AbstractABSTRACT:  Increased enviro

Abstract
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
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), andKhwaldia 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.