PRODUCTION OF SUSTAINABLE BIOPLASTIC
MATERIALS FROM WHEAT GLUTEN PROTEINS
Introductory Paper at the Faculty of Landscape Planning, Horticulture
and Agricultural Science 2011:4
The Swedish University of Agricultural Sciences
Alnarp, October, 2011
PRODUCTION OF SUSTAINABLE BIOPLASTIC
MATERIALS FROM WHEAT GLUTEN PROTEINS
Introductory Paper at the Faculty of Landscape Planning, Horticulture
and Agricultural Science 2011:4
The Swedish University of Agricultural Sciences
Alnarp, October, 2011
© By the author
Figure 1 and 2 are used with the kind permission of Peter Shewry, firstname.lastname@example.org
and the journal Philosopical Transaction of Royal Society B.
Figure 3 is used with the kind permission of Bernard Cuq, email@example.com and also from
the journal of Cereal Chemistry where it was published.
Petroleum-based products are creating a number of environmental problems. Petroleum and
oil resources are also threatened to become depleted due to the massive utilization. Therefore,
it is important to replace the petroleum-based products with products that are instead derived
from renewable resources e.g. the replacement of petro-based plastics with bioplastics can be
a good option. Wheat gluten proteins might be a promising solution to use for production of
bioplastics. Wheat gluten is a cheap by-product from the bio-ethanol industry, thereby largely
available and beside that, these proteins have interesting viscoelastic and thermoplastic
properties. Gliadin and glutenin i.e. the two major types of gluten protein and their behavior
when used to produce bio-based material are discussed in this paper.
The main emphasis of this introductory paper is to highlight the importance of bioplastics
production as a substitute of petro-based plastics as the later type is a risk to environment, land
and water ecosystems. Among renewable sources for bioplastics production the importance of
cereal proteins especially wheat gluten regarding bioplastics production, acceptance, socio-
economical and environmental impacts are also discussed in this paper.
Contents Page No.
Petrochemical products 7
Petro-based plastics 7
Impacts of petro-based plastics 8
Utilization of petroleum and energy resources 8
Emission of heat and green house gases 8
Health hazards 9
Plastics disposal problems and inability of biodegradation 9
Bioplastics-An alternative to petroleum-based plastics 10
Sustainable production 10
Importance of proteins in bioplastics and agriculture packaging materials 12
Wheat proteins and their types 14
Gluten proteins and their properties 15
Structure of α/β -, γ- and ω-types gliadins 15
Low molecular weight glutenin subunits 16
High molecular weight glutenin subunits and their importance 17
Structure of high molecular weight glutenin subunits 17
Genes for gluten proteins 20
Polymerization process of wheat gluten proteins 21
Importance of wheat gluten proteins for bioplastic industry 22
Gluten-based bioplastics 23
Gluten-based bioplastics with improved qualities 24
Processing of gluten 26
Genetic engineering of wheat gluten 27
Use of heterologous expression systems/bacterial system for gluten protein 28
Sustainability of gluten-based bioplastics 30
Bioplastics versus petro-based plastics 33
Social and environmental impacts 33
Reduced CO2 emission 33
Rising fuel prices and depletion of fossil fuels 34
Economic benefits 34
Sustainable activity 34
Challenges for the bioplastics industry 35
Development and progress of the chemical industry in the mid 19th century is directly related
to the discoveries of fossil reserves which provide raw material for the synthesis of all
petrochemical products (Mecking, 2004). Overall, 90% of the raw material for the chemical
industry is currently produced by fossil feed stocks in the form of petroleum and gas. This
puts chemical industry in the third position as a user of oil and gas, while energy generation
and transportation holds the first and second position, respectively (Mecking, 2004). As to
chemical industry, the highest priority area for the utilization of fossil feed stocks is for
polymer fabrication (Mecking, 2004). The annual plastic production will be increased up to
300 million tonnes by the year 2010 (Thompson et al., 2009). This is due to the success of
synthetic polymer to produce plastics for manufacturing of a range of household and industrial
products over the last 50 years.
Petro-based plastics are playing a key role in modern society due to their versatile nature
(Momani, 2009). The petro-based plastics possess a range of divergent properties e.g. they can
be rigid or elastic, breakable or resilient, transparent or brightly colored, and can have many
added advantageous properties (e.g. cheap, recyclable, insulators) (Momani, 2009).
The first polymer, polyvinyl chloride (PVC) was synthesized accidentally in 1838 but was not
fabricated into applicable plastic polymer at that time (Wade, 2006). The first successfully
applicable polymer named bakelite, was synthesized in 1910 by Leo Baekeland
(http://polymer.w99of.com/history-of-polymers/). Due to the possession of a wide range of
properties almost every household as well as almost every construction equipment contain
commodities that are fully or partly made up of petro-based plastic. The huge production and
utilization of petro-based plastics has become a giant threat for the survival of the earth and its
inhabitants. This is due to the fact that petro-based plastics are enormously affecting our globe
due to its recalcitrance and disposal problems (Barnes et al., 2009).
Impacts of petro-based plastics
Utilization of petroleum and energy resources
Plastic production is directly affecting the petroleum consumption due to the fact that tonnes
of plastics are fabricated from petroleum products every year. Many types of plastics e.g.
ethylene, propylene, and styrene are directly extracted from crude oil enhancing crude oil
consumption (Gervet, 2007).
In year 2009, the total world’s petroleum consumption was 98.3 million barrels per day. If this
rate of consumption persists, the known oil reserves which are almost 1.24 trillion barrels will
last for 41 years (Momani, 2009). The amount of oil used to accomplish plastics products is
4% of the absolute oil utilization (Hopewell et al., 2009). Thus, by keeping in view the bulk
petroleum utilization of the world, 4% results in a large amount of oil used for plastic
The European contribution to the World’s plastic production is 25%, resulting in 60 million
tones of plastics per year (www.plasticisland.org).
Significant amount of energy is required for the synthesis of petro-based plastics and plastic-
based products. About 6% of all the energy used by American industries is utilized by the
plastic industry (Momani, 2009). This results into 1.3% heat loss and it in turns make an
addition of 0.5% global warming (Gervet, 2007). Amount of energy used to manufacture
chemicals and petrochemicals from 1971-2004 raised by 206% i.e. 33.6 EJ/year globally
Emissions of heat and green house gas
Beside the release of heat from the plastics industry, also CO2 is released from the plastic
industry further contributing to global warming. The release of CO2 from plastic industry to
the environment was increased by 160% from 1971-2004 resulting in a release of 1.0 Gt/year
(Gielen et al., 2008). As the manufacturing of plastics and plastic products is increasing day
by day it is obvious that CO2 production will also be increasing. Plastic production and
manufacturing produces heat that contributes to the global warming (Nordell, 2003).
Use of crude oil in plastic manufacturing on average has produced approximately 0.38x1014
kWh heat from 1939-2000 and it has reached to 0.49 x 1014 kWh in 2004.
Increased use of petro-based plastics has created many health hazards. The major health risks
that are associated with petro-based plastics are mainly result of monomers present in the
plastics. These monomers are trapped in the polymer matrix during the process of fabrication
and then, under certain conditions e.g. heating, may leak out (Momani 2009). For instance,
styrene, a monomer can leak out from its polymer polystyrene when subjected to heating and
it is assumed to be involved in endocrine disorders and cancer
Another example is the release of bisphenol-A on heating from the thermoplastic polymer,
lexan. Bisphenol-A can be mixed up into food and is carcinogenic as well as can cause
hormone disruption (McRandle, 2004).
Plastics disposal problems and inability of biodegradation
All over the world, considerable quantity of waste streams is produced from manufacturing
and utilization of petro-based plastics. Municipal waste products in US comprise almost 10%
of plastic (Barnes et al., 2009).
The huge amount of plastics that is discarded every year end up in landfills and water. These
dumps of plastics are contaminating almost every ecosystem including marine, fresh water,
terrestrial and deserts posing numerous environmental problems (Thompson et al., 2009).
Plastics are generally resistant to microbial degradation making them even more hazardous to
the ecosystems (Domenek et al., 2004). Further, the presence of plastics everywhere is a threat
to the existence of wildlife. It is estimated that more than 260 species of insects, birds, reptiles
and mammals have perceived disabilities in movement, feeding habits, sterility and even death
due to ingestion of plastics or because they have been intertwined in plastic debris (Gregory,
The health and environmental hazards of petro-based plastics have resulted in prohibition of
use of plastic bags in a large part of the European Union (EU) countries, Australia, China, the
city of San Francisco and the ban was also tried in the California (Mooney, 2009).
Bioplastics- an alternative to petroleum-based plastics
Bioplastics (also known as biopolymers) are derivatives of renewable biomass resources e.g.
plant proteins and starch. These biopolymers can be fabricated in many different organisms
e.g. plants and microbes. These biopolymers do not cause risksdue to the fact these are
biocompatible to the hosts (e.g. polyhydroxyalkonates (PHA) synthesized in bacterias).
Biopolymers synthesized in microbes are mostly lipid in nature and accumulated in the form
of mobile granules and help microbes to survive under stress conditions (Barnard and Sander,
1989; Sudesh et al., 2000).
The scientific community is not only concentrating on exploration of resources that can be a
substitute to petroleum derived plastics, but the focus has also been turning into consequences
of the biodegradability of the plastics. Many research groups are continuing their efforts in
order to investigate options of making bio-based plastics photodegradable. However, such
plastics will not be suitable for landfill disposals as these are continuously exposed to sunlight
(Zan et al., 2006). One of the important objectives of the research related to synthesis of
biodegradable plastics is the creation of biodegradation ability in composters or metropolitan
Sustainable production enhances the quality and quantity of environmentally friendly goods
and services for human kind. This is done through minimal utilization of natural resources,
effective utilization of raw materials with decreased waste production. Sustainable production
is possible only with the collaboration of government, industries and consumers (Falkman,
The requirements for petro-based products are expected to be doubled in upcoming years. So
it is expected that the production of plastics will also increase and it will increase the
environmental pollution as well. In order to tackle with the problem of environmental
pollution, plastic production by utilization of renewable resources should be increased
(Thompson et al., 2009). It will reduce the reliability of people on fossil fuels and other
natural resources which are exhaustible (Willke and Vorlop, 2004). For the sake of
conservation of energy resources and natural raw materials, together with decreasing of the
global warming, it is now the time to replace non-renewable resources with renewable
resources. Some manufacturers are also showing interests in utilization of renewable resources
for plastic production. One example is from one of the largest plastic manufacturers
“DuPont”, aimed to fabricate 25% of their materials by utilizing renewable resources by the
year 2010 (Dupont, 2010). Plant-based bioplastics (e.g. wheat gluten-, ligno-cellulose-, and
cellulose-derived bioplastics) are valuable alternatives to petroleum-based plastics due to their
specific polymer formation and biodegradable abilities (Wretfors et al., 2009).
Proteins are one of the most vital nutrients, being of essential importance for human survival
and life. Proteins are natural heteropolymers made up from 22 different amino acids arranged
in different combinations giving rise to thousands of different proteins (Guilbert and Cuq,
Proteins in wheat and their content and composition i.e. monomeric and polymeric proteins,
amount and size distribution of these proteins, etc. are the main detriments of bread-making
quality (Johansson et al., 1993, 2001; Gupta et al., 1993).
Proteins (plants or animal derived) are the ideal raw material for bioplastics production due to
the presence of many polar and non-polar amino acids providing a broad spectrum of
functional and structural properties (Guilbert and Cuq, 2005). In addition to this, proteins are
easily processable and can adhere to various substrates, so proteins can be utilized to produce
blends or composites of desirable characters (Vaz et al., 2003).
Importance of proteins in bioplastics and agriculture packaging materials
The utilization of proteins for the formation of plastic materials was initially started in 1930’s
and remained active till 1940’s. The principle raw proteins for plastic production during this
era were mainly milk casein, soy and corn zein (Verbeek and Berg, 2009). However, the use
of proteins for plastic production was slowed down because of the discovery of the
opportunities to use petroleum for plastic production. The diverse utilization of petroleum for
a number of products has caused scaricity of these resources at present. This situation has
resulted again in an increased interest for development of proteinaceous bioplastic materials
and agricultural packaging films (Cuq et al., 1998).
Proteins are a competent choice for bioplastics production as the proteins are able to offer
opportunities for a wide range of chemical utilities. Protein serves as an ideal biomaterial with
a possibility of formation of many different kinds of polymer networks (Guilbert and Cuq,
2005). Ability to form polymers with a wide range of functional properties and structural
confirmations makes proteins suitable to form packaging films (Cuq et al., 1998). Proteins
derived from plants are low in cost, renewable, abundant and biodegradable. Therefore, they
are suitable as raw material to be used in the bioplastic industry (Irissin-Mangata et al., 2001).
At present, a vast variety of agricultural and medicinal films are being made from a large
number of plants (gluten, zein etc.) and animal’s derived (collagen, gelatin, myofibril etc.)
proteins. For example films made from corn zein proteins are being utilized to preserve dry
and fresh foods, vitamins and for medicinal purposes (controlled release of active
compounds). One interesting example of such films based on zein protein is “Optaglaze”
commercially produced by Opta Food Ingredients, Inc (Cuq et al., 1998). Mixtures of soy
proteins and phenol formaldehyde resins are also used to make parts of automobiles (Jane et
Toxic impacts of chemical fertilizers are strongly polluting the soil and underground water.
One solution for this problem is to entrap or spread the fertilizers in coatings with
biodegradable materials like proteins (Devassine et al., 2002). There will be many additional
benefits of using protein coated fertilizers; protein can swell and in turns can save a large
number of water molecules. The proteins may also serve as an additional nitrogen source after
degradation at a very low cost (Montain et al., 2004, Yuan et al., 2010).
Wheat (Triticum aestivum L.) is one of the largest staple food source in the world and it is a
universally grown crop (Carter, 2002).
A large part of the grown wheat (90%) is utilized for human consumption and only 10% is
utilized as seeds for next crop and for industrial purposes e.g. for the production of starch,
gluten, malt, dextrose (Hussain and Qamar, 2007). Wheat production reached 600 million
tonnes in the year 2005. Wheat consumption has increased over time and it is predicted that its
production will increase up to 840 million metric tonnes by the year 2020 (Rajaram, 2005).
Wheat provides a number of nutritional components such as protein, carbohydrates, vitamin E
and minerals etc. to the daily diet of the world’s population. Besides the nutritional
importance of wheat, it is increasingly used by the industries. Currently, wheat starch and
proteins are both being utilized to make biofuel and bioplastics respectively (Cuq et al., 1998).
Wheat proteins and their types
According to solubility properties, wheat proteins are grouped into two major classes; non-
gluten proteins and gluten proteins (Shewry et al., 1986). Various protein types and properties
are also listed in Table 1.
Non-gluten proteins are albumin and globulin comprising 15% of total wheat proteins.
Albumins are soluble in water while the globulins are soluble in dilute salt solutions (Obsorne,
Wheat gluten proteins can be further subdivided into two groups based on their solubility and
extractability in alcohols i.e. alcohol soluble (gliadins) and alcohol insoluble (glutenins)
(Shewry et al., 2002; Wieser 2007). Gluten proteins show low solubility in water or dilute salt
solutions due to presence of non-polar amino acids as compared to amino acids with polar
ionizable side chains. Low solubility behavior of gluten proteins is also due to the presence of
high amount of glutamine and proline residues as these residues contain non-polar side chains
(Hernandez-Munoz et al., 2003; Lagrain et al., 2010).
Table 1. Wheat protein classification based on solubility (Osborne, 1924; Aykroyd and
Doughty, 1970; Krull and Inglett, 1971; Wieser, 2007)
Gluten proteins and their properties
Gluten is a cohesive, visco-elastic proteinaceous material with strong thermoplastic properties
which is isolated from wheat (Day et al., 2006). Gluten can be separated out as a byproduct in
the isolation of starch from wheat flour by simply washing the dough with water. Gluten
proteins were first isolated from wheat flour three centuries ago (Bailey, 1941).
Gluten proteins i.e. gliadins and glutenins were also named as prolamins i.e. due to presence
of high number of proline and glutamine amino acids (Gianibelli et al., 2001). Gluten is a
huge and complex network of proteins which can be separated into almost 50 different types
of proteins with two dimensional isoelectric focusing or sodium dodecyl sulphate
polyacrylamide gel electrophoresis (SDS-PAGE) (Wrigley and Bietz, 1988; Shewry et al.,
Gliadins are one of the two protein types that are present in wheat gluten. Gliadins are
monomeric single chained proteins and their molecular weights range from 30,000-60,000
Daltons (Veraverbeke and Delcour, 2002). Gliadins are further divided into three structurally
distinct groups i.e. α/β-, γ- and ω-types (Wieser, 2007).
Structure of α/β -, γ- and ω-types gliadins
The structural differences among the three groups of gliadins are small due to a difference in
one amino acid residue. This amino acid residue difference has arisen by insertion,
substitution and deletion of the amino acid (Wieser, 2007).
Two classes of gliadins i.e. α/β- and γ- gliadins have overlapping molecular weights in the
range of 28000 to 35000 Daltons (Wieser, 2007). Both of these classes have distinct C- and N-
terminal domains. The repetitive sequences rich in tyrosine, phenylalanine, glutamine and
proline are frequently occurring in the N-terminal domain of the α/β - and γ-gliadins. The C-
terminal domains of α/β - and γ- glidin are homologous, without repetitive structures and with
very low amount of glutamine and proline residues (Grosch and Wieser, 1999). However the
C-terminal domains of α/β- and γ- gliadins are rich in cystein residues i.e. α/β- with six and γ-
gliadins with eight cystein residues (Grosch and Wieser, 1999). The structural confirmation of
the N-terminal domain of the α/β- and γ-gliadins is similar to ω-gliadins, both of these groups
have β turns in their secondary structure (Tatham and Shewry, 1985). C-terminal domain of
α/β- and γ- gliadins occurs in the form of α-helix and β-turns in their secondary structures.
ω-gliadins have higher molecular weights than α/β- and γ-gliadins in range of 40,000-55000
Daltons (Wieser, 2007). The ω-gliadins consist of the sequence units PQQPFPQQ and these
units are rich in glutamine and proline residues (Grosch and Wieser, 1999).
Glutenins are the other protein type present in wheat gluten and consist of a mixture of
polypeptides. The glutenin polymer is considered the largest protein in nature (Wrigley,
1996). Molecular weight of glutenin ranges from 8000 to several millions Daltons
(Veraverbeke and Delcour, 2002).
Structurally, glutenins appears as polypeptides, thus peptide chains are interconnected via
inter and sometimes intrachain disulphide linkages (Shewry and Tatham, 1997; Lagrain et al.,
Glutenins are further classified into two subunits; low molecular weight glutenin subunits
(LMW-GS) and high molecular weight glutenin subunits (HMW-GS) (Ye et al., 2006;
Low molecular weight glutenins subunits
LMW-GS comprises 30% of all the wheat gluten proteins (Laszity et al., 2000). The LMW-
GS consist of two domains, the N- terminal domain which is enriched of glutamine and
proline residues and with sequence repetitive motifs i.e. QQQPPFS, and the C-terminal
domain which is similar in structure and amino acid composition to α-, β- and γ- gliadins.
LMW-GS has eight cysteine residues (Grosch and Wieser, 1999). Six of these are forming
intrachain bonds and their positions are homologous to α-, β- and γ- gliadins. Two additional
cysteine residues are unique to LMW-GS and are not involved in any type of bonding
High molecular weight glutenin subunits and their importance
The average amount of HMW-GS in the wheat grain is estimated to be 10-12% (Shewry et al.,
2002; Weiser, 2007). However, the amount can vary due to genotypic variations e.g. gene
silencing, polymorphism etc. (Shewry et al., 2002).
The viscoelastic properties of wheat gluten have to a large extent been attributed to the HMW-
GS. Studies of wheat lines only differing in HMW-GS subunit composition have proven the
role of HMW-GS in determining the viscoelastic properties of the wheat dough (Branlard and
Dardevet 1985; Payne et al. 1988, Shewry et al., 2002). Variation in HMW-GS composition
accounts for 45-70% of the variation in baking quality (Shewry et al., 2002).
The importance of HMW-GS to impart wheat gluten viscoelastic properties also indicate that
transformation of wheat cultivars with increased number of HMW subunit genes can improve
the strength and structure of wheat gluten (Shewry et al., 2002). Moreover, the use of genetic
engineering and transformation of wheat cultivars with multiple copies of HMW-GS can be an
important tool to improve the gluten utilization in the bioplastic industry.
Structure of HMW-GS
The HMW-GS have an extensive rod-like structure of 50-60 nm length (Li et al., 2006) with
three discrete domains. The two N- and C- terminal domains have the form of an α- helix and
posses interchain crosslinks of disulphide bonds due to the presence of cysteine residues. The
third domain comprises a large central repetitive unit with conformation of regular β-turn
repeats (Shewry et al., 1992).
The amino acid composition of the N-terminal domain consists of 81-89 and 104 amino acid
residues for the x- and y-type subunit, respectively. This difference in number of amino acids
residues results in a total of three cystein residues in the x-type and five in the y-type subunits
(Tatham et al. 1984; Van Dijk et al. 1998).
The C-terminal domain of all HMW glutenin subunits has equal number of amino acid
residues i.e. 42 with one cysteine (Shewry et al., 2002). The structure of the HMW-GS
central repetitive domain has been studied extensively by using X-ray crystallography but
these experiments were not able to generate clear diffraction pattern (Shewry et al., 2002).
Hydrodynamic and detailed spectroscopic studies have depicted the formation of β-reverse
turns in the central domain (Gilbert et al., 2000). Further, it has been hypothesized that the
central repetitive unit of HMW-GS has adopted a regular β-spiral structure (figure 1), which
has also been confirmed by molecular modeling, viscometric analysis, and scanning tunneling
microscopy (Shewry et al 2002).
Keeping in view the elasticity of gluten, the loop and train model for the HMW-GS has been
presented by Belton in 1999. This model predicts that there is a struggle for making more
hydrogen bonds between glutamine and water residue upon hydration. As the level of
hydration increases, this competition will lead to the formation of mobile loop structures via
the formation of hydrogen bonds between glutamine and water. The confirmation of mobile
loops segments is in the form of β-turn structures. This result in breakage of many but not all
interchain hydrogen bonds of adjacent HMW-GS, which results in the formation of β-sheets
called as “trains” (figure 2).
Figure 1: Molecular model developed for β- spiral structure based on the amino-acid
sequence of a repetitive motifs of HMW-GS subunit Atoms are shown in white (carbon), blue
(nitrogen), red (oxygen) and grey (hydrogen) (With permission Shewry et al., 2009)
Figure 2: Model for the effect of hydration on the loop to train ratio of HMW-GS subunits.
(a) Low hydration, disordered, close interactions; (b) intermediate hydration, low loop to train
ratio; (c) high hydration, high loop to train ratio (With permission from Shewry et al., 2002
and Journal of Phil. Trans R. Soc. Lond. B).
Genes for gluten proteins
Hexaploid bread wheat species contain three set of genomes i.e. A, B and D. In wheat seed,
nine loci have been found to be involved in the synthesis of gluten proteins.
Three loci Glu-A1, Glu-B1 and Glu-D1 on long arm of chromosomes 1A, 1B and 1D encodes
the synthesis of HMW-GS (Halford et al. 1992; Seilmeier et al. 1991). Each locus with two
genes thus encodes two different subunits designated x- and y-type subunits (Payne et al.,
1987). Due to gene silencing mechanisms, bread-wheat cultivars contain 3-6 HMW-GS.
(Payne et al., 1987; Halford et al. 1992; Seilmeier et al. 1991).
Among the two types of HMW-GS subunits the x-type is considered to be a more important
contributor to enhance the viscoelastic and hydration properties of glutenin proteins than the
y-type subunit (Wieser and Kieffer, 2001).
The same chromosomes i.e. 1A, 1B and 1D also contain three loci Gli-A1, Gli-B1 and Gli-D1
at their short arms. The three major gene families located on these loci are encoding the
synthesis of ω-gliadins, γ-gliadins and LMW-GS subunits (Payne et al., 1982).
The short arms of chromosomes 6A, 6B and 6D carry three loci, Gli-A2, Gli-B2 and Gli-D2.
Each of these loci encodes the synthesis of α- and β-gliadins (Bietz et al., 1976).
Polymerization process of wheat gluten proteins
Wheat gluten is actually a complex mixture of proteins, containing 50-100 different types of
proteins. Several research groups (Sartor and Johari, 1996; Weegles et al., 1996; Feeney et al.,
2003; Wellner et al., 2005; Li et al., 2006) have focused on the understanding of the detailed
polymerization of gliadin and glutenin proteins. However still, it is very difficult to provide a
complete and clear picture about the polymerization process of wheat gluten due to the
complex network of the proteins. The improved understanding of the polymerization process
is important in order to determine the best suitable combination of gluten protein for the
development of bioplastics. The structural confirmation of gliadin and glutenin proteins has
been studied as well. The primary, secondary and tertiary structures have been clearly
depicted (Shewry et al., 2002).
Polymeric proteins, i.e. LMW-GS (with more than one cystein residue) and HMW-GS are
regarded as chain extender proteins and this may be a reason to their strengthening effect on
wheat gluten and may contribute to stability of gluten derived products (Lee et al., 1999).
LMW-GS with only one cystein are also identified and these LMW-GS are known as
polymeric chain terminators (Tamas et al., 1998). In a recent study conducted by Hernandez-
Munoz and co workers (2004), disulphide linkages were introduced in monomeric gliadins.
Gliadins were polymerized with the addition of cystein and effects of this induced
polymerization were evaluated by the functional properties of the derived films. These films
possesed improved water vapor resistance properties due to development of intra- and
interchain disulphide linkages of the gliadin proteins (Hernandez-Munozet al., 2004).
Wretfors et al., (2010) was able to show that addition of diamine in blends of gluten and hemp
fibre increased the protein polymerization.
A complete understanding of gluten protein polymerization is required in order to develop
superior quality of gluten-based plastic products.
Bioplastics can be defined as derivatives of renewable biomass resources, which are largely
biodegradable and may provide similar functional advantages (e.g. packaging materials) as
those of traditional plastics (Song et al., 2009).
At present, there is an increasing interest for the development of biodegradable plastics and
agricultural packaging films derived from renewable biomass (Ye et al., 2006). Biopolymers
(lipids, proteins, and polysaccharides) constitute the principle raw material for bioplastic
production. Bioplastics derived from these biopolymers are biodegradable, renewable and
environment friendly materials as compared to petro-based plastics (Murphy and Bartle,
Importance of wheat gluten proteins for bioplastic industry
Wheat gluten has a wide range of uses in food and non-food industries. Among non-food uses,
gluten is used in production of cosmetics, detergents, rubber and polymer fabrication
(Magnuson, 1985; Bietz and Lookhart, 1996). Gluten forms a soft and elastic solid material
when it is plasticized with the addition of glycerol. Thus, the complex of wheat gluten and
glycerol shows pseudo-plastic properties (Guilbert and Cuq, 2005). Wheat gluten based
bioplastics are harmless and environment friendly biomaterials regardless of the technical
process utilized to fabricate them (Domenek et al., 2004). Moreover, wheat gluten is annually
renewable and a low cost material for utilization in the bioplastics industry (Lagrain et al.,
Wheat gluten possesses good oxygen barrier properties and thermostability when compared to
other renewable materials like starch, cellulose, oils etc. (Krull and Inglett, 1971; Bietz and
Lookhart, 1996; Hernandez-Munoz et al., 2003; Woerdeman et al., 2004). These properties
make wheat gluten proteins a suitable raw material for the production of bioplastics. However,
the water absorbing property of gluten protein material is a problem resulting in a low water
vapor barrier (Cho et al., 2010). The low water vapour barrier might be overcomes by
processing at high temperature which increases the high crosslinking (Pommet et al., 2005).
Another way of reducing water absorption might be to use hydrophobic plasticizers e.g.
palmitic acid chloride and succinic acid (Brauer et al., 2007). Lamination of poly lacticacid
(PLA) to wheat gluten has improved water vapor barrier of the plastics and can be a suitable
solution for packaging of dry foods under moist or dry conditions (Cho et al., 2010).
Amino acid composition, hydration responses, various structural analysis and proposed
structural/conformational models has shown considerable variations in the structure of
plasticized wheat gluten. Due to amino acid compositions as well as nature and energy
variations, a large number of chemical reactions are possible among gluten proteins. Thus,
composition of the protein in the material enhances the possibilities of great functional
variations (Pommet et al., 2003). These variations (e.g. elasticity, cohesiveness,
biodegradability) can be utilized in the bioplastic industry. Both gliadins and glutenins impart
their effects on dough made from wheat flour (Wieser, 2007). The glutenins have been found
the most important determinant of gluten elastic and cohesive properties, while the gliadins
are more viscous and act as a plasticizer upon hydration (Wieser, 2007).
Abundant quantity of wheat gluten is obtained as a byproduct of food processing industry at a
very low cost i.e. less than $1.00/kg (Ye et al., 2006). A significant quantity of wheat gluten is
obtained as a byproduct when wheat is utilized for the production of bioethanol. The market
for the production of biofuel from wheat gluten is expected to increase over the time and so
the availability of wheat gluten will also increase (Cho et al., 2010). At present, wheat gluten
is a readily available raw material with an annual production of almost 500,000 tonnes (Reddy
and Yang, 2007).
The degradation rate of wheat gluten is among the fast degrading polymers and it has been
confirmed experimentally that when gluten derived products were buried in farmland soil,
these were completely degraded in 50 days (Domenek et al., 2004). Due to favorable
properties of wheat gluten to be fabricated into biopolymer, it is becoming a material of
choice for the production of plastics and packaging films (Jerez et al., 2005). Formation of
complex networks with disulphide linkages upon thermosetting is the innate property of wheat
gluten and this property facilitates its processing into films and plastics when it is plasticized
(Cuq et al., 1998).
Amino acids like cysteins and dialdehydes offer crosslinking reactions between gliadins and
glutenins. Water resistance and tensile strength of casting films can be improved by these
crosslinking reactions. Crosslinking is useful in order to lower the water permeability of
gluten-based bioplastics (Hernandez-Munoz et al., 2003, 2004).
In thermoplastic processing, bioplastic production require a complete premixing of all the
components including biopolymer (e.g. protein), water and plasticizer to obtain a dough like
material (Jerez et al., 2005). Bioplastics have been prepared from gluten (Domenek et al.,
2003; Gomez-Martinez et al., 2009; Song et al., 2009), gliadin-rich fraction and glutenin-rich
fraction (Hernandez-Munoz et al., 2003; Song and Zheng, 2008; Song et al., 2009) by using
different plasticizers like glycerol (Domenek et al., 2003; Sun et al., 2007) and water
(Domenek et al., 2003; Gomez-Martinez et al., 2009). Wheat gluten films and plastics without
the addition of plasticizers are delicate and brittle. The use of plasticizer contributes to the
elasticity and extensibility of gluten protein plastic materials. Plasticizers impart these effects
by dropping intermolecular forces and by increasing the mobility of polymeric chains
(Gontard et al., 1992; Gennadios, 2002).
Hernandez-Munoz et al., (2004) has developed gliadin rich films with improved water barrier
properties. This has been done by cleaving the intramolecular disulphide bonds and
rearrangement of structures by the formation of intermolecular disulphide bonds.
Hernandez-Munoz et al., (2003), has found that biodegradable wheat films obtained from
glutenin rich fractions are stronger, possess higher tensile strength and lower water vapor
permeability properties as compared to those derived from gliadin rich fractions.
Gluten-based bioplastics with improved qualities
Diverse characteristics can be imparted to the bioplastics according to processing conditions
and chosen formulations. Gomez-Martinez et al., (2009) has shown that an increase in
compression-molding temperature results in bioplastics having improved elastic properties
with higher viscoelastic modulus. If the bioplastics are being designed to use in agriculture
industry, addition of citric acid is advantageous to increase water absorption capacity (Gomez-
Martinez et al., 2009).
Preferable methods for protein processing are extrusion and compression-molding, providing
fast routes for production of bioplastics (Ullsten et al., 2006). However, the processing
window of gluten is limited for these methods and has to be improved in order to make the
technique more useful. Ullsten et al., 2006 has increased the processing window of wheat
gluten by the use of salicylic acid (SA). SA possesses radical scavenging activities. It
slowered the rate of crosslinking reactions and enhanced the processing window (Ullsten et
al., 2006). SA possesses germicidal activity in addition to radical scavenging activity and
expected to enhance the shelf life of gluten-based products (Brabias and Swiatek., 1998).
Increase in molding temperature from 25 to 125 Co has been shown to have promising effects
on the three dimensional structural networks of gluten by increasing protein crosslinking
density via disulphide linkages (Sun et al., 2007). Addition of tri-thiol can also strengthen the
delicate wheat gluten (Woerdeman et al., 2004).
The choice of plasticizer is important and some plasticizers cause aging of the protein-based
films due to loss over time (Olabarrieta et al., 2006). Blends of natural polymers (proteins,
starch) and synthetic polymers (polycaprolactone, polylactic acid) provide an opportunity to
produce biodegradable bioplastics without the addition of plasticizers (Ramkumar et al.,
1996). Synthetic and natural polymers, when to be blended, must be compatible in order to
manufacture products with superior properties. John et al., (1998) has produced biodegradable
blends by mixing wheat gluten and modified polycaprolactone (PCL). PCL is a natural
aliphatic polyester and it is biodegradable. PCL was modified by incorporating a reactive
functional group ‘anhydride’. Anhydride increases the compatibility and reactive blending of
PCL with proteins. The gluten composition in these blends was 65% and 75%, fixed by
weight. The blends of gluten and modified PCL showed improved physical properties
(morphology, viscosity, biodegradability) over the simple mixtures of gluten and PCL. The
materials made from these blends showed stable characteristics when studied under extreme
conditions of temperature in oven and freezer (John et al., 1998).
Cho et al., (2010) has developed compression-molded glycerol plasticized wheat gluten films.
These films were laminated with polylactic acid (PLA). In addition to mechanical
strengthening, PLA coating imparts two additional benefits to glycerol plasticized wheat
gluten i.e. good oxygen barrier properties and prevention of loss of glycerol plasticizer over
time (Cho et al., 2010).
Processing of gluten
Polymer processing can be defined as the mixing and shaping of raw materials to convert
them into required products with suitable properties according to end use purpose (Verbeek
and Berg, 2009).
Processing routes to derive bioplastics from biomaterials (proteins, starch) with similar
functional properties as petro-based plastics is important to understand. Processing, which is
mostly accomplished by the application of heat and pressure is mainly dependent on the
nature of the biomaterial and whether it is a thermoplastic or a thermoset material. Wheat
gluten proteins possess thermoplastic properties. In a thermoplastic process, the biomaterial is
first degraded or melted and thereafter shaped according to the requirement and finally cooled
to set it into its new form (figure 3). Heat source for melting can be provided by radiations,
conduction or mechanical work (Verbeek and Berg, 2009). Two key methods i.e. solvent-
based and dry processing techniques are mainly employed for protein processing.
Commonly utilized thermoplastic techniques are extrusion, thermo-forming and injection
molding. Extrusion is a dry processing method and it is the most widely used technique for the
gluten-based bioplastic production. Jerez et al., (2005) has compared the properties of wheat
gluten films and bioplastics developed through a thermo mechanical and casting process.
Bioplastics and films processed by thermo mechanical techniques possess higher thermal
susceptibility as compared to casting processed bioplastics. However, bioplastics obtained
from either of the process, possess a similar mechanical spectra i.e. microstructure and
rheological behaviour (Jerez et al., 2005).
Figure 3. Thermoplastic processing of wheat gluten to form agro-packaging materials
(Reproduced with permission from Cuq et al., 1998 and Journal of Cereal Chem.)
Genetic engineering of wheat gluten
Genetic engineering techniques have been used to modify the properties of wheat gluten and
its subfractions gliadins and glutenins. These techniques are used to increase or modify the
expression of gluten proteins according to the need of its utilization and end products (Shewry
et al., 2002).
Rooke et al., (1999) has developed a transgenic wheat line B73-6-1 with multiple genes of
1Dx5 encoding HMW-GS. The result is a four fold increase in expression of HMW-GS and
gluten protein resulting in a dough with more strength and elasticity (Rooke et al., 1999).
Increased expression of 1Dx5 protein leads to formation of a highly crosslinked protein
networks with increased gluten strength and elasticity (Popineau et al., 2001). Mutant forms
of wheat gluten have been created to study the detailed structural and functional
characteristics of total gluten. At present, there are several studies conducted to evaluate the
synthesis of novel peptides based on gliadins and glutenin repeat motifs by the application of a
heterologous expression system. As the HMW-GS is mainly related to elastic properties of
wheat gluten, it is a good target for modifications and genetic engineering of wheat proteins
(Vasil and Anderson, 1997). The clear genome, amino acid sequence and structural analysis of
HMW-GS have facilitated the construction of mutants and their study in expression systems.
Use of heterologous expression systems/bacterial system for gluten protein production
Heterologous expression systems are used to study the structural and functional properties of
proteins. These are the expression systems of a protein into an organism from which it is not
originated (Tamas and Shewry, 2006). Traditionally, three types of expression systems as a
host are being used for recombinant protein synthesis viz; E.coli, yeasts and cultured cells
(Tamas and Shewry, 2006). The expression system must be able to grow readily in culture and
should be able to reproduce proteins in bulk quantities for commercial applications.
Galili, (1989) has developed a high level expression system for the production of wheat
glutenin in E.coli by the application of pET vectors. Maruyama et al., (1998) has developed
expression systems for the production of α-, γ- and ω-gliadins and LMW and HMW-GS
subunits. This system is very useful for the expression of repetitive subunits of gluten protein.
Thompson et al., (1994) used baculovirus expression system for the production of gluten.
However, E.coli based expression systems are preferable due to ease of its use, cost
effectiveness and availability of large number of host strains. They give high yield and fusion
technology can be applied easily. Presence of inclusion bodies in E.coli can also facilitate the
protein purification. E.coli expression system is not involved in unnecessary protein
modification like glycosylation and post translational modifications. Clarke et al., (2003) has
used E.coli with pET11d vector for the production of LMW-GS. HMW-GS expression studies
have been carried out by several workers (Anderson et al., 1996; Feeney et al., 2001) for
detailed structural studies. pET vectors have also been employed by Elmorjani et al., (1997)
for the expression of gliadin motif (Pro-Gln-Gln, ), about 32 copies of the motif was expressed
in this expression system (Elmorjani et al., 1997). Detailed overview of the past studies
related to expression of recombinant gluten protein subunits have been given in table 2.
However, it is notable that until now, no research group all over the world has developed an
expression system for the bulk production of wheat gluten.
Table 2: Overview of important studies of expression system used and further characterization
of recombinant proteins (Reproduced from Tamas and Shewry, 2006).
Blechl et al., (1992)
Pratt et al., (1991)
Patacchini et al.,
7% of total
D’Ovidio et al.,
Bekes et al.,(1994)
and Dowd and
Buonocore et al.,
and Gilbert et al.,
Elmorjani et al.,
and Sourice et al.,
CD and FT-IR
Feeney et al.,
Wellner et al.,
Biocomposites are blends of two biomaterials, one is a biodegradable polymer and the other is
a biodegradable filler. Biocomposites are made in order to achieve improved performance,
which is not possible by either of the component alone. There is considerable interest of
several research groups to make biodegradable composites from biopolymers like starch
(Gaspar et al., 2005) and wheat gluten (Yang et al., 2011; Ye et al., 2006).
Ye et al., (2006) has manufactured biodegradable composites of wheat gluten and basalt
fibres. El-Wakil, (2009), has studied the formation of biocomposites formed by the
combination of wheat gluten, alkalized lignin and sodium silicates. The resulting materials
exhibits increased tensile strength, uniformity, low thermal expansion and high glass transition
Kim, (2008) has developed a new technology for the formation of biocomposites at the room
temperature without the need of extrusion or processing at high temperature. It requires very
minute amount of biomaterials like wheat gluten. This is possible by utilizing the strong
adhesive properties of corn protein ‘zein’. This technology saves time, energy and cost of
production due to minimal utilization of resources (Kim, 2008). Yang et al., (2011) has
studied and prepared biocomposites materials by mixing wheat gluten and rice proteins. In
this study, reducing and crosslinking agents were used to improve the crosslinking and tensile
strength of the two blended proteins.
Sustainability of gluten-based bioplastics
As in the initial sections related to petro-based plastics, sustainability problems of petro-based
plastics have been clearly depicted. Plastics, derived from fossil fuels are largely
unsustainable due to their social, environmental and health damaging effects (Poole et al.,
2009). So, considerable efforts are made on the development of sustainable bioplastics
production from biomaterials (Nordhoff et al., 2007). To be regarded as a truly sustainable
resource, the biomaterial must possess the following properties.
• Limited utilization of resources (energy, cost, biomaterial)
• Material should be renewable
• Should be biodegradable or compostable
• Should be able to produce locally to avoid environment and economic effects of
• Must possess sustainable character throughout the lifecycle of material; growing of
biomass resource, polymer production, conversion to biodegradable plastic product,
end user consumption (Sustainable bioplastic guidelines, 2007).
As described in figure 4, sustainability of bioplastics must be depicted at all the three levels;
Social, economic and environment. So, the gluten as a resource material and its derived
biodegradable films and bioplastics could be sustainable as they confer no long term effects
on environment and could be adorable at social and economic levels.
Figure 4: Concept of sustainability at economic, social and environmental level
Bioplastics vs. petroleum-based plastics
Bioplastics must confer some advantages in order to be used as commodity products over
petroleum derived plastics. Table 3 represents a brief comparison of bioplastics with
petroleum derived plastics as related to sustainability.
Table 3: Comparison of bioplastics with petroleum derived plastics (Industrial use for
crops: Bioplastics www.hgca.com)
Yes or partially
Break down in the
Some degradable by
Biopolymers (lipids, proteins,
starch etc.), Bacterial polymers
Green house gas emissions
Utilization of fossil fuels
Agriculture land utilization
Expected to increase
Social and environmental impacts of bioplastics
Utilization of bioplastics has a number of advantages over petroleum-based plastics.
Bioplastics confers important beneficial effects both at the social and environmental level.
Some of their impacts and benefits to the environment and communities are listed below.
Reduced CO2 emission
Petroleum derived synthetic polymers and plastic products are a big source of pollution, so
biodegradable plastics derived from renewable biomass resources are an excellent alternative
(Sun et al., 2008). The amount of CO2 released from one metric ton of bioplastic is 0.8-3.2
metric tonnes less than that released by petroleum derived plastics (Heath, 2007).
Rising fuel prices and depletion of fossil fuels
Reliance on fossil fuels for various industrial and domestic purposes is increasing. As a
consequence their cost is increasing and their availability as a raw material will be decreased
soon. So, development and innovations in bioplastic industry is essential in order to cope with
the shortage of fossil fuels (Garrain et al. 2007). It is estimated that if the total amount of
plastic which is utilized by the world is replaced with bioplastics, a total of 3.5 million barrels
of oil reserves can be saved per day (Momani, 2009).
Bioplastics are economically beneficial because they can be fabricated by utilizing the
available machinery used to synthesize the traditional plastics (Gomez-Martinez et al., 2009).
Production of bioplastics can be cost effective if the amount of biopolymer produced in plant
is high and the biomass which is left over is utilized for energy generation (Kurdikar et al.,
2001). Gluten-based bioplastics are highly cost effective due to low price and abundant
availability of the gluten and the option to utilize the existing plastic processing machinery. It
is predictable that price of bioplastics and its derived products will keep on decreasing with
time due to its competition with conventional plastic industries and development of new
processing routes (European bioplastics, 2008). The bioplastic industry can also contribute to
create the new job potentials and can boost up the rural economy due to increasing demand of
Bioplastics are debated strongly to be sustainable, however there processing toward
sustainability is slow. It may be due to the fact, that raw material for bioplastic production is
derived from crops, so at present it is in limited supply as compared to raw material of
conventional plastics. The use of genetically engineered plants and bacteria for the raw
material production is arguable for public acceptance as they may create instability in
ecosystems (Gaskell et al., 2006).
Biodegradation of bioplastics means degradation of materials in nature by the action of
microbes via enzymatic reactions (Mostafa et al., 2010). The use of biopolymer at industrial
level can be approved more environment friendly when compared to synthetic petro-based
polymers due to its biodegradability (Grifin, 1994). Different biopolymers undergo different
changes in the biodegradation process according to their chemical structures and the type of
soil in which they are buried (Mostafa et al., 2010).
Challenges for the bioplastic industry
Bioplastics also possess some disadvantages and at present are not providing a perfect solution
to the problems created by petroleum derived products. The most challenging point for
bioplastic production is not to violate the potential food sources. This obligation can be
overcome by utilizing the non-food resources for the purpose. These are called as second
generation bioplastics. However, these must be processable via common processing routes
like extrusion, compression and injection-molding (Verbeek and Berg, 2009). Some
bioplastics (e.g. derived from bacterial polymer polylactic acid) are only biodegradable in
controlled conditions of temperature and humidity. This limitation must be overcome and
bioplastics must be able to degrade in landfills (Matsuura et al., 2008). Cost of aliphatic
polymers like polylactic acid must be reduced (present cost is between $2 and $5/1b) in order
to compete with synthetic polymers (Yang et al., 1996). However, agriculture raw materials
like wheat gluten, starch, corn zein and soy proteins are cheap and available in large quantity,
but plastics produced from them are still brittle, highly viscous and hydrophilic. Therefore,
they must be produced with a plasticizer (John et al., 1998). Bioplastics, when subjected to
biodegradation under anaerobic conditions release methane in landfills. In order to compete
with the problem and to produce valuable composts for the soil improvement, bioplastic
products should be collected separately from other non-biodegradable materials and then can
be composed at industrial level (Song et al., 2009).
I am extremely grateful to almighty Allah, The supreme power, Who conferred upon mankind
the knowledge and sagacity and enable me to write this manuscript. Countless praises for
Prophet Muhammad (PBUH) who enabled me to recognize my Creator.
I am cordially thankful to my cooperative, encouraging and supporting supervisors Prof. Eva
Johansson and Ramune Kuktaite for their valuable comments and suggestions to improve this
I am also thankful to my colleague Ali Hafeez Malik for his positive criticism, consistent help and
kind guidance during the writing of this paper. Many thanks for William Roy Newson for
photographic assistance to make the cover picture of this introductory picture.
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