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New plasticizers for wheat gluten films
European Polymer Journal 37 (2001) 1533±1541
www.elsevier.nl/locate/europolj
New plasticizers for wheat gluten ®lms Josiane Irissin-Mangata a, Gerard Bauduin a, Bernard Boutevin a,*, Nathalie Gontard b a
ENSCM ± Laboratoire de Chimie Macromol eculaire, UMR CNRS 5076, 8 Rue de LÕEcole Normale, F-34296 Montpellier Cedex 5, France b Unit e d'Ing enierie des Syst emes Macromol eculaires LGBSA, cc023 Universit e Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 5, France Received 14 November 2000; received in revised form 2 February 2001; accepted 14 February 2001
Abstract Diethanolamine and triethanolamine were selected among several low and high molecular weight polyols and amines tested as potential plasticizers of wheat gluten ®lm. In comparison with glycerol, their use did not signi®cantly aect the solubility in water, the opacity of the ®lm (both increased very slightly) and the water vapor barrier properties. However, signi®cant changes in mechanical properties were observed, i.e., increase in extensibility and elasticity. At 58% relative humidity (RH) and 20 g/100 g dry matter of di or triethanolamine, the elongation was ®ve times higher than ®lm plasticized with glycerol. At 98% RH, water appeared to compete with the amines to act as the plasticizer. Dynamic mechanical thermal analyses showed a glass transition temperature similar to that of the glycerol plasticized ®lm. However, the observed broadening of thermal transition and the decrease in height of tan d peak in the presence of amines, were related to dierences in the type of the interactions with the proteins and to the basicity of the amino compounds. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Wheat gluten; Protein ®lms; Plasticizer; Tensile strength; Water vapor permeability; Thermal analysis
1. Introduction For the last twenty years, research interest into the use of natural biopolymers for manufacturing edible or biodegradable materials for packaging and preservation, has greatly increased. Proteins, lipids and polysaccharides have been used as ®lm-forming agents [1±6]. Proteins are thermoplastic heteropolymers of both polar and non-polar amino acids that are able to form numerous intermolecular linkages and undergo various interactions, allowing a wide range of potential functional properties [7,8]. Moreover, plant proteins are an inexpensive, renewable, and abundant raw material. Edible ®lms and coatings aord numerous advantages over conventional (non-edible) polymeric packaging. *
Corresponding author. Fax: +33-4-6714-7220. E-mail address: [email protected] (B. Boutevin).
Protein based ®lms have been used to protect pharmaceuticals and to improve the shelf life of food products. Some commercialization of protein ®lms has been realized in collagen sausage casing [9], gelatin pharmaceutical capsules [10] and corn zein based protective coatings for nutmeats and candies [11,12]. In fact, wheat gluten is a major functional food ingredient, especially in baked goods [13,14], and an excellent ®lm-forming agent. The formation and property evaluation of wheat gluten ®lms has been a major part of several studies [15± 23]. In all of them, a laboratory scale method was developed, and consisted of drying casted aqueous ethanol solutions of wheat gluten. In addition to the ®lm-forming biopolymer, a major component of protein-based ®lms formation is the plasticizer. Without a plasticizing agent, gluten ®lms are brittle and thus dicult to handle [20,21]. The use of a plasticizer reduces the intermolecular forces and increases the mobility of polymeric chains, thereby
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 1 ) 0 0 0 3 9 - 8
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improving the ¯exibility and the extensibility of the ®lm [21]. Hence, plasticizer insures the formation of freestanding ®lms [24] and helps avoid chipping or cracking of the ®lm during subsequent handling and storage, which could decrease ®lm barrier properties. Moreover, plasticizers decrease the glass transition temperature of thermoplastic wheat gluten proteins. This result is interesting and allows one to consider thermoplastic formation like extrusion for processing wheat gluten based materials, avoiding blushing and thermal degradation [25]. Finally, plasticizers generally increase the gas, water vapor and solute permeability of the ®lm and can decrease elasticity and cohesion [21]. The plasticizer must be miscible with the polymer and be present at concentrations ranging from 10 to 60 g/100 g of dry matter (dm) depending upon polymer rigidity [1]. Water is the most ubiquitous and uncontrollable plasticizer of wheat gluten proteins because of its ability to modify the structure of this natural polymer [20,21]. Glycerol was introduced in most of the hydrocolloid ®lms as a plasticizing agent. The action of water and glycerol on gluten ®lm properties were extensively studied by Gontard [21]. Other compounds have also been tested for gluten ®lm plasticization. The most commonly studied plasticizers are polyols, mono, di or oligosaccharides, lipids and their derivatives [21]. According to Gontard [25], hydrophilic compounds such as polyols (glycerol, sorbitol, propanediol) and lactic acid were the only substances tested able to notably plasticize gluten ®lm, whereas tested amphiphilic substances (i.e., glycol monostearate, acetic ester of monoglyceride, sucrose ester of stearic acid and diacetyl tartaric ester of monoglyceride) had no substantial plasticizing eect. The hydrophobic substances investigated, including beeswax and fatty acids (lauric, stearic and oleic acids), had an antiplasticizing eect on gluten ®lms, thus decreasing ¯exibility [25]. Wheat gluten ®lms in a dry state are very eective oxygen barriers, and exhibit a very high selectivity value at high moisture conditions (the ratio of CO2 permeability to O2 is 28 at 24°C, 100% relative humidity (RH)), which could be quite promising for the preservation of fresh or minimally processed fruit and vegetables under modi®ed atmosphere [26]. However, wheat gluten ®lms and protein ®lms in general are poor water vapor barriers because of the inherent hydrophilicity of the proteins [3]. The use of glycerol as a plasticizer not only increases extensibility, but also causes the ®lm to have a very high water vapor permeability (WVP). Modi®cation of the amount of glycerol and pH can decrease permeability, but not to a desirable level [21]. The incorporation of lipid compounds (waxes and fatty acids) to gluten ®lms, either as a composite or separate layer, helped in limiting
the moisture migration [27,28]. The use of original synthetic hydrophobic ®lms in bilayer structures has been previously investigated to help to improve the water vapor barrier of gluten ®lms [29,30]. However, low molecular weight compound such as glycerol may migrate, and cause the loss of desirable mechanical properties of the ®lm. This is shown in the reduction of the elongation value during aging, which is a consequence of plasticizer migration [31]. In response to the problems caused by the use of a low molecular weight plasticizer (increase in diusion of gas and water vapor through the ®lm and migration of the plasticizer), it is possible that replacing glycerol with a higher molecular weight compound that has hydrophobic substituents may help to decrease WVP and plasticizer migration, and hence improve the extensibility of gluten ®lms [24,31]. It was also interesting to check hydrophilic compounds containing amino functional groups and not only hydroxyl functional groups such as the usual polyols for these desirable properties. New types of interactions between such components and gluten proteins are expected which could modify the properties of gluten ®lms. The aim of this study consists of investigating the eect of new compounds (dierent molecular weight polyols and amines) used as plasticizers, on the mechanical, thermomechanical, and water vapor barrier properties of gluten ®lms. 2. Experimental 2.1. Reagents Vital wheat gluten (7.9 wt.% water content) was provided by Ogilvie Aquitaine (Bordeaux, France). The other ingredients used to prepare gluten ®lms were distilled water, sodium sul®te, ethanol, acetic acid, and formaldehyde (Sigma Aldrich, St. Quentin Fallavier, France). Various substances were used as plasticizer: anhydrous glycerol (Sigma Aldrich, St. Quentin Fallavier, France), polyethyleneglycols of molecular weight (MW) 200, 400, 1500 and 3400 (Lambert Riviere Company, France), polypropyleneglycols of MW 400, 2000, 3000 and 4000 (Lambert Riviere Company, France), trimethylolpropane, diethanolamine, dimethylethanolamine, triethanolamine, (Sigma Aldrich, St. Quentin Fallavier, France), polyethyleneimine 700, and polyvinylalcohol 8000. 2.2. Preparation of wheat gluten ®lms Films based on wheat gluten were prepared according to a slightly modi®ed GontardÕs procedure [25]. 100 ml of ®lm-forming solution was prepared in several steps. First, sodium sul®te (0.3 g/100 g dry gluten matter)
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dissolved in a sucient quantity of distilled water was added to 15 g of wheat gluten in order to cut the disul®de bonds of the gluten proteins. The components were mixed using a spatula until completely homogeneous (no insoluble particles present). In the second step, ethanol (45 ml), plasticizer, acetic acid (to adjust the pH of the solution to 4), and formaldehyde (0.48 g/100 g dm) were added and mixed with a magnetic stirrer. Finally, distilled water (8 ml) was added to obtain 100 ml of solution. The solution was heated at 40°C and then vigorously mixed for 10 min to improve the dispersion of gluten proteins. The ®lm-forming solution was then spread onto a polyvinylchloride plate using a thin layer chromatography applicator (Braive Instruments, Checy, France) and dried at room temperature for 24 h. Initially, a solution of thickness 0.5 mm was spread to obtain a ®nal ®lm thickness of approximately 50 lm. 2.3. Film thickness Film thickness was measured to the nearest 1 lm with a hand-held micrometer (Braive Instruments, Checy, France). Five measurements were taken at random positions. The results used in this report are the calculated averages of these values. 2.4. Film opacity Film opacity was assessed using a modi®ed standard procedure BSI [32]. The ®lm sample was cut into rectangle (3 20 mm) shaped strips and placed onto the internal side of a spectrophotometer cell (Ultrospec 2000 spectrometer). The ®lm opacity was calculated from the recorded absorbance spectrum and was expressed as an absorbance value (A) wavelength product (A nm). 2.5. Weight loss in water The ®lm dispersion in water was de®ned by the content of dm that was lost after 24 h of immersion. Two discs of each ®lm were cut and weighed. One was dried at 104°C for 24 h to determine the percentage of initial dry matter. The other disc was immersed in 50 ml of water, containing sodium azide (0.02% wt./vol) to prevent growth of microorganisms. After 24 h immersion at 25°C with occasional stirring, the piece of ®lm was removed and dried to a constant weight at 104°C for 24 h to determine the weight of dm that was dispersed in water. The ®nal result of the ®lm dispersion in water calculation was the average value of ®ve measurements. 2.6. Mechanical properties Tensile strength tests were carried out using a stable microsystem TAXT2 texture analyzer (model Cham-
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plan, France) and in accordance with the ASTM standard method [33]. Samples were prepared using the NFT 54-102 standard and conditioned at two RHs. They were placed in a desiccator with a saturated solution of NaBr (58% RHs) or of K2 SO4 (98% RHs) and kept for 5 days in a chamber maintained at 20°C prior to being tested. Stress±strain curves of a total of 12 samples were recorded at 0.5 mm s 1 . The strength and deformation at break were used to calculate the tensile strength (Pa) and the elongation (%). The Young modulus was calculated using the slope of the curve at the origin [34]. 2.7. Water vapor permeability The WVP of ®lms was determinated gravimetrically at 20°C using a modi®ed ASTM procedure [35]. The test ®lms (discs of 40 mm diameter) were hermetically sealed (with te¯on seals and silicone grease) in a glass permeation cell containing distilled water. The permeation cell 3:4 5:2 4:0 cm3 was placed in a desiccator containing silicagel to obtain a RH gradient equal to 100%. The desiccator was placed in a room maintained at 20°C and air circulation around the cells was maintained by an electric fan to prevent the formation of a stagnant air layer on the surface [36]. The water vapor transfer through the exposed ®lm area (9.1 cm2 ) was measured from the cell weight loss as a function of time. The cells were weighed (to the nearest 0.1 mg) initially and then every 12 h over a 5 day period, after steady-state vapor ¯ow had been attained. At least three samples of each type of ®lm were tested and permeability was calculated from the following equation: WVP Dw x=A Dt Dp
mol m 1 s
1
Pa 1
where Dw is the weight loss of the permeation cell in the steady state (mol), x is the ®lm thickness (m), Dt is the time of weight loss (s), A is the area of exposed ®lm (m2 ), and Dp (Pa) is the water vapor pressure dierential across the ®lm (at 20°C, Dp 2:33 103 Pa assuming that the RH on the silicagel is negligible). The slope of the curve representing the weight loss of cell versus time permits one to calculate the WVP. 2.8. Dynamic mechanical thermal analyses Dynamic mechanical thermal analyses (DMTA) of the gluten ®lms were carried out using a dynamic mechanical thermal analyzer MK III (Rheometric Scienti®c, Piscataway, USA). A sinusoidal mechanical stress was applied to the ®lm sample frequency 1 Hz to produce a sinusoidal strain of preselected amplitude (16 lm peak-to-peak). The extension of the deformation was measured for a given geometry of the ®lm sample (20 mm length and 4 mm width). Gluten ®lms (at least 100 lm thick) plasticized with amine (diethanolamine,
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triethanolamine) or glycerol were equilibrated at 0% RH over P2 O5 at 25°C for ®ve days before testing, in order to obtain completely dry ®lms (i.e., not plasticized with water). The scans (from 20°C to 180°C) were performed at a heating rate of 3°C/min. The furnace temperature was calibrated with indium (156.6°C, Rheometric Scienti®c Standard) and ¯ushed with dry nitrogen during analysis in order to avoid rehydratation of the ®lms. Three replicate ®lm samples were tested. For each analysis, the storage modulus E0 and the loss angle tan d (ratio of loss modulus (E00 ) to storage modulus (E0 )) were given. The glass transition temperature (Tg ) was de®ned by the tan d peak or the beginning of the drop in E0 . 3. Results and discussion Dierent molecular weight polyols and amines (three hydroxyamines and one polyimine) were tested as plasticizers for gluten ®lms. Their eciency was compared with that of glycerol, the most commonly used plasticizer. 3.1. Preliminary screening of dierent substances as plasticizer: preparation and observation of the wheat gluten ®lms The polyols used were: glycerol, polyethyleneglycols (MW 200, 400, 1500, and 3400), polypropyleneglycols (MW 400, 2000, 3000, 4000), and the polyvinylalcohol 8000. The amines used were: diethanolamine, dimethylethanolamine, triethanolamine and polyethyleneimine 700. Using conditions of plasticizers: In the ®rst step, the solubility of the dierent compounds in the gluten proteins solvents (ethanol, acetic acid), and in water was considered. All of the substances were soluble, except polyvinylalcohol which was soluble in warm distilled water only. Gluten ®lms were then prepared by the casting and drying of a ®lm-forming solution containing 15 g/100 g dm of each of the chosen substances. In the presence of trimethylolpropane, high molecular weight polyethyleneglycol, the polypropyleneglycol of MW 400 or polyvinylalcohol, the gluten ®lms were brittle. Moreover, they were opaque in the presence of high MW polyethyleneglycols and polypropyleneglycols. Thus, these substances were likely not to be miscible with wheat gluten proteins. With the polyethyleneglycols 200 and 400, the gluten ®lms were continuous, and homogeneous, but not ¯exible enough. The best interpretation of these results is that polyols have a plasticizing eect that depends on their chain length. So, in the case of trimethylolpropane, the chain
length would be too short, whereas it would be too long in the case of high MW polyethyleneglycols, polypropyleneglycols and polyvinylalcohol. Moreover, the polyvinylalcohol contains only secondary alcohol functions. Glycerol seems to be the polyol having the most adequate chain length. Among the substances containing an amino functional group, N,N 0 -dimethylethanolamine led to a gluten ®lm with continuity and homogeneity that depended on the thickness of the casted ®lm-forming solution. Moreover, gluten ®lm plasticized with polyethyleneimine 700 had a yellow appearance that is undesirable for gluten ®lm packaging applications. Finally, after considering some of the visual and tactile characteristics of the ®lms (continuity, homogeneity, handling and transparency), two of the novel substances were selected for a complete study: diethanolamine and triethanolamine. 3.2. Properties of the wheat gluten based ®lms as aected by diethanolamine and triethanolamine Wheat gluten ®lms were prepared with glycerol, diethanolamine or triethanolamine as the plasticizing agent. The in¯uence of these substances on the gluten ®lm properties were studied at two RH conditions (58% and 98%) and for two amounts of plasticizer (10 g and 20 g/100 g dm). 3.2.1. Opacity All prepared gluten ®lms showed a low opacity value (between 83 and 140 A nm) which corresponded to a relative good transparency. These results (Table 1) were in good agreement with those obtained by Gontard et al. [20] for gluten ®lms prepared in similar conditions (between 90 and 140 A nm). The optical properties of biopolymer based ®lms are dependent on the nature of the ®lm-forming agent and additives used, as well as their preparation [20]. For example, wheat gluten based ®lms are less transparent than those prepared from myo®brillar proteins by Cuq et al. [37]. It is also known from the literature that ideal dispersion conditions of wheat gluten proteins in the ®lm-forming solution allows one to obtain transparent and homogeneous ®lm [20]. It should also be noted (Table 1) that the opacity values of prepared ®lms were more aected by the nature of the plasticizer than by its concentration. The gluten ®lms plasticized with glycerol were sligthly more transparent than those plasticized with either of the amines studied. 3.2.2. Weight loss in water The plasticized ®lms were only slightly aected by 24 h of immersion in water. The solubility values (Table 1)
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Table 1 In¯uence of the type and concentration of the plasticizer on the solubility and opacity of gluten ®lms (standard deviations are reported in parentheses) Plasticizer
Thickness (lm)
Opacity (A nm)
Weight loss in water (g/100 g dm)
10 20
56 53
88.4 (1.1) 83.5 (7.3)
8.0 (0.6) 8.9 (0.5)
Diethanolamine
10 20
45 57
106.8 (2.3) 138.9 (14.8)
10.7 (0.9) 15.4 (0.6)
Triethanolamine
10 20
53 54
108.7 (9.9) 108.7 (3.1)
10.6 (0.4) 14.8 (0.3)
Compound
Concentration (g/100 g dm)
Glycerol
were between 8 and 15 g dissolved per 100 g dm. Moreover, they had no visual loss of integrity. This relative insolubility in water is consistent with the observation that in general wheat gluten ®lms are largely insoluble [38]. The high interaction density and more importantly, the presence of intermolecular covalent bonds or physical knots (i.e., chain entanglements) could be partly responsible for the insolubility of these ®lms [39]. The gluten ®lms are partially insoluble in water because of the low content of ionized polar amino acids, the numerous hydrophobic interactions between the apolar amino acids, and the presence of disul®de bonds [40]. The percentage solubility of gluten ®lms prepared without crosslinking agent were higher, between 31.5 and 100 g/100 g dm, depending on the preparation conditions [20]. The addition of formaldehyde as the crosslinking agent decreased the solubility of gluten proteins in water. Its very low content (0.48 g/100 g dm) was apparently sucient to form intermolecular covalent bonds. Thus, the abnormally low solubility of gluten ®lm plasticized by 20 g/100 g dm glycerol could be explained by the fact that there are some molecules of plasticizer trapped in the crosslinked network and unable to escape into solution. An increase in plasticizer content led to an increase in the amount of water soluble dm (Table 1). According to Cuq et al. [41], the dm that were solubilized in water is likely to consist of the plasticizer and some protein chains of low molecular weight. The weight losses obtained from the amines were slightly higher than those from glycerol, showing a greater solubility. A comparison with a gluten ®lm that did not contain plasticizer was impossible because the ®lm was brittle and dicult to handle [20,21].
plasticizer concentrations (10 and 20 g/100 g dm) and at two RH conditions, 58% RHs and 98% RHs. The values are summarized in Table 2. Plasticizing ®lms of wheat gluten proteins with glycerol or amino compounds (diethanolamine or triethanolamine) induced important modi®cations to their mechanical properties which can be clearly quanti®ed at 58% RHs (Table 2, Panel A). When the samples of gluten ®lms plasticized with glycerol or amines were conditioned at 58% RHs, it was noted that an increase in the content of plasticizer led to a decrease in mechanical resistance (decrease in tensile strength at break), an increase in elasticity (decrease in apparent Young modulus) and an increase in extensibility (increase in elongation at break). Similar changes in the mechanical properties of hydrocolloid based ®lms plasticized with hydrophilic compounds such as glycerol have been reported [21,24,41±44]. The comparison between Panels A and B of Table 2 clearly showed the plasticizing eect of water. Furthermore, water seemed to be the most active plastizicer at 98% RHs. The variations of the mechanical properties which correspond to a classical plasticizing eect were not clear at 98% RHs. The results at 98% RHs could be explained by the overlapping plasticizing eect of water. So the eect of the plasticizer was reduced in the presence of large amounts of water. Finally, the comparison between glycerol and amine plasticized gluten ®lms showed that the use of amino compounds instead of glycerol improved the mechanical properties of the wheat gluten ®lm. At 58% RHs and at 20 g/100 g dm plastizicer content, the use of diethanolamine or triethanolamine showed a similar increase in the extensibility and the elasticity of the gluten ®lm.
3.2.3. Mechanical properties The mechanical properties of ®lms plasticized by glycerol, diethanolamine or triethanolamine were assessed by measuring their tensile strength (rR ), elongation at break (eR ), and Young modulus (E) for two
3.2.4. Water vapor permeability WVP was evaluated at 20°C and a RHs gradient (0±100%) through gluten ®lms plasticized by glycerol, diethanolamine or triethanolamine at two dierent concentrations: 10 g/100 g dm and 20 g/100 g dm (Table 3).
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Table 2 In¯uence of the plasticizer concentration (g/100 g dm) and relative humidity (% RHs) on the mechanical properties of gluten ®lms (standard deviations are reported in parentheses) Plasticizer
rR (MPa)
eR (%)
E (MPa)
10 20
12.1 (2.3) 2.6 (1.1)
4 (1) 22 (10)
5.7 (1.9) 2.6 (1.1)
Diethanolamine
10 20
7.5 (2.9) 4.1 (1.9)
9 (5) 125 (8)
2.3 (1.6) 0.1 (<0.1)
Triethanolamine
10 20
10.1 (1.5) 5.3 (1.2)
6 (2) 114 (8)
0.1 (0.1) 0.1 (0.1)
10 20
0.4 (0.1) 0.9 (0.2)
11 (4) 30 (6)
0.1 (<0.1) 0.1 (<0.1)
Diethanolamine
10 20
1.0 (0.1) 0.8 (0.3)
61 (11) 58 (11)
<0.1 (<0.1) <0.1 (<0.1)
Triethanolamine
10 20
1.3 (0.9) 0.8 (0.2)
103 (10) 54 (9)
<0.1 (<0.1) <0.1 (<0.1)
Compound
Concentration (g/100 g dm)
Panel A: Results at 58% RHs Glycerol
Panel B: Results at 98% RHs Glycerol
rR : tensile strength at break; eR : elongation at break; E: Young modulus; Table 3 Water vapor permeabilities of various ®lms T (°C)
RHs gradient (%)
WVP (10 12 mol m 1 s 1 Pa 1 )
Reference
20 20 20 20 20 20
100±0 100±0 100±0 100±0 100±0 100±0
3.77 4.75 4.48 4.59 3.41 4.40
Present Present Present Present Present Present
Wheat gluten glycerol Fish myo®brillar proteins Corn zein
30 25 21
100±0 100±0 85±0
5.08 3.91 6.45
[21] [44] [18]
Low density polyethylene Polyvinylchloride Aluminium foil
25 25 38
95±0 95±0 95±0
0.0305 0.0395 0.00028
[45] [46] [45]
Film Wheat Wheat Wheat Wheat Wheat Wheat
gluten gluten gluten gluten gluten gluten
10% 20% 10% 20% 10% 20%
glycerol glycerol diethanolamine diethanolamine triethanolamine triethanolamine
Permeability data for proteins based ®lms and synthetic ®lms are also included in Table 3. Generally, WVP of gluten ®lms in this study (mainly for 20 g/100 g dm amount of glycerol or amine) were somewhat similar to that of glycerol plasticized gluten ®lms studied by Gontard et al. [21] and to that of other types of proteins based ®lms [18,45] (Table 3). All gluten ®lms had a lower moisture barrier than the synthetic ®lms [46,47]. For example, the gluten ®lm plasticized with 20 g/100 g dm of glycerol was 150 times less resistant moisture to than a synthetic ®lm of low density polyethylene. Furthermore, the WVP of gluten ®lms increased with an increasing concentration of plasticizer, especially
study study study study study study
with glycerol (usual plasticizer) and triethanolamine. This tendency could be explained by structural modi®cations of the protein network. The network may become less dense because of an increase in the mobility of the polymeric chains and in the free volume of the ®lm. These consequences of the plasticizing action of glycerol or amino compounds are favorable to the adsorption or desorption of water molecules [21,48]. Furthermore, the increase of WVP with plasticizer content could be related to the hydrophilicity of all tested plasticizers. It is well known that the presence of plasticizers increase the concentration of polar residues in hydrocolloids based ®lms and, thus consequently the solubility of water [21,36,49]. Since permeability is de®ned as the product of
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the diusion constant and the solubility coecient, the WVP is substantially increased. Concerning the nature of the plasticizers, the use of both of the studied amino compounds did not really improve the water vapor barrier properties of gluten ®lms compared to the use of glycerol. Notwithstanding that the WVP value of the gluten ®lm plasticized with triethanolamine was slightly lower. 3.2.5. Dynamic mechanical thermal analyses For proteins and agropolymers in general, the glass transition temperature range is broad and that of the change in heat capacity small: DMTA are thus more appropriate than dierential scanning calorimetry [50]. Wheat gluten ®lm samples were dried before the analyses in order to compare the in¯uence of amines on the gluten ®lm glass transition temperature, with that of glycerol. Thus, the well-known depression eect of water on the glass transition temperature of gluten (and hydrophilic polymers in general) did not need to be taken into account [24,51]. DMTA were so carried out on dried gluten ®lms plasticized with diethanolamine, triethanolamine, or glycerol, at equal concentrations (20 g/100 g dm). The DMTA graphs obtained for each type of ®lm, showing E0 (storage modulus) and tan d (loss modulus/storage modulus) as a function of temperature, are represented in Fig. 1. Wheat gluten clearly displayed the behavior of an amorphous polymer, since the decrease in the storage modulus and the peak in the tan d corresponds to a typical transition from a glassy to a rubbery state [24,52]. Similar variations have been observed for amorphous biopolymers and are mainly due to the a relaxation phenomenon in polymers which is associated with the glassy transition notion of amorphous materials [53±57]. The drop in the storage modulus on an increase in temperature is relatively small compared to that of synthetic polymers [52]. These moderate variations of E0 were previously observed for various protein materials [51,57], and can be explained by the presence of chemical or strong physical crosslinks inside the polymeric material [52]. The broadening of the tan d peaks, compared to those of synthetic polymers, has already been described by Gontard and Ring [51] for glycerol plasticized ®lms. It is due to the very wide distribution of protein molecular weights (typically 30 000 to millions). According to the tan d plot, the Tg of plasticized gluten ®lms can be estimated at 77°C, 82°C and 84°C for glycerol, diethanolamine and triethanolamine respectively. Thus, there are few dierences between these three plasticizers at the same weight concentration. However the plasticizing eect of glycerol, sorbitol or sucrose in ®sh myo®brillar proteins [40], and water,
Fig. 1. Dynamic mechanical analysis thermograms of dried wheat gluten ®lms as a function of the plasticizer used at 20 g/ 100 g dm. Storage modulus (E0 ), and loss angle ( tand) are plotted against temperature.
glycerol, sorbitol or sucrose in wheat gluten [52], as well as fructose, sucrose or glucose in wheat gluten [57] has already been studied. It was thus shown that the eect is essentially depending on the molar content of the plasticizer, whatever their molecular weight and the number (at least two) of polar hydroxy functions in their molecule. As glycerol, diethanolamine and triethanolamine have increasing MW (92, 105 and 149 respectively), the molar content of the ®lms, for a given weight concentration slightly decreases, in accordance with the measured values of Tg for each plasticizer. 4. Conclusions This study shows that di- and tri-ethanolamine can be considered at least as eective, and in regards to some
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®lm properties, superior to glycerol, the established plasticizer for wheat gluten ®lms. Their use permitted us to create ®lms with many similar properties to ®lms plasticized with glycerol, i.e., solubility in water, transparency and WVP. The largest dierences were mechanical properties, especially the higher values of elongation at break in the presence of amines and in the thermomechanical behavior. Thus, even though the glass transition temperatures were similar, the broadening of thermal transition peak and a decrease in tan d peak were observed with amines. Some hypotheses can be advanced to explain these dierences: possible ionic bonds between amines and the free carboxyl acid groups of proteins in addition to hydrogen bonds (with contribution from the amino or alcohol functional groups of amines) and a modi®cation of basicity of the casting solution by the presence of amines. Further investigations should be based on the molar concentration of the plasticizer, attempting to correlate the plasticizing eect to the number and nature of polar functional groups, and other aspects of the chemical structure.
References [1] Guilbert S. In: Mathlouthi M, editor. Food packaging and preservation: theorie and practice. New York: Elsevier; 1986. p. 371. [2] Kester JJ, Fennema O. Food Technol 1986;40(12):47. [3] Krochta JM. In: Sing RP, Wirakartakusumah MA, editors. Advances in food engineering. Boca Raton, FL: CRC Press; 1992. p. 517. [4] Nawrath C, Poirier Y, Somerville C. Mol Breed 1995;1:105±22. [5] Gontard N, Guilbert S. In: Mathlouthi M, editor. Food packaging and preservation. Glasgow: Blackie; 1994. p. 159±81. [6] Cuq B, Gontard N, Guilbert S. Polymer 1997;38:4071±8. [7] Guilbert S, Graille J. In: Gueguen J, editor. Les colloques, vol. 71. Paris: INRA editions, 1994. p. 195±206. [8] Marquie C, Aymard C, Cuq JL, Guilbert S. J Agric Food Chem 1995;43:2762±7. [9] Hood LL. Adv Meat Res 1987;4:109. [10] Rose PI. In: Mark HF, Bikales HF, Overberger CG, Menges G, editors. Encyclopedia of polymer science and engineering, vol. 17. New York: Wiley; 1987. p. 488±513. [11] Aklanis JJ. Candy Technology Av: Westport, CT, 1979. [12] Andres C. Food Process 1984;45(13):48. [13] Kasarda DD, Bernardin JE, Nimma CC. In: Pomeranz Y, editor. Advances in cereal and technology. American Association of Cereal Chemist, 1976. p. 158. [14] Wall JS. In: Laidman DL, Win Jones RG, editors. Recent advances in the biochemistry of cereal. London: Academic Press; 1979. p. 275. [15] Wall JS, Beckwith AC. Cereal Sci Today 1969;14:16. [16] Anker CA, Foster GA, Loader MA. US Patent 1969;14:16. [17] Okamoto S. Cereal Foods World 1978;23:256.
[18] Park HJ, Chinnan MS. Am Soc Agric Engng, ASAE Paper 90-6510, St Joseph, MI, 1990. [19] Aydt TP, Weller CL, Testin RF. Trans ASAE 1991;34: 207. [20] Gontard N, Guilbert S, Cuq JL. J Food Sci 1992;57:190. [21] Gontard N, Guilbert S, Cuq JL. J Food Sci 1993;58:206. [22] Gennadios A, Park HJ, Weller CL. Trans ASAE 1993;36:1867. [23] Gennadios A, Weller CL, Testin RF. Cereal Chem 1993;70:426. [24] Cherian G, Gennadios A, Weller C, Chinachoti P. Cereal Chem 1995;72(1):1±6. [25] Gontard N. ScD Thesis. Universite de Montpellier II, France, 1991. [26] Gontard N, Thibault R, Cuq B, Guilbert S. J Agric Food Chem 1996;44(4):1064. [27] Gontard N, Duchez C, Cuq JL, Guilbert S. Int J Food Sci Tech 1994;20:30. [28] Gontard N, Marchessau S, Cuq JL, Guilbert S. Int J Food Sci Tech 1995;30:49. [29] Irissin-Mangata J, Boutevin B, Bauduin G. Polym Bull 1999;43:441±8. [30] Irissin-Mangata J, Bauduin G, Boutevin B. Polym Bull 2000;44:409±16. [31] Park HJ, Bunn JM, Weller CL, Vergano PJ, Testin RF. Am Soc Agric Engng 1994;37(4):1281±5. [32] BSI. BS 4432. London: British Standard Institution; 1968. [33] ASTM D882-88, 1989. [34] Nussinovitch A, Kopelman IJ, Mizrahi S. Food Hydrocolloid 1990;4 (4):257±65. [35] ASTM Method 15-09: E96, Philadelphia PA: American Society for Testing and Materials, 1983. [36] Mc Hugh Th, AvenaBustillos RJ, Krochta JM. J Food Sci 1993;58(4):889±903. [37] Cuq B, Gontard N, Cuq JL, Guilbert S. J Food Sci 1996;61(3):1369±74. [38] Micard V, Guilbert S. Rapport semestriel contrat europeen ERB FAIRCT 96 1979, 1997. [39] Fukushima D, Van J. Cereal Chem 1970;47:687±96. [40] Reiners RA, Wall JS, Inglett GE. In: Pomerang Y, editor. Industrial use of cereals. St Paul, MN: AACC, Inc.; 1973. p. 285±302. [41] Cuq B, Gontard N, Cuq JL, Guilbert S. J Agric Food Chem 1997;45:622±6. [42] Somathan N, Naresh ND, Arumugan V, Ranganathan TS, Sanjeevi R. Polym J 1992;24:603±11. [43] Fairley P, Monathan FJ, German JB, Krochta JM. J Agric Food Chem 1996;44:438±43. [44] Debeaufort F, Voiley A. J Food Chem 1997;45:658±89. [45] Cuq B, Aymard C, Cuq JL, Guilbert S. J Food Sci 1995;60:1369±74. [46] Myers AW, Meyer JA, Rogers CE, Stanett V, Szwarc M. Studies in the gas and vapor permeability of plastic ®lms and coated papers VI. TAPPI 1961;44:58. [47] Doty P. J Chem Phys 1946;14:244. [48] Lieberman ER, Guilbert S. J Polym Sci 1973;44:33±43. [49] Banker GS. J Pharm Sci 1966;55:143±88. [50] Kalichevsky MT, Blanchard JM, Marsh RD. In: Blanshard IMV, Lillford PJ, editors. In the glassy state in foods. Nottingham: University Press; 1993. [51] Gontard N, Ring S. J Agric Food Chem 1996;44:3474±8.
J. Irissin-Mangata et al. / European Polymer Journal 37 (2001) 1533±1541 [52] Pouplin M, Redl A, Gontard N. J Agric Food Chem 1999;47:538±43. [53] Cuq B, Gontard N, Guilbert S. Polymer 1997;38:2399±405. [54] Di Gioia L, Cuq B, Guilbert S. Cereal Chem 1998;75: 514±9.
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[55] Galietta G, Di Gioia L, Guilbert S, Cuq B. J Dairy Sci 1998;81:3123±30. [56] Slader L, Levine H. Carbohydr Polym 1998;21:105±31. [57] Kalichevsky MJ, Jareszkievuez EM, Blanchard JM. Int J Biol Macromol 1992;14(10):257±66.
www.elsevier.nl/locate/europolj
New plasticizers for wheat gluten ®lms Josiane Irissin-Mangata a, Gerard Bauduin a, Bernard Boutevin a,*, Nathalie Gontard b a
ENSCM ± Laboratoire de Chimie Macromol eculaire, UMR CNRS 5076, 8 Rue de LÕEcole Normale, F-34296 Montpellier Cedex 5, France b Unit e d'Ing enierie des Syst emes Macromol eculaires LGBSA, cc023 Universit e Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 5, France Received 14 November 2000; received in revised form 2 February 2001; accepted 14 February 2001
Abstract Diethanolamine and triethanolamine were selected among several low and high molecular weight polyols and amines tested as potential plasticizers of wheat gluten ®lm. In comparison with glycerol, their use did not signi®cantly aect the solubility in water, the opacity of the ®lm (both increased very slightly) and the water vapor barrier properties. However, signi®cant changes in mechanical properties were observed, i.e., increase in extensibility and elasticity. At 58% relative humidity (RH) and 20 g/100 g dry matter of di or triethanolamine, the elongation was ®ve times higher than ®lm plasticized with glycerol. At 98% RH, water appeared to compete with the amines to act as the plasticizer. Dynamic mechanical thermal analyses showed a glass transition temperature similar to that of the glycerol plasticized ®lm. However, the observed broadening of thermal transition and the decrease in height of tan d peak in the presence of amines, were related to dierences in the type of the interactions with the proteins and to the basicity of the amino compounds. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Wheat gluten; Protein ®lms; Plasticizer; Tensile strength; Water vapor permeability; Thermal analysis
1. Introduction For the last twenty years, research interest into the use of natural biopolymers for manufacturing edible or biodegradable materials for packaging and preservation, has greatly increased. Proteins, lipids and polysaccharides have been used as ®lm-forming agents [1±6]. Proteins are thermoplastic heteropolymers of both polar and non-polar amino acids that are able to form numerous intermolecular linkages and undergo various interactions, allowing a wide range of potential functional properties [7,8]. Moreover, plant proteins are an inexpensive, renewable, and abundant raw material. Edible ®lms and coatings aord numerous advantages over conventional (non-edible) polymeric packaging. *
Corresponding author. Fax: +33-4-6714-7220. E-mail address: [email protected] (B. Boutevin).
Protein based ®lms have been used to protect pharmaceuticals and to improve the shelf life of food products. Some commercialization of protein ®lms has been realized in collagen sausage casing [9], gelatin pharmaceutical capsules [10] and corn zein based protective coatings for nutmeats and candies [11,12]. In fact, wheat gluten is a major functional food ingredient, especially in baked goods [13,14], and an excellent ®lm-forming agent. The formation and property evaluation of wheat gluten ®lms has been a major part of several studies [15± 23]. In all of them, a laboratory scale method was developed, and consisted of drying casted aqueous ethanol solutions of wheat gluten. In addition to the ®lm-forming biopolymer, a major component of protein-based ®lms formation is the plasticizer. Without a plasticizing agent, gluten ®lms are brittle and thus dicult to handle [20,21]. The use of a plasticizer reduces the intermolecular forces and increases the mobility of polymeric chains, thereby
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 1 ) 0 0 0 3 9 - 8
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improving the ¯exibility and the extensibility of the ®lm [21]. Hence, plasticizer insures the formation of freestanding ®lms [24] and helps avoid chipping or cracking of the ®lm during subsequent handling and storage, which could decrease ®lm barrier properties. Moreover, plasticizers decrease the glass transition temperature of thermoplastic wheat gluten proteins. This result is interesting and allows one to consider thermoplastic formation like extrusion for processing wheat gluten based materials, avoiding blushing and thermal degradation [25]. Finally, plasticizers generally increase the gas, water vapor and solute permeability of the ®lm and can decrease elasticity and cohesion [21]. The plasticizer must be miscible with the polymer and be present at concentrations ranging from 10 to 60 g/100 g of dry matter (dm) depending upon polymer rigidity [1]. Water is the most ubiquitous and uncontrollable plasticizer of wheat gluten proteins because of its ability to modify the structure of this natural polymer [20,21]. Glycerol was introduced in most of the hydrocolloid ®lms as a plasticizing agent. The action of water and glycerol on gluten ®lm properties were extensively studied by Gontard [21]. Other compounds have also been tested for gluten ®lm plasticization. The most commonly studied plasticizers are polyols, mono, di or oligosaccharides, lipids and their derivatives [21]. According to Gontard [25], hydrophilic compounds such as polyols (glycerol, sorbitol, propanediol) and lactic acid were the only substances tested able to notably plasticize gluten ®lm, whereas tested amphiphilic substances (i.e., glycol monostearate, acetic ester of monoglyceride, sucrose ester of stearic acid and diacetyl tartaric ester of monoglyceride) had no substantial plasticizing eect. The hydrophobic substances investigated, including beeswax and fatty acids (lauric, stearic and oleic acids), had an antiplasticizing eect on gluten ®lms, thus decreasing ¯exibility [25]. Wheat gluten ®lms in a dry state are very eective oxygen barriers, and exhibit a very high selectivity value at high moisture conditions (the ratio of CO2 permeability to O2 is 28 at 24°C, 100% relative humidity (RH)), which could be quite promising for the preservation of fresh or minimally processed fruit and vegetables under modi®ed atmosphere [26]. However, wheat gluten ®lms and protein ®lms in general are poor water vapor barriers because of the inherent hydrophilicity of the proteins [3]. The use of glycerol as a plasticizer not only increases extensibility, but also causes the ®lm to have a very high water vapor permeability (WVP). Modi®cation of the amount of glycerol and pH can decrease permeability, but not to a desirable level [21]. The incorporation of lipid compounds (waxes and fatty acids) to gluten ®lms, either as a composite or separate layer, helped in limiting
the moisture migration [27,28]. The use of original synthetic hydrophobic ®lms in bilayer structures has been previously investigated to help to improve the water vapor barrier of gluten ®lms [29,30]. However, low molecular weight compound such as glycerol may migrate, and cause the loss of desirable mechanical properties of the ®lm. This is shown in the reduction of the elongation value during aging, which is a consequence of plasticizer migration [31]. In response to the problems caused by the use of a low molecular weight plasticizer (increase in diusion of gas and water vapor through the ®lm and migration of the plasticizer), it is possible that replacing glycerol with a higher molecular weight compound that has hydrophobic substituents may help to decrease WVP and plasticizer migration, and hence improve the extensibility of gluten ®lms [24,31]. It was also interesting to check hydrophilic compounds containing amino functional groups and not only hydroxyl functional groups such as the usual polyols for these desirable properties. New types of interactions between such components and gluten proteins are expected which could modify the properties of gluten ®lms. The aim of this study consists of investigating the eect of new compounds (dierent molecular weight polyols and amines) used as plasticizers, on the mechanical, thermomechanical, and water vapor barrier properties of gluten ®lms. 2. Experimental 2.1. Reagents Vital wheat gluten (7.9 wt.% water content) was provided by Ogilvie Aquitaine (Bordeaux, France). The other ingredients used to prepare gluten ®lms were distilled water, sodium sul®te, ethanol, acetic acid, and formaldehyde (Sigma Aldrich, St. Quentin Fallavier, France). Various substances were used as plasticizer: anhydrous glycerol (Sigma Aldrich, St. Quentin Fallavier, France), polyethyleneglycols of molecular weight (MW) 200, 400, 1500 and 3400 (Lambert Riviere Company, France), polypropyleneglycols of MW 400, 2000, 3000 and 4000 (Lambert Riviere Company, France), trimethylolpropane, diethanolamine, dimethylethanolamine, triethanolamine, (Sigma Aldrich, St. Quentin Fallavier, France), polyethyleneimine 700, and polyvinylalcohol 8000. 2.2. Preparation of wheat gluten ®lms Films based on wheat gluten were prepared according to a slightly modi®ed GontardÕs procedure [25]. 100 ml of ®lm-forming solution was prepared in several steps. First, sodium sul®te (0.3 g/100 g dry gluten matter)
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dissolved in a sucient quantity of distilled water was added to 15 g of wheat gluten in order to cut the disul®de bonds of the gluten proteins. The components were mixed using a spatula until completely homogeneous (no insoluble particles present). In the second step, ethanol (45 ml), plasticizer, acetic acid (to adjust the pH of the solution to 4), and formaldehyde (0.48 g/100 g dm) were added and mixed with a magnetic stirrer. Finally, distilled water (8 ml) was added to obtain 100 ml of solution. The solution was heated at 40°C and then vigorously mixed for 10 min to improve the dispersion of gluten proteins. The ®lm-forming solution was then spread onto a polyvinylchloride plate using a thin layer chromatography applicator (Braive Instruments, Checy, France) and dried at room temperature for 24 h. Initially, a solution of thickness 0.5 mm was spread to obtain a ®nal ®lm thickness of approximately 50 lm. 2.3. Film thickness Film thickness was measured to the nearest 1 lm with a hand-held micrometer (Braive Instruments, Checy, France). Five measurements were taken at random positions. The results used in this report are the calculated averages of these values. 2.4. Film opacity Film opacity was assessed using a modi®ed standard procedure BSI [32]. The ®lm sample was cut into rectangle (3 20 mm) shaped strips and placed onto the internal side of a spectrophotometer cell (Ultrospec 2000 spectrometer). The ®lm opacity was calculated from the recorded absorbance spectrum and was expressed as an absorbance value (A) wavelength product (A nm). 2.5. Weight loss in water The ®lm dispersion in water was de®ned by the content of dm that was lost after 24 h of immersion. Two discs of each ®lm were cut and weighed. One was dried at 104°C for 24 h to determine the percentage of initial dry matter. The other disc was immersed in 50 ml of water, containing sodium azide (0.02% wt./vol) to prevent growth of microorganisms. After 24 h immersion at 25°C with occasional stirring, the piece of ®lm was removed and dried to a constant weight at 104°C for 24 h to determine the weight of dm that was dispersed in water. The ®nal result of the ®lm dispersion in water calculation was the average value of ®ve measurements. 2.6. Mechanical properties Tensile strength tests were carried out using a stable microsystem TAXT2 texture analyzer (model Cham-
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plan, France) and in accordance with the ASTM standard method [33]. Samples were prepared using the NFT 54-102 standard and conditioned at two RHs. They were placed in a desiccator with a saturated solution of NaBr (58% RHs) or of K2 SO4 (98% RHs) and kept for 5 days in a chamber maintained at 20°C prior to being tested. Stress±strain curves of a total of 12 samples were recorded at 0.5 mm s 1 . The strength and deformation at break were used to calculate the tensile strength (Pa) and the elongation (%). The Young modulus was calculated using the slope of the curve at the origin [34]. 2.7. Water vapor permeability The WVP of ®lms was determinated gravimetrically at 20°C using a modi®ed ASTM procedure [35]. The test ®lms (discs of 40 mm diameter) were hermetically sealed (with te¯on seals and silicone grease) in a glass permeation cell containing distilled water. The permeation cell 3:4 5:2 4:0 cm3 was placed in a desiccator containing silicagel to obtain a RH gradient equal to 100%. The desiccator was placed in a room maintained at 20°C and air circulation around the cells was maintained by an electric fan to prevent the formation of a stagnant air layer on the surface [36]. The water vapor transfer through the exposed ®lm area (9.1 cm2 ) was measured from the cell weight loss as a function of time. The cells were weighed (to the nearest 0.1 mg) initially and then every 12 h over a 5 day period, after steady-state vapor ¯ow had been attained. At least three samples of each type of ®lm were tested and permeability was calculated from the following equation: WVP Dw x=A Dt Dp
mol m 1 s
1
Pa 1
where Dw is the weight loss of the permeation cell in the steady state (mol), x is the ®lm thickness (m), Dt is the time of weight loss (s), A is the area of exposed ®lm (m2 ), and Dp (Pa) is the water vapor pressure dierential across the ®lm (at 20°C, Dp 2:33 103 Pa assuming that the RH on the silicagel is negligible). The slope of the curve representing the weight loss of cell versus time permits one to calculate the WVP. 2.8. Dynamic mechanical thermal analyses Dynamic mechanical thermal analyses (DMTA) of the gluten ®lms were carried out using a dynamic mechanical thermal analyzer MK III (Rheometric Scienti®c, Piscataway, USA). A sinusoidal mechanical stress was applied to the ®lm sample frequency 1 Hz to produce a sinusoidal strain of preselected amplitude (16 lm peak-to-peak). The extension of the deformation was measured for a given geometry of the ®lm sample (20 mm length and 4 mm width). Gluten ®lms (at least 100 lm thick) plasticized with amine (diethanolamine,
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triethanolamine) or glycerol were equilibrated at 0% RH over P2 O5 at 25°C for ®ve days before testing, in order to obtain completely dry ®lms (i.e., not plasticized with water). The scans (from 20°C to 180°C) were performed at a heating rate of 3°C/min. The furnace temperature was calibrated with indium (156.6°C, Rheometric Scienti®c Standard) and ¯ushed with dry nitrogen during analysis in order to avoid rehydratation of the ®lms. Three replicate ®lm samples were tested. For each analysis, the storage modulus E0 and the loss angle tan d (ratio of loss modulus (E00 ) to storage modulus (E0 )) were given. The glass transition temperature (Tg ) was de®ned by the tan d peak or the beginning of the drop in E0 . 3. Results and discussion Dierent molecular weight polyols and amines (three hydroxyamines and one polyimine) were tested as plasticizers for gluten ®lms. Their eciency was compared with that of glycerol, the most commonly used plasticizer. 3.1. Preliminary screening of dierent substances as plasticizer: preparation and observation of the wheat gluten ®lms The polyols used were: glycerol, polyethyleneglycols (MW 200, 400, 1500, and 3400), polypropyleneglycols (MW 400, 2000, 3000, 4000), and the polyvinylalcohol 8000. The amines used were: diethanolamine, dimethylethanolamine, triethanolamine and polyethyleneimine 700. Using conditions of plasticizers: In the ®rst step, the solubility of the dierent compounds in the gluten proteins solvents (ethanol, acetic acid), and in water was considered. All of the substances were soluble, except polyvinylalcohol which was soluble in warm distilled water only. Gluten ®lms were then prepared by the casting and drying of a ®lm-forming solution containing 15 g/100 g dm of each of the chosen substances. In the presence of trimethylolpropane, high molecular weight polyethyleneglycol, the polypropyleneglycol of MW 400 or polyvinylalcohol, the gluten ®lms were brittle. Moreover, they were opaque in the presence of high MW polyethyleneglycols and polypropyleneglycols. Thus, these substances were likely not to be miscible with wheat gluten proteins. With the polyethyleneglycols 200 and 400, the gluten ®lms were continuous, and homogeneous, but not ¯exible enough. The best interpretation of these results is that polyols have a plasticizing eect that depends on their chain length. So, in the case of trimethylolpropane, the chain
length would be too short, whereas it would be too long in the case of high MW polyethyleneglycols, polypropyleneglycols and polyvinylalcohol. Moreover, the polyvinylalcohol contains only secondary alcohol functions. Glycerol seems to be the polyol having the most adequate chain length. Among the substances containing an amino functional group, N,N 0 -dimethylethanolamine led to a gluten ®lm with continuity and homogeneity that depended on the thickness of the casted ®lm-forming solution. Moreover, gluten ®lm plasticized with polyethyleneimine 700 had a yellow appearance that is undesirable for gluten ®lm packaging applications. Finally, after considering some of the visual and tactile characteristics of the ®lms (continuity, homogeneity, handling and transparency), two of the novel substances were selected for a complete study: diethanolamine and triethanolamine. 3.2. Properties of the wheat gluten based ®lms as aected by diethanolamine and triethanolamine Wheat gluten ®lms were prepared with glycerol, diethanolamine or triethanolamine as the plasticizing agent. The in¯uence of these substances on the gluten ®lm properties were studied at two RH conditions (58% and 98%) and for two amounts of plasticizer (10 g and 20 g/100 g dm). 3.2.1. Opacity All prepared gluten ®lms showed a low opacity value (between 83 and 140 A nm) which corresponded to a relative good transparency. These results (Table 1) were in good agreement with those obtained by Gontard et al. [20] for gluten ®lms prepared in similar conditions (between 90 and 140 A nm). The optical properties of biopolymer based ®lms are dependent on the nature of the ®lm-forming agent and additives used, as well as their preparation [20]. For example, wheat gluten based ®lms are less transparent than those prepared from myo®brillar proteins by Cuq et al. [37]. It is also known from the literature that ideal dispersion conditions of wheat gluten proteins in the ®lm-forming solution allows one to obtain transparent and homogeneous ®lm [20]. It should also be noted (Table 1) that the opacity values of prepared ®lms were more aected by the nature of the plasticizer than by its concentration. The gluten ®lms plasticized with glycerol were sligthly more transparent than those plasticized with either of the amines studied. 3.2.2. Weight loss in water The plasticized ®lms were only slightly aected by 24 h of immersion in water. The solubility values (Table 1)
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Table 1 In¯uence of the type and concentration of the plasticizer on the solubility and opacity of gluten ®lms (standard deviations are reported in parentheses) Plasticizer
Thickness (lm)
Opacity (A nm)
Weight loss in water (g/100 g dm)
10 20
56 53
88.4 (1.1) 83.5 (7.3)
8.0 (0.6) 8.9 (0.5)
Diethanolamine
10 20
45 57
106.8 (2.3) 138.9 (14.8)
10.7 (0.9) 15.4 (0.6)
Triethanolamine
10 20
53 54
108.7 (9.9) 108.7 (3.1)
10.6 (0.4) 14.8 (0.3)
Compound
Concentration (g/100 g dm)
Glycerol
were between 8 and 15 g dissolved per 100 g dm. Moreover, they had no visual loss of integrity. This relative insolubility in water is consistent with the observation that in general wheat gluten ®lms are largely insoluble [38]. The high interaction density and more importantly, the presence of intermolecular covalent bonds or physical knots (i.e., chain entanglements) could be partly responsible for the insolubility of these ®lms [39]. The gluten ®lms are partially insoluble in water because of the low content of ionized polar amino acids, the numerous hydrophobic interactions between the apolar amino acids, and the presence of disul®de bonds [40]. The percentage solubility of gluten ®lms prepared without crosslinking agent were higher, between 31.5 and 100 g/100 g dm, depending on the preparation conditions [20]. The addition of formaldehyde as the crosslinking agent decreased the solubility of gluten proteins in water. Its very low content (0.48 g/100 g dm) was apparently sucient to form intermolecular covalent bonds. Thus, the abnormally low solubility of gluten ®lm plasticized by 20 g/100 g dm glycerol could be explained by the fact that there are some molecules of plasticizer trapped in the crosslinked network and unable to escape into solution. An increase in plasticizer content led to an increase in the amount of water soluble dm (Table 1). According to Cuq et al. [41], the dm that were solubilized in water is likely to consist of the plasticizer and some protein chains of low molecular weight. The weight losses obtained from the amines were slightly higher than those from glycerol, showing a greater solubility. A comparison with a gluten ®lm that did not contain plasticizer was impossible because the ®lm was brittle and dicult to handle [20,21].
plasticizer concentrations (10 and 20 g/100 g dm) and at two RH conditions, 58% RHs and 98% RHs. The values are summarized in Table 2. Plasticizing ®lms of wheat gluten proteins with glycerol or amino compounds (diethanolamine or triethanolamine) induced important modi®cations to their mechanical properties which can be clearly quanti®ed at 58% RHs (Table 2, Panel A). When the samples of gluten ®lms plasticized with glycerol or amines were conditioned at 58% RHs, it was noted that an increase in the content of plasticizer led to a decrease in mechanical resistance (decrease in tensile strength at break), an increase in elasticity (decrease in apparent Young modulus) and an increase in extensibility (increase in elongation at break). Similar changes in the mechanical properties of hydrocolloid based ®lms plasticized with hydrophilic compounds such as glycerol have been reported [21,24,41±44]. The comparison between Panels A and B of Table 2 clearly showed the plasticizing eect of water. Furthermore, water seemed to be the most active plastizicer at 98% RHs. The variations of the mechanical properties which correspond to a classical plasticizing eect were not clear at 98% RHs. The results at 98% RHs could be explained by the overlapping plasticizing eect of water. So the eect of the plasticizer was reduced in the presence of large amounts of water. Finally, the comparison between glycerol and amine plasticized gluten ®lms showed that the use of amino compounds instead of glycerol improved the mechanical properties of the wheat gluten ®lm. At 58% RHs and at 20 g/100 g dm plastizicer content, the use of diethanolamine or triethanolamine showed a similar increase in the extensibility and the elasticity of the gluten ®lm.
3.2.3. Mechanical properties The mechanical properties of ®lms plasticized by glycerol, diethanolamine or triethanolamine were assessed by measuring their tensile strength (rR ), elongation at break (eR ), and Young modulus (E) for two
3.2.4. Water vapor permeability WVP was evaluated at 20°C and a RHs gradient (0±100%) through gluten ®lms plasticized by glycerol, diethanolamine or triethanolamine at two dierent concentrations: 10 g/100 g dm and 20 g/100 g dm (Table 3).
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Table 2 In¯uence of the plasticizer concentration (g/100 g dm) and relative humidity (% RHs) on the mechanical properties of gluten ®lms (standard deviations are reported in parentheses) Plasticizer
rR (MPa)
eR (%)
E (MPa)
10 20
12.1 (2.3) 2.6 (1.1)
4 (1) 22 (10)
5.7 (1.9) 2.6 (1.1)
Diethanolamine
10 20
7.5 (2.9) 4.1 (1.9)
9 (5) 125 (8)
2.3 (1.6) 0.1 (<0.1)
Triethanolamine
10 20
10.1 (1.5) 5.3 (1.2)
6 (2) 114 (8)
0.1 (0.1) 0.1 (0.1)
10 20
0.4 (0.1) 0.9 (0.2)
11 (4) 30 (6)
0.1 (<0.1) 0.1 (<0.1)
Diethanolamine
10 20
1.0 (0.1) 0.8 (0.3)
61 (11) 58 (11)
<0.1 (<0.1) <0.1 (<0.1)
Triethanolamine
10 20
1.3 (0.9) 0.8 (0.2)
103 (10) 54 (9)
<0.1 (<0.1) <0.1 (<0.1)
Compound
Concentration (g/100 g dm)
Panel A: Results at 58% RHs Glycerol
Panel B: Results at 98% RHs Glycerol
rR : tensile strength at break; eR : elongation at break; E: Young modulus; Table 3 Water vapor permeabilities of various ®lms T (°C)
RHs gradient (%)
WVP (10 12 mol m 1 s 1 Pa 1 )
Reference
20 20 20 20 20 20
100±0 100±0 100±0 100±0 100±0 100±0
3.77 4.75 4.48 4.59 3.41 4.40
Present Present Present Present Present Present
Wheat gluten glycerol Fish myo®brillar proteins Corn zein
30 25 21
100±0 100±0 85±0
5.08 3.91 6.45
[21] [44] [18]
Low density polyethylene Polyvinylchloride Aluminium foil
25 25 38
95±0 95±0 95±0
0.0305 0.0395 0.00028
[45] [46] [45]
Film Wheat Wheat Wheat Wheat Wheat Wheat
gluten gluten gluten gluten gluten gluten
10% 20% 10% 20% 10% 20%
glycerol glycerol diethanolamine diethanolamine triethanolamine triethanolamine
Permeability data for proteins based ®lms and synthetic ®lms are also included in Table 3. Generally, WVP of gluten ®lms in this study (mainly for 20 g/100 g dm amount of glycerol or amine) were somewhat similar to that of glycerol plasticized gluten ®lms studied by Gontard et al. [21] and to that of other types of proteins based ®lms [18,45] (Table 3). All gluten ®lms had a lower moisture barrier than the synthetic ®lms [46,47]. For example, the gluten ®lm plasticized with 20 g/100 g dm of glycerol was 150 times less resistant moisture to than a synthetic ®lm of low density polyethylene. Furthermore, the WVP of gluten ®lms increased with an increasing concentration of plasticizer, especially
study study study study study study
with glycerol (usual plasticizer) and triethanolamine. This tendency could be explained by structural modi®cations of the protein network. The network may become less dense because of an increase in the mobility of the polymeric chains and in the free volume of the ®lm. These consequences of the plasticizing action of glycerol or amino compounds are favorable to the adsorption or desorption of water molecules [21,48]. Furthermore, the increase of WVP with plasticizer content could be related to the hydrophilicity of all tested plasticizers. It is well known that the presence of plasticizers increase the concentration of polar residues in hydrocolloids based ®lms and, thus consequently the solubility of water [21,36,49]. Since permeability is de®ned as the product of
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the diusion constant and the solubility coecient, the WVP is substantially increased. Concerning the nature of the plasticizers, the use of both of the studied amino compounds did not really improve the water vapor barrier properties of gluten ®lms compared to the use of glycerol. Notwithstanding that the WVP value of the gluten ®lm plasticized with triethanolamine was slightly lower. 3.2.5. Dynamic mechanical thermal analyses For proteins and agropolymers in general, the glass transition temperature range is broad and that of the change in heat capacity small: DMTA are thus more appropriate than dierential scanning calorimetry [50]. Wheat gluten ®lm samples were dried before the analyses in order to compare the in¯uence of amines on the gluten ®lm glass transition temperature, with that of glycerol. Thus, the well-known depression eect of water on the glass transition temperature of gluten (and hydrophilic polymers in general) did not need to be taken into account [24,51]. DMTA were so carried out on dried gluten ®lms plasticized with diethanolamine, triethanolamine, or glycerol, at equal concentrations (20 g/100 g dm). The DMTA graphs obtained for each type of ®lm, showing E0 (storage modulus) and tan d (loss modulus/storage modulus) as a function of temperature, are represented in Fig. 1. Wheat gluten clearly displayed the behavior of an amorphous polymer, since the decrease in the storage modulus and the peak in the tan d corresponds to a typical transition from a glassy to a rubbery state [24,52]. Similar variations have been observed for amorphous biopolymers and are mainly due to the a relaxation phenomenon in polymers which is associated with the glassy transition notion of amorphous materials [53±57]. The drop in the storage modulus on an increase in temperature is relatively small compared to that of synthetic polymers [52]. These moderate variations of E0 were previously observed for various protein materials [51,57], and can be explained by the presence of chemical or strong physical crosslinks inside the polymeric material [52]. The broadening of the tan d peaks, compared to those of synthetic polymers, has already been described by Gontard and Ring [51] for glycerol plasticized ®lms. It is due to the very wide distribution of protein molecular weights (typically 30 000 to millions). According to the tan d plot, the Tg of plasticized gluten ®lms can be estimated at 77°C, 82°C and 84°C for glycerol, diethanolamine and triethanolamine respectively. Thus, there are few dierences between these three plasticizers at the same weight concentration. However the plasticizing eect of glycerol, sorbitol or sucrose in ®sh myo®brillar proteins [40], and water,
Fig. 1. Dynamic mechanical analysis thermograms of dried wheat gluten ®lms as a function of the plasticizer used at 20 g/ 100 g dm. Storage modulus (E0 ), and loss angle ( tand) are plotted against temperature.
glycerol, sorbitol or sucrose in wheat gluten [52], as well as fructose, sucrose or glucose in wheat gluten [57] has already been studied. It was thus shown that the eect is essentially depending on the molar content of the plasticizer, whatever their molecular weight and the number (at least two) of polar hydroxy functions in their molecule. As glycerol, diethanolamine and triethanolamine have increasing MW (92, 105 and 149 respectively), the molar content of the ®lms, for a given weight concentration slightly decreases, in accordance with the measured values of Tg for each plasticizer. 4. Conclusions This study shows that di- and tri-ethanolamine can be considered at least as eective, and in regards to some
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®lm properties, superior to glycerol, the established plasticizer for wheat gluten ®lms. Their use permitted us to create ®lms with many similar properties to ®lms plasticized with glycerol, i.e., solubility in water, transparency and WVP. The largest dierences were mechanical properties, especially the higher values of elongation at break in the presence of amines and in the thermomechanical behavior. Thus, even though the glass transition temperatures were similar, the broadening of thermal transition peak and a decrease in tan d peak were observed with amines. Some hypotheses can be advanced to explain these dierences: possible ionic bonds between amines and the free carboxyl acid groups of proteins in addition to hydrogen bonds (with contribution from the amino or alcohol functional groups of amines) and a modi®cation of basicity of the casting solution by the presence of amines. Further investigations should be based on the molar concentration of the plasticizer, attempting to correlate the plasticizing eect to the number and nature of polar functional groups, and other aspects of the chemical structure.
References [1] Guilbert S. In: Mathlouthi M, editor. Food packaging and preservation: theorie and practice. New York: Elsevier; 1986. p. 371. [2] Kester JJ, Fennema O. Food Technol 1986;40(12):47. [3] Krochta JM. In: Sing RP, Wirakartakusumah MA, editors. Advances in food engineering. Boca Raton, FL: CRC Press; 1992. p. 517. [4] Nawrath C, Poirier Y, Somerville C. Mol Breed 1995;1:105±22. [5] Gontard N, Guilbert S. In: Mathlouthi M, editor. Food packaging and preservation. Glasgow: Blackie; 1994. p. 159±81. [6] Cuq B, Gontard N, Guilbert S. Polymer 1997;38:4071±8. [7] Guilbert S, Graille J. In: Gueguen J, editor. Les colloques, vol. 71. Paris: INRA editions, 1994. p. 195±206. [8] Marquie C, Aymard C, Cuq JL, Guilbert S. J Agric Food Chem 1995;43:2762±7. [9] Hood LL. Adv Meat Res 1987;4:109. [10] Rose PI. In: Mark HF, Bikales HF, Overberger CG, Menges G, editors. Encyclopedia of polymer science and engineering, vol. 17. New York: Wiley; 1987. p. 488±513. [11] Aklanis JJ. Candy Technology Av: Westport, CT, 1979. [12] Andres C. Food Process 1984;45(13):48. [13] Kasarda DD, Bernardin JE, Nimma CC. In: Pomeranz Y, editor. Advances in cereal and technology. American Association of Cereal Chemist, 1976. p. 158. [14] Wall JS. In: Laidman DL, Win Jones RG, editors. Recent advances in the biochemistry of cereal. London: Academic Press; 1979. p. 275. [15] Wall JS, Beckwith AC. Cereal Sci Today 1969;14:16. [16] Anker CA, Foster GA, Loader MA. US Patent 1969;14:16. [17] Okamoto S. Cereal Foods World 1978;23:256.
[18] Park HJ, Chinnan MS. Am Soc Agric Engng, ASAE Paper 90-6510, St Joseph, MI, 1990. [19] Aydt TP, Weller CL, Testin RF. Trans ASAE 1991;34: 207. [20] Gontard N, Guilbert S, Cuq JL. J Food Sci 1992;57:190. [21] Gontard N, Guilbert S, Cuq JL. J Food Sci 1993;58:206. [22] Gennadios A, Park HJ, Weller CL. Trans ASAE 1993;36:1867. [23] Gennadios A, Weller CL, Testin RF. Cereal Chem 1993;70:426. [24] Cherian G, Gennadios A, Weller C, Chinachoti P. Cereal Chem 1995;72(1):1±6. [25] Gontard N. ScD Thesis. Universite de Montpellier II, France, 1991. [26] Gontard N, Thibault R, Cuq B, Guilbert S. J Agric Food Chem 1996;44(4):1064. [27] Gontard N, Duchez C, Cuq JL, Guilbert S. Int J Food Sci Tech 1994;20:30. [28] Gontard N, Marchessau S, Cuq JL, Guilbert S. Int J Food Sci Tech 1995;30:49. [29] Irissin-Mangata J, Boutevin B, Bauduin G. Polym Bull 1999;43:441±8. [30] Irissin-Mangata J, Bauduin G, Boutevin B. Polym Bull 2000;44:409±16. [31] Park HJ, Bunn JM, Weller CL, Vergano PJ, Testin RF. Am Soc Agric Engng 1994;37(4):1281±5. [32] BSI. BS 4432. London: British Standard Institution; 1968. [33] ASTM D882-88, 1989. [34] Nussinovitch A, Kopelman IJ, Mizrahi S. Food Hydrocolloid 1990;4 (4):257±65. [35] ASTM Method 15-09: E96, Philadelphia PA: American Society for Testing and Materials, 1983. [36] Mc Hugh Th, AvenaBustillos RJ, Krochta JM. J Food Sci 1993;58(4):889±903. [37] Cuq B, Gontard N, Cuq JL, Guilbert S. J Food Sci 1996;61(3):1369±74. [38] Micard V, Guilbert S. Rapport semestriel contrat europeen ERB FAIRCT 96 1979, 1997. [39] Fukushima D, Van J. Cereal Chem 1970;47:687±96. [40] Reiners RA, Wall JS, Inglett GE. In: Pomerang Y, editor. Industrial use of cereals. St Paul, MN: AACC, Inc.; 1973. p. 285±302. [41] Cuq B, Gontard N, Cuq JL, Guilbert S. J Agric Food Chem 1997;45:622±6. [42] Somathan N, Naresh ND, Arumugan V, Ranganathan TS, Sanjeevi R. Polym J 1992;24:603±11. [43] Fairley P, Monathan FJ, German JB, Krochta JM. J Agric Food Chem 1996;44:438±43. [44] Debeaufort F, Voiley A. J Food Chem 1997;45:658±89. [45] Cuq B, Aymard C, Cuq JL, Guilbert S. J Food Sci 1995;60:1369±74. [46] Myers AW, Meyer JA, Rogers CE, Stanett V, Szwarc M. Studies in the gas and vapor permeability of plastic ®lms and coated papers VI. TAPPI 1961;44:58. [47] Doty P. J Chem Phys 1946;14:244. [48] Lieberman ER, Guilbert S. J Polym Sci 1973;44:33±43. [49] Banker GS. J Pharm Sci 1966;55:143±88. [50] Kalichevsky MT, Blanchard JM, Marsh RD. In: Blanshard IMV, Lillford PJ, editors. In the glassy state in foods. Nottingham: University Press; 1993. [51] Gontard N, Ring S. J Agric Food Chem 1996;44:3474±8.
J. Irissin-Mangata et al. / European Polymer Journal 37 (2001) 1533±1541 [52] Pouplin M, Redl A, Gontard N. J Agric Food Chem 1999;47:538±43. [53] Cuq B, Gontard N, Guilbert S. Polymer 1997;38:2399±405. [54] Di Gioia L, Cuq B, Guilbert S. Cereal Chem 1998;75: 514±9.
1541
[55] Galietta G, Di Gioia L, Guilbert S, Cuq B. J Dairy Sci 1998;81:3123±30. [56] Slader L, Levine H. Carbohydr Polym 1998;21:105±31. [57] Kalichevsky MJ, Jareszkievuez EM, Blanchard JM. Int J Biol Macromol 1992;14(10):257±66.