Molecular Basis of Processing Wheat Gluten toward Biobased Materials

Biomacromolecules 2010, 11, 533–541

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Molecular Basis of Processing Wheat Gluten toward Biobased Materials Bert Lagrain,*,† Bart Goderis,‡ Kristof Brijs,† and Jan A. Delcour† Laboratory of Food Chemistry and Biochemistry and Leuven Food Science and Nutrition Research Centre (LFoRCe), Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, B-3001 Leuven, Belgium, and Molecular and Nanomaterials, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium Received January 4, 2010; Revised Manuscript Received January 27, 2010

The unique properties of the wheat grain reside primarily in the gluten-forming storage proteins of its endosperm. Wheat gluten’s structural and functional properties have led to an expanding diversity of applications in food products. However, its viscoelastic properties and low water solubility also are very interesting features for nonfood applications. Moreover, gluten is annually renewable and perfectly biodegradable. In the processing and setting of gluten containing products, temperature plays a very important role. In this review, the structure and reactivity of gluten are discussed and the importance of sulfhydryl (SH) and disulfide (SS) groups is demonstrated. Wheat gluten aggregation upon thermosetting proceeds through direct covalent cross-linking in and between its protein groups, glutenin and gliadin. Predominant reactions include SH oxidation and SH/SS interchange reactions leading to the formation of SS cross-links. Additionally, thermal treatment of gluten can result in the formation of other than SS covalent bonds. We here review two main technological approaches to make gluten-based materials: wet processes resulting in thin films and dry processes, such as extrusion or compression molding, exploiting the thermoplastic properties of proteins under low moisture conditions and potentially resulting in very useful materials. Gluten bioplastics can also be reinforced with natural fibers, resulting in biocomposites. Although a lot of progress has been made the past decade, the current gluten materials are still outperformed by their synthetic polymer counterparts.

Introduction Of all cereal grains, wheat is the most important human food grain. It ranks second in total production as a cereal crop, with maize being the first.1 Wheat is unique because its flour can form dough that exhibits important properties required for the wider diversity of well-known food products. However, these unique properties have also allowed developing nonfood applications that take advantage of these attributes. The unique properties of the wheat grain reside primarily in the gluten-forming storage proteins of its endosperm. Gluten proteins have specific viscoelastic properties.2 In contrast to the nongluten proteins, they show very low solubility in water or dilute salt solutions. Factors that contribute to their low solubility include a low content of amino acids with ionizable side chains and high contents of nonpolar amino acids and glutamine. The latter has a high hydrogen-bonding potential. Gluten proteins consist of monomeric gliadins with a molecular weight (MW) between 30000 and 60000 and a mixture of glutenin polymers with a MW ranging from about 80000 to several million.2 Although proteins themselves are already heteropolymers, with R-amino acids being their monomer units, the terms monomeric and polymeric refer in this case to the quaternary structure of the proteins. Gliadins represent a heterogeneous mixture of single-chained or monomeric gluten proteins, while glutenin proteins consist of more peptide chains (subunits) associated through interchain disulfide (SS) bonds. Three structurally distinct groups of gliadins, that is, R-, γ-, * To whom correspondence should be addressed. Tel.: +32 16321634. Fax: +32 16321997. E-mail: [email protected]. † Laboratory of Food Chemistry and Biochemistry and LFoRCe. ‡ Molecular and Nanomaterials.

and ω-types, can be distinguished. Cysteine residues in R-type (six cysteine residues) and γ-type (eight cysteine residues) gliadins are located at highly conserved positions and are all involved in conserved intrachain SS bonds. In contrast, ω-type gliadins lack cysteine residues and also contain a very low level of methionine.3,4 While gliadins are readily extractable in aqueous alcohols, glutenin is partly insoluble in most common solvents due to its large size. However, its subunit building blocks have solubilities comparable to those of gliadins. The glutenin subunits (GS) can be obtained by treatment of glutenin with a disulfide (SS) reactive agent such as β-mercapto-ethanol or dithiothreitol (DTT). High molecular weight glutenin subunits (HMW-GS) and low molecular weight glutenin subunits (LMWGS) are distinguished. Cysteine residues occur both in gliadin and glutenin. They either occur as a free sulfhydryl (SH) group, are involved in SS bonds within the same polypeptide (intrachain SS bonds), or in SS bonds between different polypeptides (interchain SS bonds).4 The above shows that the structure of the gluten constituent varies considerably. Of further importance is that also the molecular diversity of the amino acids goes hand in hand with a large variety of possible chemical reactions that differ with respect to their position, nature, and energy. Moreover, as thermoplastic heteropolymers, proteins have the potential to react in cross-linking reactions, yielding a wide range of potential functional properties.5 All these unique structural and functional properties of wheat gluten have led to diverse applications. Besides its presence in many products as a constituent of wheat flour, wheat gluten is also industrially prepared from flour during gluten-starch separation.6 Wheat gluten is extensively used in both food and nonfood applications (cosmetic and hair products, detergents,

10.1021/bm100008p  2010 American Chemical Society Published on Web 02/08/2010

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rubber, and polymer products).7,8 The use of proteins for nonfood applications is part of a trend to produce biodegradable materials with a large range of functional properties. New biodegradable materials not only help reduce waste, they can also offer an alternative for synthetic petrochemical polymers. Wheat gluten has been widely investigated as a protein source because it is annually renewable and readily available as a lowcost raw material. Moreover, wheat gluten plastics are fully biodegradable without releasing toxic products irrespective of the technological process applied for their production.9 Eventually, gluten-based plastics could replace cellulose-based materials and find applications in composite materials, films for agricultural use, or molded objects.5,9 The processing of film coatings or other protein-based materials mostly requires the following three main steps: breaking of intermolecular bonds (noncovalent and covalent, if necessary) by using chemical or physical rupturing agents; arranging and orienting mobile polymer chains in the desired conformation; and, finally, allowing the formation of new intermolecular bonds and interactions stabilizing the threedimensional network.10 A downside to the use of plant-based protein material is that their inherent properties are inferior to those of petrochemicalbased systems. Among the different structure modifications tailoring protein properties, such as water resistance and mechanical performance, modification of the amino acid functional groups through physical (temperature, pressure) and chemical (cross-linking) methods is a very powerful and versatile tool.11 We focus here on the most common ways to alter gluten protein functionality through processes involving heat treatment often combined with pressure and shear. More in particular, we first provide a basic overview of the molecular changes in gluten proteins during thermal processing, which predominantly involves cross-linking. In a second part, we indicate how gluten cross-linking can be exploited to create Bioplastic materials.

Physical and Chemical Changes of Gluten during Processing The changes of gluten proteins during processing have been studied in various model experiments. These model systems focus on reaction conditions necessary for further gluten polymerization and cross-linking, namely, heat, sometimes in combination with shear and pressure. Effect of Heat on Physical Properties of Gluten. Temperature plays an important role and is often essential in the processing for gluten-based products. On a molecular level, conformations of proteins, their polymeric state, and their interaction behavior are affected by temperature. Gluten and its subfractions, gliadin and glutenin, exhibit a glass transition,12 that is, the transition from a glassy to a rubberlike state at a certain temperature (Tg). Literature has reported a broad range of Tg values for dry gliadin, glutenin, and gluten, that is, from 120 to 180 °C.13-15 This Tg decreases with increasing plasticizer (moisture) contents. The phase diagram of glutenin in Figure 1 shows three physical states, namely, a glassy state below Tg, a rubbery state, and a range where the entangled polymers flow and react. At temperatures exceeding Tg, gluten polymers acquire mobility necessary to react.16,17 Whether or not the polymers actually flow in this rubbery state depends on the cross-linking degree. Highly cross-linked polymers do not flow and remain rubbery until they eventually degrade in the heating process. At room temperature, the

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Figure 1. State diagram for glutenin. At moisture contents exceeding 20%, all gluten proteins are in a rubbery state above 0 °C, entangled polymer flow is possible at temperatures beyond 30 °C, reactions take place when temperature increases to 70 °C, and at temperatures exceeding 130 °C, gluten proteins soften [Reprinted with permission from Kokini, J. L.; Cocero, A. M.; Madeka, H.; Degraaf, E. The development of state diagrams for cereal proteins. Trends in Food Science and Technology 1994, 5 (9), 281-288, Copyright 1994 Elsevier (http://www.sciencedirect.com/science/Trends in Food Science & Technology/09242244)].

transition temperatures of both gliadin and glutenin become moisture independent at moisture contents exceeding 20%, which corresponds to full hydration of the gluten proteins.13,15,18 Below 40 °C, thermal treatment affects chain mobility and possibly hydrogen bonds but not the chemical structure of plasticized gluten.19 At increasing temperatures, chemical changes may be induced in the so-called reactive and softening zones (Figure 1). For wet gluten, Guerrieri and co-workers20 observed that, above 45 °C, gluten hydrophobicity changes, indicating unfolding of the gluten polymers, exposing hydrophobic groups, and decreasing the extractability in different solvents. The decrease in extractability in solvents containing denaturing agents, such as sodium dodecyl sulfate (SDS) or urea, is often taken as a measure of the protein aggregation.21 In this reactive state, above 50 °C, cross-links are formed up to 130 °C. Due to these cross-linking reactions, gluten viscosity levels off or increases upon heating.13,22 The glutenin fraction of gluten is more sensitive to extractability loss during heating than its gliadin portion. At temperatures exceeding 55 °C, the molecular size of glutenin increases and, hence, its extractability decreases.23,24 At these elevated temperatures, protein extractability loss is a direct function of the increasing density of network cross-links. As such, gliadins also react with glutenin polymers as cross-linking agents and drastically decrease glutenin polymer mobility.25 In addition to the covalent incorporation of gliadin, physical entrapment of gliadin in the network of glutenin polymers has also been suggested.26 The kinetics of gluten aggregation can be described by a simple first order law.21 The activation energy for the precipitation reactions of gluten proteins, fully plasticized at room temperature, is 172-183 kJ/mol.27-29 At temperatures exceeding 130-150 °C, the polymer softens as a result of molecular breakdown via thermal degradation13,29,30 (Figure 1). Not only increased temperature, but also mechanical shear, involved in the mixing or extrusion of flour or gluten with water or another plasticizer, plays a role in the polymerization of gluten proteins. Shearing or agitating wet gluten likely increases the molecular mobility of proteins and, thus, enhances the exposure of reactive sites.21 This increases the cross-linking reactivity of gluten proteins and lowers the temperature dependence of gluten aggregation.21,26,30,31 Shear stress, from mixing or extrusion,

Processing Wheat Gluten toward Biobased Materials

decreases the activation energy for protein cross-linking at higher temperatures.31 Upon mixing up to 100 rpm in the presence of 30% glycerol, the loss of gluten extractability showed an Arrhenius-type temperature dependence with an activation energy of 33.7 kJ/mol instead of the more than 100 kJ/mol reported for heat-induced gluten protein extractability loss without shear.26,31 The structural and viscoelastic properties of plasticized (wet) gluten can also irreversibly be changed by a combination of hydrostatic pressure (HP) and heat treatment. Pressure can unfold or partially denature proteins even at room temperature.32 Increased HP and heating increase gluten strength and eventually lead to a loss of cohesivity at 800 MPa and 60 °C.33 Effect of Heat Treatment on Chemical Properties of Gluten. In the following paragraph, the chemical changes in plasticized gluten (mostly wet gluten with typical moisture contents above 30%) at elevated temperatures are discussed. Gluten proteins contain a high content of the amino acid glutamine, which is responsible for substantial hydrogen bonding, resulting in noncovalent bonds between glutenin and gliadin at ambient conditions.34 Heating readily breaks hydrogen bonds and, thus, initially decreases the degree to which glutenin and gliadin interact in gluten.20,35 SS bonds play a key role in the further polymerization and the thermosetting of the gluten network during heating. As such, the polymerization of glutenin at temperatures below 100 °C may involve oxidation of SH groups as postulated by Weegels and co-workers18,36 based on their finding that heating leads to additional SS bridges,37 but evidence for SH/SS interchange reactions has also been reported.24,35 Already a small increase in the level of SS bonds can result in a large increase in network formation. Indeed, the formation of a single SS bond between proteins that are originally soluble in the extraction solvent and insoluble proteins suffices to ensure loss of extractability.27,30 Heating to at least 90 °C leads to SS-bond-linked aggregates between gliadin and glutenin. The incorporation of gliadins into the glutenin structure at higher temperatures is reported to result from the SH/SS interchange.24,38,39 This exchange reaction is catalyzed by SH groups and readily occurs in (other) proteins at higher temperatures.40 The free SH group carries out nucleophilic attack on the sulfur atom of a disulfide. This type of reaction is not favored in acidic conditions, with the pKa value of cysteine being about 8.5.38,40,41 Figure 2 illustrates the polymerization mechanism. The reaction of gliadin with the large glutenin molecules leads to a three-dimensional structure and can be fitted successfully with a first-order rate law.27,28,42 The folded conformation of gliadins influences their SS bond reactivity. R-Gliadins have less intramolecular SS bonds than γ-gliadins. This may be the reason for the lower thermal stability of the former,20,42 which is reflected in the differences in activation energy between R(110 kJ/mol) and γ-gliadins (146 kJ/mol) in the presence of glutenin.42 It is postulated that during mixing and extrusion, the SS interchange reactions are mediated by thiyl radicals from the shear-mediated scission of gluten disulfide bonds. Thiyl radicals, instead of free SH groups, would interchange rapidly and account for the formation of additional intermolecular SS bonds between gluten proteins. After heating at 110 °C for 18 h, gluten insolubility can no longer be reversed by the addition of DTT, indicating permanent cross-links other than and in addition to SS bonds.20 Indeed, although most of the gluten cross-linking during heat treatment occurs through formation of additional SS bonds, cross-links between other amino acids may occur. Tilley and co-workers43 reported an increase in dityrosine bond levels after bread baking. At higher pH (g8) and higher


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Figure 2. Model for gliadin-glutenin cross-linking through SH/SS exchange reactions during hydrothermal treatment. (I.1) Heating to the critical temperature (Tc) leads to conformational changes, exposing previously unavailable free SH groups and polymerization of glutenin with oxidation of SH groups. (I.2) Glutenin can link to gliadin at temperatures exceeding Tc through a SH/SS exchange reaction, and the generated free SH group can react further with either gliadin or glutenin [reprinted with permission from Lagrain, B.; Thewissen, B. G.; Brijs, K.; Delcour, J. A. Mechanism of gliadin-glutenin crosslinking during hydrothermal treatment. Food Chemistry 2008, 107 (2), 753-760, Copyright 2008 Elsevier (http://www.sciencedirect.com/ science/Food Chemistry/03088146)].

temperatures (g70 °C), lysinoalanine (LAL) is formed.44-46 LAL is formed primarily at very high pH (g13) through the reaction between dehydrolalanine and lysine residues of gluten proteins.44 Heating proteins in a dry state at neutral pH may lead to isopeptide bonds between the ε-amino groups of lysine residues and the β- or γ-carboxamide groups of asparagine and glutamine residues.46 However, the occurrence and importance of isopeptide bond formation during thermal treatment of gluten proteins is not clear at present. Redox agents impact the SH functionality in proteins. They modify the structure and functional properties of wheat gluten proteins and may affect the capacity of gluten proteins to associate during heating. Reducing (SH containing) agents cause a loss of gluten structure prior to heat-setting. During heatsetting, however, the formation of a network structure is promoted.22,47 This effect can be explained through an increase in the level of free SH groups which initially increases the flexibility of dissociated glutenin chains but later on initiates the cross-linking reactions at high temperatures. After all, free

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SH groups are obligatory intermediates in interchanges with SS groups present in e.g. gliadins.38,47 The addition of oxidants leads to a more heat stable protein structure as a result of the formation of SS bonds.48-50 Removing free SH groups beforehand by addition of an oxidant or a SH blocking agent strongly reduces the extent to which gluten proteins become unextractable during thermal treatment, while, as mentioned above, increasing the levels of free SH by addition of reducing agents has the opposite effect.38,47 Cooling heat-treated gluten favors formation of low energy interactions, such as hydrogen bonds.19,32,51

Functional Applications Two technological approaches can be used to make materials based on proteins. Wet processes lead to film formation. Dry processes, using the thermoplastic properties of proteins under low moisture conditions, lead to materials with different properties.52 To overcome the rather low stiffness and strength properties of the biobased materials these can be reinforced with natural fibers to prepare biocomposites. Whatever the technique used, the processing of wheat gluten generally involves plasticizing, heat treatment (curing), shaping (e.g., extrusion and/ or injection molding), cooling and deplasticizing (e.g., drying) of the proteins. The difference between wet and dry processing lies in the amount of plasticizer that is used during the processing step. In this context, a plasticizer is a small molecule of low volatility which, when added to polymeric materials, modifies their three-dimensional organization, decreases attractive intermolecular forces, and increases free volume and chain mobility. A low melting point and a moderate hydrophobicity are critical characteristics of good wheat gluten plasticizers.41 Glycerol is the most effective plasticizer and imparts flexibility and extensibility to gluten based materials.53 In dry processing the addition or removal of excess plasticizer is avoided as much as possible. The direct consequence is that shaping needs to occur at temperatures exceeding the product Tg. Among others, water is an efficient plasticizer, which explains the terminology adopted here. Dry processing typically happens at water contents below 10%. In wet processing, such high amounts of plasticizer are used that it is often referred to as a “solvent” rather than a “plasticizer”. Wet Processes. Film formation through a wet process is based on separation of proteins from a solvent phase by precipitation or phase changes induced by altering the solvent quality, by thermal treatment, and by solvent removal. The resulting film can be defined as a thin layer of edible material, the purpose of which is to inhibit migration of moisture, gases, lipids, to carry food ingredients, and to improve mechanical integrity or handling characteristics of the food. Biodegradable edible films from wheat proteins are useful for food packaging provided they are flexible, strong, heat sealable, and relatively transparent.54 In that case, they could be used as wraps, pouches, bags, casings, and sachets to protect foods, reduce waste, and improve package recyclability.55 Generally, wheat gluten films have poor water barrier properties but remarkable gas barrier properties (oxygen and carbon dioxide) due to their exceptional gas selectivity. The latter could help preserve fresh and minimally processed vegetables.52 Traditionally, wheat gluten-based films are obtained by casting and drying aqueous alcoholic protein solutions in a thin layer. The solvents used to prepare protein film-forming solutions are generally based on water and ethanol and occasionally also acetone. Dispersing gluten proteins in solvents may require altering of solvent quality by addition of disruptive

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agents, pH adjustment, and ionic strength control.52 Film and film-forming properties of proteins are normally inferior at a pH of the dispersion close to the isoelectric point, which, for wheat gluten, is of the order of 7.5.56 Wheat gluten can be cast from solutions below or above the isoelectric point, but overall, alkaline conditions result in stronger films.56-58 This may be due to increased gluten polymerization at alkaline pH and the formation of irreversible intermolecular cross-links.56,57 The film formation by solvent removal is due to increased polymer concentration in the medium, inducing bonds, and forming a three-dimensional network that, at low solvent content, vitrifies. Casting of gluten probably leads to an open and loose network structure with high thermosetting and cross-linking potential.59,60 In almost all cases, a given amount of plasticizer is left in the final protein-based films and coatings to improve the film flexibility and make it easy to handle. Because of the high glutamine content of wheat gluten proteins, numerous hydrogen bonds between protein chains can be formed. This may well contribute to highly cooperative protein-protein interactions and thus to cohesiveness and low flexibility of unplasticized gluten film.53 The addition of a polar plasticizing agent that breaks the hydrogen bonds overcomes film brittleness, which otherwise would be caused by extensive intermolecular forces.53 Films can also be formed by collecting the skin formed after thermal treatment by boiling protein dispersions. Significant reduction in water vapor permeability, but also an increase in tensile strength and a decrease in the elongation at break, can be obtained with increasing curing temperature and heat exposure time.61 As also mentioned before, severe thermal treatment induces drastic loss of gluten extractability.62 Heating (under alkaline conditions) of film forming solutions denatures gluten proteins. Most likely SH/SS interchange reactions contribute to protein cross-linking by SS bonds that yield better film properties.63 At higher temperatures and pH 11, the formation of LAL is also one of the ways in which irreversible cross-links are introduced in gluten films.57 When the formation or rearrangement of SS bonds is disturbed by the addition of an SH blocking agent, such as N-ethylmaleimide, protein network development is prevented and the film weakens.58,64 However, an increase in free SH groups with the addition of LMW-thiols results in a remarkable increase of strength.58 As such, cysteine can act as a cross-linking agent, promoting SH/ SS interchange reactions that increase both tensile strength and modulus of elasticity.54,62,65 Thermal treatment can effectively be applied to optimize the use and application of gliadin- and glutenin-rich films due to the promotion of cross-linking of polymer chains through SS bonds.66,67 Gliadin and glutenin films differ significantly in physical properties.58 HMW proteins, such as glutenin, generally form films with good mechanical properties.52 Therefore, films from glutenins are stronger and have better barrier properties than films from gliadins or whole gluten. Gliadin films present better transparency but are not water resistant. However, cysteine-mediated polymerization of gliadins improves the water vapor resistance of films to almost match that of glutenin films.68 Aging of gluten films involves oxidation of SH groups and increases tensile strength and stiffness. Although often used, solution casting is rather inefficient and solvent consuming.69,70 Therefore, faster techniques avoiding the usage of solvents and allowing continuous and more efficient processing have been developed. These are discussed in the next part. Dry Processes. Instead of producing gluten-based materials with a wet process based on dispersion or solubilization of proteins, materials can also be manufactured with a dry process


Figure 3. Mechanical properties of a typical, contemporary wheat gluten material after dry processing73 compared to the properties of currently available (synthetic) polymeric materials.

based on thermoplastic properties of proteins under low water content conditions. In that regard, protein-based materials can be shaped by existing plastics processing machinery including thermoforming, compression molding, extrusion (films and fibers), roller milling, or extrusion coating and lamination (traditionally called thermoplastic processing technologies).52,71 Gluten-based materials can now be converted into a usable plastic with properties approaching those of commercial polymeric materials such as polypropylene and epoxies72,73 (Figure 3). Thermoplastic processing is used for efficient large scale production due to the low moisture levels, high temperatures, high pressures, and short times used.55 The thermomolded materials obtained may present a tensile strength higher than that of films casted with the same amount of plasticizer.64 Extrusion is one of the most important polymer processing techniques in use today and offers several advantages over solution casting. Most synthetic plastics, such as low density polyethylene films, are produced in extruders.55 In contrast to what can be observed for thermoplastic materials, gluten viscosity does not decrease upon heating, but rather levels off or increases due to cross-linking reactions, as is obvious from the state diagram illustrated in Figure 1.13,22 Extrusion of gluten proteins is, therefore, in general, only possible in a limited window of operating conditions ranging from the onset of protein flow to aggregation in the reaction zone and eventually extensive depolymerization in the softening zone,25,70 which occurs at rather low temperatures when compared to most synthetic polymers. Gluten materials are thus usually dry processed between 80 and 130 °C and the material properties of extrudates depend on the processing conditions in a quite complex way.25 Formation of the final molecular network involves the dissociation and unraveling of the gluten proteins, which allows both glutenin and gliadin to recombine and crosslink through specific linkages in an oriented pattern.74 Overall, gluten-based materials could be defined as a stable threedimensional macromolecular network stabilized by low-energy interactions and strengthened by covalent bonds such as SS bonds between cysteine residues.5 Moderate plasticization is often necessary for the interaction of proteins to form a continuous network from powdered raw materials.52 As mentioned before, water and glycerol are very common and effective plasticizers of gluten. However, glycerol migrates to the surface during storage, causing wheat gluten films to lose flexibility.11 Other hydrophilic compounds such


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as polyols, oligosaccharides, and lactic acid can also be used to plasticize gluten. However, acidic plasticizers inhibit gluten association by thermal treatment,41 probably due to their inhibition of the SH/SS exchange reaction. In agreement with the observations for solution casting, alkaline conditions enhance intermolecular interactions during dry processing resulting in strong adhesion among the different components in gluten.75 Dry processing of wheat gluten without a plasticizer (except for a low amount of unavoidable water) at 150 °C and 500 kPa for 5 min yields a rigid plastic-like material with high stiffness (1-3.5 GPa, depending on the actual water content) approaching that of epoxy and a reasonable strength (20-50 MPa) when compared to that of other bioplastics.73 However, without a plasticizer, gluten materials can be quite brittle and sometimes difficult to handle.53 The irreversible aggregation reactions of gluten proteins during dry processing result in the formation of a structured and cross-linked network structure in a similar way as can be observed during the curing of epoxy resin or the vulcanization of rubber.29,60,76 As discussed above, cross-linking of wheat gluten is a temperature-controlled phenomenon.5 The increase in treatment (pressing) temperature increases the stiffness in a similar way, as observed for rubber, with increasing crosslinking agent concentrations.77 Increasing the temperature of thermomolding to temperatures up to 150 °C also strongly reduces protein extractability.5,60,73,77-79 As explained above, the variations of extractability with treatment temperature are related to an increasing participation of SS bonds that contribute to the protein network.77 Hence, wheat gluten aggregation upon thermosetting proceeds through direct covalent cross-linking between glutenin oligomers and the gluten macropolymer involving SH/SS interchange reactions.78 Although protein extractability loss mainly occurs due to the formation of SS cross-links, high-temperature thermomolding of gluten can result in the formation of additional covalent bonds (e.g., isopeptide bonds) rendering proteins unextractable.60,78 Thermal energy thus has a strong structuring influence on the protein network and will govern the material properties of gluten products.5,78 In that regard, Tg of gluten increases due to heat cross-linking when it is processed dry.60 Thermomolded bioplastics can also be formed from gliadin- and glutenin-rich fractions separately.80,81 Reducing agents can be applied to reduce the stiffness of plasticized gluten bioresin (by reducing the glutenin SS crosslinks, cfr. supra) and to improve the Young’s modulus of crosslinked bioplastics80 (by promoting the SS-SH interchange reaction during high temperature curing, cfr. supra). The reducing agent cysteine has been reported to promote crosslinking after thermomolding and can improve strength and elasticity of gluten plastics.82 Similarly, thiol-terminated, starbranched molecules increase the cross-link density of compression molded gluten proteins and reduce their water absorption.73 In this regard, a multifunctional macromolecular thiol was designed for gluten cross-linking using poly(vinyl alcohol) esterified with 3-mercaptopropionic acid.11 Biocomposites. The application of bioplastics is severely limited due to their low stiffness and strength properties as well as their strong tendency to absorb moisture. Natural fibers or inorganic particles have been used to reinforce plant proteins to prepare green biocomposites with improved mechanical properties and water resistance.83 Wheat gluten/glycerol-based materials have been reinforced through natural fibers (chitin, cellulose, lignin) from grass, hemp, ramie, bran, or nanoclay particle addition.83-86 Biocomposites have been prepared by conventional blending wheat gluten as matrix, the fiber as filler

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and glycerol as plasticizer followed by thermomolding of the mixture to induce cross-linking of the matrix. Fiber addition increases the composite tensile strength and stiffness but decreases elongation at break. Resulting materials exhibit lower water sensitivity than those from the matrix without fibers alone. Fiber addition does not modify the protein aggregation.83-85 The improvement of material properties is not only the result of a reinforcing effect, but also originates from a deplasticizing effect. Often the fibers exhibit some affinity toward the plasticizer and can absorb it.87 An important parameter affecting composite mechanical properties is the adhesion between matrix and fibers, as a good adhesion ensures a good stress transfer from the matrix to the fiber. Fiber/matrix adhesion can result from a physical adhesion or from a chemical cross-linking. Plasticized gluten behaves as a pressure sensitive adhesive. This implies that it can adhere on a surface under a slight pressure.79 This property is affected by the temperature during processing. Thermal treatment at 100 °C leads to better adhesive properties than a treatment at 130 °C.87 In addition to physical adhesion, chemical bonding can also strongly affect the quality of the interface. Fibers may contain polyphenolic compounds (lignin) on their surface. These compounds have the potential to link to proteins88 and, hence, also to gluten proteins.87

Mechanical Properties of Gluten Materials Figure 3 plots the stiffness (Young’s modulus) of a selection of common synthetic polymers and composites as a function of the elongation at break in a tensile experiment. Highperformance materials display a high toughness, which is the energy absorbed prior to failure and, to a first approximation, results from a combined high elastic modulus and a reasonable elongation at break. Therefore, ultimate high-performance materials should reside in the upper right corner of the graph. On the opposite side, most current gluten materials73,89 are found and are, in this context, outperformed by any of the available synthetic polymer materials, commodity plastics included. Different methods are available to improve their mechanical performance. Increasing the elongation at break of glassy, amorphous polymers (such as gluten) can be realized by adding plasticizers or by introducing small (rubbery) inclusions, which accounts for the difference between polystyrene (PS) and highimpact PS (HIPS).90 Unfortunately, such operations go at the expense of the elastic modulus (Figure 3). This can also be observed for gluten plastics containing increasing concentrations of glycerol (Figure 4). The elastic modulus of isotropic amorphous polymers depends on the molecular characteristics and can be increased by acting on the molecular orientation, increasing the molar mass, or improving intermolecular interactions.90 Therefore, performance improvements may be induced by properly tuning the gluten molecular composition. Alternatively, adding high-modulus fibers readily inflates the material modulus,90 as illustrated for poly(butylene terephthalate), with 30% of glass fibers (PBT 30% GF in Figure 3), but may reduce the elongation at break as seen in glass fiber composites of polyamide 6 (PA6), but also in wheat gluten-based biocomposites.83,85 The performance of polymer-fiber composites is influenced by the polymer matrix and fiber properties, fiber distribution, and interfacial characteristics, as mentioned in the previous part. Although both Young’s modulus and tensile strength increase in gluten biocomposites,83,85 their final material properties (e.g., modulus of 105 MPa, elongation of break of 70% in nanocomposites85) are nowhere near those of conventional synthetic composites as shown in Figure 3.

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Figure 4. Mechanical properties of thermomolded wheat gluten materials after dry processing (0% G)73 and after processing with increasing levels of glycerol (25-40% G)69 in comparison to the properties of some currently available synthetic polymeric materials with similar properties.

Besides the mechanical performance, the heat distortion temperature, TD, is also relevant, as this determines the upper temperature for application.90 For glassy polymers, TD coincides with the glass transition temperature, Tg. The Tg of dry gluten can be 180 °C, but is reduced to less than 60 °C in the presence of 10% of the plasticizer water (Figure 1). Similarly, water acts as a plasticizer to PA6 (Figure 3, PA6 vs PA6 at 50% humidity), leading to an unwanted decrease of TD and the stiffness. Figure 3 also shows that it is hard to find biodegradable polymers at this moment with properties like, for example, HIPS (TD ) 95 °C). However, biodegradable plastics offer specific advantages in medical use and, occasionally, in packaging and waste management. Gluten is intrinsically biodegradable and so should be the additives (plasticizers, rubber particles) and reinforcements (natural fibers or mineral particles) that may be considered to boost the gluten performance to meet the properties of HIPS.

Future Perspectives To convert gluten into materials that, in terms of performance, can compete, for example, with HIPS, research needs to address more than one challenge. Important gaps of knowledge remain on the native structure and functionality of gluten proteins, in spite of the large number of excellent studies dedicated to that topic.91 As described above, polymeric glutenin is a mixture of subunits held together by SS bonds. Because these polymers have molecular sizes ranging up to tens of millions of Daltons, it has been difficult to determine the precise structure and MW distribution.92 Until now, only hypothetical structures of wheat glutenin have been proposed.91,93 Nevertheless, understanding the native cross-linking mechanism is of great value to control end-use quality and processing properties. One of the priorities should be to get more insight into changes of the SS structure beginning from the synthesis of proteins in the growing plant and ending in the processed products. A better understanding of the relations between structure and functionality of gluten proteins would lift the gluten processing research from the current empirical approach to a more rational implementation of process principles. Also, the chemical and physical aspects of gluten processing and the mechanical performance of gluten plastics need to be better understood and improved to be reliable competitors of their synthetic high-performance counterparts. To enhance the intrinsic material properties of gluten bioplastics, both processing and material stiffness, strain to failure, and toughness need to be improved. As already mentioned, the elastic modulus of


gluten materials can be increased by increasing the molar mass of the proteins by introducing cross-links. Gluten network formation during processing has now mainly been attributed to formation of intermolecular SS bonds by oxidation of SH groups of cysteine and SH/SS-exchange reactions. Such reactions lead to further glutenin cross-linking at moderate heating and, at higher temperatures, also involve gliadin. Therefore, controlling the thiol functionality of gluten during processing is essential. In that regard, the use of multifunctional macromolecular thiols as an additive to improve gluten mechanical properties seems very promising.11,73,94 Although of major importance, not all phenomena during thermal processing of gluten can be fully explained by reactions involving SS bonds. Especially gluten interactions at alkaline pH are of particular importance because they clearly enhance the thermal cross-linking of wheat gluten and, hence, also material properties.75 They lead to unreducible covalent cross-links such as LAL. However, further research is necessary to identify all possible unreducible bonds, to understand and control the chemistry behind their formation, and to determine their importance for the final gluten network properties. Another major issue is to avoid the impact of environmental conditions (e.g., humidity) on processing and material thermomechanical performance. Ways to counter gluten plastic hydrobiodegradability are the use of hydrophobic liquids89,95 (e.g., castor or silicone oil) or blending gluten with hydrophobic polymers, such as polyvinylalcohol.11,94 With these treatments, the extent of biodegradability or recyclability of the final material needs to be carefully monitored. Finally, gluten-based materials need to be cheaper or to display other assets (such as biodegradability) than their petrochemical counterparts to penetrate the markets that are currently reserved for products such as from HIPS (toys and product casings). Dry gluten is generally cheaper compared to styrene, the monomer, and, thus, resource of polystyrene and HIPS. Nevertheless, current drawbacks are that, besides the poor performance, current processing routes toward suitable gluten materials are often unpractical and cost inefficient. Therefore, both performance optimization and process efficiency need to be considered. Overall, future challenges include knowledge on the characteristics of gluten proteins, on chemical reactions during processin, and expertise on polymer mechanical performance. Hence, input from organic chemistry, polymer physical chemistry, and polymer processing is required to design and deliver the envisaged high-performance, gluten-based polymer materials. Clearly, this complex task needs to be tackled by a multidisciplinary team. Acknowledgment. B.L. wishes to acknowledge the Research Foundation - Flanders (FWO, Brussels, Belgium). K.B. wishes to acknowledge the Industrial Research Fund (IOF, K.U. Leuven, Leuven, Belgium). This work is a part of the IOF knowledge platform “Wheat gluten: from an agro-industrial coproduct to biobased high performance materials”.

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Molecular Basis of Processing Wheat Gluten toward Biobased Materials

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