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Brazilian Journal of Chemical Engineering

Print version ISSN 0104-6632On-line version ISSN 1678-4383

Braz. J. Chem. Eng. vol. 15 n. 4 São Paulo Dec. 1998

https://doi.org/10.1590/S0104-66321998000400006 

EFFECT OF SOME EXTRUSION VARIABLES ON RHEOLOGICAL PROPERTIES AND PHYSICOCHEMICAL CHANGES OF CORNMEAL EXTRUDED BY TWIN SCREW EXTRUDER

 

Y.K. CHANG1,*, F. MARTÍNEZ-BUSTOS2 and H. LARA1
1Faculdade de Engenharía de Alimentos, Departamento de Tecnología de Alimentos, Universidade Estadual de Campinas. Caixa Postal 6121-13083-Campinas-SP, Brazil. yokic@fea.unicamp.br  Phone/Fax 55192393617
2Laboratorio de Investigación en Materiales Centro de Investigación y de Estudios Avanzados del I.P.N. Queretaro, Qro. México.

 

(Received: December 18, 1997; Accepted: October 9, 1998)

 

Abstract - The effect of extrusion variables, such as barrel temperature (100 to 170ºC), feed rate (100 to 500 g/min), feed moisture (20 to 40 g/100 g wet basis), screw speed rate (from 100 to 500 rpm), and slit die rheometer configuration (0.15 and 0.30 cm height) were studied using a co-rotating intermeshing twin-screw extruder coupled to a slit die rheometer on the rheological properties of yellow cornmeal. An increase in feed rate decreased WAI and WSI, but increased the viscosity values. The temperature interacts strongly with screw speed in affecting the WSI. The most important factor in starch degradation was the screw speed. Increasing the screw speed completely modifies the organised structure of starch (crystalline region).
Keywords: Extrusion variables, rheological properties, screw speed.

 

 

INTRODUCTION

Thermoplastic extrusion is a process by which the raw material, formed basically by starch and protein is subject to structural modifications and through which proteinaceous material is modified and starch is gelatinised losing their native organised structure, and forming a plastic and viscous mass as it goes through the extruder. Knowledge of the properties of the raw material and the nature of its flow under the complete range of conditions within the extruder, mainly in the transition zone of the screw (high pressure, and metering zone) and also in the die zone is very important to control the extruder behaviour for producing good quality, extruded end products. Rheology provides an effective method for characterising changes in the material during extrusion and may indicate time, temperature, and shear history effects. Some researchers have measured the effects of process parameters on the molecular changes seen in the structure of the starch granule (Linko, 1992). Others have proposed physical and chemical property changes in the starch granule during extrusion (Linko et al.,1980, Linko et al.,1984; Davidson et al.,1984). Other methods such as die pressure may, however, be used as an index of the viscosity of the melt (Tadmor and Gogos, 1979). Tang and Ding (1992) proposed a mathematical model that relates the expansion ratios and the extent of molecular degradation of corn starch. During extrusion, the introduction of specific thermal and mechanical energy causes various changes in starch structure. The principal effect of this thermomechanical treatment is to rupture the granular structure of starch. The partial or complete destruction of the crystalline structure of starch is shown by X-ray diffraction patterns (Mercier et al., 1980) and scanning electron micrographs (Chinnaswamy et al., 1989). Many authors have also studied the changes of starch macromolecular structure due to extrusion (Colonna and Mercier, 1983; Ding and Tang, 1990; Tang, 1991; Davidson, 1992). Kalentunc and Breslauer (1993) proposed the use of glass transition temperature measurements on extrudates as a criterion for adjusting operating extruder conditions. 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 feed rate, screw speed and geometry, and process temperature affected the physical properties of extruded wheat flour. The higher the Specific Mechanical Energy input, the higher the extent of starch molecular size reduction (Zhen et al., 1995). Van Zuilichem et al. (1984) compared the engineering aspects of single- and twin-screw extruders during the extrusion-cooking of biopolymers. In addition to extruder characteristics, the specific power consumption, throughput and extrudate properties were expressed as functions of parameters such as die diameter, screw speed, barrel temperature and feed moisture. In the field of extruded starchy products, some important progress has recently been made to explain product properties in terms of molecular transformations. However, little has been done on the on-line measurement using slit die during extrusion to evaluate the functional and physical characteristics of starch. Current product transformation is best correlated with the energy provided to the product. It is clear that research is needed to develop direct and more precise methods for the measurement of the rheological properties of biopolymers. In this study one approach was attempted in order to measure some functional properties by on-line measurement using a slit die during extrusion processing and evaluating the structural characteristics of extruded cornmeal.

 

MATERIALS AND METHODS

Raw Material

The yellow cornmeal sample used in this study was obtained from Lauhoff Grain Co. (Danville, Illinois USA). Due to the complexity of the food systems and considering the proximate composition of cornmeal, this material was selected as a model system for this study. The proximate composition of cornmeal was 12.5 g/100 g moisture on a wet basis; 7.5 g/100g protein, 0.75 g/100g fat, 0.56 g/100g fibre, and 76.5 g/100g starch. The distribution of the cornmeal fractions were: > 850, 0.3%; 710-850, 8.7%; 600-710, 43.0%; 425-600, 45.0%; 300-425, 2.0%; < 300, 1.0%. Cornmeal is comprised of a large proportion of starch, moderate amounts of protein and small amounts of lipids, fibre and minor components.

Extruder and Extrusion Conditions

The cornmeal samples were extruded in a Krupp Werner & Pfleiderer ZSK-30 co-rotating, intermeshing, twin screw extruder coupled with a rectangular slit die rheometer and equipped with a K-Tron loss-in-weight feeder. The L/D ratio of the barrel was 29, and the screw configuration is shown in Figure 1.

 

Experimental Design for Extrusion Variables

The experimental design considered the following variables: internal mass temperature of extruder (100 to 170ºC), feed moisture content (30 g/100g wet basis), screw speed rate (100 to 500 rpm) and slit die rheometer A (0.15 cm height) B (0.30 cm height) (Figure 2).

The extruder was fitted axially with dual purpose Dynisco temperature and pressure transducers along the barrel length. Accurate determinations of PS/2 mass pressure and mass temperature (±1ºC) were carried out using the Data Acquisition and Control System Keitheley Series 500 DAC system coupled to an IBM/PC-PS-230.

The rheological properties were measured using a slit die rheometer attached to the exit die of the extruder with the following dimensions: 0.15 or 0.30 cm height, 2.0 cm width, and 14.8 cm length, with provisions for measuring pressures at four axial locations.

The Dynisco pressure/temperature transducers (Model TPT 463 E) were used with a range of 0-500, 0-1500 and 0-1000 psi and placed in the die extruder entrance, in the centre and in the exit of the slit die with L/D: 31: 1, 56: 1, 82: 1, and 90: 1

Analytical Methods

Moisture content, protein (N x 6.25), fat (ether extract), starch, fibre, and ash contents were determined using standard AACC (1983) procedures. The flour particle size was determined using USA standard screens # 40, 60, 80, 100 and 120 in a Rotap-shaker for 10 min. The viscoamylograph Brabender was used to measure paste viscosity characteristics according to the AACC method (1983). The following parameters were determined from the viscoamylograph curve (viscosity in amylograph unit, B.U. vs. time in minutes): Initial viscosity was defined as the initial viscosity of the suspension when starts the heating cycle (25° C); viscosity at 90° C was defined as the maximum viscosity at 90° C during heating cycle; final viscosity was defined as the viscosity value at the end of the cooling cycle (50° C). The modification of the physical structure of the starch was determined following the method of X-ray diffraction proposed by Mercier et al. (1980). Water Absorption Index (WAI) and Water Solubility Index (WSI) were determined following the method of Linko et al. (1980).

 

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.

 

 

Figure 2: Schematic diagram of slit rheometer.

 

RESULTS AND DISCUSSION

Effect of Extrusion Operational Parameters on Water Absorption Index and Water Solubility Index of Extruded Products

The effects of feed rate (from 100 to 500 g/min), screw speed (300 and 500 rpm), and barrel temperature (140 and 160°C) using a slit rheometer (0.15 and 0.30 cm height) on WAI and WSI of extruded products at 30 g/100g (wet basis) moisture content are shown in Figures 3 and 4 respectively. Under all the conditions studied, increasing the feed rate resulted in a trend towards decreasing the WAI and WSI. Van Lengerich (1984) reported that the hydration capacity in wheat starch first increased with increasing energy and then decreased up to a total loss of hydration capacity. By increasing the feed rate, the shear rate and extrusion pressure increased resulting in a lower WAI and WSI. The feed rate and screw speed affected the key parameters associated with a greater hydrolysis of starch such as the water solubility index and the water absorption index. A breakdown of polymers 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). Kim and Rottier (1980) reported that only damaged starch granules absorb water at room temperature and swell, an increase in the viscosity. After reaching a maximum, with respect to the degree of starch damage, WAI decreased with the onset of dextrinisation. An inverse effect was observed on the WSI. Owusu-Ansah et al. (1983) reported that as the gelatinisation results, the interaction between temperature and moisture was the most significant variable. When this is removed, temperature becomes the next most significant variable (P £ 0.01), followed by moisture and then screw speed. The second order terms of moisture and temperature, and the interaction between temperature and screw speed were also significant at the 5% level.

The WSI reduced as the feed rate increased. At a given feed rate the WAI and WSI were lower at 500 rpm than at 300 rpm screw speed. Also, the lowest values for WSI were lower at the lowest mass temperatures. This indicates that an increase in feed rate and screw speed and a decrease in mass temperature results in a more fully developed flow along the slit die increasing starch breakdown. These observations were similar to results reported by Mason and Hoseney (1986) and Tang and Ding (1994). The low water accessibility of extruded starches can be ascribed to their compact structure, whereas the solubility may be related to the lower molecular weight of the starch components, which can be separated quite easily from each other owing to more limited entanglement (Doubter et al., 1986). The water solubility index and extrudate slurry flow behaviour indices were also both correlated to the extent of molecular degradation (Tang and Ding 1994).

 

 

 

 

Effect of Extrusion Operational Parameters on the Viscosity Characteristics of the Extruded Products

The effects of feed rate (from 100 to 500 g/min), screw speed (300 and 500 rpm), temperature (160 and 140°C) using a slit rheometer (0.15 and 0.30 cm height) on the viscosity characteristics of extruded products at 30 g/100g (wet basis) moisture content are shown in Figures 5, 6 and 7. The initial viscosity, viscosity at 90°C and final viscosity at 50°C of the extrudates increased linearly with feed rate. An increase in screw speed decreased the initial viscosity and viscosity at 90°C of the extruded products. The change in screw speed affected the degree of full along the slit. As the feed rate and screw speed increased, a full flow developed along the slit die increasing the viscosity. The viscosity determined at 90°C was higher at barrel temperature of 160°C with the 0.15 cm slit rheometer than at 140°C with the 0.30 cm slit rheometer. A small decrease in die resistance changed the hold up, increased the residence time and therefore increased the viscosity. On the other hand, mechanical and thermal energy contributed to the reduction in viscosity. The viscosity at 50°C, using the 0.30 cm slit rheometer showed a sharp increase with an increase in feed rate from 300 to 500 g/min. The viscosity at 50°C was higher at 300 rpm than that observed at 500 rpm with the 0.3 cm slit die rheometer and a mass temperature of 140°C . The viscosity at 90ºC showed lower values than those observed at 50ºC with a greater tendency to retrogradation during the cooling cycle. The primary effect of increasing the feed rate is to decrease the Specific Mechanical Energy (SME) input and to decrease the product transformation, the counteracting effect being a small increase in the residence time (Colonna et al., 1989). Mercier et al. (1975) showed that when the extrusion temperature is increased, the final cooked paste consistency (viscosity at 50°C) decreases. Chiang and Johnson (1977) studied the influence of extrusion variables on the gelatinisation of wheat flour. At temperatures above 80°C gelatinisation of starch in the sample sharply increased, and the final viscosity at 50ºC decreased. Anderson et al. (1969) reported that the high initial viscosity is typical of gelatinised products while lower viscosities indicate the breakdown of polymers. The absence of a peak at 60-70°C reveals that no intact starch granules were present in the extrudates, otherwise they would have gelatinised increasing the paste viscosity. At a given moisture content, the hot-paste viscosity increases up to a maximum (Launay and Lisch, 1983; Mason and Hoseney, 1986) and then decreases as the extrusion temperature increases. At a given extrusion temperature, the hot-paste viscosity increases as the moisture of the starch increases (El-Dash et al.,1984). An increase in feed rate produces three major effects. The first one is to change the working point of the extruder with a pressure increase at the die (Van Zuilichem et al.,1984; Tayeb et al.,1989). The second one is to induce a temperature decrease due to a reduction in SME input; this has been observed by Fletcher et al. (1984) and Della Valle et al. (1987). The third is that the average residence time does not change significantly when the feed rate is changed (Bounié and Cheftel, 1986). Llo et al. (1996) reported that the apparent viscosity, SME, and product properties of extruded maize grits were found to be very dependent on the feed moisture.

 

 

 

 

 

 

 

Effects of Extrusion Parameters and Variation of Slit Die Rheometer Height (0.15 and 0.30 cm) on the Crystalline Structure of Extruded Cornmeal Starch

Macromolecular degradation during extrusion has been demonstrated for pure starches (Mercier, 1977; Launay and Koné, 1984; Diosady et al., 1985) and flours (Scheweizer and Reimann, 1986). Charbonniere et al. (1973) used X-ray diffraction techniques to study the effects of extrusion cooking on the organised structure of starch and concluded that this physical technique is the best instrumental method for the estimation of crystallinity. The effect of the variation of slit die rheometer height (0.15 and 0.30 cm) on the crystalline structure of extruded starch which was processed with 30 g/100g (wet basis) moisture content, 160°C barrel temperature, 500 rpm screw speed and 300 g/min feed rate is illustrated in Figure 8. Increase in slit die height at least somewhat hydrolysed the starch and increased starch degradation. X-ray diffraction already showed a significant loss in crystallinity at very low energy inputs. Van Lengerich (1984) reported that even at low thermal and mechanical energy inputs in the extruder, the amount of crystallinity remaining in the granule was reduced dramatically The difractogram of sample A (0.30 cm slit die height) showed minor peak at 2q = 13° , and strong peaks at 2q = 18.7° , and 2q = 22.6° This pattern closely matches reported values for A-type cereal starches (Charbonniere et al., 1973; Zobel, 1988; Bhatnagar and Hanna, 1994). According to Mercier et al. (1979) cereal starches behave quite differently. Commercial corn starch, at 22 g/100g (wet basis) moisture and extruded at 135°C, formed a new type of structure characterised by three main peaks on the X-ray diffraction pattern. The X-diffractograms of sample B (0.15 cm slit die height) showed that the organised structure of starch was destroyed in the extruder to a great extent. It is evident that the reduction of slit die height led to a remarkably macromolecular degradation during the extrusion process and modified their functional properties such as viscosity. This could be related to the reduction in slit die height and an increase in viscosity due to higher shear rates, resulting in higher mechanical damage of the starch. The loss of the organised crystalline structure of the starch granules on extrusion cooking can be demonstrated by X-ray techniques. It is quite clear that starch granules may survive extrusion under conditions of high moisture or low shear (Tang and Ding, 1994), but with the increasing severity of thermal treatment (19 g/100g wet basis moisture, 150° C), the granules lose their organised structure (Lai and Sarkanen, 1969).

Effects of Variation of Feed Rate (from 100 to 400 g/ min) and Extrusion Parameters on the Crystalline Structure of Extruded Cornmeal Starch

The effect of variation of feed rate on the structure of the starch of cornmeal samples extruded with the 0.15 cm slit die rheometer, 30 g/100g wet basis moisture content, 160°C barrel temperature, and 500 rpm screw speed is shown in Figure 9. An increase in the feed rate produces greater degradation of the starch crystalline structure. The samples extruded varying the feed rate from 100 to 300 g/min did not show peaks. However, the increase in the feed rate at 400 g/min showed a peak at 2q = 19.8° , indicating that only a few small molecular breakdown products were formed. The extent of starch degradation was shown to be dependent on the variation of the feed rate. Rodis et al. (1993) showed that the amount of fragmentation is highly affected by the chemical nature of the extrudate, the design and configuration of the extruder and the extruder operating conditions. Chinnaswamy et al. (1989) and Bhatnagar and Hanna (1994) indicated that peaks located at 2q of 12.7 and 19.8 A resulted as a formation of complexes between starch and native lipids. Della Valle et al. (1989) reported that as a result of increasing the feed rate, the Specific Mechanical Energy input decreased, and, when the SME was below 250-360 kJ. kg-1, product transformation decreased with extrudates showing large amounts of flattened and sheared granules. With SME values higher than 500-600 kJ. kg-1 important granule damage occurred. Cai and Diosady (1993) developed models for the mechanical degradation of wheat starch in single screw and twin screw extruders. The models correlated the extent of molecular degradation of extruded starch with residence time, nominal shear stress, action on the starch macromolecules and the degree of cooking. Extruded cassava starches have a lower gelling and thickening power due to molecular reorganisation and/or depolymerisation (Perez-Sira and Gonzalez-Parada, 1997).

 

Image343.gif (1503 bytes)

Figure 8: Effect of variation of slit die rheometer height on x-ray diffraction patterns of cornmeal starch. A : 0.15 cm; B: 0.30 cm; 30g/100g moisture content, 160° C barrel temperature, 500 rpm screw speed, 300 g/min feed rate.

 

 

Figure 9: Effect of variation of feed rate on x-ray diffraction patterns of cormeal starch.
A : 100 g/min; B: 200 g/min; C: 300 g/min; D: 400 g/min; 30 g/100g moisture content, 160° C barrel temperature, 0.15 slit die rheometer height.

 

Effects of Variation of Temperature (160 and 180°C) and Extrusion Variables on the Crystalline Structure of Extruded Starch

Figure 10 shows the effect of the variation of barrel temperature on the structure of the starch of cornmeal samples extruded using 0.15 cm slit die rheometer, 30 g/100g wet basis moisture content, 500 rpm screw speed, and a feed rate of 400g/min. The evaluated extruded conditions led to the granules losing their organised structure. However, the increase in barrel temperature from 160 to 180°C showed a lower modifying degree, showing a peak at 2q = 19.8 A. These results are in concordance with those determined by the viscosity characteristics, where an increase in barrel temperature decreased the viscosity. Extrusion-cooking formed complexes between starch and native lipids with a peak at 2q = 19.8 A (Mercier et al., 1980). A known effect of the mass temperature on product transformation is the enhancement of the disruption of the granules by the gelatinisation process. This phenomenon occurs at various temperatures depending on the water content; a high water content permits a lower gelatinisation temperature (Colonna and Mercier, 1983). From the product temperature effect, an increase in barrel temperature tends to induce a more transformed product but to a limited extent only. The decrease in viscosity decreases mechanical heat generation, as confirmed by Diosady et al. (1985), who observed a decrease in intrinsic viscosity of wheat starch with barrel temperature increases in a single-screw extruder. Also, with the increasing severity of thermal treatment the granules lose their organised structure (Richmond and Smith, 1985). Charbonniere et al. (1973) demonstrated that at higher extrusion temperatures, the structure of cassava starch is completely destroyed, leading to an X-ray pattern typical of an amorphous state. Also these authors observed reduced crystallinity in potato and cassava starch which are free of lipids, and in waxy corn starch, which is free of amylose at an extrusion temperature as low as 70ºC. At higher temperatures, the structure is completely destroyed, leading to an X-ray pattern typical of an amorphous state. Wen et al. (1990) studied the extrusion of corn meal under 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. Starch molecules containing low-moisture content can be depolymerised easily by heated or heated-sheared treatment, and the shear force in addition to the heat treatment contributed significantly to molecular cleavage (Fujio et al., 1995).

Effects of Variation of Screw Speed (150, 300, and 500 rpm) and Operational Parameters on the Crystalline Structure of Extruded Cornmeal Starch

The effect on the variation of screw speed on the structure of starch of extruded samples using a slit die rheometer at a height of 0.15 cm and processing with a 30 g/100g wet basis moisture content, 160° barrel temperature, feed rate of 300 g/min and screw speeds of 150, 300, and 500 rpm are shown in Figure 11. When the screw speed was low (150 and 300 rpm), starch degradation was incomplete. Only partial disappearance of the crystalline structure of the starch took place (samples A, and B). The modification of the physical structure of granular starch as observed by X-ray diffraction showed that with a higher screw speed (500 rpm), the organised structure of starch (crystalline region) was completely destroyed. The extent of starch modification was dependent on the residence time (screw speed). Diosady et al. (1985) concluded that degradation is almost exclusively due to the product of the shear stress and the time spent in the active zone. Product properties depend mostly on the molecular transformations (disruption of the starch granules, depolymerisation of amylose and amylopectin); these transformations are generated in the various parts of the extruder by temperature, pressure, shearing and residence time (Colonna et al., 1989).

 

Figure 10: Effect of variation of barrel temperature on x-ray diffraction patterns on cornmeal starch. A : 160° C ; B: 180° C ; 30 g/100g moisture content, 500 rpm speed , 0.15 slit die rheometer height, 400 g/min feed rate.

 

 

Figure 11: Effect of screw speed on x-ray diffraction patterns of cornmeal starch.
A : 150 rpm; B: 300 rpm; C: 500 rpm; 30 g/100g moisture content, 160° C barrel temperature, 0.15 slit die rheometer height, 300g/min feed rate.

 

CONCLUSION

The use of a slit die rheometer coupled to a co-rotating intermeshing twin screw extruder was successful in the direct measurement of some rheological properties of cornmeal. An increase in feed rate decreased the WAI and WSI, but increased the initial viscosity, viscosity at 90°C, and viscosity at 50°C. The temperature and the slit rheometer height interacted strongly with screw speed in affecting the WSI. The lowest values of initial viscosity were at the lowest moisture content. Twin-screw extrusion cooking of cornmeal destroys the organised crystalline structure either partially or completely, depending on extrusion variables such as screw speed, moisture, temperature, feed rate, and slit die rheometer height. According to the results the most important factor in starch degradation was the screw speed. Variation of operating parameters during twin-screw extrusion processing enables extrudates to be used for various industrial applications with specific technological properties.

 

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