicrobial activity of essential oils such as thyme,cinnamon, clove, oregano, and their major components aremainly related to their high small terpenoid and phenolic con-tents (Helander and others 1998).

Carvacrol, which is a major component of oregano essentialoil, has been incorporated in SPI coatings on paper (Ben Arfa andothers 2007a). According to these authors, better carvacrol reten-tion was observed when the SPI-coating solution was preparedat 25 ◦C. SPI-carvacrol-coated papers containing various residualcarvacrol quantities were tested, at different times of the kineticrelease, to assess their antimicrobial activity. They demonstratedthat the carvacrol quantity from coated paper necessary to induceE. coli growth inhibition is equal to or greater than 1.1 g/m2. Inanother study, Arfa and others (2007b) designed antimicrobial pa-per based on a SPI or modified starch coating including carvacroland cinnamaldehyde. They investigated the effect of the coatingand drying processes on the ability of these matrices to retaincarvacrol and cinnamaldehyde. Antimicrobial compound losseswere higher for modified starch-coated papers than for SPI-coatedpapers. The antimicrobial properties of the coated papers wereshown against the bacterium E. coli and the mold Botrytis cinereadue to the fast active agent release by the matrices in favorableconditions (high humidity). Coated paper containing 60% (w/w)



of carvacrol or 10% (w/w) of cinnamaldehyde induced E. coligrowth inhibition from 4 to 5 log and a growth delay up to 21 dfor B. cinerea, whatever the coating matrix.

The ability of coating matrices (coated papers) to release activecompounds may depend on matrix nature, the compound natureand concentration, and the environmental conditions such astemperature and relative humidity (Whorton 1995; Chalier andothers 2009). Chalier and others (2009) investigated the com-bined effect of temperature and relative humidity on carvacrolrelease from SPI-coated paper. According to these researchers,increasing storage temperature and relative humidity led to anincrease in carvacrol diffusivities. At 30 ◦C, a significant increasein carvacrol diffusivity of about 81 times was observed by in-creasing relative humidity from 60% to 100%. The effect of these2 parameters (on carvacrol release) could be related to the glasstransition changes of the protein matrix.

Rodriguez and others (2007) have tested the activity of a newactive paper packaging material manufactured by adding es-sential oils to the wax coating formulation against a wide ar-ray of foodborne microorganisms. Essential oils tested in theirstudy included clove, cinnamon, oregano, and cinnamaldehyde-enriched cinnamon essential oil. The use of paper packagingwith an active coating provided an attractive option for protect-ing food from fungal infestation, which also showed promise forprotection against Gram-negative bacteria. The antimicrobial ac-tivities of active wax coatings were affected by the concentrationof the essential oil in the coating. The ability of the developedactive packaging materials were assessed to preserve 2 varietiesof strawberries, since these fruits are usually packaged in paperor board and are prone to fungal spoilage. Complete protec-tion was obtained, during 7 d storage at 4 ◦C, for strawberriesstored in packaging with an active coating containing 4% (w/w)cinnamaldehyde-enriched cinnamon essential oil.

Despite the good results achieved with the incorporation ofessential oils into coating formulations, the major drawback istheir strong flavor, which could change the original taste of foods.The implications on sensory characteristics of food products areof great merit for future research.

Weak organic acids, which are the most common classicalpreservative agents, inhibit the outgrowth of both bacterial andfungal cells. Fungistatic wrappers were developed by coatinggrease-proof paper with an aqueous dispersion of sorbic acidin 2% carboxymethyl cellulose solution. This sorbic-acid treatedpaper could preserve foods that are generally prone to spoilageby mold for a minimum of 10 d (Ghosh and others 1977). Onthe other hand, sorbic acid has also been incorporated into awax-based coating on paper. The active packaging materials de-veloped were used to package sausages and cheeses (Labuza andBreene 1989).

Chitosan is inherently antimicrobial and has attracted attentionas a potential food preservative of natural origin due to its antimi-crobial activity against a wide range of foodborne filamentousfungi, yeasts, and bacteria (Sagoo and others 2002; Shahidi andAbuzaytoun 2005). Several hypotheses have been proposed toexplain the mechanism of the antimicrobial activity of chitosan:chitosan disrupts the barrier properties of the outer membranesof Gram-negative bacteria, which leads to the leakage of intracel-lular constituents (Young and others 1982; Helander and others2001). Chitosan can act as a chelating agent that binds trace met-als, spore elements, and essential nutrients, and thereby inhibitsthe production of toxins and microbial growth (Cuero and others1991). The antibacterial effects of chitosan are reported to be de-pendent on its molecular weight (Chen and others 1998; Jeon andothers 2001), its degree of deacetylation (Tsai and others 2002),its concentration in solution, the pH of the medium (Rabea andothers 2003), and the type of bacterium (No and others 2002).

Inherent antibacterial/antifungal properties and the film-forming ability of chitosan make it ideal for use as a biodegradableantimicrobial packaging material. Chitosan is insoluble in mostsolvents, but is soluble in dilute organic acids such as acetic,formic, succinic, lactic, and malic and forms viscous solutions.The viscosity property of chitosan solution may differ with or-ganic acid type used as a dissolving solvent, thus affecting theproperties of the resultant films or coatings. Vartiainen and oth-ers (2004) tested the effects of nisin and dissolving solvents onthe antimicrobial activity of chitosan-coated paper. Chitosan dis-solved in acetic and propionic acids and did not have any activ-ity against Bacillus subtillis. Chitosan coatings containing lacticacid, however, showed strong antimicrobial activity according toboth inhibition zone and bacteria reduction tests. The incorpora-tion of nisin, at a concentration of 0.08 g/L, in coating solutionsprepared from chitosan dissolved in different acids did not en-hance the antimicrobial activity. From a food quality perspective,chitosan does not adversely affect the quality properties of foods(that is, organoleptic, texture, and so on). However, the use ofacetic acid in the formulation should be controlled and reducedto the furthest extent possible to optimize the active coating for-mulation without affecting organoleptic properties of the foodproducts.

Nisin and chitosan have also been coated, in 3% con-centrations, onto paper with a binder medium of a vinylacetate/ethylene copolymer to provide antimicrobial activityagainst Listeria monocytogenes and/or E. coli (Lee and others2003). Combined inclusion of nisin and chitosan in the coatingimproved the microbial stability of milk and orange juice storedat 10 ◦C. Lee and others (2004) applied nisin on the surface ofpaperboard. They reported, using the coated materials with nisin,inhibition of Micrococcus flavus growth in a model emulsion andin cream (from milk).

ConclusionsBiopolymer-coating on paper packaging materials are very

promising systems for the future improvement of food packaging.They have potential environmental advantages over conventionalsynthetic paper coatings. The use of such biopackagings will openup potential economic benefits to farmers and agricultural pro-cessors. Extensive research is needed on the development of newcoating materials, methods of coating formation, methods to im-prove coating properties, and potential applications.

Biopolymer coatings on paper packaging materials are po-tential inclusion matrices of antimicrobial agents to developbiodegradable active packaging. Due to its potential to providequality and safety benefits, antimicrobial packaging is expectedto grow in the next decade with the advent of new polymericmaterials and antimicrobials. Further research is needed to gainmore knowledge regarding the interactions between the coatingmatrix, active compounds, and target microorganisms to evalu-ate the materials’ performance and to optimize the compositionsof active coatings. Furthermore, the antimicrobial properties ofcoated papers are of great merit for future research. For foodproduct applications, research is essential to evaluate the impactof active agents on organoleptic properties of the packaged foodproducts.

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Biopolymer Coatings on Paper Packaging Materials