Gluten-glycerol dough was extruded under a variety of processing condi- tions using a corotating self-wiping twin-screw extruder. Influence of feed rate, screw ...
Extrusion of Wheat Gluten Plasticized with Glycerol: Influence of Process Conditions on Flow Behavior, Rheological Properties, and Molecular Size Distribution Andreas Redl,1,2 Marie Hélène Morel,1 Joëlle Bonicel,1 Bruno Vergnes,3 and Stephane Guilbert1 ABSTRACT
Cereal Chem. 76(3):361–370
Gluten-glycerol dough was extruded under a variety of processing conditions using a corotating self-wiping twin-screw extruder. Influence of feed rate, screw speed, and barrel temperature on processing parameters (die pressure, product temperature, residence time, specific energy) were examined. Use of flow modeling was successful for describing the evolution of the main flow parameters during processing. Rheological properties of extruded samples exhibited network-like behavior and were characterized and modeled by Cole-Cole distributions. Changes in molecular sizes of proteins during extrusion were measured by chromatography and appeared to be correlated to molecular size between network strands, as derived from the rheological properties of the materials obtained. Depending on operating conditions,
extrudates presented very different surface aspects, ranging from very smooth-surfaced extrudates with high swell to completely broken extrudates. The results indicated that extrudate breakup was caused by increasing network density, and some gliadins may have acted as cross-linking agents. Increasing network density resulted in decreasing mobility of polymeric chains, and “protein melt” may no longer have been able to support the strain experienced during extrusion through the die. Increasing network density was reflected in increased plateau modulus and molecular size of protein aggregates. Increasing network structure appeared to be induced by the severity of the thermomechanical treatment, as indicated by specific mechanical energy input and maximum temperature reached.
Extrusion is a highly efficient method for continuous shaping of thermoplastic materials. Using extrusion technology on a renewable, agricultural raw product to produce material with a low environmental impact is a challenge in producing “bioplastics.” Recently, interest has been focused on thermoplastic starch, and efforts have been made to investigate the influence of extrusion conditions and type of plasticizer on the macromolecular structure of plasticized starch (Della Valle et al 1996, 1998; Myllymäki et al 1997; Soest and Vliegenhart 1997). The evolution of starch structure along screws during extrusion has been studied using clam shell extruders and “dead stop” operations (Colonna et al 1982, Cai and Diosady 1993). Mechanical properties of thermoplastic starch materials seem to depend on amylose content and its ability to form a network structure governed by crystallinity (Soest 1997, Della Valle et al 1998). The high water affinity or solubility and aging of extruded thermoplastic starch materials appears to be the main limiting factor for largescale industrial use (Soest 1997, Soest and Vliegenhart 1997). To decrease water solubility, many chemical modifications have been proposed (reviewed by Fritz et al 1994), but the hydroxyl sugar group is the only reactive site, and therefore the possible chemical reactions are limited. Proteins as heteropolymers offer a much larger array of possible interactions and chemical reactions. The multitude of possible interactions may be one reason why, in spite of the rapid development of extrusion technology during recent years, information about the molecular mechanisms of protein interactions remains limited. Extrusion of proteins has been studied mainly for food use as textured vegetable proteins, primarily based on soy proteins (Camire 1991, Dahl and Villota 1991, Horvath and Czukor 1993) or soy protein-carbohydrate blends (Bhattachararya et al 1988). Extrusion of proteins recently was reviewed by Areas (1992). The structure formation of extrudates is believed to result from a complete restructuring of polymeric material in an oriented pattern. Formation of the final molecular network involves the dissociation and unraveling of macromolecules, which allows macromolecules to recombine and cross-link through specific linkages (Areas 1992). However,
the way in which proteins interact with proteins in an extrusion process is unclear. Based on investigations of wheat-flour protein solubility after extrusion at different temperatures, Li and Lee (1996a,b) emphasized the key role of hydrophobic interactions in disulfide cross-linking of wheat proteins during extrusion. According to Li and Lee (1996a,b), denatured proteins first associate by hydrophobic interaction of residues, and the sulfhydryl groups in associated proteins react with each other to form interchain disulfide bonds. Little research has been conducted on use of extrusion technology for nonfood applications of proteins. Proteins, especially from wheat, offer very interesting features for nonfood applications (reviewed by Gennadios et al 1994, Guilbert et al 1997, Cuq et al 1998). In contrast to starchy products, the viscosity of “protein melt” may increase with temperature due to cross-linking reactions and restrict operating conditions. To decrease melt viscosity, high levels of plasticizer may be necessary, which may result in undesired material properties. To cope with problems of increasing viscosity due to cross-linking reactions, Jane and Wang (1996) proposed the use of different reducing agents in an invention that provides a thermoplastic material made from soybean protein for obtaining solid plastic materials by extrusion and injection molding. In previous studies (Redl et al 1997, Redl 1998), the influences of temperature, plasticizer content, and mechanical energy on the rheological properties of a batch-mixed gluten-glycerol dough were investigated, and a generalized viscosity model was proposed. The object of the current study was to obtain a better understanding of the feasibility of shaping gluten-glycerol doughs by extrusion and to investigate the flow behavior of the resulting material and its developing structure during extrusion under a variety of conditions.
1 Unité
de Technologie des Céréales et des Agropolymères, ENSA.M-INRA, 2 place Viala, 34060 Montpellier Cedex 1, France. 2 Corresponding author. Phone: +33499612477. Fax: +33467522094. E-mail: REDL@ ENSAM.INRA.FR 3 Ecole de Mines de Paris, CEMEF, UMR CNRS 7635, BP 207, 06904 Sophia Antipolis, France. Publication no. C-1999-0415-07R. © 1999 American Association of Cereal Chemists, Inc.
MATERIALS AND METHODS Commercial vital wheat gluten was obtained from Amylum Aquitaine (Bordeaux, France). Protein content was 79.8% dry mass (dm), and water content was 6.2% dm. Food-grade glycerol was used (SA Pieri Chimie, Portes les Valence, France). Extrusion Extrusion was performed with a corotating, self-wiping twin-screw extruder with a barrel diameter of 25 mm (DS 25 Brabender O.H.G., Duisburg, Germany) connected to a computer interface and control unit (Brabender PL 2000). The extruder barrel consisted of three zones 150 mm long; each zone was equipped with an independent temperature control based on resistive heaters and water circulation in a double jacket. The second zone was equipped with an opening. Vol. 76, No. 3, 1999
361
The feeding zone was 100 mm long and cooled by water circulation. The total length of the screw was 550 mm. Each screw was composed of double-flighted right-handed screw elements with different pitches and a kneading section of five paddles with a righthanded staggering of 45°. The barrel was followed by a converging section and a die made up of three successive tubes: 19, 8, and 5 mm in diameter and 30, 69, and 23 mm long, respectively. Figure 1 is a schematic representation of the extruder and screw configuration. The pressure probes were strain-gauge sensors with a precision of 0.5% of the measuring range (Dynisco PT422 A: 0– 345 bars; Gneuss DA 150: 0–800 bars). Temperature was measured in the converging section with a flush-mounted temperature probe (Brabender 673276) and by introducing a manual thermocouple (G99858, k type, Bioblock Scientific, Illkirch, France) 1.6 mm diameter into the die for 5, 50, and 100 mm. Gluten powder was fed with a single-screw volumetric feeder (Afrem, Lyon, France), and glycerol was fed with a peristaltic pump (Bioblock Scientific, Masterflex M 39681, pump head M51005). The feed rate of gluten was determined at the beginning and end of each experiment. Glycerol feed rate was recorded continuously with a balance and continuous data acquisition system (Precisa 3600F connected to an IBM 286 PC and INRA software). Flow rate was determined by weighing samples collected for 1 min, and five readings were done every 10 min. Residence time distribution was determined by introducing 0.2 g of colored gluten-water dough (Coomassie brilliant blue G, SigmaAldrich, St. Louis, MO). Intermediate mean residence time was estimated by visual evaluation of maximum blue coloration at the opening. Residence time distribution was measured by collecting extrudate every 10 sec and evaluating the intenseness of blue col-
oration with a colorimeter (Minolta CR310) (Abecassis et al 1994). Residence time was determined in triplicate at 10-min intervals. Specific mechanical energy (SME) was determined by dividing net mechanical energy input to the extruder by extrudate flow rate. Net mechanical energy input was calculated using torque and angular velocity measurements provided by the continuous data acquisition system. Before beginning the experiment, the torque measuring device was calibrated with Brabender software, the screw speed was adjusted to set point, and gluten and glycerol feeds were started simultaneously at set point. After at least 30 min of stable torque and pressure conditions, feed and screw rotations were stopped simultaneously, the die with the converging section was dismounted, and the screws were extracted from the barrel. Samples for biochemical analyses were collected in the mixing section of the screw, in the beginning of the completely filled section, and in the converging section of the die. The gluten dough in the die was in a solid state and did not adhere to the surface. Therefore, it could be collected very easily without damage, and weighed. Division of the sample weight (30.2 ± 0.2 g) by the flow rate indicated residence time in the die. A piece of material collected in the 20-mm section of die was cut into three parallel disks 3 mm high for triplicate rheological analyses. Density of the material was determined by weighing a cylindrical specimen collected in the 8-mm section of die (ρ = 1.194 ± 0.006 × 106 g/m3). Extrudate dimensions were measured using a digital caliper square. Extrudate swell (ES) was calculated as: ES = d/d0
(1)
where d is extrudate diameter and d0 is die diameter (d0 = 5 mm).
Fig. 1. Brabender DS25 extruder. screw configuration defined (from hopper to die) as length: 187.5, 75, 37.5, 225, and 25 mm; pitch: 37.5 and 25 mm; and kneading block: 37.5 and 25 mm. TABLE I Operating Conditions and Measured Values of Extrusion Trials Run Under Stable Conditionsa
Trial
Speed (rpm)
Operating Conditions Feed Rate Tr (kg/hr) (°C)
SFL (g/rev)
Torque (Nm)
SME (kJ/kg)
P1 (bar)
Measurement T1 (°C)
T3 (°C)
MRT2 (sec)
62.7 (3.7) 69.2 (5.0) 93.8 (2.1) 97.0 (5.6) 93.9 (3.9) 94.5 (5.3) 114.7 (8.4) 104.4 (5.6)
637.9
50.8 (5.1) 45.4 (5.3) 38.8 (1.6) 69.2 (6.7) 59.1 (4.6) 67.5 (4.7) 101.4 (8.3) 83.2 (5.3)
86.7 (0.7) 86.6 (1.3) 91.9 (1.0) 89.5 (1.5) 101.4 (1.7) 106.4 (4.6) 78.0 (1.5) 91.5 (1.5)
97.3 (0.7) 110.86 (1.4) 133.7 (1.4) 107.65 (1.6) 138.8 (1.2) nd nd 101 (1.5) 123.7 (1.5)
153 (3.1) 147 (3.2) 123 (4.3) 75 (3.5) 59 (3.7) 46 (3.1) 80 (1.2) 60 (1.5)
A
50.0
1.9
80
0.62
B
100.0
1.9
80
0.32
C
200.0
1.9
80
0.16
D
100.0
4.9
80
0.83
E
200.0
4.9
80
0.40
F
200.0
8.1
80
0.68
G
100.0
4.9
60
0.83
H
200.0
4.9
60
0.42
a
1,357.2 3,699.9 734.5 1,476.4 877.9 864.9 1,545.5
ES 1.58 1.38 Disrupted 1.58 Disrupted 1.42 1.85 Irregular
SFL, specific feeding load (g/revolution); SME, specific mechanical energy; P1, pressure in die (Fig. 1); Tl and T3, temperatures measured with manual thermocouple introduced into the die for 5 and 100 mm, respectively; MRT1, mean residence time at opening; MRT2, total mean residence time; ES, extrudate swell (irregular and disrupted values could not be measured). Standard deviation of continuous data acquisition during at least 30 min for triplicate readings indicated in parenthesis.
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CEREAL CHEMISTRY
The experimental design was based on investigation of the influence of feed rate (Q = 1.9 ± 0.04, 4.9 ± 0.1, and 8.1 kg/hr) and screw speed (N = 50, 100, and 200 rpm) at a constant barrel temperature (Tr = 80°C) and the influence of barrel temperature (Tr = 40, 60, and 80°C) and screw speed (N = 50, 100, and 200 rpm) at a constant feed rate (Q = 4.9 ± 0.1 kg/hr). Glycerol content was kept constant at 53 ± 1.2%. Extrusion Modeling To obtain more information about the evolution of the main process parameters along the screws, we used twin-screw extrusion software (Ludovic) developed for computing extrusion-cooking processes (Della Valle et al 1993, Vergnes et al 1998). Ludovic is based on a 1D nonisothermal approach along an entire screw and allows users to calculate the main process parameters such as pressure, temperature, residence time, and filling ratio. The flow path along the screws followed a figure-eight pattern and was composed of a succession of flows along C-shaped chambers and flows in the intermeshing area between adjacent screws. Thus, pressure-flow rate relationships were developed for the two flow categories. Flows in kneading disks were modeled separately. The elementary models were linked to obtain a global description of the flow field along the extruder. It is assumed in the global model that melting is instantaneous and takes place before the first restrictive element of the screw profile. Afterward, the material is presumed to be fully molten and can fill the screw channel according to local geometry and flow conditions. Because screws are starve fed, the filling ratio of the system is unknown, so the computation has to be started from the die and proceed backward. However, because the final product temperature is unknown, an iterative procedure must be used. The model has been validated through comparisons among experiments on both natural and synthetic polymers (Vergnes et al 1998).
300 mm) was used with a TSK 3000-SW guard column (7.5 × 75 mm) (Beckman Instruments, Gagny, France). Columns were eluted isocratically by 0.1M sodium phosphate buffer (pH 6.9) containing 0.1% SDS. The flow rate was 0.7 mL/min at ambient temperature, and effluent was recorded at 214 nm. The apparent molecular weights of five major peaks (F1–F5) were estimated by calibrating the column with protein standards according to Dachkevitch and Autran (1989). The first peaks (F1 and F2) in the SE-HPLC profile obtained for the first extract corresponded to protein fractions ranging from Mr = 7 × 106 (the claimed molecular weight exclusion limit of the column used) to Mr = 150 × 103. The range corresponded to the known glutenin polymer molecular weight range (Kasarda et al 1976). Peaks F3 and F4 corresponded to proteins ranging from Mr = 150 to 20 × 103 and therefore could be assimilated to gliadins (Singh et al 1990). The second extract (after solubilization by sonication) allowed characterization of insoluble proteins (Fi) whose molecular weight exceeded 7 × 106 before sonication. Total extracted proteins (TEP) was estimated from the total surface of SE-HPLC profiles (first and second extracts). For native gluten, TEP was 16.34 ± 0.48 V/sec. Combining all values obtained for extrudate samples gave an average value of 10.86 ± 0.36 V/sec. Because of the glycerol content of extrudates (35.06 mg/100 mg), a TEP of 10.61 V/sec would be expected if all gluten proteins were extracted. The calculated value was similar to the experimental value, illustrating the efficiency of sonication in promoting solubilization of gluten proteins from extrudates. Peak fractions F1–F4 and Fi for the total area of the second extract were expressed as percent TEP. RESULTS AND DISCUSSION
Rheological Measurements Rheological measurements were taken in triplicate with a dynamic spectrometer (Mk III torsion head, Rheometrics, Piscataway, NJ) equipped with two parallel plates (20 mm diameter). The experiment was begun in the constant force mode of the rheometer, and the sample was compressed under 5 N until zero displacement; the experiment was stopped and restarted in the constant gap mode, and force was recorded. A time sweep was run for 60 min at 11 discrete frequencies of 0.03–30 Hz, which resulted in seven successive frequency sweeps. The measurement enabled us to check the stability of the product. The observed variation coefficient of the evolution within time was
