ABSTRACT
Figure 13.2 Chemical approaches in processing wheat gluten-based polymer materials (Zhang, 2014).feedstock. It can be realized from the fact that protein polymers can be sourced from agricultural co-products avoiding the need of designated crops for protein polymer extraction. An attractive proposal is the utilization of co-products generated by the soy-and corn-based bio-fuels industry to enable a better economic and viable feedstock for plant proteins as a bioplastics. The utilization of the co-products also improves the efficiency of the bio-fuel process, thus reducing their process and raw material cost. A certain number of proteins have received much attention as biodegradable polymers (Zhang and Zeng, 2008), but few have led to actual industrial scale-up due to the high production cost and the low product performance. Proteins are thermoplastic heteropolymers constituted by both different polar and non-polar a-amino acids able to form a lot of intermolecular linkages resulting in different interactions. These offer a wide possibility of chemical functionalities and functional properties (Fig. 13.2). Protein-based materials have greatly drawn attention during the past two decades and some material applications have already been found for them (Guillaume et al., 2011). Most of the proteins are neither soluble nor fusible, especially fibrous proteins such as silk, wool, and collagen, so they are used in their natural form. The classical way to process protein-based bioplastics is the thermoplastic processing, which consists of mixing proteins and plasticizers (Gao et al., 2006; Song and Zheng, 2008). Heat-induced cross-linking of the proteins creates a thermoplastic polymer based on three-dimensional networks of disulfide bonds, as well as hydrophobic interactions and hydrogen bonding. Addition of plasticizers, such as glycerol, improves the structural properties of the film. Although these films are poor moisture barriers, they are
good oxygen barriers and so can be used as a layer in a multilayer sheet to prevent oxidation of packaged food (Gennadios et al., 1993). In terms of potential sources, soy protein, corn protein (zein) and wheat proteins (gluten) are among the main plant proteins. 13.2.1.1 Soy proteinTypically, soybean proteins (the isolated proteins from soybean) contain 38-42% crude protein, 16-20% triglycerides, and around 33% carbohydrates, on dry basis, while in the case of corn proteins, they form about 9% of the dry weight of corns. They are mainly composed of zein (a highly hydrophobic protein, soluble in alcohols), glutelin (soluble in aqueous alkaline solutions), albumins, and globulins. Soy protein-based products suffer from a number of technical problems. The moisture absorption properties of soy proteins lead to problems with microbial growth and adhesion stability in soy-based plastics and glues. The other major drawbacks of soy protein, which have severely limited its commercial applications, are its high brittleness. Soy protein contains several kinds of amino acids with hydrophilic groups, which make soy protein products moisture sensitive (Kumar et al., 2002). Without plasticizers, the brittleness of soy protein products makes them very difficult to process (Liu et al., 2007). Because of their brittleness, both fracture strain and strength of pure soy proteins are low. Extensive research has been conducted to modify commercially available soy protein products (Kumar et al., 2008). The physicochemical and functional properties of soy proteinbased products can be modified by physical, chemical, and enzymatic treatments. These treatments include heating, pH adjustment, blending, hydrolysis, and the covalent cross-linking attachment of other constituents. Usually, low-molecular-weight plasticizers are added to proteins to facilitate processing as they help in decreasing attractive intermolecular forces, modifying the three dimensional conformation, and increasing the chain mobility and the free volume (Lagrain et al., 2010; Shi and Dumont, 2014). The functional effects of plasticizers on the properties of protein-based polymers include a decrease in rigidity and barrier properties and an increase in flexibility, mechanical resistance, and elongation properties. Water is the most used plasticizer; however, it has a low boiling point and may not be efficient in lowering the viscosity of materials
when high-temperature processing is required. Therefore, organic plasticizers may be required for high-temperature processing such as extrusion. Examples of compounds that meet these requirements are polyalcohols (polyols) and urea. Two different approaches can be considered for the processing of proteins: wet or dry. Wet processing consists of solubilizing proteins in large amounts of solvent, casting, and drying the material (Micard et al., 2001). It is generally performed under alkaline conditions to unfold proteins (Swain et al., 2004), it is generally slow, requires large volumes of solvents, and thus may be considered commercially less interesting than solid-state or dry processing. Dry processing is indeed a more conventional approach where the proteins are mixed with plasticizers and additives. The mixture is then thermomechanically shaped by compression molding, extrusion, or injection molding. Since proteins are heat sensitive, plasticizers must be added to the protein bulk. Otherwise, proteins can undergo degradation as a result of the deformation of the network under heat treatment, which decreases the chain’s mobility and thus increases the viscosity of the system. The melting temperature of proteins is above their thermal decomposition temperature (Zarate-Ramirez et al., 2011). Therefore, the specific purpose of the plasticizer is to decrease the Tg of proteins (Kumar et al., 2008; Reddy et al., 2010). 13.2.1.2 Zein
The development of bio-based films from proteins for their utilization as packaging materials and composites has gained popularity due to the availability, renewable nature, and biodegradability of proteins. The most studied of these polymers is certainly sourced from soy, however, other sources of proteins show great commercial potential when transformed into films. As such, zein from corn can be plasticized into films that exhibit good mechanical properties. Zein is mainly isolated from corn gluten meal, the by-product of corn wet milling, it is insoluble in water, unless alcohol or alkali (no less than pH 11) or high concentration of urea or anionic detergents or acid (less than pH 2) is present in the solution (Shukla and Cheryan, 2001). In its wet processing, the literature reports that combining plasticizers within the polymeric system can greatly improve the properties of the
resulting matrices. The use of hydrophilic plasticizer may increase the water content of the matrix as it can facilitate water absorption from air, which confers flexibility to the matrices at low relative humidity (RH) as compared to hydrophobic plasticizers. This characteristic may become a severe inconvenience when matrices are exposed to high RH, as the increase in water content renders the matrices very weak (Lawton, 2004). On the other hand, hydrophobic plasticizers decrease the water content of zein matrices and increase their rigidity (Shi et al., 2012). Similar to wet processing, various plasticizers, solvents, additives, and methods were utilized for the preparation of zein films through dry processing. Several studies were done on the properties of moldable resins plasticized with various fatty acids and processed by rolling, extrusion, or hot pressing (Wang and Padua, 2004). Glyoxal was also used as cross-linking agent to improve the physical properties of thermoplastic zein, as it modified the structure by reacting with the functional groups of lysine, arginine, and the N-terminal amine in solution or in the melted state (Selling, Woods, Biswas, and Willett, 2009). 13.2.1.3 Wheat glutenWheat gluten is a protein carbohydrate complex of which proteins are the major component. Two main fractions are present: the first fraction is represented by gliadins, soluble in neutral 70% ethanol, made of single chain polypeptides with an average molecular weight of 25,000-100,000 g. mol-1 by intramolecular disulphide bonds. The second fraction is represented by glutenins, which is an alcohol-insoluble fraction consisting of gliadin-like sub-units stabilized by intermolecular disulphide bonds in large aggregates with molecular weight greater than 100,000 g . mol-1. The amount, size distribution, and macromolecular architecture of glutenins and gliadins greatly influence the rheological, processing, mechanical, and physicochemical properties of the gluten (Domenek et al., 2004; Kim, 2008). As a co-product of the industrial gluten-starch separation, gluten is available in large quantities and used both in the food and in the non-food industry. In absence or at low concentrations of plasticizer, high-temperature compression molded wheat gluten vitrifies into a rigid, glassy material upon cooling (Attenburrow et al., 1990). It was shown
that by plasticizing gluten with glycerol, a malleable phase can be obtained (Domenek et al., 2004; Jerez et al., 2005). 13.2.1.4 Animal proteinsAnother abundant protein that can be used for fabrication of economically feasible bioplastics is the whey protein, which is a mixture of globular proteins isolated from milk as a by-product of the manufacture of cheese or casein (Morra and Haa, 1993). Whey protein has been already explored in packaging applications because of its excellent oxygen barrier properties and abundant availability (50 million ton of annual unprocessed whey) (Schmid et al., 2012). Edible films based on whey protein and casein were also reported to be flavorless, tasteless, flexible and has desirable film forming and barrier properties, which compared well to petroleum based products (McHugh and Krochta, 1994). However, they have low tensile strength and high water vapor permeability (Zhao et al., 2008). Regrettably, the use of biodegradable films for food packaging has been strongly limited so far because of the poor barrier properties and weak mechanical properties shown by kind of natural polymers. For this reason, natural polymers are frequently blended with other synthetic polymers or, less frequently, chemically modified with the aim of extending their applications in more special or severe circumstances. The application of nanocomposites promises to expand the use of edible and biodegradable films that reduce the packaging waste associated with processed foods (Sorrentino et al., 2007). Recently, casein-based biopolymers have already been successfully processed by the film extrusion technique combined with the reactive extrusion by the application of enzymatic cross-linking. However, processing window was described to be narrow due to the necessity of a relatively long residence time of the material within the system. Investigations on the extrusion attempt of casein and the description of the processing issues are mostly related to the food industry or to the production of blends with other biopolymers (Fernández-Gutiérrez et al., 2004).Another source of animal protein for the preparation of biopolymeric matrices is gelatin, a protein resulting from partial thermal or chemical hydrolysis of collagen polypeptide chains from animal skin, bones, and cartilages. For commercial production of gelatin, two processes are mainly used, differing by the pH
conditions; type A gelatin involves an acid extraction, whereas type B gelatin is obtained by an alkaline lime followed by a solubilization at near neutral pH at 60-90°C (Arvanitoyannis, 2002). Gelatin has been successfully used to form casted films that are transparent, flexible, water-resistant, and impermeable to oxygen (Cuq et al., 1998). Although gelatin can be used as a valuable biopolymer in tissue engineering, its poor mechanical properties, especially in the wet state, and its high water sensitivity, revealed by a high water swelling, restrict its application as a structural biomaterial such as osteosynthetic devices. 13.2.2 StarchStarch is one of the cheapest and most abundant agricultural products and it is completely degradable in a number of environments. Starch is mainly extracted from cereals (wheat, corn, rice, etc.) and from tubers (potatoes, manioc, etc.). It is stocked in seeds or roots and represents the main plant energy reserve. It is composed of amylose (poly-a-1,4-D-glucopyranoside), a linear and crystalline polymer and amylopectine (poly-a-1,4-Dglucopyranoside and a-1,6-D-glucopyranoside), a branched and amorphous polymer. The starch granule organization consists in alternation of crystalline and amorphous areas leading to a concentric structure (Fig. 13.3a).
