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.
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.
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).
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).