Advances in Biorefineries || The use of biomass for packaging films and coatings

819

© 2014 Woodhead Publishing Limited

26 The use of biomass for packaging fi lms

and coatings

H. M. C. D E A Z E R E D O , Institute of Food Research , UK , M. F. R O S A and M. D E S Á M . S O U Z A F I L H O ,

Embrapa Tropical Agroindustry , Brazil and K. W. WA L D R O N , Institute of Food Research , UK

DOI : 10.1533/9780857097385.2.819

Abstract : 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 fi lms 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 fi lms and coatings), their properties and interactions, and how they infl uence the packaging performance.

Key words : polysaccharides , proteins , lignocellulose , nanocomposites.

26.1 Introduction

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 diffi culties. 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 suffi ciently rapid to create new enzymes capable of degrading synthetic polymers during their relatively

820 Advances in Biorefi neries

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 fl exible 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 (fi lms 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 fi lms (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, O 2 , CO 2 , lipids and fl avour compounds between the food and the surrounding environment, thus increasing food stability. So, they help to improve the effi ciency 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 use of biomass for packaging fi lms and coatings 821

The barrier requirements of an edible fi lm 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 O 2 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-fl avor formation. Nuts are especially susceptible to oxidation, thus the barrier against O 2 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 fi nal fat content; in this case, coatings should be highly hydrophilic to have a good barrier against the hydrophobic oils. Edible fi lms 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 fi lms 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 fi lms or coatings are supposed to have a characteristic desirable fl avour, which should be compatible with the food to be coated. This is the case, for instance, for fruit purée-based edible fi lms (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 fi lms (Park and Zhao, 2006 ), and vegetable purée edible fi lms (Wang et al ., 2011 ).

There are a number of methods for evaluating the physical properties of the resulting fi lm 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 fi lms (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 fi lms and coatings), their basic properties and interactions, and how they affect the end materials.


26.2 Components of packaging fi lms and coatings

from the biomass

All packaging fi lms 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 fi lms 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 fi lms and coatings.

26.3 Processes for producing bio-based fi lms

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 fi lm forming dispersion (containing at least a biopolymer matrix, a solvent and usually a plasticizer) on a suitable base material (from which the fi lm

26.1 Basic and optional components for fi lms and coatings.

COMPONENTS for FILMS and COATINGS

MATRIX(one or more must be used)

CROSSLINKERS(one or more can be used)

REINFORCEMENT(one or more can be used)

PLASTICIZER(one or more must be used)

Lignocellulose

Derived Polymers

Proteins

Polyols

Lipids

Mono- and

disaccharides

PhenolsNanocellulose

Starch Nanostructures

Chitin/chitosanNanostructures

Aldehydes

Genipin

Lipids(can’t be usedalone for films)

Other

Polysaccharides


can be easily peeled off) and later drying to a moisture content (usually 5–8%) which is optimal for peeling the fi lm 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 fi lm casting methods are typically used to manufacture biopolymer fi lms 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 fi lm 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 effi ciency, while eliminating expense of a separate substrate such as polyester fi lm or coated paper. In (b), known as web coating, solutions are spread uniformly onto a carrier web or substrate, usually a polyester fi lm or coated paper, and the coated substrate is passed through a drying chamber. The dry fi lm is wound into rolls while still adhering to the substrate, and is usually separated in a secondary operation (Rossman, 2009 ).

26.2 Basic scheme of the steps of a casting process to produce fi lms.

Peel Solventevaporation Casting

HomogenizationVacuum

degassing

Film forming solution/dispersion

PlasticizerBiopolymer

matrix

Solvent


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 ( T g ) changes them into a rubbery state, making it possible to form fi lms after cooling. Although the dry processes require more extensive and advanced equipment, they are effi cient 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 fi lms cast with the use of plasticizers (Mangavel et al ., 2004 ) and lower water solubility, due to the creation of a highly crosslinked fi lm network (Rhim and Ng, 2007 ).

Most conventional plastics, such as low density polyethylene fi lms, are produced by extrusion. An extruder consists of a heated, fi xed 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 fi lms are more diffi cult to produce by dry processes when compared to petroleum-based polymers, as they do not usually have defi ned 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 fi lm performance (Rossman, 2009 ). Starch (Pushpadass et al ., 2008 ; Thunwall et al ., 2008 ) and protein fi lms (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 ).

26.3 Schematic representation of an extruder (adapted from Li et al ., 2011b ).

Powder feeding

Plasticizer

Die

Film


26.4 Processes for producing edible coatings

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 fi lm 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

26.4 Basic steps to prepare and apply a coating to a food surface.

Homogenization

Immersion Spraying

Vacuumdegassing

PlasticizerBiopolymer

matrix

Solvent


and/or when only one side of the product is supposed to be coated (such as in pizza crusts, as exemplifi ed before). Both spraying and dipping are followed by drying for polysaccharide or protein coatings, or by cooling for lipid-based coatings.

26.5 Products from biomass as fi lm

and/or coating matrices

There has been increasing interest in the search for new uses of biomass byproducts from the food industry. Many of these byproducts contain potential fi lm-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 inter-plays between different factors occur simultaneously, often making it diffi cult to establish correlations, and creating exceptions when an apparent rule was expected to be followed. For instance, since the fi rst 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.


26.5.1 Lignocellulosic biomass Cellulose and its derivatives

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 fi brils) containing crystalline and amorphous regions; the crystalline regions are maintained by inter- and intramolecular hydrogen bonding. Several elementary fi brils 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 microfi bril (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 modifi cation, and its formation of versatile semicrystalline fi bre morphologies.

Together with starch, cellulose and its derivatives (such as ethers and esters) are the most important raw materials for elaboration of biodegrad-able and edible fi lms (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 etherifi cation; the water-soluble cellulose ethers, including methyl cellulose (MC), hydroxypropyl cel-lulose (HPC), hydroxypropylmethyl cellulose (HPMC), and carboxymethyl cellulose (CMC), have good fi lm-forming properties (Cha and Chinnan, 2004 ; Janjarasskul and Krochta, 2010 ).

Cellulose fi lms 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 deriva-tives 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 fi lms have been produced by several companies such as Innovia (UK), FKuR (Germany) and Daicel Polymer (Japan).

Hemicelluloses

Hemicelluloses (HC) are heteropolysaccharides closely associated with cellulose in the plant cell walls (Mikkonen and Tenkanen, 2012 ). They are usually defi ned 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 confi guration. 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 microfi brils. The structural similarity between HC and cellulose generates a conformational homology leading to a hydrogen-bonded network between HC and cellulose microfi brils (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 fi lms. Höije et al . ( 2005 ) obtained strong but highly hygroscopic arabinoxylan fi lms from barley husks. β -glucan fi lms were reported to be more compact than arabinoxylan fi lms, with smaller nanopores, favouring the barrier properties (Ying et al ., 2011 ). Zhang et al . ( 2011 ) reported that the properties of arabinoxylan fi lms were well correlated with the arabinose/xylose (Ara/Xyl) ratios. More crystalline fi lms 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 fi lms have also been the subject of several studies. Mikkonen et al . ( 2007 ) reported that galactomannans with lower galactose content produced fi lms 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

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 esterifi ed 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-esterifi es the methyl esters to pectic acids and generates low-methoxyl pectins (LMP). Commer-cial pectins are categorized according to their degree of esterifi cation (DE), defi ned as the ratio of esterifi ed 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 esterifi ed to non-esterifi ed 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 Ca 2 + , which links adjacent LMP chains via ionic interactions, forming a tridimensional network (Janjarasskul and Krochta, 2010 ).

Pectin fi lms, similarly to other polysaccharide fi lms, have poor water resistance, and have been proposed for potential industrial uses where water binding either is not a problem or can provide specifi c advantages, such as edible bags for soup ingredients (Fishman et al ., 2000 ). LMP fi lms, 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 > pK a enables exploiting electrostatic interactions of pectin with positively charged counterparts (Farris et al ., 2009 ).

26.5 A fragment of pectin containing esterifi ed and non-esterifi ed carboxyl groups of galacturonic acid.

OH

OH OH

O

C

O

O OCH3

OHO

C

O

O OH

OH

OH

O

C

O...

O OH

...O


Lignin

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 fi lms, such as its small particle size, hydrophobicity and ability to form stable mixtures (Park et al ., 2008 ). Moreover, lignins have been shown to have effi cient antibacterial and antioxidant properties (Ugartondo et al ., 2009 ). Lignin has been used as a fi lm 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 fi lms incorporated with up to 30% lignin presented higher elongation and water resistance than control starch fi lms. On the other hand, lignin impaired the tensile strength of the fi lms at high relative humidity (71%), refl ecting the incompatibility between the hydrophilic starch and the hydrophobic lignin, which was bolstered by the water. Indeed, microscopic observations confi rmed that the material consisted of two phases – a hydrophilic starch matrix fi lled 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 fi lms revealed strong protein conformational changes induced by lignin, producing a plasticizing effect, which was refl ected in the mechanical and thermal properties (Núñez-Flores et al ., 2013 ).

26.6 Lignin monolignols (from Buranov and Mazza, 2008 ).

OH

p-coumaryl: R1=R2=Hconiferyl: R1=OCH3; R2=Hsinapyl: R1=R2=OCH3

OH

R2R1 43

2

1α

βγ

5

6


A drawback from incorporating lignin in fi lms 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.

26.5.2 Polysaccharides (other than cell wall polysaccharides)

Polysaccharides are hydrophilic polymers and therefore exhibit very low moisture barrier properties. They are of prime interest as matrices for biodegradable fi lm 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 fi lm 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 fi lms 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 fi lms provide a good barrier to CO 2 and O 2 , 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

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 fi lm-forming capacity of starches, since linear chains of amylose in solution tend to interact by hydrogen bonds and, consequently, amylose gels and fi lms are stiff, cohesive and relatively strong. On the other hand, amylopectin fi lms are brittle and non-continuous, since branched amylopectin chains in solution present little tendency to interact (Peressini et al ., 2003 ).

The fi rst studies using starch in biodegradable food packaging were focused on substituting part of the synthetic matrix (usually polyethylene) by starch, but there were diffi culties 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 fi lms. The fi rst one is that native starch com-monly 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 fi rst drawback and improve material fl exibility (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 affi nity of the fi lms (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).

26.7 Chemical structure of a starch fragment.

OH

OH

O

O

CH2OH

OH

OH OH

O

O

OH

OH

O

O

OH

OH

O

O

CH2OH CH2OH

OH

OH

O

O...

CH2OH

OH

OH

O

O

CH2OH

CH2

...O


Alginates

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 (Ca 2 + ), to produce strong gels or fi lms (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 fi lms may be altered by changing calcium concentration and temperature, among other factors (Kester and Fennema, 1986 ). Alginate fi lms 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

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

26.8 Basic structure of alginates, containing units of mannuronic (M) and guluronic (G) acids.

OH

M

COOH

OH

O

O

OH

M

COOH

COOH

OH

O

OH

G

OH

OCOOH

OH

G

OH

O

O

O

O...

...O


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 –NH 2 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 modifi cation, 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 fi lms 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 ).

Less conventional polysaccharides

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.

26.9 Chemical structure of chitosan.

OH

OH

OO

CH2OH

NH2

OHO

O

n

CH2OH

NH2

OH

OHO

CH2OH

NH2


Carrageenans

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 alternat ing (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 effi cient 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 fi rmness of strawberries when compared to starch coatings, probably refl ecting a better moisture barrier of carrageenan coatings. Moreover, carrageenan fi lms presented a signifi cantly lower oxygen permeability than starch fi lms.

Tree exudates

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 fi lms 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 fi lms crosslinked with CaCl 2 . CTG reduced tensile strength and barrier properties of the fi lms, but favoured fi lm extensibility. Some studies have described the use of mesquite gum as fi lm-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 ).

Cactus mucilages

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 fi cus-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.

Bacterial cellulose

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 effi cient producers being the Gram-negative, acetic acid bacteria Gluconacetobacter xylinum (Iguchi et al ., 2000 ; Retegi et al ., 2010 ). These microfi bril 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 fi ner 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 fi ne 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) fi lms with different porosities, generated by different compression pressures. Higher pressures were found to produce fi lms with better fi nal mechanical properties. This behaviour was ascribed to the higher densifi cation, reducing interfi brillar spaces, thus increasing the possibility of interfi brillar bonding zones.

26.5.3 Proteins

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 fi lms 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 (–NH 2 ) 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 disulfi de) 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 fi lms 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 fi lm-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 fi lm-forming properties and good adherence to hydrophilic surfaces. Protein-derived fi lms provide good barriers to O 2 and CO 2 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 fi lms 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 fi lm 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 fi nal 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 ).

Gelatin

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 (fi sh 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 fi sh industry sector has tried to fi nd new outlets for their skin and bone byproducts. However, gelatins from cold water species, representing the majority of the industrial fi sheries, 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 fi sh 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 fi sh species, like tilapia, on the other hand, have physical properties more similar to those of mam-malian gelatins (Sarabia et al ., 2000 ). Moreover, fi sh gelatins contain more hydrophobic amino acids, so their fi lms show signifi cantly lower water vapour permeability when compared to fi lms 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 fi lms 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 fi lms 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 ).


Milk proteins

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 fi lms (Zhou et al ., 2009 ). Since whey proteins have a high proportion of hydrophilic amino acid in their structure, whey protein fi lms 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 fi lms was decreased by addition of beeswax, by both increasing the hydrophobic character of the fi lm 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 fi lms 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 fi lm 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 CO 2 was reported to present lower water solubility than acid-precipitated casein (Dangaran et al ., 2006 ). Casein fi lms have been used to increase stability of bread (Schou et al ., 2005 ).

Zein

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 fi lms 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 fi lms when compared to other protein fi lms (Dangaran et al ., 2009 ; Ghanbarzadeh and Oromiehi, 2008 ).

Gluten

Wheat gluten proteins are a byproduct of starch extraction from wheat fl our, which is commonly used as a functional ingredient, especially in bakery products. Wheat gluten consists of a mixture of proteins that can be classifi ed into two types: the water insoluble glutenins, which comprise proteins with multiple peptide chains linked via interchain disulfi de 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 disulfi de 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 fl our improvement in breadmaking, but can be used for preparing plastic fi lms, presenting adequate viscoelasticity, adhesiveness, thermoplasticity, and good fi lm-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 disulfi de 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 fl ow 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 specifi c 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 ).

26.5.4 Lipids

Unlike other macromolecules, lipids are not biopolymers, being unable to form cohesive, self-supporting fi lms 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 fi lms, 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 fi nish 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 affi nity for proteins and polysaccharides when in emulsions, producing more homogeneous fi lms and avoiding the separation of a lipid phase (Fabra et al ., 2011b ).

Several types of lipids can be used for food coating or as fi lm 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., paraffi n 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 fi lms. Lipid-polysaccharide or lipid-protein emulsion fi lms


combine the complementary advantages of each component. Polysaccharides and proteins act as matrices, since they have the mechanical properties to form self-supporting fi lms, and have good barrier against gases such as O 2 and CO 2 , while lipids are useful to reduce water vapour permeability. García et al . ( 2000 ) demonstrated that lipid addition to starch fi lms decreased their crystalline-amorphous ratio, which is expected to increase fi lm diffusivity and consequently permeability. On the other hand, lipid addition also reduces the hydrophilic–hydrophobic ratio of fi lms, which decreases their water solubility and therefore water vapour permeability. Emulsion fi lms 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 ).

26.6 Products from biomass as fi lm plasticizers

Brittleness is an inherent quality attributed to most biopolymer fi lms, due to extensive intermolecular forces such as hydrogen bonding, electrostatic forces, hydrophobic bonding, and disulfi de 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 ( T g ) and reducing fi lm rigidity and brittleness.

26.10 Representative scheme of the effect of plasticizers.

Plasticizer

Polymer chains


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 fi lm 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.

26.6.1 Polyols

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 signifi cant 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 fi lms 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 affi nity for polysaccharide and protein fi lms, because of their hydrophilicity. Several studies have demonstrated the effectiveness of polyols as plasticizers for polysaccharide-based and protein-based fi lms 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.

26.6.2 Mono- and disaccharides

It is not only molecular size, confi guration and total number of functional hydroxyl groups that are important characteristics to be considered for an


effective plasticizer, but also its compatibility with the fi lm-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 fi lms 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 fi lms plasticized with monosaccharides (glucose, mannose and fructose) presented better overall physical properties when compared to starch fi lms plasticized with polyols (glycerol and sorbitol). On the other hand, polyols (especially glycerol) presented better ability to lower T g of the fi lms, 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 fi lms (Olivas and Barbosa-Cánovas, 2008 ; Piermaria et al ., 2011 ). Whey protein fi lms plasticized with sucrose presented excellent oxygen barrier properties, but sucrose tended to crystallize with time (Dangaran and Krochta, 2007 ).

26.6.3 Lipids

Generally, the purpose of adding lipids to fi lms is to reduce their water vapour permeability and/or to provide an attractive gloss. Moreover, incorporating lipids in protein- or polysaccharide-based fi lms may interfere with polymer chain-to-chain interactions and provide fl exible domains within the fi lm. Fabra et al . ( 2008 ) observed that oleic acid apparently interacted with the protein (sodium caseinate) matrix forming bonds through polar groups, modifi ying the interaction balances in the protein network. The result can be a plasticizing effect, including reduction of fi lm strength and increase of fi lm fl exibility, as described for whey protein fi lms plasticized with beeswax (Talens and Krochta, 2005 ), caseinate fi lms with oleic acid (Fabra et al ., 2008 ), and gelatin fi lms 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.

26.7 Products from biomass as crosslinking

agents for packaging 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 specifi city 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.

26.7.1 Phenolic compounds

Several natural phenolic compounds derived from plants have been used as crosslinkers to modify biopolymer fi lms. Several potential interactions may be involved, such as hydrogen bonding, ionic, hydrophobic interac-tions, 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 confi rmed 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

26.11 Schematic representation of crosslinking of polymer chains.

Crosslinked polymer networkUncrosslinked

polymer chains


gelatin fi lm crosslinked by tannic acid (3 wt%) was stable even under boiling, and that the crosslinking modifi cation enhanced the mechanical properties of the protein.

Several other studies have reported benefi cial 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 signifi cant increases in tensile strength and decreases in elongation at break resulting from adding ferulic acid to starch/chitosan fi lms and gelatin-based fi lms, 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 ).

26.7.2 Genipin

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 fi lms 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

26.12 Chemical structure of genipin (from Yuan et al ., 2007 ).

O

H


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 signifi cantly on their crosslinking pH values, being higher around pH 7.

Crosslinking with genipin has been reported to improve mechanical properties, water resistance of chitosan fi lms, 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 fi lms crosslinked with genipin presented higher Young ’ s modulus, better thermal stability (refl ected in higher denaturation temperature), and better water resistance than uncrosslinked fi lms. After 1 month of storage in buffer solution, a small gelatin amount (about 2%) was lost from the fi lms, but their mechanical, thermal and swelling properties were very close to those of gelatin fi lms previously crosslinked by glutaraldehyde (Bigi et al ., 2001 ).

Another geniposide derivative, aglycone geniposidic acid, has been used to crosslink chitosan fi lms, improving their tensile strength and water vapour barrier, although reducing their elongation (Mi et al ., 2006 ).

26.7.3 Aldehydes

Aldehydes such as glutaraldehyde, glyoxal or formaldehyde have been used as crosslinking agents to improve mechanical and barrier properties of protein fi lms. 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

26.13 Chemical structure of cinnamaldehyde (from Balaguer et al ., 2011b ).

O

H HPeriodateoxidation

Polysaccharide Polysaccharidedialdehyde

NalO4

Gelatin

Gelatin N=C

H

NH2

OHO OH


cinnamaldehyde for gliadin fi lms, 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 fi lms presented signifi cant 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 fi lms 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 fi lms. 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 fi lms (Dawlee et al ., 2005 ; Mu et al ., 2012 ). Dialdehyde starch (DAS) was used as a crosslinking agent for gelatin fi lms (Martucci and Ruseckaite, 2009 ). Crosslinking with DAS up to 10 wt% enhanced moisture resistance and barrier properties of the fi lms, 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 fi lms increased their tensile strength and thermal stability and reduced their water sensitivity, while keeping their transparency.

26.14 Periodate oxidization of a glucose residue in a polysaccharide and the Schiff ’ s reaction between gelatin and the dialdehyde polysaccharide (from Mu et al ., 2012 ).

O

OH

OH

H

H

CH2

COCH3


26.8 Products from biomass as reinforcements for

packaging materials

Polymer composites are mixtures of polymers with inorganic or organic fi llers with certain geometries (fi bres, fl akes, spheres, particulates). When the fi llers are nanoparticles, that is to say, when they have at least one nanosized dimension (up to 100 nm), the resulting material is a nanocom-posite (Alexandre and Dubois, 2000 ). Polysaccharides are good candidates for renewable and biodegradable nanofi llers, because of their partly crystalline structures, conferring good reinforcement effects (Le Corre et al ., 2010 ).

26.8.1 From micro to nanoscale

The change of fi ller 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/fi ller 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 nanofi bres, are particularly interesting because of their high specifi c 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 fi ller content, a reduction in particle size increases the number of fi ller particles, bringing them closer to one another, and causing the interface layers from adjacent particles to overlap, altering the bulk properties signifi cantly.

De Moura et al . ( 2009 ) observed that the water vapour permeability of hydroxypropyl methylcellulose (HPMC) fi lms 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.

26.8.2 Cellulosic reinforcements

Cellulose fi bres are built up by smaller and mechanically stronger entities (cellulose nanoparticles) which can be extracted under proper condi-tions. 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 nanofi brils (Azizi Samir et al ., 2005 ). The term ‘whiskers’ (or ‘nanocrystals’) is used to designate elongated crystalline rod-like nanoparticles, whereas the designation ‘nanofi brils’ is used for long fl exible 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 fi brils leaving the crystalline regions intact; the dimensions of the whiskers after hydrolysis depend on the percentage of amorphous regions in the bulk fi brils, which varies for each organism (Gardner et al ., 2008 ). The morphology of the obtained nanowhiskers is infl uenced 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 fi bres is wood, crops such as fl ax, 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 chemo-mechanical 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 microfi brils (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 T g 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 fi bril network within the polymer matrix, the cellulose fi bres probably being linked through hydrogen bonds. Barrier properties of polymer fi lms 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 fi bres 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 fi rst time, and it was an important step towards the industrialization of this technology.

26.8.3 Starch nanoreinforcement

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 fi ller 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 fi ller 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 fi lms resulted in improved tensile strength and modulus and decreased water vapour permeability. Moreover, the SNC promoted an increase in T g , 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.

26.8.4 Chitin/chitosan nanostructures

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 benefi cial effects from adding chitin whiskers to biopolymer fi lms. 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 fi lms, 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 fi lms. The incorporation of chitosan-tripolyphosphate (CS-TPP) nanoparticles signifi cantly improved mechanical and barrier properties of hydroxypropyl methylcellulose (HPMC) fi lms (De Moura et al ., 2009 ). The authors attributed such effects to the nanoparticles fi lling 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.

26.9 Future trends

26.9.1 Polymer surface modifi cations

Since most biopolymers have limitations for their applications as packaging materials, such as water sensitivity, brittleness or poor mechanical performance, chemical modifi cation 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 modifi ed zein.


Moreover, the modifi cation 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 fi lms. 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 fi lms. Moreover, the authors suggested that, once the surface has been modifi ed, 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.

26.9.2 Active and bioactive biopackaging

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 defi ned 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 benefi ts, 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 benefi ts both for food stability and for consumer health. Some studies have described the incorporation of antioxidant compounds or extracts to biopolymer fi lms and coatings, such as phenolic compounds to zein fi lms (Arcan and Yemenicioglu, 2011 ),


α -tocopherol to chitosan fi lms (Martins et al ., 2012 ), curcumin and ascorbyl dipalmitate to cellulose-based fi lms (Sonkaew et al ., 2012 ), and ferulic acid and α -tocopherol to sodium caseinate fi lms (Fabra et al ., 2011a ). Other studies have described the development of probiotic fi lms in coatings, based on the incorporation of lactic acid bacteria, such as Bifi dobacterium lactis incorporated to alginate and gellan fi lms for coating of fresh fruits (Tapia et al ., 2007 ), Lactobacillus sakei added to caseinate fi lms 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 Bifi dobacterium bifi dum incorporated to gelatin coatings for fi sh (López de Lacey et al ., 2012 ).

The active and bioactive compounds are most frequently simply added to the fi lm-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 fi lms 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 fi lms (Li et al ., 2013 ).

26.10 Conclusion

The world biomass is rich in macromolecules with fi lm-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 nanofi llers 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 nanofi llers, 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 nanofi llers to be incorporated to food packaging materials.

Finally, the biopackaging fi eld 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.

26.11 Acknowledgements

The authors wish to acknowledge fi nancial support from EMBRAPA (Brazil) and the BBSRC (UK).

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Advances in Biorefineries || The use of biomass for packaging films and coatings

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