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FAIR-CT96-1979 Wheat Gluten as Biopolymer for the Production of Renewable and Biodegradable Materials |
Source: Final Report December 2000
Introduction
The increasing demand from consumers and industry for environmentally friendly and renewable polymers explains the great interest for plant macromolecules as substitutes of synthetic, petroleum-based, polymers. Besides starch and cellulose which have been extensively studied, proteins exhibited great potential for biopackaging materials, plastic films, adhesives and disposables. The use of proteins as film forming agents concerns various applications: edible films, coatings, soluble films, biodegradable films. In the present project we want to focus on: - films for food packaging: special attention will be paid to biodegradable films with selective permeability for gases like O2 and CO2 or to water and aroma - plastic films for agricultural use - coatings for different substrates. The use of wheat gluten as a substitute for synthetic polymers appears a very interesting option for many reasons:
Wheat gluten is composed of storage proteins, the prolamins, comprising monomeric gliadins and polymeric glutenins. The latter are made of low and high molecular weight subunits associated by intermolecular disulfide bridges. These proteins form a transient network through non covalent bonds between polymers and monomers. This network is viscoelastic when hydrated. Its theological properties are influenced by the amount, the size distribution and the organisation of glutenin polymers. Monomeric gliadins act essentially as plasticizers.
Previous studies have shown that it is possible to make homogeneous, transparent and strong films from gluten proteins. Research has been done on the formation of edible gluten films for food applications, and on the improvement of the mechanical properties by use of plasticizers and crosslinkers. An expanding field of non-food applications is the development of coatings, adhesives and disposables, which can be biodegradable e.g. for use in the packaging or paper industry.
However, more knowledge is needed on the interdependence between protein composition and structure on the one hand and the processability and the final properties of gluten products on the other hand to reach large scale applications, to tailor the properties.
Objectives
The objective of the project was to investigate the potentialities of native and modified gluten protein fractions to produce biodegradable materials. In this respect research was focused on the use of wheat gluten as films for food packaging, agricultural uses, or coatings for different substrates (paper, cardboards, paints, ...) on one hand and for obtaining thermoplastic materials on the other hand. An effective utilisation of gluten as biopolymer requires to better understand the relationship between protein structures and the macroscopic properties of gluten final products. To this end the main scientific objectives of this program are summarised by the following items:
Activities
The project was divided into five tasks that corresponded to the above objectives.
Results
This project has shown that biomatetials of varying mechanical properties can be prepared from industrial gluten (native or deamidated) and from gliadins and glutenin enriched fractions using either an aqueous casting procedure or thermomoulding. Casting could be performed in aqueous, alkaline or acidic/ethanolic conditions. Depending on the casting conditions, on plasticizer and protein/plasticizer ratio, the stress and strain of films generally ranged from I to 4.5 Mpa and 100 to 800%. For industrial gluten, these values were generally comprised between I and 3 Mpa and 300-500% respectively.
Gliadin films from neutral dispersion showed a higher resistance and a smaller elongation than glutenin films. On -the other hand, gliadin films from alkaline dispersion were less resistant than glutenin films. This might be related to discrepancies in protein conformations at neutral and basic pHs.
In addition when casting of alkaline dispersion was concerned, a single relationship, depending on the plasticizer/protein ratio but not on the protein composition, was found between maximum tensile strength and maximum extension of gluten, gliadin and glutenin films. The empirical relation between tensile strength and Young's modulus was similar to that found for synthetic polymers. Both materials behave in the same way.
The feasibility of thermomoulding of native and modified gluten and of gluten fraction was also demonstrated. Mechanical spectrum of thermomoulded gluten is globally similar to that of synthetic polymers except that transition from glassy to rubbery behaviour extends over a broader range of time. Compared to cast films, thermomoulded gluten films are characterised by lower strain (200-300%) but higher stress (3-5mpa).
Changes of water content of films (40 to 60% relative humidity ) affected greatly their mechanical properties. The glass transition temperature (Tg) for gluten was shown by modulated DSC to be dramatically dependent on the water content. It dropped from approximately 110°C to 10°C when the water mass fraction increased from 2 to 20%. The deltaCp was confirmed to be 0.4J/g/°C . Glass transition of native, hydrophobized gluten and gluten fractions was shifted towards lower temperature with increasing water content. It is noticeable that the thermal parameters of hydrophobized gluten did not differ from those of native gluten. Gliadin-rich fraction displayed, however, a 10°C lower Tg and deltaCp than gluten and glutenin. Gordon-Taylor, Kwei and Couchman-Karasz equations were used to model plasticization of gluten by water. The best fit was obtain with the Kwei equation, probably because it better accounts for secondary interactions between the polymer and the plasticizer. The general viscous behaviour of gluten was described by a simple equation, which might be sufficient to model extrusion of plasticized gluten.
According to all the results obtained during the four years on the influence of gluten composition on film properties, we are able to conclude that, except for films cast from water dispersion, the differences of mechanical properties induced by the process of film preparation were larger than those arising from variations of protein composition and properties due to the wheat genotype, including durum wheat. This means that there is no need for specific breeding as far as uses of wheat proteins in non food film material concerned. Hence wheat unsuitable for bread- or pasta-making or low quality gluten could be used without trouble for preparing plastic films.
To confirm this assumption, complementary experiments were performed to examine the effect of the protein sequences by comparing the filming properties of purified components of prolamins (S-rich and S-poor gliadins, low molecular and high molecular weight glutenin subunits). Preliminary results (only casting of alkaline dispersion) showed some differences between S-poor and S-rich prolamins and between gliadins and high molecular weight glutenin subunits. This suggests that the presence of cysteine and the length and sequence of the repetitive domain are factors influencing the properties of the materials. The successful expression of strictly periodic polypeptides modelled on the repetitive domain of gliadins should help to clarify this structure / function relationship.
Considering the water sensitivity of films, some influence of the film-forming conditions was observed. Thermomoulded film exhibiting a smooth and homogeneous surface and some microstructural differences compared to the more heterogeneous aspect of cast films are characterised by lower water absorption capabilities. Alkaline process with ethanol as co-solvent also appeared very efficient in decreasing the water solubility of the films, owing to covalent linkages between polypeptide chains presumably generated during drying at pH I 1. Surprisingly, protein hydrophobation by mean of covalent grafting did not result in a significant decrease of the sensitivity of cast films to water. Finally, the best compromise was obtained by combining solid fatty acid (myristic acid) mixing with glyoxal mediated cross- linking. Moreover, this treatment which reduced water sensitivity without altering the mechanical properties of the film could be applied more easily and is cheaper than covalent grafting of hydrophobic chains. Despite the rather negative effects of chemical modification on film properties, a great experience has been acquired in the programme on the chemical modification of gluten, which could be exploited for other aims in non-food uses.
Basics concerning the network formation in the biomaterials, were gained through studies on the influence of plasticizer and effects of processes on proteins structure and interactions. The major role of hydrogen bonds in the network formation was clearly demonstrated by using plasticizer varying in structure and length; above two carbons, no effect of the plasticizer chain length was noticed. On the other hand, a threshold value of hydrogen bond ratio supplied by the plasticizer (1.4 hydrogen bonds per 100 g of film) was found above which the mechanical properties of the films remain unchanged. Moreover, from FTIR spectra it could be observed that network formation was concomitant with the appearance of hydrogen bonded anti-parallel P-sheets, whatever the process applied.
The main role of covalent bonds for increasing the stress at break was also highlighted, whatever the cross-linking agents, physical, chemical or enzymatic. Chemical modifications offer a wide spectrum of possibilities to modify either mechanical or surface properties of films. Chemical (formaldehyde, glyoxal) and physical (temperature, UV and gamma radiations) cross-linking formation was induced as a co-treatment or a post treatment of cast films. Temperature (125°C) and formaldehyde vapour showed a similar effect; the creation of new covalent bonds in the network increased tensile strength (x 4-5) and decreased extensibility.
No effect on water vapour transmission (NWT) was observed. Glyoxal had a somewhat different effect. Added at up to 10% to the filming dispersion, it increased the tensile strength (x 2-3) without changing elongation. The water content of the film was also reduced. A gliadin treatment with Mn2+ peroxidase, induced similar effects with an increase of the tensile strength and a decreasing of elongation, as expected. A more specific effect was observed with transglutaminase by grafting diamines to deamidated proteins. In this case both tensile strength and elongation were enhanced. Nevertheless, they did not exceed the values usually obtained with non-modified gluten. The limitation of these experiments was the difficulties to use enzymatic pre-treatments when the forming process is casting, because of the resulting low solubility of the treated proteins. For practical uses, heat or aldehyde treatments are better adapted.
Comparing casting and thermomoulding processes, some drawbacks and advantages have to be considered in both cases. The wet process is difficult to use with hydrophobized proteins and water immiscible hydrophobic molecules, and polymerised insoluble proteins. In this respect the dry process appears to be more versatile. Such mixing was performed in extrusion devices, opening the way to continuous processes. Nevertheless convenient films were prepared by casting. The simplest process involved only dispersion in plain water and drying at moderate temperatures. This is a very clean and environment friendly process. After pH adjustment in the acidic range, varying dispersion and drying conditions makes it possible to modulate the mechanical properties of these films. The dry process results in a shorter range of variation for the film extensibility but with a higher tensile strength. It was not possible to reach the high extensibility obtained casting. This was related to a more extensive cross-linking occurring during pressing at elevated temperature. So, the relationship between maximal tensile strength and maximal elongation established with films cast under alkaline conditions is no longer valid for thermo-moulded films.
For thermal processing, it can be concluded that the whole thermo-mechanical history of samples has to be considered, due to the induced polymerisation and cross-linking of the proteins. A critical temperature for thermoulding was identified around 125°C, where mechanical strength increases and where the characteristic shape of the tensile curve change. Some molecular information could be deduced from the modelling of tensile data by the Mooney-Rivlin approach. The drastic change of thermal sensitivity around 125°C could be explained by the hardening of one polymeric fraction, probably the gliadin fraction, which might inhibit random chain motion. Between 100 and 125°C the gliadin fraction might change its role from plasticizing function to hard filler of the system. Above 150°C , the gluten network is then stabilised through covalent linkages. However the increase of temperature involves steric hindrance effects.
On the basis of all these results, and for demonstrating a technical feasibility, wheat gluten was successfully processed by different techniques such as brushing, casting, and spraying into films and coatings on substrates as glass, polyethylenes and poly(styrene). More precise assays were performed on Leneta testcards using talc, titanium dioxide and carbon as fillers. Very good adhesion was obtained with talc which seems to have the most useful properties as fillers. Thermomoulding of native and modified gluten and of gluten fraction was also performed after controlled thermomechanical treatments either in a mixer or by extrusion. Mechanical spectrum of thermomoulded gluten is globally similar to that of synthetic polymers except that transition from glassy to rubbery behaviour extends over a broader range of time.
If the gluten biomaterials are compared to a representative panel of available polymers, including "agricultural" (starch, proteins ... ) "biotechnical" (PHB, PLA, ...), chemical biodegradable and chemical non biodegradable (LD PE, HD PE, PVC, ...), the range of elongation is very wide and encompasses the whole range of values observed with current polymers. In opposite, variability of the tensile strength is relatively limited, since it seems difficult to exceed 10 MPa. These mechanical properties, although lower than those of commercial polymers, could be sufficient for specific applications if the interesting features, positive and specific characteristics of these proteic biopolymers like biodegradability and elasticity, were to be exploited.
© Copyright 2006 Policy Statements
Updated
by CPL Press
3 July, 2007
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