InTechOpen uses cookies to offer you the best online experience. By continuing to use our site, you agree to our Privacy Policy.

Materials Science » "Viscoelastic and Viscoplastic Materials", book edited by Mohamed Fathy El-Amin, ISBN 978-953-51-2603-4, Print ISBN 978-953-51-2602-7, Published: September 21, 2016 under CC BY 3.0 license. © The Author(s).

Chapter 5

Plasticizers for Protein‐Based Materials

By N. Henrik Ullsten, Mikael Gällstedt and Mikael S. Hedenqvist
DOI: 10.5772/64073

Article top


Torque (solid line) and temperature (broken line) as a function of mixing time in the Brabender. (a) Glycerol and (b) trimethylolpropane.
Figure 1. Torque (solid line) and temperature (broken line) as a function of mixing time in the Brabender. (a) Glycerol and (b) trimethylolpropane.
Tensile curves.
Figure 2. Tensile curves.
(a) Maximum stress versus modulus and (b) fracture strain versus modulus for the additive/WG materials in Table 5.
Figure 3. (a) Maximum stress versus modulus and (b) fracture strain versus modulus for the additive/WG materials in Table 5.
Modulus versus additive (a) molecular weight, (b) critical molar volume, (c) total polar surface area, and (d) molar refractivity.
Figure 4. Modulus versus additive (a) molecular weight, (b) critical molar volume, (c) total polar surface area, and (d) molar refractivity.
Maximum stress versus additive (a) molecular weight, (b) critical molar volume, (c) total polar surface area, and (d) molar refractivity.
Figure 5. Maximum stress versus additive (a) molecular weight, (b) critical molar volume, (c) total polar surface area, and (d) molar refractivity.

Plasticizers for Protein‐Based Materials

N. Henrik Ullsten1, 3, Mikael Gällstedt1 and Mikael S. Hedenqvist2
Show details


This study presents a practical approach to select plasticizers for proteins. It is a case study on thermoformed wheat gluten, considered here as a model protein, and it involved 30 plasticizer candidates. The approach consisted of selecting plasticisers (30 wt%) based on visual examination, rheological and molding behavior of the dough, and finally tensile data. There was no unique relationship between the torque behavior of the dough and the mechanical properties of the films. Nevertheless, the extensibility and dough analysis indicated that the most promising plasticizers were as follows: glycerol, linear glycols, ethanol amines, diols, and trimethylolpropane. Further, considering also the stiffness, it was concluded that the most efficient plasticisers were those that contained three hydroxyl groups and the linear glycols of intermediate size. Out of those, glycerol stood out as having the highest extensibility and lowest stiffness and strength. In an attempt to predict the mechanical properties of the films based on several physical data of the compounds, it was observed that there was a weak nonlinear relationship between the stiffness/strength and the size (molecular weight/molar volume), polarity and molar refractivity of the compound. The stiffness/strength decreased with an increase in these physical parameters.

Keywords: proteins, wheat gluten, plasticisers, compression molding, mixing

1. Introduction

Protein‐based films have generally low oxygen permeability in dry conditions due to the high amount of hydrogen bonds. However, the hydrogen bonds also make the films brittle in dry conditions, and a plasticizer is needed for the film to have desirable ductility [14]. Several factors must be taken into account when choosing between different possible plasticizers, and the perfect plasticizer is probably not to be found. Plasticizer and protein materials must have similar polarity to be compatible. Insufficient dispersion of the plasticizer in the protein matrix results in a material with properties depending on the internal variation in plasticizer concentration. The molecular weight and chemistry/polarity of the plasticizer affects its diffusion properties and therefore the migration from the protein matrix and the long‐term properties of the protein material. There are numerous reports on plasticized protein‐based films, including matrices from plants such as soy, pea, sunflower, and wheat proteins and zein [5]. Animal‐based protein matrices include sodium caseinate, keratin, gelatin, collagen, and whey and myofibrillar proteins. Plasticisers have also been an important factor to consider in biocomposites [6].

The most commonly used plasticizer for protein films is glycerol, which is miscible in most proteins, but several other plasticizers have also been studied [712]. Examples are polyfunctional alcohols such as sorbitol, propylene glycol, and di‐ and triethanolamine [1, 4, 7, 8, 10, 11, 1320]. Often, more than one plasticizer has been used, for example water and glycerol [21], glycerol and trehalose [22], and glycerol and dendritic polyglycol [23]. An interesting direction toward new types of plasticisers is low‐molar‐mass proteins (hydrolysates). However, as reported by Nuanmano et al. [24] on the plasticization of fish myofibrillar proteins, that glycerol is more effective than the hydrolysates (gelatin‐based) at similar contents. Both glycerol and sorbitol are harmless as plasticizers for films in contact with foodstuff and are also frequently used as sweeteners in foodstuffs [13]. It was interesting to compare the number of papers with the search words “glycerol” and “plasticizer” with those on “phthalate” and “plasticizer”. The total number in the former and latter case was 48,600 and 89,800 (Google Scholar). In the years 1980, 1990, 2000, 2010, and 2015, the papers on glycerol plasticizer were 330, 520, 1270, 3640, and 5150. The same numbers for PVC plasticizer were 800, 1310, 2450, 4490, and 6020. It shows that glycerol, as a plasticizer for bio‐based materials, is investigated to almost the same extent as the phthalates, being a common plasticizer for petroleum‐based plastics (mainly PVC). The interest in glycerol is also increasing with time.

The potential migration of plasticizers is an important aspect to consider when choosing plasticizer. Migration of plasticizers leads to a decrease in fracture strain [11, 12]. Even though the film becomes more ductile with increasing plasticizer content, it is important to keep the content as low as possible because of barrier property issues; the gas/vapor permeabilities, generally, increase with increasing plasticizer content [25]. In general, a compromise between permeability and extensibility has to be made [11, 2628]. In fact, a compromise has to be made also between ductility and stiffness/strength, since they normally go in opposite directions with increasing plasticizer content. Several less hygroscopic plasticizers, such as mono‐, di‐, or oligosaccharides and urea, have also been studied in solution cast films. Amphipolar plasticizers such as octanoic and palmitic acids, dibutyl tartrate and phthalate, and mono, di‐, and tri‐glyceride esters have also been studied in solvent cast protein films such as WG and zein. Still, glycerol and triethanolamine have so far seemed to be the most appropriate plasticisers for films of proteins such as whey or wheat gluten [7, 11, 12]; however, sorbitol is also a commonly used plasticizer [29]. These plasticizers have mostly been evaluated for solution cast films [30, 31], and only a limited number of studies have been reported for thermoformed gluten‐based matrices with plasticizers [3239]. In fact, for the most common plastic processing method (injection molding), only a few studies have been performed on the effects of plasticizers [40, 41].

There is no study, to our knowledge, that compares a very large set of different types of potential plasticizers for thermoformed wheat gluten films. This study focuses on a practical evaluation of potentially interesting plasticizers for use in thermoformed wheat gluten films. The outcome should be valid not just to WG, but also to proteins of different origins like pure or modified whey [42] and blood meal [43] proteins. The number of potential plasticizers is high, and these have to be carefully selected to avoid poor film properties and issues relevant, for e.g., the barrier properties, migration, aging behavior, sealing properties [44], printing properties, and cost. Which properties that are most interesting depend on the specific application, but the thermoforming and mechanical properties are of fundamental importance for most applications. This is also why these properties have been compared and evaluated, as a function of plasticizer type, in this study.

2. Experimental

2.1. Materials

The wheat gluten powder was supplied by Reppe AB, Lidköping, Sweden. The additives added for plasticizing purposes are presented in Table 1. The selection was based on polarity, melting, and boiling temperatures.

2.2. Blending procedure

About 350 g wheat gluten powder and 150 g of plasticizer were blended using a food processor (WATT; DUKA AB, Sweden) at the lowest speed, “Speed 1” (about 95 rpm), for 60 s. About 40 g of the blend was then transferred to a Brabender plasticorder PL2000 with a M50EHT measuring head and kneaded at 50 rpm for 2 min. The torque and the temperature were monitored with Brabender correlation program version 2.2. The starting temperature was set to 50°C.

2.3. Water adding procedure

Gluconic acid lactone, PEG 150, PEG 200, PEG 400, sorbitol, and xylose, respectively, were also mixed with small amounts of water (20% of the plasticizer content) for improved miscibility with the gluten powder. The water used was deionized and was blended in with the plasticizer.

2.4. Visual evaluation of the mixtures

The mixtures were visually evaluated during mixing. The focus was on crack formation, visible separation of the additive and the protein, and also on “apparent” dough brittleness. The visually “most promising” additive/gluten mixtures were chosen for compression molding.

AdditiveTma (°C)Tba (°C)PuritySupplier
1,2‐propanediol-59 187.698% Fluka, The Netherlands
1,3‐propanediol-27 214–21699.6% Aldrich, USA
1,4‐butanediol16 230≥99.9% Merck KGaA, Germany
2‐propanol-89 82.4≥99.9% Sigma‐Aldrich GmbH, Germany
Adipic acid151–154 265≥99.9% Sigma‐Aldrich GmbH, Germany
Diethanolamine28 21799% Sigma‐Aldrich
Gluconic acid lactone117 ≥99.9% Sigma‐Aldrich GmbH, Germany
Glycerol18 29099.5% Karlshamns Tefac AB, Karlshamn, Sweden
Lactic acid53 ≥99.9% Sigma‐Aldrich GmbH, Germany
Myvacet™ 5‐072941–46 Quest International Inc., USA
Myvacet™ 9‐0829-12 to -14 Quest International Inc., USA
Myvacet™ 9‐45294–12 Quest International Inc., USA
Octanoic acid16–17 237≥99.9% Sigma‐Aldrich GmbH, Germany
Diethylene glycol-1024599% Sigma‐Aldrich
Triethylene glycol-727899% Sigma‐Aldrich
Tetraethylene glycol-631499% Sigma‐Aldrich
Octaethylene glycol-4327≥99% Sigma‐Aldrich GmbH, Germany
Ethylene glycol-1319799.5% Merck KGaA, Germany
SAIB™ 90EA30–83°C 78°CEastman Chm. Co., USA
Sorbitol95296≥98% Sigma‐Aldrich, USA
Succinic acid185–187 235≥99.9% Coleman & Bell Co., USA
Trimethylolpropane58292–297≥99% Perstorp Specialty Chemicals AB, Perstorp, Sweden
Triethanolamine21208≥99.9% Riedel‐de Haen GmbH, Germany
Xylose144–145 ≥99% Sigma‐Aldrich GmbH, Germany

Table 1.

Additives that were mixed with the wheat gluten powder.

[i] - aTm and Tb are melting and boiling points.

2.5. Preparation of compression‐molded films

The wheat gluten powder and the additives were blended using a food processor (WATT; DUKA AB, Sweden) at the lowest speed, “Speed 1” (about 95 rpm), for 30 s and thereafter at “Speed 3” (about 200 rpm) for 1 min. Five grams of the mixture was then put in a frame and pressed into 0.5 mm thick films at 120°C for 3 min at a pressure of 100 bar. The compression‐molded gluten sheet was cut into a square following the frame and put on Mylar films for 1 h. The press used was a Laboratory Press Polystat 200T #7105, Servitec Maschinenservice GmbH, Wustermark, Germany.

2.6. Tensile testing

The Young's modulus, tensile stress, and fracture strain (maximum tensile strain) were measured on samples, punched from the pressed gluten/additive films. Ten specimens, with a size described by EP 04/ISO 37‐3 (with a test area of 4 × 40 mm), from each sample were measured by a Zwick Z010 tensile tester using the sensor 0.5 kN, controlled by TestXpert 7.11. A preload of 0.5 N applied with a speed of 100 mm/min was used. The entire tests were also performed at 100 mm/min. The tensile test and the 24‐h preconditioning were performed at 50% RH and 23°C. The average values for each sample are presented.

3. Results and discussion

The molecular structure and properties of the compounds evaluated here are given in Tables 2 and 3. To narrow down the number of potential plasticisers the mixing/compounding and molding properties were first evaluated before going further with mechanical characterization of the compression‐molded films. Very early six different compounds (four different starch syrups, octanoic acid, and pentaerythritol) were discarded due to very poor mixing with WG.

AdditiveMolecular structureVan der Waals volume
Ethylene glycolmedia/ufig3.pngmedia/ufig4.png
Diethylene glycolmedia/ufig5.pngmedia/ufig6.png
Triethylene glycolmedia/ufig7.pngmedia/ufig8.png
Tetraethylene glycolmedia/ufig9.pngmedia/ufig10.png
Octaethylene glycolmedia/ufig11.pngmedia/ufig12.png
Lactic acidmedia/ufig31.pngmedia/ufig32.png
Succinic acidmedia/ufig33.pngmedia/ufig34.png
Adipic acidmedia/ufig35.pngmedia/ufig36.png
Octanoic acidmedia/ufig37.pngmedia/ufig38.png
Gluconic acid lactonemedia/ufig39.pngmedia/ufig40.png
SAIB™ 90EAmedia/ufig41.pngmedia/ufig42.png
Myvacet™ 5‐07media/ufig43.pngR1: fatty acid chains based on 30% palmitic acid and 70% stearic acid
Myvacet™ 9‐08As 5‐07R1: fatty acid chains based on 47% lauric acid, 18% myristic acid, 12% stearic acid, 10% palmitic acid, 13% others
Myvacet™ 9‐45As 5‐07R1: fatty acid chains based on 73% oleic acid (isomers), 18% myristic acid, 13% stearic acid, 10% palmitic acid, 4% others

Table 2.

Additive properties.

Additive (0.43 g additive/g WG)Densitya (g/cm3)Molecular weight
Crit. molar volumeb (cm3/mol)Specific volume (cm3/g)Amount addedc (mol/g WG)Volume addedd (cm3/g WG)Functional groups addede (mol/g WG)Log PftPSAgMRh (cm3/mol)
Ethylene glycol1.20062.1185.50.8250.00690.3550.0138-0.7940.4614.21
Diethylene glycol1.120106.1315.50.8930.00900.3840.0189-0.9549.6925.14
Triethylene glycol1.127150.2445.50.8870.00290.3820.0057-1.158.9236.06
Tetraethylene glycol1.129194.2575.50.8860.00220.3810.0044-1.2668.1546.99
Octaethylene glycol1.130370.41095.50.8850.00120.3810.0023-1.88105.0790.68
Lactic acid1.20090.1240.50.8330.00480.3580.0143-0.5157.5318.65
Succinic acid1.572118.1307.50.6360.00360.2740.0073-0.6474.621.93
Adipic acid1.360146.1419.50.7350.00290.3160.00590.274.631.13
Octanoic acid0.910144.2507.51.0990.00300.4730.00302.4337.339.7
Gluconic acid lactone1.610178.1411.50.6210.00240.2670.0121-2.44107.2235.58
SAIB™ 90EA301.110846.90.9010.387

Table 3.

Additive properties.

[i] - aIf not stated otherwise, the density was obtained from PubChem and at 15–25°C. The value for 1,3‐propanediol was obtained from

[ii] - bCritical molar volume.

[iii] - cAmount of additive added in moles.

[iv] - dAmount (volume) of additive added based on the specific volume and molecular weight.

[v] - eAmount of functional groups added based on the molecular weight and the number of functional groups per molecule.

[vi] - fLogarithm of the partition coefficient in n‐octanol/water. The ratio of the solubility of the additive in n‐octanol and water, the larger the more hydrophobic the additive is. Log P, tPSA and MR obtained from CS Chembiodraw Ultra.

[vii] - gTotal polar surface area.

[viii] - hMolar refractivity.

3.1. Additive/WG blends that were evaluated only qualitatively (visually and by hand) during and after mixing and compression molding, due to poor miscibility/mixing and/or brittleness

Xylose and gluconic acid lactone, with and without water, gave both an inhomogeneous WG dough with a clear phase separation. The resulting compression‐molded films were very brittle with properties similar to the molded pure wheat gluten powder. The use of SAIB™ 90EA [45], which is a low‐viscous liquid with 90% sucrose acetate isobutyrate and 10% ethyl acetate that is used in, for example, coatings and in nail‐polish lacquers, resulted in a homogeneous dough with potential for further development. However, the molded films were too brittle. Three grades of acetylated monoglyceride (AMG, Myvacet™ 5‐07, 9‐08, and 9‐45 [46, 47]) were mixed with WG. The 5‐07 grade (which is an AMG with 50% acetylation) has been used previously in WG formulations, although not with the primary purpose of plasticicizing the film [48]. This grade gave an inhomogeneous dough, and the resulting compression‐molded film was brittle. The mechanical flexibility was too poor to proceed with further film development at this stage. Myvacet™ 9‐08 and Myvacet™ 9‐45 (fully acetylated using, respectively, hydrogenated coconut oil and partially hydrogenated soybean oil as fat sources) did not dissolve in the WG. The resulting molded films fell apart due to extensive brittleness. Lactic acid made the dough highly viscous and sticky, which prevented longer mixing in the Brabender. The pressed films were too soft and sticky to be further tested.

The use of succinic and adipic acid resulted in doughs that were quite homogeneous and viscous. The molded films were at warm conditions (when they were released from the compression molder) flexible. However, they turned out to be inhomogeneous with a significant amount of trapped air bubbles and turned brittle when cooled to room temperature. The 2‐propanol did not yield a dough when mixed with WG. The blend could be described as a wet powder rather than a dough. The compression‐molded films were very brittle and could not be tensile tested. The largest glycol tested here, octaethylene glycol (PEG 400), yielded an inhomogeneous dough and was only very poorly mixed with the WG powder. The film had an uneven surface but parts existed which were homogeneous and smooth. However, the overall impression was, however, that the films were like pressed powder. The compression‐molded films had similar appearance to molded WG powder without plasticizer and were too brittle to be tensile tested.

3.2. Additive/WG blends that were evaluated with respect to rheological and tensile properties

Glycerol serves here as a reference plasticizer due to its well‐known excellent plasticizer efficiency. Its torque and temperature evolution during the 2‐min mixing in the Brabender are given in Figure 1a. The temperature was steadily increasing from 50°C and reached the maximum temperature after 2 min (76°C). The torque was also steadily increasing reaching a maximum torque of 18.4 Nm at the end of the experiment.

The use of ethylene glycol (PEG 60) and diethylene glycol (PEG 100) resulted in doughs that seemed appropriate for, for example, extrusion. The resulting maximum and final torque values were high (similar to glycerol, Table 4), and the PEG 60 mixed faster with the WG powder (a faster rise in torque) then PEG 100. The reason to that is most likely the lower melting temperature of PEG 60. The maximum temperature during the 2‐min mixing occurred close to or at the end of the experiment and was, for both glycols, slightly lower than for glycerol. The resulting compression‐molded films were homogeneous and mechanically flexible. Triethylene glycol (PEG 150) yielded a dough that was sticky and that had a low viscosity/torque throughout the mixing step (Table 4). The resulting temperature increase was also low. An even constantly lower torque, with no temperature increase, was observed when mixing tetraethylene glycol (PEG 200) with WG. Still, a homogeneous dough and a flexible film were obtained.

The diols 1,2‐propanediol, 1,3‐propanediol, and 1,4‐butanediol all gave homogeneous doughs and initially flexible films. However, they became less flexible after 1 day (see the mechanical section below). There were no clear correlation between molecular size and rheological behavior in this group (Table 4). The largest torque values and temperature increase were observed for 1,3‐propanediol, whereas the lowest torque values and temperature increase were observed for 1,4‐butanediol.

Diethanolamine and triethanolamine gave homogeneous doughs and flexible films. The torque remained very low throughout the mixing step and the temperature remained low. A similar torque and temperature behavior were observed for trimethylolpropane (TMP) (Figure 1b), which also resulted in a homogeneous dough and flexible film. Based on the different rheological behaviors and the apparent plasticizer efficiencies of the analyzed additive/WG combinations, it is concluded that it is not possible to predict the plasticizer efficiency based on the torque/temperature behavior during mixing. This is most clearly illustrated in Figure 1 where two of the most efficient plasticisers gave two very different torque/temperature‐time curves.


Figure 1.

Torque (solid line) and temperature (broken line) as a function of mixing time in the Brabender. (a) Glycerol and (b) trimethylolpropane.

AdditiveτmaxaT at τmaxbTime to τmaxcτFinaldT finale
Ethylene glycol17.2711.815.271
Diethylene glycol15.2702.015.270
Triethylene glycol8.4592.08.459
Tetraethylene glycol0.5502.00.550

Table 4.

The thermomixing properties of wheat gluten with plasticisers.

[i] - aMaximum torque (Nm).

[ii] - bTemperature at maximum torque (°C).

[iii] - cTime to maximum torque (min).

[iv] - dFinal torque (Nm).

[v] - eFinal temperature (°C).

In Figure 2, the tensile curves of the compression‐molded films with the most interesting additives are displayed, and in Table 5, their mechanical properties are given. The WG with glycerol showed the greatest fracture strain and the lowest modulus and maximum stress. The curve shape (Figure 2a) indicated a clear yielding (non‐linearity in the curve) before fracture. The scatter in the tensile properties was also among the lowest, indicating good mixing/high miscibility (Table 5). TMP, not tested in wheat gluten before, showed a fracture strain above 100% and somewhat higher modulus and maximum stress as compared to glycerol. The scatter in these parameters was also higher than for glycerol. Triethanolamine and, in particular, diethanolamine were not as effective plasticisers as glycerol and TMP. However, the scatter in data (mixing efficiency) was similar to that of glycerol in the case of maximum stress and fracture strain (triethanolamine). It seemed as if the additives with three hydroxyl‐terminated arms (glycerol, TMP, and triethanolamine) were better plasticizers than the linear diethanolamine.

The range in mechanical properties was large within the oligoethylene glycol family (Figure 2b and Table 5). The highest extensibility (fracture strain) was observed for triethylene glycol (Figure 2b), which value was close to that of TMP. In fact, its modulus was lower than that of TMP, but higher than that of glycerol. The smallest glycol (ethylene glycol) showed poor plasticizing properties with a high modulus and maximum stress and low fracture strain. The scatter in data within this family was lowest for the two most effective plasticisers (triethylene and diethylene glycols). Their scatter in modulus and maximum stress was on the same order as that of glycerol.

As for ethylene glycol, the propanediols and the 1,4‐butanediol had high modulus, maximum stress, and low fracture strain (Figure 2c and Table 5). It is amazing how the plasticizing efficiency increases when a “flexible” ether linkage is inserted in the middle of the molecule (compare mechanical data of 1,4‐butanediol and diethylene glycol, Tables 2 and 4). The effect is somewhat larger than putting an N–H group in the same place (compare 1,4‐butanediol and diethyleneamine). If a third hydroxyl group is placed on the central carbon, a sizeable improvement is observed (compare 1,3‐propanediol and glycerol).


Figure 2.

Tensile curves.

In general, the modulus and yield stress of tough polymers go hand in hand whereas the fracture strain is not a simple function of these [49]. Here, we show that the modulus and strength (maximum stress) are basically linearly related (Figure 3a). For those blends that show a clear yielding, it is also obvious that the stiffer materials also show larger yield stresses (Figure 2). There does, in general, not exist a clear correlation between stiffness/strength and fracture strain when tough and brittle materials of different types are investigated together. A material can be stiff and brittle or stiff and tough. Here, however, the fracture strain decreases in a nonlinear fashion with increasing stiffness (Figure 3b) or strength (not shown), where there is a lesser correlation at higher stiffness and strength. This shows that for the same polymer matrix (WG), the addition of additives that alter the mechanical properties, such as those with a higher or lower plasticizing ability, the four mechanical parameters (modulus, yield stress, maximum stress, and fracture strain) are clearly interrelated.

AdditiveEaR. EdσmaxbR. σeεcR. εbf
Glycerol27 ± 272.4 ± 0.28152 ± 107
Ethylene glycol1532 ± 2181421.5 ± 4.2202 ± 0.525
Diethylene glycol133 ± 1083.4 ± 0.2666 ± 914
Triethylene glycol77 ± 9122.8 ± 0.2799 ± 1212
Tetraethylene glycol206 ± 68334.4 ± 0.92034 ± 2059
1,2‐propanediol1068 ± 1031013.2 ± 0.972 ± 0.210
1,3‐propanediol647 ± 63109.0 ± 1113 ± 0.930
1,4‐butanediol875 ± 5569.9 ± 0.992 ± 0.315
Diethanolamine280 ± 39144.5 ± 0.3716 ± 425
Triethanolamine151 ± 24165.7 ± 0.4778 ± 68
Trimethylolpropane108 ± 21192.9 ± 0.621104 ± 2322

Table 5.

Mechanical properties of wheat gluten materials.

[i] - aYoung's modulus (MPa).

[ii] - bMaximum stress (MPa).

[iii] - cElongation at break (%).

[iv] - dRelative standard deviation of modulus data.

[v] - eRelative standard deviation of stress data.

[vi] - fRelative standard deviation of strain at break data.


Figure 3.

(a) Maximum stress versus modulus and (b) fracture strain versus modulus for the additive/WG materials in Table 5.

It was important to see whether it was possible to predict the plasticizer efficiency, or its trends, from any easily obtained parameter characterizing the additive. The mechanical parameters (Table 5) were correlated with all the physical parameters in Table 3. No specific correlation was observed between the mechanical properties and the density, specific volume, amount/volume of additive added (mol/g WG or cm3/g WG), functional groups added, and log P. In fact, none of the physical parameters in Table 3 could be used to predict the trends in the observed fracture strains. On the other hand, when plotting the size and the modulus against the critical molar volume, a trend was observed (Figure 4a and b). The stiffness decreased in a nonlinear way with increasing size of the additive, with a vanishing correlation at larger additive sizes. The modulus was correlated with the additive polarity, in terms of the tPSA (total polar surface area) (Figure 4c). The correlation was less than with the molecular size; however, two regions were clearly separated, a low polarity region (low tPSA) with high modulus and a high polarity region with low modulus. Finally, in Figure 4d, the modulus was plotted against the molar refractivity (MR), which is also a measure of the polarity (and the size) of the additive. Here, the correlation was similar as for molecular size (Figure 4a and b), with a nonlinear decrease in stiffness with increasing modulus. When plotting the same type of data for maximum stress rather than modulus, the same relationships were observed, which was not a surprise since stiffness and strength were linearly dependent (Figure 5). To conclude, of all the parameters listed in Table 2, only the molecular size and/or the polarity was affecting the mechanical properties (only stiffness and strength) in a systematic way, although the correlation was relatively poor.


Figure 4.

Modulus versus additive (a) molecular weight, (b) critical molar volume, (c) total polar surface area, and (d) molar refractivity.


Figure 5.

Maximum stress versus additive (a) molecular weight, (b) critical molar volume, (c) total polar surface area, and (d) molar refractivity.

4. Conclusions

Of the extensive number of additives/plasticizers that were tested, having different molecular weights, polarity, melting and boiling temperatures, glycerol was shown to be the most efficient plasticizer for thermoformed gluten films. The most efficient plasticisers, considering stiffness and extensibility, were those that contained three hydroxyl groups and the linear glycols of intermediate size. It should be stated, though, that only the short‐term mechanical data were analyzed, and no aging and relative humidity effects were explored. All thermoformed plasticizer/gluten mixtures were studied at equal mass concentration of plasticizer, an issue that can be further elaborated; different plasticizers have different efficiency and should thus also be further studied as a function of plasticizer concentration. Still, several plasticizers in this study seemed to be fully blended with gluten after <2 min of thermomixing. Regrettably, there did not seem to be a unique relationship between the torque behavior and the final mechanical properties of the films. However, there was a weak nonlinear relationship between the stiffness/strength and the size (molecular weight, molar volume) and the polarity/polarizability (tPSA and MR) of the compound. The mixing conditions (i.e. shear forces, time, and temperature) may differ from extrusion; hence, this study should thus be considered as a first attempt to determine appropriate plasticizers for thermoformed gluten films.


Vinnova, the Swedish Governmental Agency for Innovation Systems and the “Glupack” consortium are thanked for financial support.


1 - Muzzarelli RAA, Peter MG. Chitin handbook. Grottammare: European Chitin Society; 1997. ISBN 88‐86889‐01‐1
2 - Anker M. Edible and biodegradable whey protein films as barriers in foods and food packaging [Thesis]. Göteborg: Chalmers University of Technology; 2000
3 - Gontard N, Marchesseau S, Cuq J‐L, Guilbert S. Water vapour permeability of edible bilayer films of wheat gluten and lipids. Int J Food Sci Tech. 1995;30:49–56. doi:10.1111/j.1365‐2621.1995.tb01945.x
4 - Shaw NB, Monahan FJ, O’Riordan ED, O’Sullivan M. Physical properties of WPI films plasticized with glycerol, xylitol, or sorbitol. J Food Sci. 2002;67:164–167. doi:10.1111/j.1365‐2621.2002.tb11377.x
5 - Gómez‐Estaca J, Gavara R, Catalá R, Hernández‐Muñoz P. The potential of proteins for producing food packaging materials: a review. Packag Technol Sci. 2016. doi:10.1002/pts
6 - Jagadeesh D, Kanny K, Prashantha K. A review on research and development of green composites from plant protein‐based polymers. Polym Comp. 2015;1–15. doi:10.1002/pc.23718
7 - Pommet M, Redl A, Morel MH, Guilbert S. Study of wheat gluten plasticization with fatty acids. Polymer. 2003;44:115–122. doi:10.1016/S0032‐3861(02)00746‐2
8 - Pouplin M, Redl A, Gontard N. Glass transition of wheat gluten plasticized with water, glycerol, or sorbitol. J Agric Food Chem. 1999;47:538–543. doi:10.1021/jf980697w
9 - Gennadios A, Weller CL, Testin RF. Property modification of edible wheat gluten‐based films. T ASABE. 1993;36:465–470. doi:10.13031/2013.28360
10 - Sánchez AC, Popineau Y, Mangavel C, Larré C, Guéguen J. Effect of different plasticizers on the mechanical and surface properties of wheat gliadin film. J Food Chem. 1998;46:4539–4544. doi:10.1021/jf980375s
11 - Hernández‐Muñoz P, Hernández RJ. Effect of storage and hydrophilic plasticizers on functional properties of wheat gluten glutenin fraction films. In: Worldpak2002, 13th Proceedings of IAPRI Conference, vol 2. CRC Press LLC. 2002. pp. 481–493
12 - Irissin‐Mangata J, Bauduin G, Boutevin B, Gontard N. New plasticizers for wheat gluten films. Eur Polym J. 2001;37:1533–1541. doi:10.1016/S0014‐3057(01)00039‐8
13 - McHugh TH, Krochta JM. Sorbitol‐ vs glycerol‐plasti‐cized whey protein edible films: integrated oxygen permeability and tensile property evaluation. J Agric Food Chem. 1994;42:841–845. doi:10.1021/jf00040a001
14 - McHugh TH, Aujard J‐F, Krochta JM. Plasticized whey protein edible films: water vapor permeability properties. J Food Sci. 1994;59:416–419. doi:10.1111/j.1365‐2621.1994.tb06980.x
15 - Alcantara CR, Rumsey TR, Krochta JM. Drying rate effect on the properties of whey protein films. J Food Process Eng. 1998;21:387–405. doi:10.1111/j.17454530.1998.tb00460.x
16 - McHugh TH, Krochta JM. Water vapor permeability properties of edible whey protein‐lipid emulsion films. JAOCS. 1994;71:307–312. doi:10.1007/BF02638058
17 - Fairley P, Krochta JM, German JB. Interfacial interactions in edible emulsion films from whey protein isolate. Food Hydrocoll. 1997;11:245. doi:10.1016/S0268005X(97)80053‐7
18 - Shellhammer TH, Krochta JM. Whey protein emulsion film performance as affected by lipid type and amount. J Food Sci. 1997;62:390. doi:10.1111/j.1365‐2621.1997.tb04008.x
19 - Mahmoud R, Savello PA. Mechanical properties of and water vapor transferability through whey protein films. J Dairy Sci. 1992;75:942–946. doi:10.3168/jds.S0022‐0302(92)77834‐5
20 - Coltelli M‐B, Wild F, Bugnicourt E, Cinelli P, Lindner M, Schmid M, Weckel V, Müller K, Rodriguez P, Staebler A, Rodríguez‐Turienzo L, Lazzeri A. State of the art in the development and properties of protein‐based films and coatings and their applicability to cellulose based products: an extensive review. Coatings. 2016;6:1–59. doi:10.3390/coatings6010001
21 - Duval A, Molina‐Boisseau S, Chirat C, Morel M‐H. Dynamic mechanical analysis of the multiple glass transitions of plasticized wheat gluten biopolymer. J Appl Polym Sci. 2016;43254(1–11). doi:10.1002/APP.43254
22 - Perez LM, Piccirilli GN, Delorenzi NJ, Verdini RA. Effect of different combinations of glycerol and/or trehalose on physical and structural properties of whey protein concentrate‐based edible films. Food Hydrocoll. 2016;56:352–359. doi:10.1016/j.foodhyd.2015.12.037
23 - Bozic M, Majeric M, Denac M, Kokol V. Mechanical and barrier properties of soy protein isolate films plasticized with a mixture of glycerol and dendritic polyglycerol. J Appl Polym Sci. 2015;132(1–12):41837. doi:10.1002/app.41837
24 - Nuanmano S, Prodpran T, Benjakul S. Potential use of gelatin hydrolysate as plasticizer in fish myofibrillar protein film. Food Hydrocoll. 2015;47:61–68. doi:10.1016/j.foodhyd.2015.01.005
25 - Tong Q, Xiao Q, Lim LT. Effects of glycerol, sorbitol, xylitol and fructose plasticisers on mechanical and moisture barrier properties of pullulan–alginate–carboxymethylcellulose blend films. Int J Food Sci Techn. 2013;48:870–878. doi:10.1111/ijfs.12039
26 - Ali Y, Ghorpade VM, Hanna MA. Properties of thermally‐treated wheat gluten films. Ind Crop Prod. 1997;6:177–184. doi:10.1016/S0926‐6690(97)00216‐1
27 - Gontard N, Ring S. Edible wheat gluten film: influence of water content on glass transition temperature. J Agric Food Chem. 1996;44:3474–3478. doi:10.1021/jf960230q
28 - Gennadios A, Brandenburg AH, Park JW, Weller CL, Testin RF, Water vapor permeability of wheat gluten and soy protein isolate films. Ind Crop Prod. 1994;2:189–195. doi:10.1016/0926‐6690(94)90035‐3
29 - Jost V, Stramm C. Influence of plasticizers on the mechanical and barrier properties of cast biopolymer film. J Appl Polym Sci. 2015;133(1–9):42513. doi:10.1002/app.42513
30 - Olabarrieta I, Cho S‐W, Gällstedt M, Sarasua J‐R, Johansson E, Hedenqvist MS. Aging properties of films of plasticized vital wheat gluten cast from acidic and basic solutions. Biomacromolecules. 2006;7:1657–1664. doi:10.1021/bm0600973
31 - Olabarrieta I, Gällstedt M, Ispizua I, Sarasua J‐R, Hedenqvist MS. Properties of aged montmorillonite–wheat gluten composite films. J Agric Food Chem. 2006;54:1283–1288. doi:10.1021/jf0522614
32 - Ullsten NH, Gällstedt M, Johansson E, Gräslund A, Hedenqvist MS. Enlarged processing window of plasticized wheat gluten using salicylic acid. Biomacromolecules. 2006;7:771–776. doi:10.1021/bm050822u
33 - Ullsten NH, Gällstedt M, Cho S‐W, Johansson E, Hedenqvist MS. Properties of extruded vital wheat gluten sheets with sodium hydroxide and salicylic acid. Biomacromolecules. 2009;10:479–488. doi:10.1021/bm800691h
34 - Gällstedt M, Mattozzi A, Johansson E, Hedenqvist MS. Transport and tensile properties of compression‐molded wheat gluten films. Biomacromolecules. 2004;5:2020–2028. doi:10.1021/bm040044q
35 - Redl A, Gontard N, Guilbert S. Determination of sorbic acid diffusivity in edible wheat gluten and lipid based films. J Food Sci. 1996;61:116–120. doi:10.1111/j.1365‐2621.1996.tb14739.x
36 - Zárate‐Ramírez LS., Martínez I, Romero A, Partal P, Guerrero A. Wheat gluten‐based materials plasticised with glycerol and water by thermoplastic mixing and thermomoulding. J Sci Food Agric. 2011;91:625–633. doi:10.1002/jsfa.4224
37 - Zárate‐Ramírez LS, Romero A, Bengoechea C, Partal P, Guerrero A. Thermo‐mechanical and hydrophilic properties of polysaccharide/gluten‐based bioplastics. Carbohyd Polym. 2014;112:24–31. doi:10.1016/j.carbpol.2014.05.055
38 - Zárate‐Ramírez LS, Romero A, Martínez I, Bengoechea C, Partal P, Guerrero A. Effect of aldehydes on thermomechanical properties of gluten‐based bioplastics. Food Bioprod Proc. 2014;92:20–29. doi:10.1016/j.fbp.2013.07.007
39 - Zubeldía F, Ansorena MR, Marcovich NE. Wheat gluten films obtained by compression molding. Polym Test. 2015;43:68–77. doi:10.1016/j.polymertesting.2015.02.001
40 - Perez V, Felix M, Romero A, Guerrero A. Characterization of pea protein‐based bioplastics processed by injection moulding. Food Bioprod Process. 2016;97:100–108. doi:10.1016/j.fbp.2015.12.004
41 - Cho S‐W, Hedenqvist MS, Johansson E, Gällstedt M. Injection‐molded nanocomposites and materials based on wheat gluten. Int J Biolog Macromol. 2010;48:146–152. doi:10.1016/j.ijbiomac.2010.10.012
42 - Winkler H, Vorwerg W, Schmid M. Synthesis of hydrophobic whey protein isolate by acylation with fatty acids. Eur Polym J. 2015;62:10–18. doi:10.1016/j.eurpolymj.2014.10.014
43 - Bier JM, Verbeek CJR, Lay MCJ. Using synchrotron FTIR spectroscopy to determine secondary structure changes and distribution in thermoplastic protein. J Appl Polym Sci. 2013;130:359–369. doi:10.1002/APP.39134
44 - Cho S‐W, Ullsten H, Gällstedt M, Hedenqvist MS. Heat‐sealing properties of compression‐molded wheat gluten films. J Biobased Mater Bioenergy. 2007;1:56–63. doi:10.1166/jbmb.2007.006
45 - SAIB data sheet: sucrose acetate isobutyrate. Eastman Chemical Company. USA. 1994
46 - Product sheet: MyvacetTM. Quest International. The Netherlands. 2000.
47 - US patent 2010/0044924 A1. Cap DS. Candle refill kit and method of use.
48 - Gennadios A, Weller CL, Testin RF. Property modification of edible wheat gluten‐based films. Trans ASAE. 1993;36:465–470. doi:10.13031/2013.28360
49 - Hedenqvist MS, Ritums JE, Condé‐Brana M, Bergman G. Sorption and desorption of tetrachloroethylene in fluoropolymers: effects of the chemical structure and crystallinity. J Appl Polym Sci. 2003;87:1474. doi:10.1002/app.11645