versão impressa ISSN 0104-6632
Braz. J. Chem. Eng. v.16 n.3 São Paulo set. 1999
The influence of specific mechanical energy on cornmeal viscosity measured by an on-line system during twin-screw extrusion
Y. K. CHANG1 , F. MARTINEZ-BUSTOS 2, T. S. PARK3 and J .L. KOKINI4
1Faculdade de Engenharia de Alimentos, Departamento de Tecnologia de Alimentos, Universidade Estadual de Campinas, Caixa Postal 6121,13083-970, Campinas - SP, Brazil. E-mail: firstname.lastname@example.org
2Laboratorio de Investigación en Materiales Centro de Investigación y de Estudios Avanzados del I.P.N., Queretaro, Qro., México.
3Department of Food Science, Rutgers University New, Brunswick, NJ 08903, USA.
4Center for Advanced Food Technology (CAFT), Rutgers University, NJ 08903, USA.
(Received: October 11, 1998; Accepted: May 28, 1999)
Abstract -The influence of specific mechanical energy (SME) on cornmeal viscosity during the twin-screw extrusion at feed moisture contents of 25 and 30% and screw speeds in the range from 100 to 500 rpm was measured. Cornmeal was extruded in a co-rotating, intermeshing twin-screw coupled to a slit die rheometer. One approach to the on-line rheological measurement is to use a slit die with the extruder. In the present work it was show that shear viscosity decreased as a function of SME. The viscosity of cornmeal at the exit die was influenced by screw speed, rate of total mass flow, mass temperature inside the extruder and SME. An increase in screw speed resulted in an increase in SME and a decrease in viscosity. A reduction in slit die height and an increase in screw speed and mass temperature led to a remarkable macromolecular degradation of the starch, as evidenced by the decrease in viscosity.
Keywords: Twin-screw extruder, specific mechanical energy, slit die rheometer, cornmeal viscosity.
In recent years, food extrusion has developed into a versatile processing technique. Many food or feed products such as breakfast cereals, snacks, modified starches, baby foods and pet foods are produced using food extrusion. Modern extruders are high-temperature, short- time (HTST) food processing units and have the ability to continuously mix, knead, shear, cook, cool, form and puff the extrudates (Harper, 1981).
Extrusion cooking of starch materials has been widely studied during the last ten years. Conversion of starch properties in the extruder depends on a large number of variables in the machine and raw material control parameters. Independent process parameters include screw speed, screw configuration, product moisture content, temperature, total mass flow rate and die configuration. These independent parameters affect such system parameters as residence time distribution, the energy requirement of the process, the pressure profile along the barrel and the pressure drop in the die (Meuser et al., 1987). Methods commonly used to determine the rheological properties of viscoelastic materials include the use of conventional rheometers (Launay and Buré, 1973; Harmann and Harper, 1974; Tsao et al., 1978; Cervone and Harper, 1978; Remsen and Clark, 1978). However they are unsuitable because their shear rates do not cover the full range encountered during extrusion processing. Therefore, flow reproduction such as that occurring within the extruder is difficult to create with stabilized rheometers. Thus, the rheological properties of food materials undergoing extrusion cooking must be measured on-line. Theoretical details on slit rheometry were outlined by Han (1976). The exit pressure method was used by Han et al. (1972) to obtain reliable rheological information on viscosity and the first normal stress difference, which is the result of viscoelastic characteristics, when the polymer melts. Wen et al. (1990) studied the extrusion of corn meal with various moisture contents, screw speeds, and temperature conditions. These investigators concluded that maximal fragmentation of starch generally occurred when mechanical energy was at its peak. Polymer breakdown during extrusion is considered to be the result of mechanical rather than thermal degradation, although thermal degradation is known to occur during prolonged heating processes (Kalentunc and Breslauer, 1993). Ryu and Walker (1995) cited that feed moisture content and rate, screw speed and geometry, and process temperature affected the physical properties of extruded wheat flour. Fujio et al. (1995) reported that the huge molecule of starch can be depolymerised by a physical force such as a shearing force. The higher the SEM input, the higher the extent of starch granular size reduction (Zhen et al., 1995). Starch molecules having low-moisture contents can be depolymerised easily by heated or heated-sheared treatment and the shearing force in addition to heat treatment contributed significantly to molecular cleavage (Fujio et al., 1995).
The objective of this research was to understand quantitatively the effect of specific mechanical energy (SME) and other parameters on the viscosity (on-line measurement) of cornmeal during the twin-screw extrusion process.
MATERIALS AND METHODS
The yellow cornmeal sample used in this study was obtained from Lauhoff Grain Co. (Danville, Illinois, USA).
Extruder and Extrusion Conditions
The cornmeal samples were extruded in a Werner Pfleider ZSK-30 co-rotating, intermeshing twin-screw extruder with rectangular slit die rheometer attached to the exit die and equipped with a K-Tron loss-in-weight feeder. The L/D ratio was 29 and the screw configuration used is shown in Figure 1.
Figure 1: Screw Configuration. 1-9 Barrel Sections, a) mixing element, b), c), d) and f) kneading element, e) reverse element, g) reverse kneading element.
Experimental Design for Extrusion Variables
The experimental design considered the following variables: internal mass temperature before extruder exit (100 to 170ºC), feed moisture content (20 to 40% w.b.), screw speed rate (100 to 500 rpm) and slit die rheometers A (0.15 cm height) and B (0.30 cm height) (Figure 2).
|Figure 2: Schematic Diagram of Slit Rheometer|
The extruder was fitted axially along the barrel length with dual purpose Dynisco temperature and pressure transducers. Accurate determinations of PS/2 of mass pressure and mass temperature (±1ºC) were carried out using a Data Acquisition and Control System with a Keitheley Series 500 DAC system coupled to an IBM/PC-PS-230.
Rheological properties were measured using a slit die rheometer with heights of 0.15 and 0.30 cm, a width of 2.00 cm, and a length of 14.8 cm, with provisions for measuring pressures at four axial locations.
The Dynisco pressure/temperature transducers (Model TPT 463 E) had ranges of 0 to 500, 0 to1500 and 0 to1000 psi and were placed at the die extruder entrance, at the center and at the exit of the slit die with L/D value of 31:1, 56:1, 82:1, and 90:1.
In all the experiments reported here, the temperature was set to the required value in such a way that the extrudate temperature in all the last four zones of the slit die reached a constant desired temperature. The slit die rheometer was heated using glass fiber insulated heating tapes.
The moisture content during extrusion was adjusted by adding water using a water pump. Actual readings and measurements were recorded at steady state, was achieved, which was determined by pressure, temperature and torque measurements.
Temperature and pressure were recorded every 3 seconds in the DAC system connected to the PR685 Dynisco microprocessing pressure indicators. The shear stress and the shear rate at the wall were determined using the principles of slit rheometry (Han, 1988).
Shear Stress (tw)
The shear stress at the wall of the barrel and die was determined using the following equation:
where tw = shear stress at the wall, H = height of the channel (cm) of the slit die, DP = pressure drop of slit die and L = length of the die (cm).
The shear rate at the wall (g·wall) was calculated as follows:
With this equation, includes the Rabinowitch-Mooney correction for non-Newtonian fluids, which the apparent shear rate (g·app.) can be calculated as follows:
where g·app. = apparent shear rate, W = width of the slit die (m), Q = material flow (m3/sec) and H = height of the channel (m) slit die rheometer.
Apparent viscosity (m app.) as determined using the following equation:
Specific Mechanical Energy
Specific mechanical energy (SME) could be determined by measuring the torque and the screw speed at a constant mass flow rate. The equation for specific mechanical energy is as follows:
Torque x Screw Speed (2p * RPM)
SME (W hr / kg ) = ------------------------------------------------
Mass Flow Rate
RESULTS AND DISCUSSION
Product Exit Pressure Profile Measured by the Slit Die Rheometer
Typical pressure profiles for cornmeal at mass temperatures of 140°C and 160°C; screw speeds of 150 and 500 rpm, respectively; a moisture content of 30% (w.b.) and total mass flow rates of 100, 200, 300, 400 and 500 g/min are shown in Figures 3 and 4. The first important issue is the linearity of the pressure profiles obtained. The pressure profiles along the slit die for cornmeal taken at die temperatures of 140 and 160°C and under different extrusion conditions were studied. The exit pressure in this study was obtained by a linear extrapolation of the pressure profile from the region of fully developed flow. Interestingly, as viscosity increased with a decrease in temperature, an increase in the extent of linearity of the pressure profiles was observed. This indicated that an increase in viscosity resulted in a more fully developed flow along the slit die. Extrusion pressure increased as the feed rate increased and the axial length of the slit die decreased. The exit pressure values at a screw speed of 150 rpm and 140ºC were higher than those at 500 rpm and 160ºC. The results of the linear regression of the pressure profiles obtained in this study with correlation coefficients larger than 0.9999 show that all four pressure transducers were basically located in the fully developed flow region for the materials studied. The extent of physical and chemical changes is strongly dependent on both residence time and specific mechanical energy (the mechanical work done on the material per unit mass feed rate) (Wang et al., 1990). A small decrease in throughput (e. g., by a small increase in die resistance) may change the hold up, increase residence time, and therefore increase viscosity. If the increase in viscosity strongly affects the back flow, the process become stabler (Janssen, 1986). The residence time distribution for a power law fluid depends on the power law index, n, and the magnitude of the pressure flow (Bigg and Middleman, 1974). As the power law index decreases, the cumulative exit age distribution becomes broader, tending to ward the limit for complete mixing. As the back pressure in the extruder increases, the distribution tends to resemble a plug flow curve. These effects are relevant to the operation of food extruders since food melts have been characterized as pseudoplastic materials (Harper, 1981). Extruded starchy material exhibits non-Newtonian (pseudoplastic) flow behavior. For a similar time temperature history, its apparent viscosity decreases with an increase in temperature or shear rate (Remsen and Clark, 1978). The change in screw speed significantly affected the degree of fill. Lower screw speeds resulted in a higher degree of fill. A change in the degree of fill significantly changed the energy transport between the food materials and the extruders. Newton's theory predicts that average residence time is inversely proportional to screw rotation speed and independent of all other operating parameters. It is determined only by geometrical factors and screw speed (Davidson et al., 1983).
Figure 3: Pressure Profile of Cornmeal along Slit Die. 30% moisture content, 140° C mass temperature, 150 rpm screw speed.
Figure 4: Pressure Profile of Cornmeal along Slit Die. 30% moisture content, 160° C mass temperature, 150 rpm screw speed.
Effect of Specific Mechanical Energy (SME) and Operational Extrusion Parameters on Shear Rate at the Wall
Figure 5 illustrates the SME for cornmeal plotted against the shear rate at the wall. At a constant extruder screw speed, an increase in the mass flow rate is associated with an increase in the shear rate and a decrease in the SME. Increasing screw speed causes an increase in the volumetric flow rate that results in an increased shear rate. At a constant mass flow rate, an increase in extruder speed is associated with an increase in SME. The same effect was found by Van Lengerich (1984) with extruded wheat starch. This author reported that increasing the screw speed causes increases in SME values, attributed to the higher frictional heat and the simultaneous increase in temperature. What is clear from this figure is that shear rate and SME are not independent. Clearly, they are non-linearly related. It is therefore necessary to devise a technique to keep SME constant and estimate viscosity as a function of shear rate under constant SME conditions. In twin-screw extruders SME decreases when the screw speed decreases (Meuser et al., 1982, Fletcher et al., 1985, Della Valle et al., 1989). Tsao et al. (1978) and Della Valle (1989) observed that SME increased at higher screw speeds.
Figure 5: Specific Mechanical Energy vs Shear Rate at Wall of Cornmeal. 35% moisture content, 115° C mass temperature.
Effect of Specific Mechanical Energy (SME) and Operational Extrusion Parameters on Shear Viscosity
Figures 6 and 7 show the effect of SME on the shear viscosity of cornmeal at mass temperatures of 140 and 160°C, respectively. Shear viscosity data for cornmeal with a moisture content of 30%; 100, 200, 300, 400 and 500 g/min feed rates; 150, 300 and 500 rpm screw speeds and a mass temperature of 140°C showed that increasing SME resulted in lower viscosities. This is consistent with the findings of Wang et al. (1990), who showed that extensive conversion and degradation of starch was observed as a function of SME. In fact they developed a kinetic model to predict the conversion of starch during extrusion due to mechanical energy. This suggests that the starch component plays a key role in terms of affecting the rheological properties of cornmeal. Also, temperature and molecular weight affect the mechanical degradation principally due to their contribution to viscosity. As molecular weight increases, so does the melt viscosity and therefore the shear stress at equivalent shear rates. Mechanical degradation decreases with temperatures up to a maximum temperature. The negative temperature coefficient reflects the effect of temperature on viscosity and is a critical factor in a mechanochemical reaction (Casale and Porter, 1971). Maga and Fapojuwo (1986) and Bhattacharya and Hanna (1987) concluded that in high-shear extruders the heat input into the product is by mechanical dissipation and the heat from thermal energy is not significant. The SME input also depends on the exact composition of the product being extruded (Maga and Fapojuwo, 1986) and increases as starch content increases (Meuser and Wiedmann, 1989).
Figure 6: Shear Viscosity vs Specific Mechanical Energy of Cornmeal. 30% moisture content, 140° C mass temperature.
Figure 7: Shear Viscosity vs Specific Mechanical Energy of Cornmeal. 30% moisture content, 160° C mass temperature, 100-500 g/min feed rate.
The effect of an increase in temperature (160°C) on the shear viscosity of the extruded samples(Figure 7) shows that higher extrudate temperatures result in lower shear viscosities. Mechanical and thermal energy effectively contributed to the reduction of product viscosity. Van Lengerich (1984) cited that shear stress in the slit die was found to depend mainly upon the water content of the product and the barrel temperature in the extruder, which in turn influence product temperature. Also, these results are consistent with those obtained by Meuser et al. (1987) and Davidson et al. (1984). Higher temperatures increase product temperature, and in turn, decrease viscosity and pressure. This was confirmed by temperature and pressure measurements (Mosso et al., 1982, Meuser and Van Lengerich 1984, Fletcher et al., 1985, Meuser et al., 1987, 1989). The effect of increasing the screw rotation rate is an increase in shear rate and a decrease in residence time in the extruder but an increase in the intensity of the mechanical treatment. Basedow (1979) reported that shear stress was the controlling factor in the degradation process and that the constant rate was a function of shear stress. In a twin-screw extrusion at a constant mass flow rate, one obtains a constant shear rate. Variation in RPM does not result in a change in shear rate because the volumetric flow rate is controlled by the mass flow rate. However, variation in revolutions per minute (rpm) results in changes in SME at a constant shear rate. Therefore it is possible to study the effect of SME at a constant shear rate.
Effect of Slit Die Rheometer Height on Shear Viscosity and Shear Rate at the Wall
The effects of the slit die rheometer height (0.15 and 0.30 cm), mass temperature (125, 140 and 160°C) feed rate (100, 200, 300, 400 and 500 g/min), screw speed (100, 300 and 500 rpm) and moisture content (30%) on the viscosity characteristics of extruded products are shown in Figures 8, 9 and 10.
Figure 8: Shear Viscosity vs Shear Rate of Cornmeal. 30% moisture content, 150 rpm screw speed.
Figure 9: Shear Viscosity vs Shear Rate of Cornmeal. 30% moisture content, 140° C mass temperature, 100-500 g/min feed rate.
Figure 10: Shear Viscosity vs Shear Rate of Cornmeal. 30% moisture content, 160° C mass temperature, 100-500 g/min feed rate.
Shear viscosity decreases logarithmically as shear rate at the wall increases at both die heights (0.15 and 0.30 cm) and all three mass temperatures (125, 140 and 160°C); this characterizes the classical shear-thinning pseudoplastic behavior of cornmeal dough during extrusion. The combination of processing parameters and slit height contributed to significantly lower viscosities with the 0.15 cm die as compared to the 0.30 cm die. It is evident that the reduction in slit die height, the increase in screw speed and the increase in mass temperature led to a remarkably macromolecular degradation of starch during the extrusion process, thereby modifying the functional properties such as viscosity. Also, increases in barrel temperature caused increases in product temperature, which led to the lower viscosities of the product inside the extruder and the introduction of a lower mechanical energy. Decreasing the die hole diameter increased the shear stress on the mass of wheat starch in the extruder; this increase resulted in a higher torque at the screw shafts and therefore increased the SME (Van Lengerich 1984). During extrusion the introduction of specific thermal and mechanical energy causes various changes in the structure of starch. The principal effect of this thermomechanical treatment is to rupture the granular structure (Mercier et al 1980, Ding and Tang 1990, Tang 1991, Davidson 1992, Kalentunc and Breslauer 1993). Rodis et al. (1993) indicated that the amount of fragmentation is highly affected by the chemical nature of the extrudate, design and configuration of the extruder and extruder operating conditions. Ryu and Walker (1995) cited that feed moisture content and rate, screw speed and geometry, and process temperature affected the physical properties of extruded wheat flour.
On-line rheological measurements using a slit die in conjunction with a co-rotating, intermeshing twin-screw extruder were done for cornmeal. It was clearly shown that the shear viscosity of cornmeal is significantly influenced by extruder processing parameters such as screw speed, total mass flow rates and extrudate temperatures as well as by varying the specific mechanical energy consumption during extrusion.
An increase in screw speed caused an increase in SME input which resulted in a structural breakdown of the starch resulting in a lower viscosity. Also, the use of slit dies with heights of 0.15 or 0.30 cm and mass temperatures of 125, 140 or 160°C decreases shear viscosity .
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