H.M.C. De Azeredo, Institute of Food Research, UK
M.F. Rosa, M. De Sá and M. Souza Filho, Embrapa Tropical Agroindustry, Brazil
K.W. Waldron, Institute of Food Research, UK
Because of concerns involving the continual disposal of huge volumes of non-biodegradable food packaging materials, there has been an increasing tendency to replace petroleum-derived polymers with bio-based, environmentally friendly biodegradable macromolecules. There are a variety of biomass-derived structures which can be used as packaging materials, especially as films and coatings. Moreover, there are several biomass-derived compounds which can be used as additives for these materials such as plasticizers, crosslinking agents, and reinforcements, which can enhance physical properties and applicability of the materials for food packaging purposes. This review focuses on biomaterials which can be used to develop food packaging structures (especially films and coatings), their properties and interactions, and how they influence the packaging performance.
polysaccharides; proteins; lignocellulose; nanocomposites
People have been using natural polymeric materials such as silk, wool, cotton, wood and leather for centuries, but the advent of the petroleum-derived plastics at the beginning of the twentieth century provided the food industry with an increasingly wide variety of synthetic materials to be used as food packaging. However, fossil fuels are limited and non-renewable, and recycling is limited because of technical as well as economic difficulties. Less than 3% of the waste plastic worldwide gets recycled, compared with recycling rates of 30% for paper, 35% for metals and 18% for glass, according to Helmut Kaiser Consultancy (2012). Moreover, once discarded, petroleum-based plastics are generally non-biodegradable, in that they are resistant to microbial attack. This is due to their water insolubility, and the problem that evolutionary processes have not been sufficiently rapid to create new enzymes capable of degrading synthetic polymers during their relatively short existence in the natural environment (Mueller, 2006). In particular, the accumulated waste generated by the continuous and extensive disposal of food packaging has raised considerable concerns over their deleterious effects on wildlife and the environment. Even their incineration can produce toxic compounds such as furans and dioxins produced from burning polyvinylchloride (PVC) (Jayasekara et al., 2005). In recent decades, the concerns surrounding conventional petroleum-based plastics have stimulated a focus of attention on natural macromolecules such as polysaccharides and proteins, because of their sustainable supply and biodegradability (Verbeek and van den Berg, 2010).
In 2011, the global use of biodegradable plastics was 0.85 million metric tons. BCC Research expects that the use of bioplastics will increase up to 3.7 million metric tons by 2016, a compound annual growth rate of 34.3% (BCC Research, 2012). According to studies by Helmut Kaiser Consultancy (2012), bioplastics are expected to cover approximately 25–30% of the total plastics market by 2020, and the market itself is estimated to reach over US$10 billion by 2020. Europe is one of the most important markets, partly due to an increasing environmental awareness, but also due to the limited amount of crude oil reserves.
Biodegradable materials and particularly renewable materials have been promoted as materials for use in food packaging, especially as flexible packaging, although applications for rigid packaging materials have also been mentioned (Mohareb and Mittal, 2007; Stepto, 2009).
Some biodegradable materials (actually most proteins and polysaccharides) are also edible, and can be used to develop edible packaging (films and coatings). Edible packaging materials are intended to be integral parts of foods and to be eaten with the products, thus they are also inherently biodegradable (Krochta, 2002). Edible films (or sheets) are stand-alone structures that are preformed separately from the food and then applied on the food surface or between food components, or sealed into edible pouches. Edible coatings, on the other hand, are formed directly onto the surface of the food products (Krochta and De Mulder-Johnston, 1997). Gel capsules, microcapsules and tablet coatings made from edible materials could also be considered as edible packaging (Janjarasskul and Krochta, 2010). Although edible packaging is not expected to replace conventional plastic packaging, they have an important role to play in the whole development of renewable packaging. This is because they can be used to extend food stability by reducing exchange of moisture, O2, CO2, lipids and flavour compounds between the food and the surrounding environment, thus increasing food stability. So, they help to improve the efficiency of food packaging and therefore reduce the amount of conventional packaging materials required for each application. Hence they have been included in this review.
The barrier requirements of an edible film or coating depend on their application and the properties of the food they are supposed to protect. For example, fresh fruits and vegetables are alive, respiring foods. Films or coatings intended for use on them should have low water vapour permeability so they reduce the desiccation rates, while the O2 permeability should be low enough to reduce the respiration rates, extending the produce shelf life, but not low enough to create anaerobic conditions, leading to ethanol production and off-flavor formation. Nuts are especially susceptible to oxidation, thus the barrier against O2 is the most important factor to provide an extended shelf life; barrier against UV light is also helpful to reduce oxidation rates, and moisture barrier reduces the absorption of water, which can lead to loss of the crunchy texture. Some edible coatings can be applied to food to be fried, so that the coatings act as a barrier to frying oil, reducing the oil absorption by the product and consequently its final fat content; in this case, coatings should be highly hydrophilic to have a good barrier against the hydrophobic oils. Edible films and coatings can also be applied between food components, such as between the crust and the sauce and toppings of a pizza, minimizing the moisture transfer from the sauce and toppings to the crust.
Another requirement for edible films and coatings is that they are tasteless, and do not interfere with the sensory characteristics of the food product. But there are cases where the edible films or coatings are supposed to have a characteristic desirable flavour, which should be compatible with the food to be coated. This is the case, for instance, for fruit purée-based edible films (Azeredo et al., 2009; Mild et al., 2011; Sothornvit and Pitak, 2007) and coatings (Azeredo et al., 2012b; Sothornvit and Rodsamran, 2008), fruit pomace-based edible films (Park and Zhao, 2006), and vegetable purée edible films (Wang et al., 2011).
There are a number of methods for evaluating the physical properties of the resulting film and coatings materials, such as tensile test analyses and dynamic mechanical thermal analysis (DMTA) for mechanical properties (Moates et al., 2001; Siracusa et al., 2008), determination of barrier properties (Siracusa et al., 2008), differential scanning calorimetry (DSC) for thermal properties (Abdorreza et al., 2011), FTIR spectroscopy for interactions between components (Paes et al., 2010; Yakimets et al., 2007), scanning electron microscopy for ultrastructural analyses (Bilbao-Sáinz et al., 2010), NMR spectroscopy for studying crosslinking mechanism (Zhang et al., 2010a), polymer–permeant interactions and their effects on polymer organization (Karbowiak et al., 2008, 2009), and dielectric thermal analysis (DETA) for dielectric behaviour of the films (Moates et al., 2001). However, those techniques have not been covered in this review, which is rather focused on the biomaterials which can be used for food packaging (especially films and coatings), their basic properties and interactions, and how they affect the end materials.
All packaging films and coatings should have at least two components: a matrix, which usually consists of a macromolecule able to form a cohesive structure, and a plasticizer, usually required for reducing rigidity and brittleness inherent to most matrices. Additionally, some other components can be incorporated to improve the physical properties of films and coatings, such as their barrier and mechanical properties, or their resistance to moisture. Figure 26.1 presents a scheme of the basic and optional components of biomass-derived films and coatings.
Two basic technological approaches comprising wet and dry processes are generally used to produce biomaterials for packaging purposes.
The wet processes are based on separation of macromolecules from a solvent phase, usually by evaporation of the solvent. Usually the wet processes consist of casting a previously homogenized and vacuum degassed film forming dispersion (containing at least a biopolymer matrix, a solvent and usually a plasticizer) on a suitable base material (from which the film can be easily peeled off) and later drying to a moisture content (usually 5–8%) which is optimal for peeling the film away (Lagrain et al., 2010; Tharanathan, 2003), as indicated in Fig. 26.2. Film formation generally involves inter- and intramolecular associations or crosslinking of polymer chains forming a network that entraps and immobilizes the solvent. The degree of cohesion depends on polymer structure, solvent used, temperature and the presence of other molecules such as plasticizers, reinforcements, etc. (Tharanathan, 2003).
Two continuous film casting methods are typically used to manufacture biopolymer films by wet processes: (a) casting on steel belt conveyors and (b) casting on a disposable substrate on a coating line. In (a), solutions are spread uniformly on a continuous steel belt that passes through a drying chamber. The dry film is then stripped from the steel belt and wound into mill rolls for later conversion. One of the advantages of steel belt conveyors is the ability to cast aqueous solutions directly onto the belt surface, optimizing uniformity, heat transfer and drying efficiency, while eliminating expense of a separate substrate such as polyester film or coated paper. In (b), known as web coating, solutions are spread uniformly onto a carrier web or substrate, usually a polyester film or coated paper, and the coated substrate is passed through a drying chamber. The dry film is wound into rolls while still adhering to the substrate, and is usually separated in a secondary operation (Rossman, 2009).
In contrast, the dry processes use the thermoplastic properties of the macromolecules under low moisture conditions. The biomaterials can be shaped by existing plastic processing techniques (the so-called thermoplastic processing technologies), including thermoforming, compression moulding, extrusion, roller milling, or extrusion coating and lamination (Lagrain et al., 2010). Heating amorphous polymers above their glass transition temperature (Tg) changes them into a rubbery state, making it possible to form films after cooling. Although the dry processes require more extensive and advanced equipment, they are efficient in large-scale production due to the low moisture contents, high temperatures, high pressures and short process times (Hernandez-Izquierdo and Krochta, 2008). Furthermore, the materials obtained are likely to exhibit more robust tensile characteristics compared with films cast with the use of plasticizers (Mangavel et al., 2004) and lower water solubility, due to the creation of a highly crosslinked film network (Rhim and Ng, 2007).
Most conventional plastics, such as low density polyethylene films, are produced by extrusion. An extruder consists of a heated, fixed metal barrel containing one or two screws which convey the raw material through the heated barrel, from the feed end to the die (Fig. 26.3). The screws induce shear forces and increasing pressure along the barrel (Verbeek and van den Berg, 2010). Extrusion is one of the most important polymer processing techniques, offering several advantages over solution casting (Hernandez-Izquierdo and Krochta, 2008). However, many bio-based films are more difficult to produce by dry processes when compared to petroleum-based polymers, as they do not usually have defined melting points (due to their heterogeneous nature) and undergo decomposition upon heating (Tharanathan, 2003). There is a delicate balance required so the formulations resist the process conditions and at the same time achieve the desired film performance (Rossman, 2009). Starch (Pushpadass et al., 2008; Thunwall et al., 2008) and protein films (Hochstetter et al., 2006; Hernandez-Izquierdo et al., 2008; Kumar et al., 2010) have been produced by extrusion. Some proteins which exhibit thermoplastic behaviour can be processed without further treatment, but other proteins and starch should be plasticized before processing (Rhim and Ng, 2007).
The processes described in the literature for producing and applying edible coatings are restricted to laboratory discontinuous and small-scale techniques. Similarly to a basic casting method, the coating dispersion is produced by homogenization and vacuum degassing of a film forming dispersion (or by melting, for lipid-based coatings). The dispersion is then applied directly on a food surface (or between food components) by dipping or spraying (Fig. 26.4). Dipping is more adequate when an irregular surface has to be coated. After dipping, excess coating material is allowed to drain from the product. Spraying is more adequate for thinner coatings materials and/or when only one side of the product is supposed to be coated (such as in pizza crusts, as exemplified before). Both spraying and dipping are followed by drying for polysaccharide or protein coatings, or by cooling for lipid-based coatings.
There has been increasing interest in the search for new uses of biomass byproducts from the food industry. Many of these byproducts contain potential film-forming macromolecules such as polysaccharides or proteins which present opportunities for the design of bioplastics to be used as packaging materials.
A downside to the use of biomaterials for packaging purposes is that their inherent properties are usually inferior to those of petrochemical-based systems. However, unlike the conventional polymers, they are bio-degradable. This means that they can either be disposed of through, for example, composting or anaerobic digestion, or might even be exploited as a source of fermentable sugars after enzymatic digestion. There is therefore increasing support for their use in order to reduce the huge volume of plastic waste continually generated by food packaging disposal. According to Van der Zee (2005), the correlations between polymer structure and biodegradability have been proved challenging, since interplays between different factors occur simultaneously, often making it difficult to establish correlations, and creating exceptions when an apparent rule was expected to be followed. For instance, since the first step in biodegradation involves the action of extracellular water-borne enzymes, hydrophilicity favours the biodegradation, and the semicrystalline nature tends to limit it to amorphous regions, although highly crystalline starch materials and bacterial polyesters are rapidly hydrolysed. Some chemical properties that are important include chemical bonds in the polymer backbone, position and chemical activity of side groups, and chemical activity of end groups. Linkages involving hetero atoms, such as ester and amide (or peptide) bonds are considered susceptible to enzymatic degradation, although there are exceptions such as polyamides and aromatic polyesters.
This chapter overviews some of the macromolecules that can be obtained from biomass byproducts which have been used as matrices for food packaging materials, as well as other biomass-derived compounds which can be used as additives, crosslinking agents or reinforcements to these matrices, improving their properties and potential applicability as food packaging materials.
Cellulose, the most abundant biopolymer, is formed by the repeated connection of D-glucose building blocks. Adjacent cellulose chains form a framework of aggregates (elementary fibrils) containing crystalline and amorphous regions; the crystalline regions are maintained by inter- and intramolecular hydrogen bonding. Several elementary fibrils can associate with each other to form cellulose crystallites, which are then held together by a monolayer of hemicelluloses, generating thread-like structures which are enclosed in a matrix of hemicellulose and protolignin, forming a natural composite referred to as cellulose microfibril (Ramos, 2003).
Cellulose represents about a third of the plant cell wall composition, and it is also produced by a family of sea animals called tunicates (sea squirts), by several species of algae, and by some species of bacteria and fungi (Charreau et al., 2013). Cellulose is an important structural component characterized by its hydrophilicity, chirality, biodegradability, broad capacity for chemical modification, and its formation of versatile semicrystalline fibre morphologies.
Together with starch, cellulose and its derivatives (such as ethers and esters) are the most important raw materials for elaboration of biodegradable and edible films (Peressini et al., 2003). Cellulose is an essentially linear natural polymer of (1 → 4)-β-D-glucopyranosyl units. Its tightly packed polymer chains and highly crystalline structure makes it insoluble in water. Water solubility can be conferred by etherification; the water-soluble cellulose ethers, including methyl cellulose (MC), hydroxypropyl cellulose (HPC), hydroxypropylmethyl cellulose (HPMC), and carboxymethyl cellulose (CMC), have good film-forming properties (Cha and Chinnan, 2004; Janjarasskul and Krochta, 2010).
Cellulose films prepared from aqueous alkali/urea solutions were reported to exhibit better oxygen barrier properties when compared to those of conventional cellophane and PVC (Yang et al., 2011). Cellulose derivatives have been used as coatings, extending the shelf life of avocados (Maftoonazad and Ramaswamy, 2005; Maftoonazad et al., 2008) and fresh eggs (Suppakul et al., 2010). Cellulose-based films have been produced by several companies such as Innovia (UK), FKuR (Germany) and Daicel Polymer (Japan).
Hemicelluloses (HC) are heteropolysaccharides closely associated with cellulose in the plant cell walls (Mikkonen and Tenkanen, 2012). They are usually defined as the alkali-soluble material after the removal of pectic substances from plant cell walls (Sun et al., 2004). According to Scheller and Ulvskov (2010), HC are polysaccharides in plant cell walls that have β-(1 → 4)-linked backbones with an equatorial configuration. They have different structures which may contain glucose, xylose, mannose, galactose, arabinose, fucose, as well as glucuronic and galacturonic acids in different proportions, depending on the source (Ebringerová and Heinze, 2000). Hemicelluloses (as well as lignin) cover cellulose microfibrils. The structural similarity between HC and cellulose generates a conformational homology leading to a hydrogen-bonded network between HC and cellulose microfibrils (O’Neill and York, 2003; Rose and Bennett, 1999).
According to their primary structure, hemicelluloses can be categorized into four main groups: xyloglycans (xylans), mannoglycans (mannans), β-glucans and xyloglucans (Ebringerová et al., 2005). Xylans usually consist of a β(1 → 4)-D-xylopyranose backbone with side groups in position 2 or 3. Non-branched homoxylans occur in certain seaweeds, and heteroxylans include glucuronoxylans, arabinoxylans and more complex structures (Hansen and Plackett, 2008). Mannans comprise galactomannans and glucomannans. Galactomannans consist of a β(1 → 4)-linked mannopyranose backbone highly substituted with β(1 → 6)-linked galactopyranose residues (Wyman et al., 2005), while glucomannans consist of alternating β-D-glucopyranosyl and β-D-mannopyranosyl units attached with (1 → 4) bonds (Mikkonen and Tenkanen, 2012). β-glucans have a D-glucopyranose backbone with mixed β linkages (1 → 3, 1 → 4) in different ratios (Ebringerová et al., 2005). Finally, xyloglucans have a backbone of β(1 → 4)-linked D-glycopyranose residues with a distribution of D-xylopyranose in position 6 (Hansen and Plackett, 2008).
Some studies have been conducted on arabinoxylan-based films. Höije et al. (2005) obtained strong but highly hygroscopic arabinoxylan films from barley husks. β-glucan films were reported to be more compact than arabinoxylan films, with smaller nanopores, favouring the barrier properties (Ying et al., 2011). Zhang et al. (2011) reported that the properties of arabinoxylan films were well correlated with the arabinose/xylose (Ara/Xyl) ratios. More crystalline films with lower water uptake resulted from lower arabinose contents. On the other hand, a lower chain mobility was observed in the amorphous parts for highly substituted xylans.
Galactomannan films have also been the subject of several studies. Mikkonen et al. (2007) reported that galactomannans with lower galactose content produced films with higher elongation at break and tensile strength, probably because galactomannans with fewer side chains can interact with other polysaccharides due to their long blocks of unsubstituted mannose units (Srivastava and Kapoor, 2005). Cerqueira et al. (2009) successfully used galactomannans to coat different tropical fruits, choosing the formulations taking into account parameters such as wettability, barrier to gases and mechanical properties. Cheeses have been reported to have their shelf life extended by application of galactomannan-based coatings (Cerqueira et al., 2010; Martins et al., 2010).
Pectins are water-soluble anionic heteropolysaccharides composed mainly of (1 → 4)-α-D-galactopyranosyluronic acid units, in which some carboxyl groups of galacturonic acid are esterified with methanol (Fig. 26.5). They are extracted from citrus peels and apple pomace by hot dilute mineral acid. Short hydrolysis times produce pectinic acids and high-methoxyl pectins (HMP), while extended acid treatment de-esterifies the methyl esters to pectic acids and generates low-methoxyl pectins (LMP). Commercial pectins are categorized according to their degree of esterification (DE), defined as the ratio of esterified to total galacturonic acid groups (Sriamornsak, 2003). HMP have a DE > 50 (usually > 69), whereas LMP have a DE < 50 (Farris et al., 2009). The ratio of esterified to non-esterified galacturonic acid determines the behaviour of pectin in food applications, since it affects solubility and gelation properties of pectin. HMP form gels with sugar and acid, whereas LMP form gels in the presence of divalent cations such as Ca2 +, which links adjacent LMP chains via ionic interactions, forming a tridimensional network (Janjarasskul and Krochta, 2010).
Pectin films, similarly to other polysaccharide films, have poor water resistance, and have been proposed for potential industrial uses where water binding either is not a problem or can provide specific advantages, such as edible bags for soup ingredients (Fishman et al., 2000). LMP films, on the other hand, when crosslinked with calcium ions, have not only improved water resistance, but also improved mechanical and barrier properties (Kang et al., 2005). Moreover, the presence of carboxyl groups carrying a negative charge at pH > pKa enables exploiting electrostatic interactions of pectin with positively charged counterparts (Farris et al., 2009).
Lignin is the second most abundant terrestrial biopolymer after cellulose, accounting for approximately 30% of the organic carbon in the biosphere (Boerjan et al., 2003). It is associated with cellulose and hemicelluloses in plant cell walls, and is an abundant waste product in the pulp and paper industry.
Lignins are complex aromatic heteropolymers derived mainly from three hydroxycinnamyl alcohol monomers (monolignols) differing in their degree of methoxylation (Fig. 26.6): p-coumaryl, coniferyl and sinapyl alcohols (Boerjan et al., 2003; Buranov and Mazza, 2008).
Lignin has some interesting properties to be used for packaging films, such as its small particle size, hydrophobicity and ability to form stable mixtures (Park et al., 2008). Moreover, lignins have been shown to have efficient antibacterial and antioxidant properties (Ugartondo et al., 2009). Lignin has been used as a film component in composites with gelatin (Núñez-Flores et al., 2013; Ojagh et al., 2011; Vengal and Srikumar, 2005), starch (Baumberger et al., 1998; Vengal and Srikumar, 2005) and chitosan (Chen et al., 2009). Baumberger et al. (1998) observed that starch films incorporated with up to 30% lignin presented higher elongation and water resistance than control starch films. On the other hand, lignin impaired the tensile strength of the films at high relative humidity (71%), reflecting the incompatibility between the hydrophilic starch and the hydrophobic lignin, which was bolstered by the water. Indeed, microscopic observations confirmed that the material consisted of two phases – a hydrophilic starch matrix filled with hydrophobic lignin aggregates. On the other hand, Chen et al. (2009) reported a good dispersion of lignin (up to 20%) in a chitosan matrix, evidenced by SEM, which was corroborated by FTIR results, indicating the existence of hydrogen bonding between chitosan and lignin. An FTIR spectroscopy study on gelatin-lignin films revealed strong protein conformational changes induced by lignin, producing a plasticizing effect, which was reflected in the mechanical and thermal properties (Núñez-Flores et al., 2013).
A drawback from incorporating lignin in films is that they acquire a brownish colour (Mishra et al., 2007; Núñez-Flores et al., 2013). On the other hand, they acquire better light barrier properties (Núñez-Flores et al., 2013), which could be of interest in food applications when ultraviolet-induced lipid oxidation is a problem.
Polysaccharides are hydrophilic polymers and therefore exhibit very low moisture barrier properties. They are of prime interest as matrices for biodegradable film formation because of their availability and rather low cost.
Most polysaccharides are neutral, although some gums are negatively charged. As a consequence of the large number of hydroxyl and other polar groups in their structure, hydrogen bonds play important roles in film formation and characteristics. Negatively charged gums, such as alginate, pectin and carboxymethyl cellulose, tend to present some different properties depending on the pH (Han and Gennadios, 2005).
Polysaccharide films are usually formed by disrupting interactions among polymer segments during coacervation and forming new inter-molecular hydrophilic and hydrogen bonds upon evaporation of the solvent (Janjarasskul and Krochta, 2010). Because of their hydrophilicity, polysaccharide films provide a good barrier to CO2 and O2, hence they retard respiration and ripening of fruits (Cha and Chinnan, 2004). On the other hand, similarly to other hydrophilic materials, their high polarity determines their poor barrier to water vapour (Park and Chinnan, 1995) as well as their sensitivity to moisture, which may affect their functional properties (Janjarasskul and Krochta, 2010).
Starches are polymers of D-glucopyranosyl, consisting of a mixture of the predominantly linear amylose and the highly branched amylopectin (Fig. 26.7). Native starch molecules arrange themselves in semi-crystalline granules in which amylose and amylopectin are linked by hydrogen bonding. When heat is applied to native starch in the presence of plasticizers such as water and glycerol, the granules swell and hydrate, which triggers the gelatinization process, characterized by loss of crystallinity and molecular order, followed by a dramatic increase in viscosity (Kramer, 2009). This transformation is named gelatinization and leads to the so-called thermo-plastic starch (TPS) (Huneault and Li, 2007).
Amylose responds to the film-forming capacity of starches, since linear chains of amylose in solution tend to interact by hydrogen bonds and, consequently, amylose gels and films are stiff, cohesive and relatively strong. On the other hand, amylopectin films are brittle and non-continuous, since branched amylopectin chains in solution present little tendency to interact (Peressini et al., 2003).
The first studies using starch in biodegradable food packaging were focused on substituting part of the synthetic matrix (usually polyethylene) by starch, but there were difficulties ascribed to chemical incompatibility between the polymers. Recently, studies on pure starch-based materials have predominated, usually focusing on two major drawbacks related to application of starch films. The first one is that native starch commonly exists as granules with about 15–45% crystallinity, and starch-based materials are susceptible to ageing and starch re-crystallization (retrogradation) (Forssell et al., 1999; Ma et al., 2006), which makes starch rigid and brittle during long-term storage, restricting its applications (Huang et al., 2005). The second is their high hydrophilicity, causing its barrier properties to decrease with increasing relative humidity. Plasticizers are used to overcome the first drawback and improve material flexibility (Moates et al., 2001; Peressini et al., 2003; Mali et al., 2004), but, since they are usually highly hydrophilic, presenting hydroxyl groups capable of interacting with water by hydrogen bonds, they tend to increase the moisture affinity of the films (Mali et al., 2005).
Some companies have commercially produced starch-based packaging materials, such as Eco-Go (Thailand), Plantic (Australia) in a joint venture with Du Pont (USA), JMP (Australia), StarchTech (USA), Biome (UK), and BASF (Germany).
Alginates, which are extracted from brown seaweeds, are salts of alginic acid, a linear co-polymer of D-mannuronic and L-guluronic acid monomers (Fig. 26.8), containing homogeneous poly-mannuronic and poly-guluronic acid blocks (M and G blocks, respectively) and MG blocks containing both uronic acids. The presence of carboxyl groups in each constituent residue (Ikeda et al., 2000) enables sodium alginate to crosslink with di- or trivalent metal cations, especially calcium ions (Ca2 +), to produce strong gels or films (Cha and Chinnan, 2004). Calcium ions pull alginate chains together via ionic interactions, after which interchain hydrogen bonding occurs (Kester and Fennema, 1986). Films can be formed either from evaporating water from an alginate gel or by a two-step procedure involving drying of alginate solution followed by treatment with a calcium salt solution to induce crosslinking (Janjarasskul and Krochta, 2010). The strength and permeability of films may be altered by changing calcium concentration and temperature, among other factors (Kester and Fennema, 1986). Alginate films have been studied as edible coatings to be applied to a variety of foods such as fruits/vegetables (Fan et al., 2009; Fayaz et al., 2009) and meat products (Marcos et al., 2008; Chidanandaiah et al., 2009).
Chitosan is a linear polysaccharide consisting of β-(1 → 4)-linked residues of N-acetyl-2-amino-2-deoxy-D-glucose (glucosamine) and 2-amino-2-deoxy-D-glucose (N-acetyl-glucosamine) (Fig. 26.9). It is produced from partial deacetylation of chitin, which is considered as the second most abundant polysaccharide in nature after cellulose (Dutta et al., 2009; Aranaz et al., 2010). Chitin is present in the exoskeleton of crustacea and insects, and can also be found in the cell wall of certain groups of fungi, particularly zygomycetes (Chatterjee et al., 2005). It is usually extracted from crab and shrimp shells as a byproduct of the seafood industry. Since the deacetylation of chitin is usually incomplete, chitosan is a copolymer comprising D-glucosamine and N-acetyl-D-glucosamine with various fractions of acetylated units (Aranaz et al., 2010).
Chitosan is soluble in diluted aqueous acidic solutions due to the protonation of –NH2 groups at the C2 position (Aranaz et al., 2010). The cationic character confers unique properties to the polymer, such as antimicrobial activity and the ability to carry and slow-release functional ingredients (Coma et al., 2002). The charge density depends on the degree of deacetylation as well as the pH. Quaternization of the nitrogen atoms of amino groups has been a usual chitosan modification, whose objective is to introduce permanent positive charges along the polymer chains, providing the molecule with a cationic character independent of the aqueous medium pH (Curti et al., 2003; Aranaz et al., 2010).
Chitosan films have been proven effective in extending the shelf life of fruits (Hernández-Muñoz et al., 2006; Chien et al., 2007; Lin et al., 2011; Ali et al., 2011) and to have retarded microbial growth on fruit surfaces (Hernández-Muñoz et al., 2006; Chien et al., 2007; Campaniello et al., 2008). The polycationic structure of chitosan probably interacts with the predominantly anionic components (lipopolysaccharides, proteins) of microbial cell membranes, especially Gram-negative bacteria (Helander et al., 2001).
Several other polysaccharides are found in nature, and it is virtually impossible to mention all of them. But some examples are given in this section of less common polysaccharides which have been tested or suggested as food packaging materials.
Some red algae species (Rhodophyta) have a family of polysaccharides called carrageenans as cell wall polysaccharides (Van de Velde et al., 2002). They are hydrophilic linear sulfated galactans consisting of alternating (1 → 3)-β-D-galactopyranose (G-units) and (1 → 4)-α-D-galactopyranose (D-units), forming a disaccharide repeating unit (Campo et al., 2009). Carrageenan-based coatings have been proven to be efficient to increased stability of fresh-cut (Bico et al., 2009) and fresh whole fruits (McGuire and Baldwin, 1998; Ribeiro et al., 2007). Ribeiro et al. (2007) observed that carrageenan coatings resulted in lower weight loss and lower loss of firmness of strawberries when compared to starch coatings, probably reflecting a better moisture barrier of carrageenan coatings. Moreover, carrageenan films presented a significantly lower oxygen permeability than starch films.
Natural gums are obtained as exudates from different tree species, including gum acacia, cashew tree gum, and mesquite gum. The tree gums have been grouped into three types based on the nature of polysaccharide type, namely, arabinogalactans, substituted glucuronomannans or substituted rhamnogalacturonans (Sims and Furneaux, 2003). Cashew tree gum (CTG) is a heteropolysaccharide exudated from the cashew tree (Anacardium occidentale) bark (Miranda, 2009), whose composition is made up of galactose, glucose, arabinose, rhamnose and glucuronic acid (De Paula et al., 1998). The greatest cultivation of cashew trees can be found in Brazil, and it is mainly focused on cashew nut production (Bezerra et al., 2007). CTG films have been obtained by Carneiro-da-Cunha et al. (2009) and suggested to be applied as apple coatings. Azeredo et al. (2012a) obtained alginate–CTG blend films crosslinked with CaCl2. CTG reduced tensile strength and barrier properties of the films, but favoured film extensibility. Some studies have described the use of mesquite gum as film-forming matrices (Osés et al., 2009; Bosquez-Molina et al., 2010). Gum acacia coatings have been proven to extend the shelf life of tomatoes (Ali et al., 2010) and shiitake mushrooms (Jiang et al., 2013).
Some hetero-polysaccharides can be obtained from cactus stems, which is waste from cactus pruning. The mucilage extracted from stems of prickly pear cactus (Opuntia ficus-indica), which constitutes about 14% of the cladode dry weight (Ginestra et al., 2009), has been reported to contain residues of D-galactose,D-xylose,L-arabinose,L-rhamnose and D-galacturonic acid (McGarvie and Parolis, 1979). Del-Valle et al. (2005) studied the use of prickly pear mucilage as an edible coating to strawberries, and reported that coated strawberries presented extended physical and sensory stability when compared to uncoated ones.
Whilst ‘cellulose’ is a word originally given to the substance which constitutes a key load-bearing component of the cell wall of higher plants, bacterial cellulose (BC) is an extracellular product synthesized by bacteria belonging to some genera, its most efficient producers being the Gram-negative, acetic acid bacteria Gluconacetobacter xylinum (Iguchi et al., 2000; Retegi et al., 2010). These microfibril bundles have excellent intrinsic properties due to their high crystallinity, including a reported elastic modulus of 78 GPa (Guhados et al., 2005). Compared with cellulose from plants, BC has important structural differences and it also possesses higher water-holding capacity, higher degree of polymerization (up to 8,000), and a finer web-like network (Klemm et al., 2005). It is produced as a gel, and, although its solid portion is less than 1%, it is almost pure cellulose containing no lignin and other foreign substances (Iguchi et al., 2000). Despite the identical chemical composition, BC is superior to plant cellulose owing to its purity, high elasticity, and nano-morphology with a large surface area (S. Chang et al., 2012). It is an interesting biomaterial thanks to its fine network, excellent mechanical properties, high water-holding capacity, crystallinity and bio-compatibility (Yan et al., 2008; Putra et al., 2008).
Retegi et al. (2010) obtained compression moulded bacterial cellulose (BC) films with different porosities, generated by different compression pressures. Higher pressures were found to produce films with better final mechanical properties. This behaviour was ascribed to the higher densification, reducing interfibrillar spaces, thus increasing the possibility of interfibrillar bonding zones.
Proteins are widely available as biomass byproducts from plants (wheat gluten, maize zein, soybean proteins) and animals (collagen, gelatin, keratin, casein, whey proteins).
Proteins are linear, random copolymers built from up to 20 different monomers. The main mechanism of formation of protein films involves denaturation of the protein initiated by heat, solvents, or change in pH, followed by association of peptide chains through new intermolecular interactions (Janjarasskul and Krochta, 2010).
Proteins are distinguished from polysaccharides in that they have approximately 20 different amino acid monomers, rather than just a few or even one monomer, such as glucose in cellulose and starch. The amino acids are similar in that they contain an amino group (–NH2) and a carboxyl group (–COOH) attached to a central carbon atom, but each amino acid has unique properties conferred by a different side group attached to the central carbon, which can be non-polar, polar uncharged, or polar (positively or negatively) charged at pH 7 (Cheftel et al., 1985; Krochta, 2002). While hydroxyl is the only reactive group in polysaccharides, proteins may be involved in several possible interactions and chemical reactions (Hernandez-Izquierdo and Krochta, 2008), such as chemical reactions through covalent (peptide and disulfide) bonds and non-covalent (ionic, hydrogen, and van der Waals) interactions. Moreover, hydrophobic interactions may occur between non-polar groups of amino acid chains (Kokini et al., 1994). In addition, protein-based films are considered to have high UV barrier properties, owing to their high content of aromatic amino acids which absorb UV light (Mu et al., 2012).
The most unique properties of proteins compared to other film-forming materials are heat denaturation, electrostatic charges, and amphiphilic character. Protein conformation can be affected by many factors, such as charge density and hydrophilic–hydrophobic balance (Han and Gennadios, 2005). Proteins have good film-forming properties and good adherence to hydrophilic surfaces. Protein-derived films provide good barriers to O2 and CO2 but not to water (Cha and Chinnan, 2004). Their barrier and mechanical properties are impaired by moisture owing to their inherent hydrophilic nature (Janjarasskul and Krochta, 2010).
The processing of protein films or other protein-based materials mostly requires three main steps: breaking of intermolecular bonds (non-covalent and covalent, if necessary) by chemical or physical rupturing agents; arranging and orienting polymer chains in the desired conformation; and allowing the formation of new intermolecular bonds and interactions stabilizing the film network (Jerez et al., 2007). Globular proteins are required to unfold and realign before a new three-dimensional network can be formed, and stabilized by new inter- and intra-molecular interactions (Verbeek and van den Berg, 2010).
A material to be extruded receives a considerable amount of mechanical energy, which may affect the characteristics of the final products. Extrusion requires the formation of a melt, which implies processing the protein above its softening point. Proteins have many different functional groups, and consequently a great variety of possible chain interactions that reduce molecular mobility and increase viscosity, resulting in a softening temperature which is often above decomposition temperature. Plasticizers can be useful to reduce the softening temperature, but protein extrusion is usually only possible within a limited range of processing conditions (Verbeek and van den Berg, 2010).
Not a naturally occurring protein, gelatin is produced by partial hydrolysis of collagen, which is the main constituent of animal skin, bone, and connective tissue. The insoluble collagen is treated with dilute acid or alkali, resulting in partial cleavage of the crosslinks, and the structure is broken down to such an extent that the soluble gelatin is formed (Karim and Bhat, 2009).
Gelatin is obtained mainly from pigskin and other mammalian sources, but the marine sources (fish skin and bone) have been increasingly looked upon as possible alternatives to bovine and porcine gelatin, as they are not associated with the risks of bovine spongiform encephalopathy (BSE, or ‘mad cow disease’) outbreaks, and because they are acceptable for religious groups which have restrictions on pig and cow derivatives (Karim and Bhat, 2009). Moreover, the fish industry sector has tried to find new outlets for their skin and bone byproducts. However, gelatins from cold water species, representing the majority of the industrial fisheries, present inferior physical properties (such as lower gelling and melting temperatures and lower modulus) when compared to mammalian gelatin (Haug et al., 2004), which has been largely related to the lower contents of hydroxyproline in collagen from cold water fish species, since hydroxyproline is involved in interchain hydrogen bonding, which stabilizes the triple helical structure of collagen (Wasswa et al., 2007). Gelatins from warm water fish species, like tilapia, on the other hand, have physical properties more similar to those of mammalian gelatins (Sarabia et al., 2000). Moreover, fish gelatins contain more hydrophobic amino acids, so their films show significantly lower water vapour permeability when compared to films produced from mammalian gelatins (Avena-Bustillos et al., 2006).
Gelatin contains a large amount of proline, hydroxyproline, lysine, and hydroxylysine, which can react in an aldol-condensation reaction to form intra- and intermolecular crosslinks among the protein chains (Dangaran et al., 2009). On cooling and dehydration, gelatin films are formed with irreversible conformational changes (Badii and Howell, 2006; Dangaran et al., 2009), with an increased concentration of triple helical structures. The triple helix structure is the basic unit of collagen, from which gelatin is derived. Thus, gelatin molecules partly revert back to the collagen structure during gelation (Gómez-Guillén et al., 2002; Chiou et al., 2006).
Several studies have described the successful use of gelatin to form films by casting (Chiou et al., 2006; Jongjareonrak et al., 2006; Hanani et al., 2012b) as well as dry methods (Hanani et al., 2012a; Krishna et al., 2012).
Total bovine milk proteins consist of about 80% casein and 20% whey proteins. Whey proteins are those which remain in milk serum after casein coagulation during cheese or casein manufacture. It is a mixture of proteins with diverse functional properties, the main ones being α-lactalbumins, β-lactoglobulins, bovine serum albumin, immunoglobulins, and proteose-peptones (Pérez-Gago and Krochta, 2002). Native whey proteins are globular complexes but become random coils upon denaturation and can form three-dimensional networks to produce biodegradable films (Zhou et al., 2009). Since whey proteins have a high proportion of hydrophilic amino acid in their structure, whey protein films have low tensile strength and high water vapour permeability (McHugh et al., 1994), but these properties might be improved by combining whey protein with materials with better tensile strength and hydrophobic properties, such as zein (Ghanbarzadeh and Oromiehi, 2008). The water vapour permeability of whey protein films was decreased by addition of beeswax, by both increasing the hydrophobic character of the film and decreasing the amount of hydrophilic plasticizer (glycerol) required (Talens and Krochta, 2005). Whey protein coatings have been used to increase the stability of fruits (Pérez-Gago et al., 2003; Reinoso et al., 2008), meat products (Shon and Chin, 2008), nuts (Min and Krochta, 2007) and eggs (Caner, 2005).
Casein is a phosphoprotein, which can be separated into various electrophoretic fractions such as α-(s1)- and α-(s2)-caseins, β-casein, and κ-casein, all of them having low solubility at low pH (pI = 4.6) (Müller-Buschbaum et al., 2006). Caseins form films from aqueous solutions without further treatment due to their random-coil nature and their ability to form extensive hydrogen bonds (Lacroix and Cooksey, 2005), as well as electrostatic interactions (Gennadios et al., 1994). Casein film solubility can be decreased by buffer treatments at the isoelectric point (Chen, 2002), by physical crosslinking using irradiation (Vachon et al., 2000) and by chemical crosslinking using aldehydes (Ghosh et al., 2009). Moreover, casein precipitated with high pressure CO2 was reported to present lower water solubility than acid-precipitated casein (Dangaran et al., 2006). Casein films have been used to increase stability of bread (Schou et al., 2005).
Zein is the name given to the prolamin (alcohol-soluble) fraction of the maize proteins (Ghanbarzadeh and Oromiehi, 2008), representing about 50% of the maize endosperm proteins (Biswas et al., 2005). Zein is commercially produced from maize gluten, a co-product of the maize starch production (Ghanbarzadeh and Oromiehi, 2008). Moreover, the worldwide growth of the bioethanol industry has resulted in a huge increase in zein availability, which has motivated the development of new applications for this protein (Biswas et al., 2009).
Zein films are usually prepared by dissolving zein in aqueous ethyl alcohol (Ghanbarzadeh et al., 2007). Zein is rich in non-polar amino acids, with low proportions of basic and acidic amino acids, which confers lower water vapour permeability and better water resistance to zein films when compared to other protein films (Dangaran et al., 2009; Ghanbarzadeh and Oromiehi, 2008).
Wheat gluten proteins are a byproduct of starch extraction from wheat flour, which is commonly used as a functional ingredient, especially in bakery products. Wheat gluten consists of a mixture of proteins that can be classified into two types: the water insoluble glutenins, which comprise proteins with multiple peptide chains linked via interchain disulfide bonds, forming a continuous network that provides strength and elasticity; and the water soluble gliadins, which consist of single polypeptide chains associated via hydrogen bonding, hydrophobic interactions and intramolecular disulfide bonds, acting as a plasticizer to glutenin network and conferring viscosity to gluten (Balaguer et al., 2011a; Goesaert et al., 2005; Hernández-Muñoz et al., 2003).
The low quality gluten is unsuitable for flour improvement in breadmaking, but can be used for preparing plastic films, presenting adequate viscoelasticity, adhesiveness, thermoplasticity, and good film-forming properties, although the glutenins tend to aggregate upon shearing and heating (Balaguer et al., 2011a).
Gluten shows very low solubility in water, because of its low content of amino acids with ionizable side chains and high contents of non-polar amino acids and glutamine, which has a high hydrogen-bonding potential (Lagrain et al., 2010). A complex solvent system with basic or acidic conditions in the presence of alcohol and disulfide bond-reducing agents is required to prepare casting solutions (Cuq et al., 1998). Some aspects regarding gluten extrusion were approached by Lagrain et al. (2010). In contrast to what happens to thermoplastic materials, gluten viscosity does not decrease upon heating, but rather levels off or even increases due to crosslinking reactions. Therefore, gluten extrusion is only possible in a limited window of operating conditions ranging from the onset of protein flow to aggregation and eventually extensive depolymerization (Redl et al., 1999; Zhang et al., 2005), which occurs at rather low temperatures when compared to most synthetic polymers. Gluten materials are thus usually extruded between 80 and 130°C. The formation of a gluten network involves the dissociation and unraveling of the gluten proteins, which allows both glutenin and gliadin to recombine and crosslink through specific linkages in an oriented pattern (Hernandez-Izquierdo and Krochta, 2008), predominant reactions including SH oxidation and SH/SS interchange reactions leading to the formation of SS crosslinks (Lagrain et al., 2010). Overall, gluten-based materials are stable three-dimensional macromolecular networks stabilized by low-energy interactions and strengthened by covalent bonds such as SS bonds between cysteine residues (Lagrain et al., 2010).
Unlike other macromolecules, lipids are not biopolymers, being unable to form cohesive, self-supporting films for packaging. So they are used either as coatings directly applied to food to provide a moisture barrier, or as components in stand-alone composite emulsion films, in which proteins or polysaccharides provide structural integrity and lipids respond for hydrophobic character, improving the water vapour barrier (Krochta, 2002). Moreover, the presence of lipids in composite formulations provides an appealing glassy finish to the material.
Polarity of lipids depends on the distribution of chemical groups, the length of aliphatic chains and presence and degree of unsaturation (Morillon et al., 2002). Lipids with longer chains and lower unsaturation and branching degrees have better barrier against water vapour, because of their lower polarity (Rhim and Shellhammer, 2005; Janjarasskul and Krochta, 2010). On the other hand, more polar lipids have more affinity for proteins and polysaccharides when in emulsions, producing more homogeneous films and avoiding the separation of a lipid phase (Fabra et al., 2011b).
Several types of lipids can be used for food coating or as film components, such as waxes, triglycerides, as well as di- and monoglycerides. Waxes, which are esters of long-chain aliphatic acids with long-chain aliphatic alcohols (Rhim and Shellhammer, 2005), are especially resistant to water diffusion, because of their very low content of polar groups (Kester and Fennema, 1986) and their high content in long-chain fatty alcohols and alkanes (Morillon et al., 2002). There are a variety of naturally occurring waxes, derived from vegetables (e.g., carnauba, candelilla and sugar cane waxes), minerals (e.g., paraffin and microcrystalline waxes), or animals – including insects (e.g., beeswax, lanolin and wool grease) (Rhim and Shellhammer, 2005). Some studies reported extension of shelf life of fruits resulting from application of wax-based coatings, including carnauba wax (Dang et al., 2008; Gonçalves et al., 2010) and candelilla wax (Saucedo-Pompa et al., 2009).
Most commonly, lipids have been used as hydrophobic components of emulsion films. Lipid-polysaccharide or lipid-protein emulsion films combine the complementary advantages of each component. Polysaccharides and proteins act as matrices, since they have the mechanical properties to form self-supporting films, and have good barrier against gases such as O2 and CO2, while lipids are useful to reduce water vapour permeability. García et al. (2000) demonstrated that lipid addition to starch films decreased their crystalline-amorphous ratio, which is expected to increase film diffusivity and consequently permeability. On the other hand, lipid addition also reduces the hydrophilic–hydrophobic ratio of films, which decreases their water solubility and therefore water vapour permeability. Emulsion films have been reported to enhance stability of fresh-cut (Chiumarelli and Hubinger, 2012) and whole fruits (Bosquez-Molina et al., 2003; Maftoonazad et al., 2007; Navarro-Tarazaga et al., 2011), vegetables (Conforti and Zinck, 2002), nuts (Mehyar et al., 2012), eggs (Wardy et al., 2011) and bakery products (Bravin et al., 2006).
Brittleness is an inherent quality attributed to most biopolymer films, due to extensive intermolecular forces such as hydrogen bonding, electrostatic forces, hydrophobic bonding, and disulfide bonding (Sothornvit and Krochta, 2005; Srinivasa et al., 2007). Plasticizers are required to break polymer–polymer interactions (such as hydrogen bonds and van der Waals forces), sometimes forming secondary bonds to polymer chains, causing the distance between adjacent chains to increase (Fig. 26.10), thus lowering the glass transition temperature (Tg) and reducing film rigidity and brittleness. Moreover, plasticizers act as processing aids, since they lower the processing temperature, reduce sticking in moulds, and enhance wetting (Sothornvit and Krochta, 2005). On the other hand, plasticizers increase film permeability, and the decrease in cohesion negatively affects mechanical properties (Sothornvit and Krochta, 2005; Vieira et al., 2011).
Thanks to the low molecular size of plasticizers, they occupy spaces between polymer chains, reducing secondary forces. Most plasticizers contain hydroxyl groups which form hydrogen bonds with biopolymers, changing the three-dimensional polymer organization, reducing the energy required for mobility and the degree of hydrogen bonding between chains, resulting in increasing free volume and molecular mobility (Sothornvit and Krochta, 2005; Vieira et al., 2011).
The increased interest in bio-based packaging materials has been followed by a search for natural-based plasticizers, similarly biodegradable and of low toxicity. Commonly used plasticizers in biodegradable food packaging are polyols, mono-, di- or oligosaccharides, and fatty acids and other lipids.
Glycerol can be produced either by microbial fermentation or synthesized chemically from petrochemical feedstock. Additionally, glycerol is a major byproduct of the increasing biodiesel production, thus creating a significant surplus resulting in a sharp decrease in glycerol prices. In this context, application of glycerol for value-added products is a necessity as well as an opportunity for the biodiesel industry (Johnson and Taconi, 2007; Yang et al., 2012). The use of glycerol as plasticizer in biopolymer-based films can be a way to help solve the existing surplus of this byproduct from biodiesel production.
Glycerol as well as other polyols, including sorbitol, propylene glycol, and polypropylene glycol, have great affinity for polysaccharide and protein films, because of their hydrophilicity. Several studies have demonstrated the effectiveness of polyols as plasticizers for polysaccharide-based and protein-based films and coatings (Bergo and Sobral, 2007; Jouki et al., 2013; Ramos et al., 2013;Tapia-Blácido et al., 2013). Qiao et al. (2011) used polyol mixtures including mixtures of glycerol and higher molecular weight polyols (HP) such as xylitol, sorbitol and maltitol. The increase of the molecular weight and the content of HP in the polyol mixture enhanced the thermal stability and mechanical strength of the resulting materials.
It is not only molecular size, configuration and total number of functional hydroxyl groups that are important characteristics to be considered for an effective plasticizer, but also its compatibility with the film-forming polymer. Polymer–plasticizer compatibility is necessary to generate a homogeneous mixture without phase separation. It has been suggested that some monosaccharides work as plasticizers in polysaccharide films more effectively than polyols, because of the structure similarity between monosaccharides and polysaccharides (Zhang and Han, 2006). Indeed, Zhang and Han (2006) observed that starch films plasticized with monosaccharides (glucose, mannose and fructose) presented better overall physical properties when compared to starch films plasticized with polyols (glycerol and sorbitol). On the other hand, polyols (especially glycerol) presented better ability to lower Tg of the films, indicating that polyols are more effective in terms of thermomechanical properties. Other studies have indicated monosaccharides and disaccharides as effective plasticizers in different polysaccharide-based films (Olivas and Barbosa-Cánovas, 2008; Piermaria et al., 2011).Whey protein films plasticized with sucrose presented excellent oxygen barrier properties, but sucrose tended to crystallize with time (Dangaran and Krochta, 2007).
Generally, the purpose of adding lipids to films is to reduce their water vapour permeability and/or to provide an attractive gloss. Moreover, incorporating lipids in protein- or polysaccharide-based films may interfere with polymer chain-to-chain interactions and provide flexible domains within the film. Fabra et al. (2008) observed that oleic acid apparently interacted with the protein (sodium caseinate) matrix forming bonds through polar groups, modifiying the interaction balances in the protein network. The result can be a plasticizing effect, including reduction of film strength and increase of film flexibility, as described for whey protein films plasticized with beeswax (Talens and Krochta, 2005), caseinate films with oleic acid (Fabra et al., 2008), and gelatin films with stearic and oleic acids (Limpisophon et al., 2010).
The major drawback of using lipids as plasticizers is their low compatibility with most biopolymer matrices, because of their hydrophobicity. On the other hand, this same characteristic of lipids represents an advantage in which they reduce the water vapour permeability and moisture sensitivity of biopolymer materials.
Chemical crosslinking is the process of linking polymer chains by covalent bondings, forming tridimensional networks (Fig. 26.11) which reduce the mobility of the structure and usually enhance its mechanical and barrier properties and its water resistance. Chemical crosslinking provides a mechanism for enhancing the performance of biopolymers. The most common crosslinking reagents are symmetrical bifunctional compounds with reactive groups with specificity for functional groups present on the matrix macromolecules (Balaguer et al., 2011a). Low toxicity crosslinking agents have been explored nowadays for use in food packaging materials, such as phenolic compounds and genipin.
Several natural phenolic compounds derived from plants have been used as crosslinkers to modify biopolymer films. Several potential interactions may be involved, such as hydrogen bonding, ionic, hydrophobic interactions, and covalent bonding, although covalent bonds are more rigid and thermally stable than other interactions (Zhang et al., 2010a). The postulated chemical pathway involves oxidization of diphenol moieties of phenolic acids or other polyphenols, under alkaline conditions, producing quinone intermediates which react with nucleophiles from reactive amino acid groups such as sulfhydryl groups of cystine, amine groups of lysine and arginine, and amide groups of asparagine and glutamine, forming covalent C–N or C–S bonds between the phenolic ring and proteins. The thus regenerated hydroquinone can be reoxidized and bind a second protein chain, resulting in a crosslink (Strauss and Gibson, 2004; Zhang et al., 2010a).
Caffeic and tannic acids were used as crosslinkers for gelatin (Zhang et al., 2010a), and the use of high-resolution NMR technique confirmed the occurrence of chemical reactions between the phenolic groups in phenolic compounds and amino groups in gelatin, forming covalent C–N bonds; the crosslinking decreased the mobility of the gelatin matrix. A similar study by the same group (Zhang et al., 2010b) indicated that the structure of a gelatin film crosslinked by tannic acid (3 wt%) was stable even under boiling, and that the crosslinking modification enhanced the mechanical properties of the protein.
Several other studies have reported beneficial effects from crosslinking protein matrices with phenolic compounds, such as tannic acid and sorghum condensed tannins (Emmambux et al., 2004), procyanidin (He et al., 2011) and ferulic acid (Ou et al., 2005; Fabra et al., 2011a).
Mathew and Abraham (2008) and Cao et al. (2007) reported significant increases in tensile strength and decreases in elongation at break resulting from adding ferulic acid to starch/chitosan films and gelatin-based films, respectively, which was ascribed to the crosslinking between ferulic acid and the polysaccharides or protein used. Ferulic acid is found as a naturally occurring component in cell walls, crosslinking polysaccharides (especially hemicelluloses) with each other and with other cell wall components such as lignin (Ng et al., 1997; Parker et al., 2005).
Genipin (Fig. 26.12) is a hydrolysis product from geniposide, which is a component of traditional Chinese medicine, isolated from gardenia fruits (Gardenial jasminoides Ellis). According to studies by Sung et al. (1999), genipin is about 10,000 times less cytotoxic than glutaraldehyde.
Genipin reacts with nucleophilic groups such as amino groups, being an adequate crosslinking agent for protein films as well as chitosan. Mi et al. (2005) studied the reaction mechanism of chitosan with genipin, and found that genipin undertakes a ring-opening reaction to form an intermediate aldehyde group resulting from the nucleophilic attack by chitosan amino groups. The genipin molecules reacting with a nucleophilic reagent may further undergo polymerization (Fernandes et al., 2013; Jin et al., 2004; Yuan et al., 2007). A dark-blue coloration appears in crosslinked materials exposed to air, which is associated with the oxygen radical-induced polymerization of genipin as well as its reaction with amino groups (Muzzarelli, 2009). Mi et al. (2005) observed that the colour of genipin crosslinked chitosan membranes varied from original transparent to bluish or brownish, depending on the pH value upon crosslinking. These authors ascribed the colour changes to establishment of different structures of crosslinked chitosan resulting from reaction of original genipin or polymerized genipin with primary amino groups on chitosan. The degrees of crosslinking of the genipin-crosslinked chitosan membranes depended significantly on their crosslinking pH values, being higher around pH 7.
Crosslinking with genipin has been reported to improve mechanical properties, water resistance of chitosan films, although they have turned dark bluish (Jin et al., 2004), which may be a market hindrance to many food packaging applications. Bigi et al. (2002) reported that gelatin films crosslinked with genipin presented higher Young’s modulus, better thermal stability (reflected in higher denaturation temperature), and better water resistance than uncrosslinked films. After 1 month of storage in buffer solution, a small gelatin amount (about 2%) was lost from the films, but their mechanical, thermal and swelling properties were very close to those of gelatin films previously crosslinked by glutaraldehyde (Bigi et al., 2001).
Another geniposide derivative, aglycone geniposidic acid, has been used to crosslink chitosan films, improving their tensile strength and water vapour barrier, although reducing their elongation (Mi et al., 2006).
Aldehydes such as glutaraldehyde, glyoxal or formaldehyde have been used as crosslinking agents to improve mechanical and barrier properties of protein films. However, due to concerns about possible toxic effects of such aldehydes, naturally occurring, less toxic aldehydes and other crosslinking agents have been explored for use in food packaging.
Cinnamaldehyde is an aromatic unsaturated aldehyde derived from cinnamon, consisting of a phenyl group attached to an unsaturated aldehyde (Fig. 26.13). It has been used as an antimicrobial agent in active packaging applications (Becerril et al., 2007), and it can also act as a crosslinking agent for proteins. Balaguer et al. (2011b) demonstrated the crosslinking effect of cinnamaldehyde for gliadin films, which was ascribed to the formation of intermolecular covalent bonds between polypeptide chains, polymerizing gliadins and reticulating the protein matrix. However, it is still uncertain which functional groups of proteins or other macromolecules have more potential to react with cinnamaldehyde, but Balaguer et al. (2011a) proposed a crosslinking mechanism involving the amino groups of proteins, although not ruling out the participation of other reactive groups.
Gliadin films presented significant improvements in their tensile strength and elastic modulus upon crosslinking with cinnamaldehyde. Such effects, as well as water resistance, were reported to be proportional to the cinnamaldehyde concentration (Balaguer et al., 2011b). The crosslinked films did not disintegrate upon a 5-month immersion in water, although a weight loss was reported, indicating that part of the material was solubilized (Balaguer et al., 2011a).
Dialdehyde polysaccharides have received attention as crosslinking agents of protein films. The oxidation of polysaccharides by periodate is characterized by the cleavage of the C2–C3 bond of glucose residues, resulting in the formation of two aldehyde groups per glucose unit, forming 2,3-dialdehyde polysaccharides (Li et al., 2011a). The aldehyde groups can crosslink with ε-amino groups by C = N bonds, as in lysine or hydroxylysine side groups of gelatin (Fig. 26.14) to improve the properties of protein films (Dawlee et al., 2005; Mu et al., 2012). Dialdehyde starch (DAS) was used as a crosslinking agent for gelatin films (Martucci and Ruseckaite, 2009). Crosslinking with DAS up to 10 wt% enhanced moisture resistance and barrier properties of the films, but higher amounts of DAS conducted to phase separation, impairing transparency and tensile properties. Mu et al. (2012) reported that the addition of dialdehyde carboxymethylcellulose (DCMC) to gelatin films increased their tensile strength and thermal stability and reduced their water sensitivity, while keeping their transparency.
Polymer composites are mixtures of polymers with inorganic or organic fillers with certain geometries (fibres, flakes, spheres, particulates). When the fillers are nanoparticles, that is to say, when they have at least one nanosized dimension (up to 100 nm), the resulting material is a nanocomposite (Alexandre and Dubois, 2000). Polysaccharides are good candidates for renewable and biodegradable nanofillers, because of their partly crystalline structures, conferring good reinforcement effects (Le Corre et al., 2010).
The change of filler dimensions from micro to nanoscale brings about important advantages concerning the resulting composite materials. Because of their size, nanoparticles have larger surface area-to-volume ratio than their microscale counterparts. A uniform dispersion of nanoparticles leads to a very large matrix/filler interfacial area, which changes the molecular mobility, improving thermal, barrier and mechanical properties of the material. Fillers with a high ratio of the largest to the smallest dimension (i.e., aspect ratio), such as nanofibres, are particularly interesting because of their high specific surface area, providing better reinforcing effects (Azizi Samir et al., 2005; Dalmas et al., 2007). In addition, an interphase region of altered mobility surrounding each nanoparticle is induced by well-dispersed nanoparticles, resulting in a percolating interphase network playing an important role in improving the nanocomposite properties (Qiao and Brinson, 2009). According to Jordan et al. (2005), for a constant filler content, a reduction in particle size increases the number of filler particles, bringing them closer to one another, and causing the interface layers from adjacent particles to overlap, altering the bulk properties significantly.
De Moura et al. (2009) observed that the water vapour permeability of hydroxypropyl methylcellulose (HPMC) films reinforced with chitosan nanoparticles was affected by the size of the CsN – the smaller the particles, the lower the permeability. According to the authors, CsN tended to occupy the empty spaces in the pores of the HPMC matrix, thereby improving tensile and barrier properties.
Cellulose fibres are built up by smaller and mechanically stronger entities (cellulose nanoparticles) which can be extracted under proper conditions. Cellulose nanoparticles (i.e., cellulose elements having at least one dimension in the 1–100 nm range, here referred to as nanocellulose), are inherently a low cost and widely available material. Moreover, they are environmentally friendly, easy to recycle by combustion, and require low energy consumption in manufacturing (Klemm et al., 2005; Charreau et al., 2013).
A uniform dispersion of cellulose nanoparticles on a polymer matrix reduces the molecular mobility, changes the relaxation behaviour, and improves the overall thermal and mechanical properties of the material (Azizi Samir et al., 2005; Charreau et al., 2013; Qiu and Hu, 2013). In this context, nanocellulose has been presented as a promising reinforcing and barrier component for elaboration of low cost, lightweight, and high-strength bionanocomposites for food packaging purposes mainly due to its compatibility with the biopolymers (Helbert et al., 1996; Podsiadlo et al., 2005; Moon et al., 2011). Basically two different classes of nanocellulose can be obtained – whiskers and nanofibrils (Azizi Samir et al., 2005). The term ‘whiskers’ (or ‘nanocrystals’) is used to designate elongated crystalline rod-like nanoparticles, whereas the designation ‘nanofibrils’ is used for long flexible nanoparticles consisting of alternating crystalline and amorphous strings (Abdul Khalil et al., 2012).
Different approaches have been introduced to produce nanocellulose: top-down (nanometric structures are obtained by size reduction of bulk materials) and bottom-up approaches (nanostructures are built from individual atoms or molecules capable of self-assembling) (Yousefi et al., 2013). Any cellulosic material can be virtually considered as a potential source for top-down approaches, including crop residues and agroindustry non-food feedstock. However, variations in cellulose source and its preparation conditions lead to a broad spectrum of structures, properties and applicability, which affect performance of cellulose nanoreinforcements (Kvien and Oksman, 2007; Azeredo, 2009; Dufresne, 2012).
Acid hydrolysis has been the primary method for isolating nanocellulose, consisting basically in removing the amorphous regions present in the fibrils leaving the crystalline regions intact; the dimensions of the whiskers after hydrolysis depend on the percentage of amorphous regions in the bulk fibrils, which varies for each organism (Gardner et al., 2008). The morphology of the obtained nanowhiskers is influenced by acid-to-pulp ratio, reaction time, temperature and cellulose source. In spite of the widely varied dimensions (of 3–70 nm widths and 35–3000 nm lengths) reported from different cellulose sources and hydrolysis conditions, cellulose nanowhiskers typically consist of structures 200–400 nm in length and with an aspect ratio of about 10 (Beck-Candanedo et al., 2005; Elazzouzi-Hafraoui et al., 2008; Rosa et al., 2010). Other methods can be used to extract nanocellulose from the lignocellulosic sources, usually based on successive chemical and mechanical treatments, including high-pressure homogenization (Zimmermann et al., 2010), electrospinning (Konwarh et al., 2013), enzymatic hydrolysis (de Campos et al., 2013), TEMPO-mediated oxidation (Isogai et al., 2011), solvent-based isolation (Yousefi et al., 2011), chemi-mechanical forces (Yousefi et al., 2013), ultrasonication (Chen et al., 2011; de Campos et al., 2013), cryo crushing (Alemdar and Sain, 2008) and steam explosion (Deepa et al., 2011). Such processes usually produce longer nanostructures (typically several micrometers in length), but with less uniform width (5–100 nm) (Siró and Plackett, 2010) and lower crystallinity (Iwamoto et al., 2007).
Although the most important industrial source of cellulosic fibres is wood, crops such as flax, cotton, hemp, sisal and others, especially from by-products of these different plants (corn, wheat, rice, sorghum, barley, sugar cane, pineapple, bananas and coconut crops) are likely to become of increasing interest as sources of nanocellulose. These non-wood plants generally contain less lignin than wood and therefore pre-treatment processes are less demanding (Siró and Plackett, 2010; Rosa et al., 2010).
In contrast to celluloses from plants, which require mechanical or chemomechanical processes to produce nanosized structures, the aforementioned bacterial cellulose (BC) is produced already as a nanomaterial by bacteria through cellulose biosynthesis and the building up of bundles of microfibrils (Nakagaito and Yano, 2005). In addition, BC is produced as a highly hydrated and relatively pure cellulose membrane and therefore no chemical treatments are needed to remove lignin and hemicelluloses, as is the case for plant celluloses (Siró and Plackett, 2010).
Nanocomposites for food packaging purposes have been developed by adding nanocellulose to polymers to enhance their physical and mechanical properties (Paula et al., 2011; Azeredo et al., 2012b; Abdollahi et al., 2013; Zainuddin et al., 2013). Nanocellulose has been reported to have a great effect in improving tensile strength and elastic modulus of polymers, especially at temperatures above the Tg of the matrix polymer (Wu et al., 2007; Azeredo et al., 2012b; Zainuddin et al., 2013; Abdollahi et al., 2013). This effect is ascribed not only to the geometry and stiffness of the nanocellulose, but also to the formation of a fibril network within the polymer matrix, the cellulose fibres probably being linked through hydrogen bonds. Barrier properties of polymer films have also been observed to be improved by cellulose nanostructures (Sanchez-Garcia et al., 2008; Paula et al., 2011; Azeredo et al., 2012b; Abdollahi et al., 2013). The presence of crystalline fibres is thought to increase the tortuosity in the materials leading to slower diffusion processes and, hence, to lower permeability (Sanchez-Garcia et al., 2008). Nanocellulose has also been reported to improve thermal properties of polymers (Petersson et al., 2007; Paula et al., 2011). The performance of nanocellulose has been reported to be strongly related to the content, dimensions and consequent aspect ratios of the nanostructures, as well as to the degree of matrix–cellulose interaction and percolation effects (Petersson and Oksman, 2006; Hubbe et al., 2008; Tang and Liu, 2008; Kim et al., 2009).
Since 2011, pilot plants have been opened for the production of nanocellulose in Sweden, Canada and the United States. These facilities make it possible to produce nanocellulose on a large scale for the first time, and it was an important step towards the industrialization of this technology.
Starch granules can be submitted to an extended-time hydrolysis at temperatures below that of gelatinization, when the amorphous regions are hydrolysed before the crystalline lamellae, which are more resistant to hydrolysis. The nanocrystals thus separated show platelet morphology with thicknesses of 6–8 nm (Kristo and Biliaderis, 2007). To prepare starch nanoparticles instead of nanocrystals, Ma et al. (2008) precipitated a starch solution within ethanol as the precipitant.
Similarly to cellulose nanocrystals, the reinforcing effect of starch nanocrystals (SNC) is usually ascribed to the formation of a percolating network maintained by hydrogen bonds above a given filler content (the percolation threshold) (Le Corre et al., 2010). Although not proven, this phenomenon was evidenced from experiments which indicated a changing behaviour above certain filler concentration (Angellier et al., 2005). It has been suggested that starch nanocrystals, similarly to nanoclays (which have a platelet morphology as well), create a tortuous diffusion pathway for permeant molecules through the nanocomposite materials, improving their barrier properties (Le Corre et al., 2010).
Kristo and Biliaderis (2007) reported that the addition of SNC to pullulan films resulted in improved tensile strength and modulus and decreased water vapour permeability. Moreover, the SNC promoted an increase in Tg, probably because of strong interactions of nanocrystals with one another and with the matrix, restricting chain mobility. Chen et al. (2008) observed that the tensile strength and elongation of polyvinyl alcohol (PVA) were only slightly improved by addition of SNC up to 10 wt% and, above this content, such properties were impaired by SNC. On the other hand, the properties of PVA nanocomposite with SNC were better than those of the composites with native starch, indicating that SNC presented a better dispersion and stronger interactions with the matrix than native starch granules.
Chitin whiskers can be prepared by acid hydrolysis of chitin (Lu et al., 2004; Sriupayo et al., 2005), and have been successfully prepared from different chitin sources such as squid pens (Paillet and Dufresne, 2001), crab shells (Nair and Dufresne, 2003), and shrimp shells (Sriupayo et al., 2005). When the acid hydrolysis is followed by mechanical ultrasonication/disruption, chitin nanoparticles (nanospheres) can be formed rather than chitin whiskers (Chang et al., 2010b). Chitosan nanoparticles can be obtained by ionic gelation of chitosan, where the cationic amino groups of chitosan form electrostatic interactions with polyanionic crosslinking agents, such as tripolyphosphate (López-León et al., 2005).
Some authors reported beneficial effects from adding chitin whiskers to biopolymer films. The chitin whiskers added by Lu et al. (2004) to soy protein isolate (SPI) greatly improved the tensile properties and water resistance of the matrix. Sriupayo et al. (2005) reported that the whiskers improved the water resistance of chitosan films, and enhanced their tensile strength until a content of 2.96%, but impaired the strength when at higher contents. Similarly, Chang et al. (2010b) reported that chitin nanoparticles were uniformly dispersed and presented good interaction with a starch matrix when at low loading levels (up to 5%), improving its mechanical, thermal and barrier properties. However, aggregation of nanoparticles occurred at higher loading, impairing the performance of the matrix.
Chitosan nanoparticles have also been proven to be effective as nanoreinforcement to bio-based films. The incorporation of chitosan-tripolyphosphate (CS-TPP) nanoparticles significantly improved mechanical and barrier properties of hydroxypropyl methylcellulose (HPMC) films (De Moura et al., 2009). The authors attributed such effects to the nanoparticles filling discontinuities in the HPMC matrix. Chang et al. (2010a) added chitosan nanoparticles to a starch matrix. The tensile, barrier and thermal properties of the matrix were improved by low contents of the nanoparticles, when they were well dispersed in the matrix. Such effects were ascribed to close interactions between the nanoparticles and the matrix, due to their chemical similarities. However, higher nanoparticle loads (8% w/w) resulted in their aggregation in the nanocomposites, impairing the physical properties of the materials.
Since most biopolymers have limitations for their applications as packaging materials, such as water sensitivity, brittleness or poor mechanical performance, chemical modification techniques can sometimes be used to generate new biomaterials with improved properties. Shi et al. (2011) grafted lauryl chloride groups onto zein molecule through an acylation reaction, and obtained seven-fold increase in elongation at break of modified zein. Moreover, the modification increased the zein hydrophobicity, suggesting that the end material presented probably decreased moisture sensitivity and water vapour permeability.
The combination between ultraviolet light and ozone (UV-O treatment) has been suggested as an effective method to modify polymer surfaces, leading to oxidation reactions at the polymer surface, while the bulk properties, such as thermal, barrier and mechanical properties of the polymer, may not be altered (Shi et al., 2009). Shi et al. (2009) used UV-O treatment to control hydrophilicity of zein films. The treatment converted some of the surface methyl groups mainly to carbonyl groups, decreasing the water contact angles and increasing the surface hydrophilicity of zein films. Moreover, the authors suggested that, once the surface has been modified, several active compounds could have been linked to the functional groups formed, and the polymer would be not only a barrier material but a carrier to active compounds, constituting an active packaging material.
Conventional food packaging systems are supposed to passively protect the food, acting as barriers between the food and the surrounding environment. On the other hand, an active food packaging may be defined as a system that not only acts as a barrier but also interacts with the food in some desirable way, for example by releasing desirable compounds (such as antimicrobial or antioxidant agents), or by removing a detrimental factor (such as oxygen or water vapour), usually to improve food stability, that is to say, to better maintain food quality and safety. The compounds added more frequently are antimicrobials, such as chitosan (Shen et al., 2010), acids (Guillard et al., 2009), phenolic compounds (Cerisuelo et al., 2012; Arrieta et al., 2013) and antimicrobial peptides (Sanjurjo et al., 2006; Gómez-Guillén et al., 2011).
More recently, the concept of bioactive packaging, in which a food packaging or coating has a potential to enhance food impact over consumer health, has been proposed by Lopez-Rubio et al. (2006). Enclosing bioactive compounds within packaging instead of directly incorporating them into food presents some industrial benefits, such as increasing retention of bioactives, reducing some incompatibility problems between the bioactive and the food matrix, and reducing changes to food sensory properties (Lopez-Rubio et al., 2006). Antioxidant packaging materials could be included in both active and bioactive packaging concepts, since antioxidant compounds usually have alleged benefits both for food stability and for consumer health. Some studies have described the incorporation of antioxidant compounds or extracts to biopolymer films and coatings, such as phenolic compounds to zein films (Arcan and Yemenicioglu, 2011), α-tocopherol to chitosan films (Martins et al., 2012), curcumin and ascorbyl dipalmitate to cellulose-based films (Sonkaew et al., 2012), and ferulic acid and α-tocopherol to sodium caseinate films (Fabra et al., 2011a). Other studies have described the development of probiotic films in coatings, based on the incorporation of lactic acid bacteria, such as Bifidobacterium lactis incorporated to alginate and gellan films for coating of fresh fruits (Tapia et al., 2007), Lactobacillus sakei added to caseinate films to control Listeria monocytogenes in fresh beef (Gialamas et al.,2010),Lactobacillus acidophilus in starch-based coatings for breads (Altamirano-Fortoul et al., 2012) and L. acidophilus and Bifidobacterium bifidum incorporated to gelatin coatings for fish (López de Lacey et al., 2012).
The active and bioactive compounds are most frequently simply added to the film-forming formulation, constituting one of the formulation components, but not being chemically linked to the matrix polymer. However, some studies have suggested the covalent immobilization of active compounds onto functionalized polymer surfaces, with many possible applications for food packaging purposes. Biopolymer films may be functionalized in two basic steps, namely, the treatment of the polymer surface in order to produce reactive functional groups, and the reaction of such groups with an active compound (Kugel et al., 2011). Sometimes the functionalization can be favoured by an intermediary step, such as grafting a polyfunctional agent onto the polymer surface, increasing the density of available functional groups, or by a spacer molecule to reduce steric hindrances (Goddard and Hotchkiss, 2007). An active compound which is covalently immobilized onto the packaging material is not supposed to be released, but becomes effective when in contact with the food surface (Han, 2003). The active compounds to be tethered can be enzymes, antimicrobials, biosensors, bioreactors, etc. Some studies have described the covalent attachment of active compounds onto the surface of biopolymer materials to be applied as food packaging or coatings, such as lysozyme onto chitosan coatings (Lian et al., 2012) and N-halamine onto chitosan films (Li et al., 2013).
The world biomass is rich in macromolecules with film-forming abilities which can be explored to develop materials for food packaging and coating purposes. Several studies have been conducted describing the development of food packaging materials from biomaterials, especially polysaccharides and proteins. However, to convert macromolecules from biomass into materials with both processability and performance compatible with petrochemical-based ones, research needs to address several challenges. Important gaps in knowledge remain on the structure and functionality of biomaterials. Moreover, chemical and physical changes during processing of biomaterials need to be better understood, so that the processing conditions are improved. Another major issue is to minimize the impact of environmental conditions (especially humidity) on material performance. Finally, biomaterials need to be compatible with their petrochemical counterparts in terms of price, in order to penetrate the market. One of the main challenges to make biomaterials commercially viable is their processing by conventional processing techniques used for petroleum-based polymers such as extrusion and injection moulding. The industrial application of edible coatings to food surfaces also requires developments of techniques and equipment adequate to each kind of food to be coated.
Moreover, nanotechnology has demonstrated a great potential to expand the use of biodegradable polymers, since the addition of nanofillers has led to improvements in overall performance of biopolymers, making them more competitive in a market dominated by non-biodegradable materials. However, there are still important safety concerns about nanotechnology applications to food contact materials. Considering the tiny dimensions of nanofillers, it is reasonable to assume that they might migrate from the packaging material to food. Although the properties and safety of most starting materials in their bulk form are usually well known, their nano-sized counterparts frequently exhibit different properties, because their small sizes would allow them to cross more barriers through the body, while their high surface area increases their reactivity. Hence, detailed information is still required to assess the potential toxicity and environmental effects of nanofillers to be incorporated to food packaging materials.
Finally, the biopackaging field has adopted technologies to improve the performance and/or to add functionalities to biopolymer-based materials, so their range of applications can be widened, and they can become more competitive in the polymer market.
The authors wish to acknowledge financial support from EMBRAPA (Brazil) and the BBSRC (UK).