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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials for Coatings
  5. Functional Properties
  6. Conclusions
  7. References

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


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials for Coatings
  5. Functional Properties
  6. Conclusions
  7. References

Paper 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 microfibrils, which are composed of long-chain cellulose molecules in a crystalline state with amorphous regions regularly disrupting the crystalline structure. The hydrophilic nature of cellulose, due to the OH sites in the basic unit of cellulose (C6H10O5) and fiber network porosity, limits the water-vapor-barrier properties of paper. Paper packaging also easily absorbs water from the environment or from the food and looses its physical and mechanical strengths. Moisture migration can occur in paper by diffusion of water vapor through the void spaces as well as in condensed form through the fiber cell walls (Bandyopadthay and others 2002).

Paper is often associated with other materials, such as plastic materials and aluminum, for their good barrier properties that could be advantageously combined with paper stiffness. Paper is coated with ethyl vinyl alcohol (EVOH), a polymer with excellent oxygen-barrier properties, when gas barrier properties are requested (Zhang and others 1999, 2001). However, because of the polar groups of EVOH at the origin of the hydrophilic character of the polymer at high relative humidity, an additional polymer layer based on polyolefins is used to prevent water sorption (Despond and others 2005). Polyolefins are generally chosen as paper coating materials to overcome porosity and hygroscopicity of paper. Unfortunately, the obtained material loses its biodegradation and recyclability characteristics due to the addition of synthetic polymer layers.

Natural polymers can be used as barrier coatings on paper packaging materials. Such biodegradable coatings have the potential to replace current synthetic paper coatings, such as polyethylene, polyvinyl alcohol, rubber latex, and fluorocarbon in food packaging applications (Chan and Krochta 2001a, 2001b). Agriculturally derived alternatives to synthetic paper coatings provide an opportunity to strengthen the agricultural economy and reduce importation of petroleum and its derivatives.

Naturally renewable biopolymers have been the focus of much research in recent years because of interest in their potential use as edible and biodegradable films and coatings for food packaging. The properties, technology, functionalities, and potential uses of biopolymer films and coatings have been extensively reviewed by Kester and Fennema (1986), Gennadios and others (1994), Gontard and Guilbert (1994), Krochta and others (1994), Anker (1996), Guilbert and others (1997), Krochta and De Mulder-Johnston (1997), Krochta (2002), and Khwaldia and others (2004a).

Biopolymer-based packaging materials originated from naturally renewable resources such as polysaccharides, proteins, and lipids or combinations of those components offer favorable environmental advantages of recyclability and reutilization compared to conventional petroleum-based synthetic polymers. Biopolymer films and coatings may also serve as gas and solute barriers and complement other types of packaging by minimizing food quality deterioration and extending the shelf life of foods (Guilbert and others 1996; Krochta and De Mulder-Johnston 1997; Debeaufort and others 1998). Moreover, biopolymer-based films and coatings can act as efficient vehicles for incorporating various additives including antimicrobials, antioxidants, coloring agents, and nutrients (Baldwin 1994; Petersen and others 1999; Ozdemir and Floros 2001; Han and Gennadios 2005).

The association of biopolymers to paper provides interesting functionalities while maintaining environment-friendly characteristic of the material. Renewable biopolymers, such as caseinates (Khwaldia and others 2005; Gastaldi and others 2007; Khwaldia 2009), whey protein isolate (WPI; Han and Krochta 1999, 2001; Lin and Krochta 2003, Gällstedt and others 2005), isolated soy protein (Park and others 2000; Rhim and others 2006), wheat gluten (Gällstedt and others 2005), corn zein (Trezza and Vergano 1994; Parris and others 1998; Trezza and others 1998), chitosan (Despond and others 2005; Ham-Pichavant and others 2005; Kjellgren and others 2006), carrageenan (Rhim and others 1998); alginate (Rhim and others 2006), and starch (Matsui and others 2004) have been investigated as paper-coating materials.

Han and Krochta (1999, 2001) showed that whey-protein-coated paper improves packaging material performance of paper by increasing oil resistance and reducing water-vapor permeability. Despond and others (2005), as well as Kjellgren and others (2006), used paper coated with chitosan or chitosan/carnauba wax to obtain a packaging material with good barrier properties towards oxygen, nitrogen, carbon dioxide, and air. Rhim and others (2006) indicated that water resistance of paper is improved by coating with soy protein isolate (SPI) or alginate.

The main objectives of this study were to review the different types of renewable biopolymers investigated as paper coating materials, to summarize the barrier, mechanical, and other properties possessed by biopolymer-coated paper, and finally to discuss existing and potential applications for bioactive coatings on paper coating materials.

Materials for Coatings

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials for Coatings
  5. Functional Properties
  6. Conclusions
  7. References

Renewable biopolymers available for forming coatings on paper packaging materials are generally made from proteins, polysaccharides, and lipids used alone or together. The choice of materials for a coating is largely dependent on its desired function. Plasticizers, regular paper pigments, antioxidants, or antimicrobial agents can be added to paper coating solutions to improve its performance properties.

Biobased polymers can be applied to paper or paperboard with different coating techniques, such as surface sizing, solution coating, compression molding, and curtain coating depending on the appropriate coating material and type of paper used. Surface sizing is one of the most frequently used processes for applying an aqueous coating to a paper substrate. In surface sizing, the solid content of the coating is limited and is typically lower than 10% to 15% (Vartiainen and others 2004). A low solid content does not yield a fully continuous coating and increases the amount of drying needed. A higher coating weight and better gas-barrier properties can be obtained using curtain-coating technique in which the paper industry has begun to show a considerable interest (Kjellgren and others 2006). A thick and continuous coating, necessary in several cases to obtain coverage of the paper, is not possible to obtain by solution coating. However, this coating technique results in interesting mechanical properties (Gällstedt and others 2005). The compression-molding technique is suitable for applications where complete coverage and thick coatings were necessary, and which, therefore, involved significantly more coating material compared to solution coating.

Protein-based coatings

Proteins 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 coatings and their film properties have been quantified (Krochta 2002; Sobral and others 2005; Gounga and others 2007). Proteins are suitable for coating fruits and vegetables, meats, eggs, nuts, other dry foods, and paper packaging. Protein coatings on paper include milk proteins, wheat gluten, gelatin, corn zein, and SPIs. Protein-derived coatings show excellent oxygen barrier property at low to intermediate relative humidity as well as fairly good mechanical properties. However, their barrier against water vapor is poor due to their hydrophilic nature (Avena-Bustillos and Krochta 1993).

Caseins and caseinates Milk proteins, such as casein, have several key physical characteristics for effective performance in edible films and coatings, such as their solubility in water and ability to act as emulsifiers (Southward 1985). Sodium caseinate (NaCAS) can easily form films from aqueous solutions because of its random coil nature and its ability to form extensive intermolecular hydrogen, electrostatic, and hydrophobic bonds, resulting in an increase of the interchain cohesion (Avena-Bustillos and Krochta 1993; McHugh and Krochta 1994; Brault and others 1997). NaCAS films appear to have lower oxygen permeabilities than nonionic polysaccharide films (Khwaldia and others 2004a). This may be related to their more polar nature and more linear (nonring) structure, leading to higher cohesive energy density and lower free volume (Miller and Krochta 1997). Moreover, they possess good mechanical properties, as has been shown in our previous research (Khwaldia and others 2004b). These properties make caseinate an attractive polymer for the coating of cellulose-based materials for food packaging purposes.

Khwaldia (2009) showed that the thickness of papers coated with NaCAS was affected by the coating weight. By increasing coating weight from 3 to 18 g/m2, the dried thickness of NaCAS-paper bilayers increased. Therefore, most of the NaCAS content formed a continuous layer on the surface of the paper, which is a porous, with rough surface, material leading to an increase of measurable paper thickness. Likewise, Gastaldi and others (2007) demonstrated through microscopic observations that a calcium caseinate coating produced a homogeneous and rather dense layer on the paper with a regular and smooth surface. Impregnation percentage of the calcium caseinate coating solutions was low (4.8%) in contrast to wheat gluten solutions that presented a higher impregnation percentage (above 50%). The performance of coated paper as packaging material is closely dependent on the integrity of the coating layer and its interface with paper. Impregnation measurements constituted an original approach to characterize the structure of interface created between paper and coating. Impregnation rates were not only related to the viscosity of coating solution applied on paper, but also to the increase of concentration of coating agent during the drying stage.

On the other hand, Khwaldia (2009) showed that NaCAS coating improved both paper strength and ductility and reduced water vapor transmission. They also found that increasing paraffin wax concentration in the coating led to an increase in tearing resistance of the resulting coated papers. Conversely, Khwaldia (2004) reported that the tearing resistance of coated paper was not affected by carnauba wax concentration.

Whey protein Whey protein, a by-product of the cheese industry, is already known as an excellent barrier to oxygen, aroma, and oil and can be used as a coating material for improving the oxygen barrier property of food packaging (Miller and Krochta 1997). The oxygen permeability of whey protein films has been reported to be very low and comparable to that of EVOH polymer at low or intermediate relative humidity conditions (McHugh and Krochta 1994). Compared to currently used sizing agents and pigment adhesives, whey protein may have some advantages. It forms an intact water-insoluble film out of aqueous solution, due to the formation of intermolecular disulfide bonds after heat denaturation (McHugh and Krochta 1994). Thus, such a whey protein film has a cross-linked structure.

Some studies have considered whey proteins as coatings on paper. Han and Krochta (1999) showed that whey-protein-coated paper improves packaging material performance of paper by increasing oil resistance and reducing water vapor permeability (WVP). Chan and Krochta (2001a) reported a significant reduction in oxygen permeability for paperboard coated with denatured and undenatured WPI. Han and Krochta (2001) studied the increase in gloss and the increase in oil resistance of paper coated with WPI. The increased gloss after WPI coating may be caused by the paper surface being more homogeneous and smoother. The increase of surface smoothness and homogeneity were also suggested by the previous research of Han and Krochta (1999). Gällstedt and others (2005) showed that WPI and whey protein concentrate (WPC) enhanced the strength and toughness of the paper. Conversely, Han and Krochta (2001) reported that whey protein coating decreased the tensile strength of the paper, because the coated paper structure has smaller interaction force between fibers because of coating interference. Chan and Krochta (2001b) pointed out that WPI coatings produce high and stable gloss values. WPI might replace commercial paperboard coatings such as polyvinyl alcohol and fluorocarbon as grease and oxygen barriers while maintaining desirable color and gloss. Although the replacement of existing polymer coating formulations for paper with biodegradable coating formulations might be possible, a reliable solution has not yet been found. Many questions and problems still have to be solved before biopolymers can be commercially used as a replacement for synthetic polymers. These questions concern the demands on the biopolymer coating and the cellulose substrate as well as on the coating process.

Soy protein Generally, soy protein films have inadequate mechanical properties and are poor moisture barriers because of the hydrophilic nature of soy protein. Researchers have attempted to improve the properties of soy protein films that have major potential applications in the food and packaging industry (Stuchell and Krochta 1994; Rangavajhyala and others 1997; Rhim and others 1999). It is estimated that in the United States, about 25000 to 50000 metric tons of soy proteins are used in paper coatings (Myers 1993). SPI-coated paper was found to impart gas and oil barrier as well as adequate mechanical properties, for extending 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 paperboards. The contact angle of water on the alginate-coated paperboards decreased more than that of the SPI-coated ones. However, water resistance of the alginate-coated paperboards posttreated with the CaCl2 solution was comparable to the SPI-coated ones. These same researchers indicated that SPI coatings cross-linked by formaldehyde posttreatment or composited with organically modified montmorillonite were more effective in decreasing the WVP of coated paperboards. The cross-linking technique is an interesting approach to enhance mechanical and water vapor barrier properties of biodegradable films and coatings for food packaging applications. The more commonly used covalent 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 highly questionable. Due to the possible toxicity of these modifying agents, further research should be done to analyze chemical residues remaining in the film and their migration in the event of these 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 of edible coatings based on WG (Gontard and others 1992, 1994; Gennadios and others 1993, 1994).

Techniques have been developed for the production of paper coating dispersions in which gluten is used as a binder. These suspensions show good film-forming properties and the resulting coating has a strong adhesion to various substrates (Derksen and others 1995). According to Gastaldi and others (2007), increasing WG concentrations from 10% to 20% in coating solutions decreased adsorption percentage of dry matter from 63.3% to 53.6%, respectively. This effect could be related to the increased solution viscosity with WG solution concentration, considering that a viscous solution would be less prone to penetrate inside fibrous 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 insolubility in water, and they form strong, glossy films resistant to grease and oxygen permeation.

Corn zein coatings do not interfere with paper recycling, do not require separating protein and paper layers, and have been suggested as an alternative to polyolefin materials Trezza and Vergano (1994). Little data are available on the biodegradability, the recyclability, and the reutilization of biopolymer-coated paper or paperboard. Research is needed to evaluate their recycling potential and their rate of biodegradation under composting conditions.

Trezza and Vergano (1994) measured the grease resistance of corn zein-coated paper with respect to coating level, plasticizer addition, and time exposure. As coating level increased, uniformity of the coating also increased. Coating uniformity and quality are necessary for good grease resistance. Zein-coated papers were as effective grease barriers as were polyethylene laminates 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 coatings

Polysaccharides are nontoxic and widely available. They also are excellent gas, aroma, and lipid barriers. They form strong films, but because of their hydrophilic nature exhibit poor water vapor barrier properties (Kester and Fennema 1986; Guilbert 1986). Many researchers have studied the film formation and the properties of several polysaccharide materials (Kamper and Fennema 1985; Martin-Polo and others 1992a, 1992b; Nisperos-Carriedo 1994). Polysaccharides most used for paper coating include starch, alginates, carrageenan, and chitosan.

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

Chitosan films are tough, long-lasting, flexible, and very difficult to tear. Most of their mechanical properties are comparable to many medium-strength commercial polymers. It has been reported that chitosan films have moderate WVP values and could be used to increase the storage life of fresh produce and foodstuffs with higher water activity values. Chitosan exhibits excellent oxygen-barrier properties due its high cristallinity and the hydrogen bonds between the molecular chains (Kittur and others 1998; Gällstedt 2001). Moreover, chitosan is a good barrier against grease (Kittur and others 1998). Due to its positive charge on the amino group under acidic conditions, chitosan binds to negatively charged molecules such as fats and lipids (Jumaa and Müller 1999; Shu and others 2001). These properties make chitosan an attractive polymer for the barrier coating of cellulose-based materials for food packaging purposes.

Chitosan has been used as a papermaking additive and for the surface 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 including chitosan as a wet-end additive were improved. The chitosan retention was also reported to be good, due to the different charges of the chitosan (cationic) and cellulose (anionic). Water-insoluble sheets of chitosan and pulp fiber have been developed to enhance the gas-barrier properties of paper (Hosokawa and others 1991; Gällstedt and Hedenqvist 2006). The printability of paper increases with the addition of chitosan due to the fact that the paper surface becomes smoother (Thomson 1985).

Studies of chitosan coatings on paper, paperboard, and cellophane have been reported (Domszy and others 1985; Dobb and others 1998; Krasavtsev and others 2002; Ho and others 2003; Vartiainen and others 2004; Kjellgren and others 2006; Bordenave and others 2007). Chitosan is readily compatible with paper and is one of the most interesting polysaccharide coating materials for paper. Bordenave and others (2007) have characterized the morphology and the microstructure of chitosan-coated papers by infrared spectroscopy and scanning electron microscopy. Their observations suggested that the chitosan penetrated deeply into the paper, embedding the cellulose fibers, instead of forming a layer on paper. The chitosan-coated materials exhibited good moisture barrier properties, but not sufficient for food applications, and their surface hydrophilicity was too high.

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

Rhim and others (2006) found a decrease in contact angle of water by coating paper with alginate. Reduction in the contact angle of water by alginate coatings indicates an increase in hydrophilicity of the surface of paperboards, which became smoother and more homogeneous resulting in an increased affinity of the paperboards to water. According to Ham-Pichavant and others (2005), incorporation of sodium alginate in chitosan formulations considerably increased the fat barrier of coated papers and, at the same time, reduced the treatment cost. Indeed, the chitosan/alginate mixture, after coating on paper, allowed fat resistance with a synergistic effect, taking into account the possible limitation of chitosan penetration into the paper and the contribution to a smoother surface due to film-forming capacities of gums at low concentration. Rhim and others (2006) reported that the tensile strength of paperboards was decreased with alginate coatings. The decrease in tensile strength of alginate-coated paperboards is mainly due to the swelling of cellulose fiber by solvent penetration during coating and may be partially due to the fact that alginate impregnated into the cellulose structure of paper and interfered with fiber-to-fiber interaction.

Starch Starch is the most commonly used agricultural raw material since it is inexpensive and widely available. Starch films have poor physical properties, but these can be improved by blending the starch with cellulose derivates and proteins (Arvanitoyannis and others 1996, 1998; Psomiadou and others 1996; Peressini and others 2004).

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

The acetylation reaction is one of the most interesting ways to decrease starch hygroscopicity. This chemical reaction allows the attainment of thermoplastic and hygroscopic materials (Graaf and others 1995; Fringant and others 1996). Larotonda and others (2005) as well as Fringant and others (1998) used papers treated with starch acetate to decrease paper hygroscopicity. Larotonda and others (2005) demonstrated that significant reductions in water adsorptivity and WVP of Kraft paper might well be achieved through starch acetate impregnation, mainly in low relative humidity conditions. Indeed, the starch acetate impregnated the paper structure and partially filled superficial and internal pores, thus decreasing the paper permeability. Furthermore, as starch acetate is much less hygroscopic than paper, its adsorptivity is reduced significantly by impregnation. Indeed, the impregnation of papers with nonhygroscopic and biodegradable materials is an interesting treatment used to reduce the hygrocopicity and the WVP of papers. This treatment could not only improve the water barrier property of paper packaging, but also its barrier properties against gas and aroma compounds maintaining the foodstuff quality during storage (Dury-Brun and others 2008). However, the impregnation did not improve the mechanical properties of the paper materials (Matsui and others 2004). The properties of impregnated papers depend on the time of immersion, the concentration of the material used for impregnation, and the impregnation procedure (with or without vacuum application).

Lipid and composite coatings

Lipid compounds, such as long-chain fatty acids and waxes, can be incorporated in the film or coating matrix because of their hydrorepellency. Waxes are the most efficient substances to reduce moisture permeability. Their high hydrophobicity is a consequence of a high content in esters of long-chain fatty alcohols and acids, as well as long-chain alkanes (Kester and Fennema 1986; Donhowe 1992; Hagenmeier and Shaw 1992).

Paper and paperboard, which are the most widely used materials in food and drink packaging, are frequently wax-coated to improve their water-resistance and increase the shelf life of the packaged products (Rodriguez and others 2007). Paraffin wax applied in a molten form was commonly used to produce a water vapor barrier. Recyclable packaging paper materials using autodispersible waxes have been reported (Back 1995).

Lipid coatings provide good moisture barrier, but they have certain disadvantages such as brittleness, lack of homogeneity, and presence of pinholes and cracks in the surface of the coating. Composite coatings or multilayer coatings, applied either in the form of an emulsion or in successive layers (multilayer coating), have been prepared to combine the good structural and gas-barrier properties of hydrocolloid coatings with the good moisture-barrier characteristics of lipids. The method of application affects the barrier properties of the coatings obtained.

Parris and others (1998) measured the water barrier and grease permeation properties of Kraft paper coated with a combination of zein and paraffin wax. Their data demonstrated that the zein layer of the bilayer coating contributes grease-proofing and the wax layer water resistance. In a previous study, WVP decreases have been documented for NaCAS-coated paper due to the addition of carnauba wax (Khwaldia and others 2005). The WVP of coated papers decreased as the amount of wax in the coating increased. The addition of hydrophobic substances (carnauba wax) to this hydrophilic matrix provides the moisture-barrier properties.

Khwaldia (2009) found that the greatest reduction in paper WVP is achieved by addition of a wax layer to the paper already coated with NaCAS, due to the high resistance to moisture transfer of the paraffin wax. Indeed, the barrier ability of bilayer coating against water vapor transfer is higher than that of an emulsion coating. Emulsion coatings have the advantage that they are easier to apply on paper materials than bilayer coatings and they need only one drying step. Moreover, no problem separation of the two layers occurs, and their both hydrophilic and lipophilic nature allows their good adhesion onto any support.

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

Functional Properties

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials for Coatings
  5. Functional Properties
  6. Conclusions
  7. References

Barrier properties

Regarding the barrier properties of packaging materials, the critical compounds that can penetrate the packaging materials and degrade food quality are water vapor and oxygen of the surrounding atmosphere. To avoid the moisture transfer that can affect food quality, WVP control is important to assure stability and safety during distribution and storage. The ingress of oxygen, which is strongly and irreversibly reacted with food components such as lipids, vitamins, flavors, and colors, leads to permanent change in the nature of food products (rancidity, vitamin loss, and microbial contamination). Good oxygen barrier properties are critical for achieving a long shelf life for the packaged product. Other important gases to which food packaging should be less permeable are carbon dioxide and nitrogen.

To meet this demand, expensive synthetic barrier polymers, including EVOH copolymers and polyvinylidene chloride are commonly used in the form of laminates as oxygen-barrier layers in food packaging materials. Such composite synthetic laminates are not biodegradable and cannot be recycled. Therefore, there is an increasing interest in the development of biodegradable polymers for packaging materials that have suitable application properties and can be disposed of after use in an economically and ecologically acceptable way. The various potential functions of biopolymers used in paper coating are summarized in Table 1.

Table 1—.   Functions of biopolymers used in paper coating.
BiopolymersFunctionsReference
WPIIncrease printability of water-based inkHan and Krochta (1999)
Grease barrierHan and Krochta (2001)
 Chan and Krochta (2001a)
NaCASOxygen barrierKhwaldia (2004)
NaCAS/paraffin wax bilayerWater vapor barrierKhwaldia (2009)
Corn zeinGrease barrierTrezza and Vergano (1994)
Minimize effects of drying and brittleness 
Corn zein/paraffin wax bilayerWater vapor barrierParris and others (1998)
Grease barrier 
SPIGas and lipid barrierPark and others (2000)
SPI with CaCl2 posttreatmentWater vapor barrierRhim and others (2006)
WGOxygen barrierGällstedt and others (2005)
CarrageenanGrease barrierRhim and others (1998)
HPMC/beeswaxWater vapor barrierSothornvit (2009)
ChitosanFat barrierHam-Pichavant and others (2005)
Gas barrierKjellgren and others (2006)
Chitosan/sodium alginate bilayerFat barrierHam-Pichavant and others (2005)
Chitosan/carnauba wax bilayerGas barrierDespond and others (2005)
Chitosan/sodium alginate bilayerFat barrierHam-Pichavant and others (2005)
Paraffin waxWater vapor barrierParris and others (1998)

Gas permeability Greaseproof paper was coated with chitosan to obtain a packaging material with good barrier properties towards oxygen, nitrogen, and carbon dioxide (Kjellgren and others 2006). The oxygen permeability in the same range as the polyethylene terephthalate was obtained at coat weights exceeding 5 g/m2. The oxygen permeability was not substantially affected by temperature changes, provided that the air permeance of the base paper was low. A barrier against nitrogen and carbon dioxide required a coat weight exceeding 5 g/m2. Trezza and others (1998) reported a reduction in the oxygen permeability of paper coated with corn zein. Furthermore, Gällstedt and others (2005) studied the effects of coating procedures on oxygen barrier properties of paper and paperboard coated with chitosan, WPI, WPC, and WG. Paper sheets were solution-coated using a hand applicator, WG was compression-molded onto paper and paperboard, and chitosan solution was also applied on paperboard using curtain-coating. The coatings on the applicator-coated sheets were too thin and discontinuous to improve the oxygen barrier properties. Because of the higher amount of WG material in the compression-molding process, coatings were thick and continuous, resulting in low oxygen permeability. Chitosan-curtain-coated paperboard showed the highest oxygen-barrier properties, which are comparable to those of commonly used packaging oxygen-barrier polymers. On the other hand, Khwaldia (2004) studied the combined effects of mica, carnauba wax, glycerol, and NaCAS concentrations on oxygen-barrier properties. Coating significantly increased oxygen-barrier property. The oxygen permeability of the coated paper was 13 to 90 times lower than that of the uncoated paper.

Water vapor permeability A water barrier can be formed by changing the wettability of the paper surface with sizing agents or through coating with hydrophobic materials. Paper is often coated with paraffin wax, applied in a molten form, to produce a water vapor barrier. Han and Krochta (1999) studied the 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 by 44.8% compared to the uncoated paper after WPI coating with 10 g/m2. The properties of the NaCAS-paper bilayers were investigated by Khwaldia (2009). The WVP of NaCAS-coated paper was decreased consistently by increasing coating weight from 3 to 18 g/m2. NaCAS coating on paper reduced WVP by 75% for 18 g/m2 coating weight compared to that of the uncoated paper. In a previous study, Khwaldia and others (2005) showed that the WVP of NaCAS-coated papers decreased as the amount of wax in the coating increased. The addition of hydrophobic substances to this hydrophilic matrix provides the moisture barrier properties. The substantial reduction in WVP of paper by incorporation of waxes was expected because waxes are most efficient substances to reduce moisture permeability due to their high hydrophobicity.

Parris and others (1998) evaluated coating formulations composed of the corn protein zein and paraffin wax for their water-vapor-barrier properties. The water vapor transmission rates for paper coated with paraffin wax were found to be significantly lower than those measured using the zein-coated paper. Coating the paper with a 2% solution of zein in paraffin wax reduced the water vapor transmission rates by approximately half the values obtained for wax-coated paper. Water vapor transmission values were strongly dependent on the amount of wax in the coating. On the other hand, Rhim and others (2006) showed that water barrier properties of paperboards increased by SPI or alginate coating with CaCl2 posttreatment. Biopolymer-coated paperboards can be used in the preparation of water-resistant corrugated fiberboard boxes for the storage of high-moisture foods. Larotonda and others (2005) reported that Kraft paper impregnation with cassava starch acetate is an interesting alternative for improving the hygroscopic properties and obtaining a waterproof paper. Furthermore, hydroxypropyl methylcellulose (HPMC)-based coatings reduced WVP and further reduction was obtained when beeswax was incorporated in the HPMC-lipid composite-coated paper (Sothornvit 2009). Using HPMC as a coating material for paper has a benefit in terms of lower concentration of coating solution, while providing desirable mechanical properties. Indeed, a low concentration of HPMC is adequate to provide the appropriate viscosity for coating on paper. Further investigation is still needed to verify the properties of HPMC-based coated paper with specific products.

Bordenave and others (2007) evaluated the barrier properties against moisture and the liquid water sensitivity of chitosan-coated papers. They showed that the chitosan coating led to a significant decrease of the paper moisture transfer but the surface hydrophilicity remained high.

Oil permeability Grease resistance is an important property of paper packaging materials used for foods containing fats or oils. Limited research has been done on quantifying the oil permeability of packaging materials. Coated paper or paperboard with a good grease barrier is important for packaging used in fast-food restaurants, as well as food-packaging applications such as cereal boxes, donut boxes, and pizza boxes. Corn zein was shown to have excellent grease resistance, both as a film and as a coating on paper. Zein coating on paper for grease barrier was compared to quick-service sandwich packaging, and it was found that zein-coated papers were as effective as polyethylene laminates used for quick-service restaurant sandwich packaging (Trezza and Vergano 1994). In their study, the zein-coated papers were not heat-sealed to a 2nd sheet of paper, as were the commercial polyethylene-laminated samples. Further research is required to evaluate the effects of heat sealing of zein coating and storage on grease properties of the coated papers.

Research results also showed that a whey protein film (De Mulder-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-coated papers was comparable to polyethylene-laminated papers, and Park and others (2000) reported that soy-protein-coated papers imparted gas and lipid barrier, as well as adequate mechanical properties.

Chan and Krochta (2001a) studied the grease barrier property of WPI-coated paperboard. They found that a good grease barrier was obtained with paperboard coated with WPI and glycerol as plasticizer. However, glycerol plasticizer may migrate into the paperboard during storage. Lin and Krochta (2003) concluded that WPC with about 80% protein coatings on paperboard gave a grease barrier comparable to WPI coatings. Sucrose-plasticized whey-protein coatings on paperboard imparted excellent grease resistance, similar to glycerol-plasticized coatings. Long-term ambient storage of WPC-coated paperboard indicated that the use of sucrose as plasticizer imparted good grease resistance and minimized plasticizer migration. On the other hand, Ham-Pichavant and others (2005) explored the ability of bilayer chitosan-coated paper as fat barrier. They also investigated the nature of interactions between fatty acids, chosen as model lipids, and chitosan. Their experiments showed a strong pH-dependent chitosan-lipid interaction. The chitosan layer could act as a lipid trap coating to decrease fat transfer if the pH of the chitosan film-forming solution was adjusted to 5.5 to 6 prior to coating. Chitosan-coated papers can be used as fat barrier packaging with a chitosan level of 5.41%. However, treatment costs remain high compared with fluorinated resins. In an attempt to decrease both treatment cost and fat transfer, chitosan was associated with various polymers. Incorporation of sodium alginate considerably increased the fat barrier of coated papers. Kjellgren and others (2006) reported that chitosan-coated greaseproof papers exhibited excellent grease resistance within the coat weight range of 2.4 to 5.2 g/m2. The air permeability of the coated material had a great influence on grease resistance.

Mechanical properties

In many packaging applications, barrier properties as well as mechanical resistance are required. In general, mechanical properties of coated/laminated films in a composite structure tend to rely strongly on the substrate or base film rather than the coating (Hong and others 2004). The mechanical properties frequently measured to characterize paper-based packaging materials are tensile strength (TS), elongation (E), elastic modulus (EM), and tearing resistance (TR). TS is a measure of the ability of a film to resist breaking under tension, which is dependent on the strength of fibers, their surface area, and length, and also the bonding strength between them. E is a quantitative representation of the film's ability to stretch. EM is the fundamental measure of film stiffness. TR corresponds to the average force applied during the tearing operation; it is likely that it relates to the fracture stress and/or fracture resistance or toughness of the material (Rabinovitch 2003).

Gällstedt and others (2005) studied the mechanical properties of paper and paperboard coated with chitosan, WPI, WPC, and WG protein. The mechanical tests of solution-coated paper showed that chitosan was the most effective coating on a coat weight basis. This was due to its high viscosity, which limited the degree of penetration into the paper. The researchers reported that the fracture stress increased with increasing coat weight for all the solution coatings. The WPI-coated sheets showed a more rapid decrease in Young's modulus and greater increase in fracture strain and tear resistance, with increasing coat weight, than the WG- and WPC-coated sheets.

According to Khwaldia (2009), the TS of papers coated with NaCAS-paraffin wax emulsion was not affected by coating weight (3 to 18 g/m2) and paraffin wax concentration (10% to 40%). Indeed, the TS of the coated paper was controlled by the TS of the base paper because the coating weights were low in comparison with the coating weight of the base paper. However, the E was increased by increasing coating weight. Han and Krochta (2001) reported that whey protein coating decreased the TS of the paper. During the coating process, WPI solution swells the cellulose fiber structure and penetrates into spaces between fibers. After drying, whey protein remains in the cellulose structure and interferes with fiber-to-fiber interaction. Because the coated paper structure has a smaller interaction force between fibers because of coating interference, the TS is decreased after coating. Conversely, in a previous study, TS and ductility increases have been documented 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 TS of the coated paper. The fracture strain was, however, slightly increased (Kjellgren and others 2006). Nevertheless, SPI coating on paperboard reduced the TS by 37.5% compared to that of the uncoated paperboard, while E increased. The ring crush strength was, however, not affected by soy protein coating (Rhim and others 2006).

The TR of coated paper was shown to be affected by both coating weight and paraffin wax concentration. NaCAS coating on paper increased the TR by 25.3% for 18 g/m2, compared to that of the uncoated paper (Khwaldia 2009). These results are in agreement with those of Gällstedt and others (2005) who showed that the WPI-coated sheets showed an increase in TR with increasing coating weight.

Bioactive coatings on paper packaging

Active packaging has become one of the major areas of research in food packaging. Principal active packaging systems, successfully developed and utilized in the U.S. and Japan, involve oxygen scavenging, moisture absorption, carbon dioxide, or ethanol generation, and antimicrobial systems. Antimicrobial packaging is of great importance because it could be a potential alternative solution to extend the shelf life and assure the innocuousness and preservation of food products. The direct incorporation of antimicrobial agents into food formulations may result in partial inactivation of the active substances by the food constituents. Indeed, their incorporation in films and coatings could maintain high concentrations on food surfaces with a low migration of active substances (Coma 2008). Antimicrobial packaging materials can be prepared by adding a sachet in the package, by incorporating bioactive agents directly into the packaging material, by coating the active compound on the surface of the packaging or by utilizing inherently antimicrobial polymers exhibiting film-forming properties (Cooksey 2001).

Biopolymer coating on paper packaging materials may serve as potential inclusion matrices of volatile and nonvolatile antimicrobial agents to develop biodegradable active packaging (Table 2). The antimicrobial agents may either be released through evaporation in the headspace (volatile substances) or migrate into the food (nonvolatile additives) through diffusion. The efficacy of biopolymer-based coatings as carriers for incorporating antimicrobials is mainly related to their good film-forming properties, high retention ability, and release ability. The biopreservatives suggested for antimicrobial packaging include organic acids such as sorbic, propionic, and benzoic, or their respective acid anhydrides (Vojdani and Torres 1990; Weng and Chen 1997; Cagri and others 2001), bacteriocins such as nisin, pediocin, and lactin (Appendini and Hotchkiss 1996; Ming and others 1997; Padgett and others 1998), volatiles from essential oils, enzymes such as lysozyme, lactoperoxidase, chitinase, and glucose oxidase (Labuza and Breene 1989; Suppakul and others 2003), and fungicides such as benomyl (Halek and Garg 1989) and imazalil (Weng and Hotchkiss 1992).

Table 2—.  Potential uses of biopolymer coatings as a carrier for bioactive compounds in active food packaging systems.
Biopolymer coating carrierActive componentTarget microorganismReference
Modified starchCinnamaldehydeE. coliBen Arfa and others (2007b)
CarvacrolB. cinerea 
SPICarvacrolE. coliBen Arfa and others (2007a)
CinnamaldehydeE. coliBen Arfa and others (2007b)
 B. cinerea 
Carboxymethyl celluloseSorbic acidMold spoilageGhosh and others (1977)
ChitosanLactic acidBacillus subtilisVartiainen and others (2004)
NisinL. monocytogenesLee and others (2003)
 E. coli 
WaxCinnamaldehyde-enriched cinnamon oilFungal spoilageRodriguez and others (2007)

The choice of active components is often limited by the incompatibility of the component with the packaging material or by its heat liability. Thus it is important to choose proper coating matrix, active agents, and plasticizers.

Essential oils and their components, which are naturally occurring antimicrobial agents, are well known for their potency against pathogenic microorganisms and spoilage microorganisms (Hammer and others 1999; Cox and others 2000; Benkeblia 2004). The antimicrobial activity of essential oils such as thyme, cinnamon, clove, oregano, and their major components are mainly related to their high small terpenoid and phenolic contents (Helander and others 1998).

Carvacrol, which is a major component of oregano essential oil, has been incorporated in SPI coatings on paper (Ben Arfa and others 2007a). According to these authors, better carvacrol retention was observed when the SPI-coating solution was prepared at 25 °C. SPI-carvacrol-coated papers containing various residual carvacrol quantities were tested, at different times of the kinetic release, to assess their antimicrobial activity. They demonstrated that the carvacrol quantity from coated paper necessary to induce E. coli growth inhibition is equal to or greater than 1.1 g/m2. In another study, Arfa and others (2007b) designed antimicrobial paper based on a SPI or modified starch coating including carvacrol and cinnamaldehyde. They investigated the effect of the coating and drying processes on the ability of these matrices to retain carvacrol and cinnamaldehyde. Antimicrobial compound losses were higher for modified starch-coated papers than for SPI-coated papers. The antimicrobial properties of the coated papers were shown against the bacterium E. coli and the mold Botrytis cinerea due to the fast active agent release by the matrices in favorable conditions (high humidity). Coated paper containing 60% (w/w) of carvacrol or 10% (w/w) of cinnamaldehyde induced E. coli growth inhibition from 4 to 5 log and a growth delay up to 21 d for B. cinerea, whatever the coating matrix.

The ability of coating matrices (coated papers) to release active compounds may depend on matrix nature, the compound nature and concentration, and the environmental conditions such as temperature and relative humidity (Whorton 1995; Chalier and others 2009). Chalier and others (2009) investigated the combined effect of temperature and relative humidity on carvacrol release from SPI-coated paper. According to these researchers, increasing storage temperature and relative humidity led to an increase in carvacrol diffusivities. At 30 °C, a significant increase in carvacrol diffusivity of about 81 times was observed by increasing relative humidity from 60% to 100%. The effect of these 2 parameters (on carvacrol release) could be related to the glass transition changes of the protein matrix.

Rodriguez and others (2007) have tested the activity of a new active paper packaging material manufactured by adding essential oils to the wax coating formulation against a wide array of foodborne microorganisms. Essential oils tested in their study included clove, cinnamon, oregano, and cinnamaldehyde-enriched cinnamon essential oil. The use of paper packaging with an active coating provided an attractive option for protecting food from fungal infestation, which also showed promise for protection against Gram-negative bacteria. The antimicrobial activities of active wax coatings were affected by the concentration of the essential oil in the coating. The ability of the developed active packaging materials were assessed to preserve 2 varieties of strawberries, since these fruits are usually packaged in paper or board and are prone to fungal spoilage. Complete protection was obtained, during 7 d storage at 4 °C, for strawberries stored in packaging with an active coating containing 4% (w/w) cinnamaldehyde-enriched cinnamon essential oil.

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

Weak organic acids, which are the most common classical preservative agents, inhibit the outgrowth of both bacterial and fungal cells. Fungistatic wrappers were developed by coating grease-proof paper with an aqueous dispersion of sorbic acid in 2% carboxymethyl cellulose solution. This sorbic-acid treated paper could preserve foods that are generally prone to spoilage by mold for a minimum of 10 d (Ghosh and others 1977). On the other hand, sorbic acid has also been incorporated into a wax-based coating on paper. The active packaging materials developed were used to package sausages and cheeses (Labuza and Breene 1989).

Chitosan is inherently antimicrobial and has attracted attention as a potential food preservative of natural origin due to its antimicrobial activity against a wide range of foodborne filamentous fungi, yeasts, and bacteria (Sagoo and others 2002; Shahidi and Abuzaytoun 2005). Several hypotheses have been proposed to explain the mechanism of the antimicrobial activity of chitosan: chitosan disrupts the barrier properties of the outer membranes of Gram-negative bacteria, which leads to the leakage of intracellular constituents (Young and others 1982; Helander and others 2001). Chitosan can act as a chelating agent that binds trace metals, spore elements, and essential nutrients, and thereby inhibits the production of toxins and microbial growth (Cuero and others 1991). The antibacterial effects of chitosan are reported to be dependent on its molecular weight (Chen and others 1998; Jeon and others 2001), its degree of deacetylation (Tsai and others 2002), its concentration in solution, the pH of the medium (Rabea and others 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 biodegradable antimicrobial packaging material. Chitosan is insoluble in most solvents, 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 organic acid type used as a dissolving solvent, thus affecting the properties of the resultant films or coatings. Vartiainen and others (2004) tested the effects of nisin and dissolving solvents on the antimicrobial activity of chitosan-coated paper. Chitosan dissolved in acetic and propionic acids and did not have any activity against Bacillus subtillis. Chitosan coatings containing lactic acid, however, showed strong antimicrobial activity according to both inhibition zone and bacteria reduction tests. The incorporation of nisin, at a concentration of 0.08 g/L, in coating solutions prepared from chitosan dissolved in different acids did not enhance 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 of acetic acid in the formulation should be controlled and reduced to the furthest extent possible to optimize the active coating formulation without affecting organoleptic properties of the food products.

Nisin and chitosan have also been coated, in 3% concentrations, onto paper with a binder medium of a vinyl acetate/ethylene copolymer to provide antimicrobial activity against Listeria monocytogenes and/or E. coli (Lee and others 2003). Combined inclusion of nisin and chitosan in the coating improved the microbial stability of milk and orange juice stored at 10 °C. Lee and others (2004) applied nisin on the surface of paperboard. They reported, using the coated materials with nisin, inhibition of Micrococcus flavus growth in a model emulsion and in cream (from milk).

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials for Coatings
  5. Functional Properties
  6. Conclusions
  7. References

Biopolymer-coating on paper packaging materials are very promising systems for the future improvement of food packaging. They have potential environmental advantages over conventional synthetic paper coatings. The use of such biopackagings will open up potential economic benefits to farmers and agricultural processors. Extensive research is needed on the development of new coating materials, methods of coating formation, methods to improve coating properties, and potential applications.

Biopolymer coatings on paper packaging materials are potential inclusion matrices of antimicrobial agents to develop biodegradable active packaging. Due to its potential to provide quality and safety benefits, antimicrobial packaging is expected to grow in the next decade with the advent of new polymeric materials and antimicrobials. Further research is needed to gain more knowledge regarding the interactions between the coating matrix, active compounds, and target microorganisms to evaluate the materials’ performance and to optimize the compositions of active coatings. Furthermore, the antimicrobial properties of coated papers are of great merit for future research. For food product applications, research is essential to evaluate the impact of active agents on organoleptic properties of the packaged food products.

References

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  3. Introduction
  4. Materials for Coatings
  5. Functional Properties
  6. Conclusions
  7. References
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