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Expansion Characteristics of Extruded Corn Grits
Lebensm.-Wiss. u.-Technol., 29, 702–707 (1996)
Expansion Characteristics of Extruded Corn Grits Yusuf Ali, Milford A. Hanna and R. Chinnaswamy Department of Biological Systems Engineering and Food Science and Technology, University of Nebraska-Lincoln, Lincoln, NE 68503-0726 (U.S.A.) (Received October 24, 1995; accepted January 3, 1996)
Yellow corn grits with 180 g/kg moisture content (dry basis) were extrusion cooked in a Brabender single-screw laboratory scale extruder with various combinations of barrel temperature (100–200 °C) and screw speed (80–200 rpm). Bulk densities, solid densities, expansion properties, and total, open and closed pore volumes were determined. Extrudate bulk density, measured by glass bead displacement, was highly correlated with the bulk density based on the actual dimensions of the extrudates. Overall and radial expansion increased with temperature and screw speed whereas axial expansion decreased. Axial expansion was affected mostly by surging. The pore volumes had an increasing trend with temperature and screw speed. ©1996 Academic Press Limited
Introduction
Materials and Methods
Extrusion cooking technology has been widely used by food industries to manufacture ready-to-eat cereals, expanded products and breakfast cereals (1). Most of these products have the common, but important, functional property of expansion volume. Expansion volume is the primary quality parameter associated with product crispiness, water absorption, water solubility and crunchiness (2). Studies have concentrated mainly on engineering properties of various cereal extrusion cooking processes (3–7). Several other studies have developed response surface methodology models for extrusion processes (8–15). Still others have reported chemical changes during extrusion cooking and related them to product functional qualities such as expansion volume, water solubility and product colour (16–26). Most of these studies used radial expansion as a measure of quality for extrudate expansion. However, studies by Launay and Lisch (27) and Lai et al. (28) showed that extrudate expansion occurred in both longitudinal and lateral directions during extrusion cooking. Therefore, it seems questionable whether the reported expansion properties using lateral or crosssectional area measurements of the extrudates really reflect the overall expansion. Thus, measurement of cereal extrudate expansion has become an issue. The objectives of this study were (i) to compare extrudate bulk density determination by glass bead displacement with actual extrudate mass and dimensions measurement and (ii) to determine the effects of temperature and screw speed on overall, radial and axial expansions and on total, open and closed pore volumes.
Sample preparation Yellow corn grits were obtained gratis from Gooch Mills of Lincoln, NE. Sufficient water was added to raise the moisture content of the corn grits to 180 g/kg dry basis. Actual moisture content of samples at the time of extrusion was 177 g/kg.
0023-6438/96/080702 + 06$25.00/0
Extrusion process A C.W. Brabender single-screw laboratory scale extruder (Model 2802) with a 1.9 cm barrel diameter was used to extrude the corn grits. The extruder screw had a length to diameter ratio of 20:1 and a compression ratio of 3:1. The die nozzle diameter and length were 3 mm and 15 mm, respectively. Extrusion cooking conditions were varied using barrel temperatures from 100 to 200 °C and screw speeds from 80 to 200 rpm. The extruder was fed full. Extruded samples were dried overnight in a convective air oven at 40 °C. Physical properties of all the samples were determined using methods described below.
Physical properties Bulk density. Two methods were used to determine the bulk density of the extrudates. In the first method, the bulk density was calculated by measuring the actual dimensions of the extrudates, as suggested by Launay and Lisch (27). The diameters of the extrudates were measured with a Vernier caliper and the lengths per unit weight (g) of samples were determined. Bulk ©1996 Academic Press Limited
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densities of the extrudates were determined using the equation: ρd = 4/πD2L
Eqn [1]
where ρd = bulk density using dimensions of the extrudates (g/cm3), D = diameter of the extrudate (cm), and L = length per gram of extrudate (cm/g). In the second method, bulk density was determined by measuring the volume of extrudates by glass bead displacement (29). Glass beads with a diameter range of 1.00 to 1.05 mm (Braun Melsungen AG., Melsungen, Germany) were used as the displacement medium. Bulk densities of the extrudates were calculated as: ρb = (Wex/Wg)ρgb
Eqn [2]
viewed as open pores, while isolated or noninterconnected pores are viewed as closed pores. The extrudate closed pore volumes were calculated as the difference between the respective apparent and true solid volumes, measured using the air comparison pycnometer. Similarly, the extrudate total pore volumes were calculated as the difference between the respective bulk volume and true solid volume. The open pore volumes were calculated as the difference between total and closed pore volumes. All pore volumes were expressed as percentages of total pore volumes. The porosity of extrudates also was determined from bulk and apparent volumes. These relationships were as follows: Closed pore volume = Apparent volume – True solid volume
where ρb = bulk density using glass bead displacement method (g/cm3), Wex = extrudate mass (g), Wg = mass of glass beads displaced (g), and ρgb = density of the glass beads (g/cm3). Solid density. The apparent solid densities of the extruded samples were defined as mass per unit volume. Volume of the extrudates was determined using an air comparison multi-volume pycnometer model 130-50000-00 (Micrometrics, Inc., Norcross, GA). This method allowed gas to penetrate into the open pores and thus helped in determining the total open pores. Extruded samples, approximately 2 cm long, were used to determine the solid densities. The true solid densities were measured by the same method but using ground samples. The samples used to determine the solid densities were ground to pass through a No. 80 mesh sieve (177 µm opening) and once again the volumes of the extrudates were determined. The differences in the solid densities and true solid densities were used to determine the total closed pores. Expansion properties. Extrudate expansion volumes were determined and expressed as overall, radial and axial expansions. Overall expansion was calculated as the ratio of true solid densities, determined by the pycnometer method, to bulk densities, determined by glass bead displacement. Radial expansion was calculated as the ratio of average cross-sectional area of the extrudates to the crosssectional area of the die nozzle. A Vernier caliper was used to measure the diameter of the extrudates. The average of ten measurements of extrudate diameter was used to calculate radial expansion. Axial expansion was calculated as the ratio of overall expansion to radial expansion. Closed and open pores. Void spaces in porous solids can have three types of pore structures: (i) interconnected pore segments, (ii) isolated or noninterconnected pore segments, and (iii) dead-end or blind pore segments. Interconnected pores are accessible on both ends, blind pores are accessible only on one end, while noninterconnected pores are inaccessible closed pores within the solid material and behave as a part of the solid (30). Thus, interconnected and dead end pores are
Eqn [3] Total pore volume = Bulk volume – True solid volume
Porosity =
Bulk volume – Apparent volume Bulk volume
Eqn [4]
Eqn [5]
Experimental design and data analysis The experimental design used in this study was a completely randomized design with treatments arranged as a full two-factorial with a single experimental unit per treatment combination. The two factors considered were six levels of barrel temperature and seven levels of screw speed. An experimental unit was an extrusion at one temperature–speed combination. Statistical analyses (31) were conducted in three steps using analysis of variance (ANOVA) and regression. First, an ANOVA model was fitted on the full factorial with the two-way interactions considered as an acceptable estimate of experimental error. Each main effect and interaction was broken into one degree of freedom orthogonal polynomial contrasts. In the second step, a new ANOVA model was fitted with all nonsignificant (P > 0.05) main effects and two-way interactions from the first step pooled with the error term. All remaining terms were tested for significance against the pooled error. In the third step, all significant terms identified in the second step were fitted in a regression model. The estimated model was then used to generate predicted points on a response surface, which was then plotted against temperature and screw speed.
Results and Discussion The physical properties of corn grits extruded at different temperatures and screw speeds are presented in Table 1.
Bulk density methods Two methods used for measuring bulk density of extrudates were compared. As shown in Table 1, the bulk densities of extrudates as determined by glass
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bead displacement varied from 0.118 g/cm3 to 0.634 g/cm3, whereas, based on actual extrudate mass and dimensions, bulk densities varied from 0.11 to 0.64 g/cm3. The regression equation for the relationship between bulk density determination by volume measurement (BDvm) and glass bead displacement (BDgb) methods was:
extrudate surface should be smooth and uniform. This method is much easier to use and is less time consuming than the glass bead displacement method. For irregular extrudate surfaces, the glass bead displacement method should be used.
BDvm = 1.007 BDgb – 0.0056 (R2 = 0.985) Eqn [6]
Effects of temperature and screw speed on expansion properties The effects of temperature and screw speed on the radial, axial and overall expansions, as well as on the bulk density of the extrudates, are presented in Table 1. Statistical analyses showed that there was a significant linear interaction, at 5% level, between barrel tem-
This shows that bulk density determination by measuring dimensions of the extrudates, as suggested by Launay and Lisch (27), was as valid as the glass bead displacement method suggested by Hwang and Hayakawa (29). However, it should be noted that the
Table 1 Physical properties of corn grits extruded at different temperatures and screw speeds Bulk density (g/cm3)
Solid density (g/cm3)
Pore volume (cm3/g)
Screw speed (rpm)
Glass bead
Dimensions measurement
Apparent
True
Overall
Radial
Axial
Total
Open Closed
100
80 100 120 140 160 180 200
0.634 0.416 0.280 0.224 0.202 0.212 0.182
0.64 0.40 0.27 0.23 0.21 0.21 0.18
1.45 1.45 1.43 1.36 1.37 1.35 1.30
1.51 1.48 1.47 1.50 1.52 1.50 1.50
2.382 3.558 5.250 6.696 7.525 7.075 8.242
3.531 5.858 8.962 10.970 12.272 11.182 12.951
0.675 0.607 0.586 0.610 0.613 0.633 0.637
0.576 0.730 0.816 0.845 0.862 0.860 0.880
0.559 0.724 0.811 0.831 0.845 0.844 0.862
0.017 0.006 0.005 0.014 0.017 0.016 0.018
120
80 100 120 140 160 180 200
0.350 0.299 0.248 0.196 0.207 0.187 0.184
0.37 0.33 0.24 0.21 0.18 0.17 0.17
1.50 1.48 1.05 1.40 1.34 1.37 1.31
1.53 1.52 1.49 1.49 1.49 1.49 1.44
4.371 5.084 6.008 7.602 7.198 7.968 7.826
5.858 6.491 8.585 10.552 11.830 12.272 12.951
0.746 0.783 0.700 0.720 0.608 0.649 0.604
0.758 0.783 0.839 0.859 0.879 0.886 0.882
0.753 0.777 0.771 0.850 0.866 0.876 0.870
0.005 0.006 0.068 0.009 0.013 0.010 0.012
140
80 100 120 140 160 180 200
0.208 0.225 0.222 0.222 0.191 0.184 0.161
0.22 0.20 0.22 0.21 0.18 0.18 0.18
1.39 1.38 1.34 1.32 1.29 1.33 1.30
1.49 1.50 1.51 1.50 1.53 1.52 1.51
7.163 6.667 6.802 6.757 8.010 8.261 9.379
8.034 9.347 9.741 10.346 12.951 11.396 12.050
0.892 0.713 0.698 0.653 0.619 0.725 0.778
0.852 0.867 0.881 0.887 0.902 0.901 0.881
0.842 0.855 0.866 0.871 0.884 0.887 0.862
0.010 0.012 0.015 0.016 0.018 0.014 0.019
160
80 100 120 140 160 180 200
0.146 0.149 0.163 0.161 0.159 0.131 0.118
0.15 0.14 0.15 0.15 0.15 0.13 0.11
1.30 1.30 1.32 1.29 1.29 1.24 1.21
1.53 1.51 1.52 1.52 1.52 1.56 1.55
10.479 10.134 9.325 10.000 9.560 11.908 13.136
8.772 9.347 9.941 9.941 9.941 10.760 10.552
1.195 1.084 0.938 1.006 0.962 1.107 1.245
0.902 0.907 0.901 0.901 0.901 0.917 0.929
0.885 0.892 0.886 0.884 0.884 0.895 0.909
0.017 0.015 0.015 0.017 0.017 0.022 0.020
180
80 100 120 140 160 180 200
0.119 0.122 0.140 0.139 0.146 0.173 0.231
0.12 0.13 0.14 0.14 0.14 0.17 0.22
1.17 1.23 1.27 1.15 1.25 1.31 1.30
1.59 1.56 1.50 1.50 1.53 1.52 1.50
13.361 12.787 10.714 10.791 10.479 8.786 6.494
6.987 7.854 8.961 9.741 10.346 10.552 10.142
1.912 1.628 1.196 1.108 1.013 0.833 0.640
0.925 0.917 0.907 0.907 0.908 0.908 0.893
0.897 0.894 0.890 0.878 0.888 0.893 0.877
0.028 0.023 0.017 0.029 0.020 0.015 0.016
200
80 100 120 140 160 180 200
0.319 0.235 0.204 0.195 0.237 0.331 0.438
0.30 0.23 0.20 0.18 0.22 0.31 0.44
1.09 1.12 1.18 1.18 1.32 1.39 1.42
1.48 1.49 1.57 1.52 1.53 1.52 1.49
4.639 6.340 7.696 7.795 6.456 4.492 3.402
3.774 4.026 4.970 6.987 8.399 7.501 6.820
1.229 1.575 1.549 1.116 0.769 0.612 0.499
0.899 0.913 0.924 0.914 0.902 0.855 0.819
0.862 0.884 0.898 0.890 0.886 0.842 0.810
0.037 0.029 0.026 0.024 0.016 0.013 0.009
Temp (˚C)
704
Expansion
Overall expansion
lwt/vol. 29 (1996) No. 8
12 10 8 6 4 2 200 180 Te 160 mp era 140 tur e ( 120 °C )
180
200
160 )
140 120 100 100 80
rpm
( eed
RE = –30.53 + 0.082 T + 2.55×10–3 T2 – 1.16×10–5 T3 + 0.462 S – 3.6×10–4 S2 – 3.97×10–3 ST + 1.17×10–5 T2 S(R2 = 0.930) Eqn [8]
p ws
e
Scr
increased the radial expansion increased up to a temperature of about 160 °C and beyond that the radial expansion decreased. When screw speed was increased beyond 160 rpm the increase in radial expansion showed a decreasing trend with temperature. The maximum radial expansion was observed to be at 100 °C and 200 rpm screw speed and the minimum radial expansion was observed to be at 100 °C and 80 rpm screw speed. The regression equation for radial expansion (RE) at any temperature (T) and screw speed (S) was:
Fig. 1 Effect of barrel temperature and screw speed on overall expansion of extruded corn grits
perature and screw speed on overall expansion of extrudates. As shown in Fig. 1, overall expansion increased linearly with increasing screw speed at temperatures up to 160 °C, while at temperatures of 180 and 200 °C the overall expansion decreased linearly with increasing screw speed. As temperature was increased from 100 to 200 °C, the overall expansion increased for all screw speeds up to a temperature of about 160 °C and beyond that it decreased. The regression equation for overall expansion (OE) at any temperature (T) and screw speed (S) was: OE = 85.98 – 2.24 T + 0.018 T2 – 4.33×10–5 T3 + 0.136 S – 8.43×10–4 TS (R2 = 0.790) Eqn [7] From the regression equation it was observed that temperature had a cubic trend, whereas screw speed had a linear trend. Radial expansion showed a significant interaction between temperature (quadratic) and screw speed (linear) at 5% level. Figure 2 shows the effect of temperature and screw speed on radial expansion of the corn grits. Radial expansion increased at any one temperature as the screw speed was increased. On the other hand, at lower screw speed, as the temperature
From the regression equation it was observed that temperature had a cubic trend, whereas screw speed had a quadratic trend. The axial expansions at six temperatures and seven screw speeds are shown in Fig. 3. Statistical analyses showed that there was a significant interaction between temperature (quadratic) and screw speed (linear). Axial expansion was maximum at a temperature of 200 °C and a screw speed of 80 rpm and minimum at 200 °C and 200 rpm. The regression equation for axial expansion (AE) at any temperature (T) and screw speed (S) was: AE = 11.89 – 0.241 T + 1.62×10-3 T2 – 3.23×10–6 T3 – 0.019 S + 3.23×1010–4 TS – 1.40×10–6 T2S (R2 = 0.798) Eqn [9] Once again the temperature had a significant cubic trend and screw speed had a significant linear trend. Comparing radial expansion and axial expansion, it was observed that the response of axial expansion with prescribed temperature and screw speed combinations was the inverse of the radial expansion. At lower temperatures and higher screw speeds, the radial expansion was high while the axial expansion was low, whereas, at higher temperatures and lower screw speeds, the trend was reversed. This phenomenon can be explained by the process of surging, which is a discontinuous extrudate flow rate. At higher temperatures the extrudates were expanded primarily in
12 10
Axial expansion
Radial expansion
14
8 6 4 2 200
200
160 Te mp 140 era tur 120 e( °C ) 100
140 120 100 80
rew
eed
1.5 1.0 0.5
180
0.0
m)
200
160
180
2.0
(rp
200
180
sp
Te m
Sc
180 160
160 pe
ra
tu
re
120
(°C
120 )
Fig. 2 Effect of barrel temperature and screw speed on radial expansion of extruded corn grits
140
140 100 100
80
rew
ed
spe
m)
(rp
Sc
Fig. 3 Effect of barrel temperature and screw speed on axial expansion of extruded corn grits
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lwt/vol. 29 (1996) No. 8
3
Open pore volume (cm /g)
the longitudinal direction. Therefore, it was apparent that the axial expansion was mostly affected by surging, which was accentuated by higher temperatures and slower screw speeds and in turn affected the radial expansion. Chinnaswamy and Hanna (32) and Lai et al. (28) observed the same trends when they added sodium bicarbonate with the starch during extrusion.
1.0 0.9 0.8 0.7 0.6 0.5 200
200
180
Pore volumes and porosity It is known that the expansion or puffing process that occurs as part of extrusion cooking creates air pockets or gas cells in a product that are reflected in increased expansion, and consequently decreased bulk product density. As described earlier, these air pockets are not always intact. Some are open and, therefore, air can move freely from one cell to another. Others are closed or intact, thereby preventing air movement between pores. The expansion of an extrudate is dependent primarily on development of these air pockets which are created when vapour expands as the extrudate leaves the die and experiences a sudden decrease in pressure.
Total pore volume (cm3/g)
Effects of temperature and screw speed on pore volume The combined effects of temperature and screw speed on total, open and closed pore volumes are presented in Table 1. Statistical analyses showed that there was a significant quadratic interaction, at 5% level, between barrel temperature and screw speed on total pore volume of extruded corn grits. The effects of temperature and screw speed on total pore volume are shown in Fig. 4. The total pore volume ranged between 0.576 cm3/g and 0.929 cm3/g. It can be seen from Fig. 4 that at 100 °C and 80 rpm the total pore volume was minimum and that it increased with increases in temperature and screw speed. The regression equation for total pore volume (TP) at any temperature (T) and screw speed (S) was: TP = –4.80 + 0.067 T – 1.97×10–4 T2 + 0.067 S – 2.00×10–4S2 – 7.98×10–4 TS + 2.43×10–6 T2S – 7.55×10–9 TS2 (R2 = 0.944) Eqn [10]
1.0 0.9 0.8 0.7 0.6 200 180 Te m 160 pe 140 ra tu re (°C 120 ) 100
200 180 160 140 120 100 80
rew
ed
spe
m)
(rp
Sc
Fig. 4 Effect of barrel temperature and screw speed on total pore volume of extruded corn grits
180 160
160
Te m
pe
tu
re
120
120
(°C
)
m)
140
140
ra
100 100
80
rew
ed
spe
(rp
Sc
Fig. 5 Effect of barrel temperature and screw speed on open pore volume of extruded corn grits
The open pore volume showed a significant quadratic interaction, at 5% level, between temperature and screw speed. Figure 5 shows the effects of temperature and screw speed on open pore volume of extruded corn grits. The open pore volume followed a trend similar to that of total pore volume and ranged between 0.559 cm3/g and 0.909 cm3/g. The magnitude of the open pore volume was in the range of 96 to 99% of the total pore volume. Regression analysis for open pore volume (OP) at any temperature (T) and screw speed (S) was: OP = – 5.91 + 0.083 T – 2.53×10–4 T2 + 0.085 S – 2.68×10–4 S2 – 1.06×10–3 TS + 3.43×10–6 T2S – 1.08×10–8TS2 (R2 = 0.938) Eqn [11] Statistical analyses conducted to determine the effects of temperature and screw speed on closed pore volumes showed no trends. This was expected as the closed pore volumes were very small as compared to total and open pore volumes. The calculation of closed pore volume was done on the basis of the difference between total pore and open pore volume.
Conclusions Extrudate bulk density, measured by glass bead displacement, was highly correlated with the bulk density determined from actual extrudates mass and dimensions. Temperature and screw speed had combined effects on overall, radial and axial expansion as well as total, open and closed pore volumes. The pore volumes had an increasing trend with temperature and screw speed. Of the total pore volume, 96 to 99% of the pores were open. Overall and radial expansions increased with screw speed and temperature, while axial expansion showed a reverse trend. The magnitude of axial expansion was low as compared to the radial expansion. Most axial expansion occurred at high temperature and was mostly affected by surging which in turn affected radial expansion. Thus, it was concluded that radial expansion of extrudates really reflected overall expansion. This is important from a process performance/
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evaluation standpoint because radial expansion can be assessed readily and used to describe overall product expansion.
Acknowledgements Journal series number 9942 of the University of Nebraska Agricultural Research Division. Part of this research paper was presented at the Mid-Central Conference of the ASAE held at St. Joseph, MO during March 13–14, 1992 and the full paper was presented at the Scanning Microscopy 1992 Meeting held at Chicago, IL during Maay 9–14, 1992.
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Expansion Characteristics of Extruded Corn Grits Yusuf Ali, Milford A. Hanna and R. Chinnaswamy Department of Biological Systems Engineering and Food Science and Technology, University of Nebraska-Lincoln, Lincoln, NE 68503-0726 (U.S.A.) (Received October 24, 1995; accepted January 3, 1996)
Yellow corn grits with 180 g/kg moisture content (dry basis) were extrusion cooked in a Brabender single-screw laboratory scale extruder with various combinations of barrel temperature (100–200 °C) and screw speed (80–200 rpm). Bulk densities, solid densities, expansion properties, and total, open and closed pore volumes were determined. Extrudate bulk density, measured by glass bead displacement, was highly correlated with the bulk density based on the actual dimensions of the extrudates. Overall and radial expansion increased with temperature and screw speed whereas axial expansion decreased. Axial expansion was affected mostly by surging. The pore volumes had an increasing trend with temperature and screw speed. ©1996 Academic Press Limited
Introduction
Materials and Methods
Extrusion cooking technology has been widely used by food industries to manufacture ready-to-eat cereals, expanded products and breakfast cereals (1). Most of these products have the common, but important, functional property of expansion volume. Expansion volume is the primary quality parameter associated with product crispiness, water absorption, water solubility and crunchiness (2). Studies have concentrated mainly on engineering properties of various cereal extrusion cooking processes (3–7). Several other studies have developed response surface methodology models for extrusion processes (8–15). Still others have reported chemical changes during extrusion cooking and related them to product functional qualities such as expansion volume, water solubility and product colour (16–26). Most of these studies used radial expansion as a measure of quality for extrudate expansion. However, studies by Launay and Lisch (27) and Lai et al. (28) showed that extrudate expansion occurred in both longitudinal and lateral directions during extrusion cooking. Therefore, it seems questionable whether the reported expansion properties using lateral or crosssectional area measurements of the extrudates really reflect the overall expansion. Thus, measurement of cereal extrudate expansion has become an issue. The objectives of this study were (i) to compare extrudate bulk density determination by glass bead displacement with actual extrudate mass and dimensions measurement and (ii) to determine the effects of temperature and screw speed on overall, radial and axial expansions and on total, open and closed pore volumes.
Sample preparation Yellow corn grits were obtained gratis from Gooch Mills of Lincoln, NE. Sufficient water was added to raise the moisture content of the corn grits to 180 g/kg dry basis. Actual moisture content of samples at the time of extrusion was 177 g/kg.
0023-6438/96/080702 + 06$25.00/0
Extrusion process A C.W. Brabender single-screw laboratory scale extruder (Model 2802) with a 1.9 cm barrel diameter was used to extrude the corn grits. The extruder screw had a length to diameter ratio of 20:1 and a compression ratio of 3:1. The die nozzle diameter and length were 3 mm and 15 mm, respectively. Extrusion cooking conditions were varied using barrel temperatures from 100 to 200 °C and screw speeds from 80 to 200 rpm. The extruder was fed full. Extruded samples were dried overnight in a convective air oven at 40 °C. Physical properties of all the samples were determined using methods described below.
Physical properties Bulk density. Two methods were used to determine the bulk density of the extrudates. In the first method, the bulk density was calculated by measuring the actual dimensions of the extrudates, as suggested by Launay and Lisch (27). The diameters of the extrudates were measured with a Vernier caliper and the lengths per unit weight (g) of samples were determined. Bulk ©1996 Academic Press Limited
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densities of the extrudates were determined using the equation: ρd = 4/πD2L
Eqn [1]
where ρd = bulk density using dimensions of the extrudates (g/cm3), D = diameter of the extrudate (cm), and L = length per gram of extrudate (cm/g). In the second method, bulk density was determined by measuring the volume of extrudates by glass bead displacement (29). Glass beads with a diameter range of 1.00 to 1.05 mm (Braun Melsungen AG., Melsungen, Germany) were used as the displacement medium. Bulk densities of the extrudates were calculated as: ρb = (Wex/Wg)ρgb
Eqn [2]
viewed as open pores, while isolated or noninterconnected pores are viewed as closed pores. The extrudate closed pore volumes were calculated as the difference between the respective apparent and true solid volumes, measured using the air comparison pycnometer. Similarly, the extrudate total pore volumes were calculated as the difference between the respective bulk volume and true solid volume. The open pore volumes were calculated as the difference between total and closed pore volumes. All pore volumes were expressed as percentages of total pore volumes. The porosity of extrudates also was determined from bulk and apparent volumes. These relationships were as follows: Closed pore volume = Apparent volume – True solid volume
where ρb = bulk density using glass bead displacement method (g/cm3), Wex = extrudate mass (g), Wg = mass of glass beads displaced (g), and ρgb = density of the glass beads (g/cm3). Solid density. The apparent solid densities of the extruded samples were defined as mass per unit volume. Volume of the extrudates was determined using an air comparison multi-volume pycnometer model 130-50000-00 (Micrometrics, Inc., Norcross, GA). This method allowed gas to penetrate into the open pores and thus helped in determining the total open pores. Extruded samples, approximately 2 cm long, were used to determine the solid densities. The true solid densities were measured by the same method but using ground samples. The samples used to determine the solid densities were ground to pass through a No. 80 mesh sieve (177 µm opening) and once again the volumes of the extrudates were determined. The differences in the solid densities and true solid densities were used to determine the total closed pores. Expansion properties. Extrudate expansion volumes were determined and expressed as overall, radial and axial expansions. Overall expansion was calculated as the ratio of true solid densities, determined by the pycnometer method, to bulk densities, determined by glass bead displacement. Radial expansion was calculated as the ratio of average cross-sectional area of the extrudates to the crosssectional area of the die nozzle. A Vernier caliper was used to measure the diameter of the extrudates. The average of ten measurements of extrudate diameter was used to calculate radial expansion. Axial expansion was calculated as the ratio of overall expansion to radial expansion. Closed and open pores. Void spaces in porous solids can have three types of pore structures: (i) interconnected pore segments, (ii) isolated or noninterconnected pore segments, and (iii) dead-end or blind pore segments. Interconnected pores are accessible on both ends, blind pores are accessible only on one end, while noninterconnected pores are inaccessible closed pores within the solid material and behave as a part of the solid (30). Thus, interconnected and dead end pores are
Eqn [3] Total pore volume = Bulk volume – True solid volume
Porosity =
Bulk volume – Apparent volume Bulk volume
Eqn [4]
Eqn [5]
Experimental design and data analysis The experimental design used in this study was a completely randomized design with treatments arranged as a full two-factorial with a single experimental unit per treatment combination. The two factors considered were six levels of barrel temperature and seven levels of screw speed. An experimental unit was an extrusion at one temperature–speed combination. Statistical analyses (31) were conducted in three steps using analysis of variance (ANOVA) and regression. First, an ANOVA model was fitted on the full factorial with the two-way interactions considered as an acceptable estimate of experimental error. Each main effect and interaction was broken into one degree of freedom orthogonal polynomial contrasts. In the second step, a new ANOVA model was fitted with all nonsignificant (P > 0.05) main effects and two-way interactions from the first step pooled with the error term. All remaining terms were tested for significance against the pooled error. In the third step, all significant terms identified in the second step were fitted in a regression model. The estimated model was then used to generate predicted points on a response surface, which was then plotted against temperature and screw speed.
Results and Discussion The physical properties of corn grits extruded at different temperatures and screw speeds are presented in Table 1.
Bulk density methods Two methods used for measuring bulk density of extrudates were compared. As shown in Table 1, the bulk densities of extrudates as determined by glass
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bead displacement varied from 0.118 g/cm3 to 0.634 g/cm3, whereas, based on actual extrudate mass and dimensions, bulk densities varied from 0.11 to 0.64 g/cm3. The regression equation for the relationship between bulk density determination by volume measurement (BDvm) and glass bead displacement (BDgb) methods was:
extrudate surface should be smooth and uniform. This method is much easier to use and is less time consuming than the glass bead displacement method. For irregular extrudate surfaces, the glass bead displacement method should be used.
BDvm = 1.007 BDgb – 0.0056 (R2 = 0.985) Eqn [6]
Effects of temperature and screw speed on expansion properties The effects of temperature and screw speed on the radial, axial and overall expansions, as well as on the bulk density of the extrudates, are presented in Table 1. Statistical analyses showed that there was a significant linear interaction, at 5% level, between barrel tem-
This shows that bulk density determination by measuring dimensions of the extrudates, as suggested by Launay and Lisch (27), was as valid as the glass bead displacement method suggested by Hwang and Hayakawa (29). However, it should be noted that the
Table 1 Physical properties of corn grits extruded at different temperatures and screw speeds Bulk density (g/cm3)
Solid density (g/cm3)
Pore volume (cm3/g)
Screw speed (rpm)
Glass bead
Dimensions measurement
Apparent
True
Overall
Radial
Axial
Total
Open Closed
100
80 100 120 140 160 180 200
0.634 0.416 0.280 0.224 0.202 0.212 0.182
0.64 0.40 0.27 0.23 0.21 0.21 0.18
1.45 1.45 1.43 1.36 1.37 1.35 1.30
1.51 1.48 1.47 1.50 1.52 1.50 1.50
2.382 3.558 5.250 6.696 7.525 7.075 8.242
3.531 5.858 8.962 10.970 12.272 11.182 12.951
0.675 0.607 0.586 0.610 0.613 0.633 0.637
0.576 0.730 0.816 0.845 0.862 0.860 0.880
0.559 0.724 0.811 0.831 0.845 0.844 0.862
0.017 0.006 0.005 0.014 0.017 0.016 0.018
120
80 100 120 140 160 180 200
0.350 0.299 0.248 0.196 0.207 0.187 0.184
0.37 0.33 0.24 0.21 0.18 0.17 0.17
1.50 1.48 1.05 1.40 1.34 1.37 1.31
1.53 1.52 1.49 1.49 1.49 1.49 1.44
4.371 5.084 6.008 7.602 7.198 7.968 7.826
5.858 6.491 8.585 10.552 11.830 12.272 12.951
0.746 0.783 0.700 0.720 0.608 0.649 0.604
0.758 0.783 0.839 0.859 0.879 0.886 0.882
0.753 0.777 0.771 0.850 0.866 0.876 0.870
0.005 0.006 0.068 0.009 0.013 0.010 0.012
140
80 100 120 140 160 180 200
0.208 0.225 0.222 0.222 0.191 0.184 0.161
0.22 0.20 0.22 0.21 0.18 0.18 0.18
1.39 1.38 1.34 1.32 1.29 1.33 1.30
1.49 1.50 1.51 1.50 1.53 1.52 1.51
7.163 6.667 6.802 6.757 8.010 8.261 9.379
8.034 9.347 9.741 10.346 12.951 11.396 12.050
0.892 0.713 0.698 0.653 0.619 0.725 0.778
0.852 0.867 0.881 0.887 0.902 0.901 0.881
0.842 0.855 0.866 0.871 0.884 0.887 0.862
0.010 0.012 0.015 0.016 0.018 0.014 0.019
160
80 100 120 140 160 180 200
0.146 0.149 0.163 0.161 0.159 0.131 0.118
0.15 0.14 0.15 0.15 0.15 0.13 0.11
1.30 1.30 1.32 1.29 1.29 1.24 1.21
1.53 1.51 1.52 1.52 1.52 1.56 1.55
10.479 10.134 9.325 10.000 9.560 11.908 13.136
8.772 9.347 9.941 9.941 9.941 10.760 10.552
1.195 1.084 0.938 1.006 0.962 1.107 1.245
0.902 0.907 0.901 0.901 0.901 0.917 0.929
0.885 0.892 0.886 0.884 0.884 0.895 0.909
0.017 0.015 0.015 0.017 0.017 0.022 0.020
180
80 100 120 140 160 180 200
0.119 0.122 0.140 0.139 0.146 0.173 0.231
0.12 0.13 0.14 0.14 0.14 0.17 0.22
1.17 1.23 1.27 1.15 1.25 1.31 1.30
1.59 1.56 1.50 1.50 1.53 1.52 1.50
13.361 12.787 10.714 10.791 10.479 8.786 6.494
6.987 7.854 8.961 9.741 10.346 10.552 10.142
1.912 1.628 1.196 1.108 1.013 0.833 0.640
0.925 0.917 0.907 0.907 0.908 0.908 0.893
0.897 0.894 0.890 0.878 0.888 0.893 0.877
0.028 0.023 0.017 0.029 0.020 0.015 0.016
200
80 100 120 140 160 180 200
0.319 0.235 0.204 0.195 0.237 0.331 0.438
0.30 0.23 0.20 0.18 0.22 0.31 0.44
1.09 1.12 1.18 1.18 1.32 1.39 1.42
1.48 1.49 1.57 1.52 1.53 1.52 1.49
4.639 6.340 7.696 7.795 6.456 4.492 3.402
3.774 4.026 4.970 6.987 8.399 7.501 6.820
1.229 1.575 1.549 1.116 0.769 0.612 0.499
0.899 0.913 0.924 0.914 0.902 0.855 0.819
0.862 0.884 0.898 0.890 0.886 0.842 0.810
0.037 0.029 0.026 0.024 0.016 0.013 0.009
Temp (˚C)
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Expansion
Overall expansion
lwt/vol. 29 (1996) No. 8
12 10 8 6 4 2 200 180 Te 160 mp era 140 tur e ( 120 °C )
180
200
160 )
140 120 100 100 80
rpm
( eed
RE = –30.53 + 0.082 T + 2.55×10–3 T2 – 1.16×10–5 T3 + 0.462 S – 3.6×10–4 S2 – 3.97×10–3 ST + 1.17×10–5 T2 S(R2 = 0.930) Eqn [8]
p ws
e
Scr
increased the radial expansion increased up to a temperature of about 160 °C and beyond that the radial expansion decreased. When screw speed was increased beyond 160 rpm the increase in radial expansion showed a decreasing trend with temperature. The maximum radial expansion was observed to be at 100 °C and 200 rpm screw speed and the minimum radial expansion was observed to be at 100 °C and 80 rpm screw speed. The regression equation for radial expansion (RE) at any temperature (T) and screw speed (S) was:
Fig. 1 Effect of barrel temperature and screw speed on overall expansion of extruded corn grits
perature and screw speed on overall expansion of extrudates. As shown in Fig. 1, overall expansion increased linearly with increasing screw speed at temperatures up to 160 °C, while at temperatures of 180 and 200 °C the overall expansion decreased linearly with increasing screw speed. As temperature was increased from 100 to 200 °C, the overall expansion increased for all screw speeds up to a temperature of about 160 °C and beyond that it decreased. The regression equation for overall expansion (OE) at any temperature (T) and screw speed (S) was: OE = 85.98 – 2.24 T + 0.018 T2 – 4.33×10–5 T3 + 0.136 S – 8.43×10–4 TS (R2 = 0.790) Eqn [7] From the regression equation it was observed that temperature had a cubic trend, whereas screw speed had a linear trend. Radial expansion showed a significant interaction between temperature (quadratic) and screw speed (linear) at 5% level. Figure 2 shows the effect of temperature and screw speed on radial expansion of the corn grits. Radial expansion increased at any one temperature as the screw speed was increased. On the other hand, at lower screw speed, as the temperature
From the regression equation it was observed that temperature had a cubic trend, whereas screw speed had a quadratic trend. The axial expansions at six temperatures and seven screw speeds are shown in Fig. 3. Statistical analyses showed that there was a significant interaction between temperature (quadratic) and screw speed (linear). Axial expansion was maximum at a temperature of 200 °C and a screw speed of 80 rpm and minimum at 200 °C and 200 rpm. The regression equation for axial expansion (AE) at any temperature (T) and screw speed (S) was: AE = 11.89 – 0.241 T + 1.62×10-3 T2 – 3.23×10–6 T3 – 0.019 S + 3.23×1010–4 TS – 1.40×10–6 T2S (R2 = 0.798) Eqn [9] Once again the temperature had a significant cubic trend and screw speed had a significant linear trend. Comparing radial expansion and axial expansion, it was observed that the response of axial expansion with prescribed temperature and screw speed combinations was the inverse of the radial expansion. At lower temperatures and higher screw speeds, the radial expansion was high while the axial expansion was low, whereas, at higher temperatures and lower screw speeds, the trend was reversed. This phenomenon can be explained by the process of surging, which is a discontinuous extrudate flow rate. At higher temperatures the extrudates were expanded primarily in
12 10
Axial expansion
Radial expansion
14
8 6 4 2 200
200
160 Te mp 140 era tur 120 e( °C ) 100
140 120 100 80
rew
eed
1.5 1.0 0.5
180
0.0
m)
200
160
180
2.0
(rp
200
180
sp
Te m
Sc
180 160
160 pe
ra
tu
re
120
(°C
120 )
Fig. 2 Effect of barrel temperature and screw speed on radial expansion of extruded corn grits
140
140 100 100
80
rew
ed
spe
m)
(rp
Sc
Fig. 3 Effect of barrel temperature and screw speed on axial expansion of extruded corn grits
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lwt/vol. 29 (1996) No. 8
3
Open pore volume (cm /g)
the longitudinal direction. Therefore, it was apparent that the axial expansion was mostly affected by surging, which was accentuated by higher temperatures and slower screw speeds and in turn affected the radial expansion. Chinnaswamy and Hanna (32) and Lai et al. (28) observed the same trends when they added sodium bicarbonate with the starch during extrusion.
1.0 0.9 0.8 0.7 0.6 0.5 200
200
180
Pore volumes and porosity It is known that the expansion or puffing process that occurs as part of extrusion cooking creates air pockets or gas cells in a product that are reflected in increased expansion, and consequently decreased bulk product density. As described earlier, these air pockets are not always intact. Some are open and, therefore, air can move freely from one cell to another. Others are closed or intact, thereby preventing air movement between pores. The expansion of an extrudate is dependent primarily on development of these air pockets which are created when vapour expands as the extrudate leaves the die and experiences a sudden decrease in pressure.
Total pore volume (cm3/g)
Effects of temperature and screw speed on pore volume The combined effects of temperature and screw speed on total, open and closed pore volumes are presented in Table 1. Statistical analyses showed that there was a significant quadratic interaction, at 5% level, between barrel temperature and screw speed on total pore volume of extruded corn grits. The effects of temperature and screw speed on total pore volume are shown in Fig. 4. The total pore volume ranged between 0.576 cm3/g and 0.929 cm3/g. It can be seen from Fig. 4 that at 100 °C and 80 rpm the total pore volume was minimum and that it increased with increases in temperature and screw speed. The regression equation for total pore volume (TP) at any temperature (T) and screw speed (S) was: TP = –4.80 + 0.067 T – 1.97×10–4 T2 + 0.067 S – 2.00×10–4S2 – 7.98×10–4 TS + 2.43×10–6 T2S – 7.55×10–9 TS2 (R2 = 0.944) Eqn [10]
1.0 0.9 0.8 0.7 0.6 200 180 Te m 160 pe 140 ra tu re (°C 120 ) 100
200 180 160 140 120 100 80
rew
ed
spe
m)
(rp
Sc
Fig. 4 Effect of barrel temperature and screw speed on total pore volume of extruded corn grits
180 160
160
Te m
pe
tu
re
120
120
(°C
)
m)
140
140
ra
100 100
80
rew
ed
spe
(rp
Sc
Fig. 5 Effect of barrel temperature and screw speed on open pore volume of extruded corn grits
The open pore volume showed a significant quadratic interaction, at 5% level, between temperature and screw speed. Figure 5 shows the effects of temperature and screw speed on open pore volume of extruded corn grits. The open pore volume followed a trend similar to that of total pore volume and ranged between 0.559 cm3/g and 0.909 cm3/g. The magnitude of the open pore volume was in the range of 96 to 99% of the total pore volume. Regression analysis for open pore volume (OP) at any temperature (T) and screw speed (S) was: OP = – 5.91 + 0.083 T – 2.53×10–4 T2 + 0.085 S – 2.68×10–4 S2 – 1.06×10–3 TS + 3.43×10–6 T2S – 1.08×10–8TS2 (R2 = 0.938) Eqn [11] Statistical analyses conducted to determine the effects of temperature and screw speed on closed pore volumes showed no trends. This was expected as the closed pore volumes were very small as compared to total and open pore volumes. The calculation of closed pore volume was done on the basis of the difference between total pore and open pore volume.
Conclusions Extrudate bulk density, measured by glass bead displacement, was highly correlated with the bulk density determined from actual extrudates mass and dimensions. Temperature and screw speed had combined effects on overall, radial and axial expansion as well as total, open and closed pore volumes. The pore volumes had an increasing trend with temperature and screw speed. Of the total pore volume, 96 to 99% of the pores were open. Overall and radial expansions increased with screw speed and temperature, while axial expansion showed a reverse trend. The magnitude of axial expansion was low as compared to the radial expansion. Most axial expansion occurred at high temperature and was mostly affected by surging which in turn affected radial expansion. Thus, it was concluded that radial expansion of extrudates really reflected overall expansion. This is important from a process performance/
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evaluation standpoint because radial expansion can be assessed readily and used to describe overall product expansion.
Acknowledgements Journal series number 9942 of the University of Nebraska Agricultural Research Division. Part of this research paper was presented at the Mid-Central Conference of the ASAE held at St. Joseph, MO during March 13–14, 1992 and the full paper was presented at the Scanning Microscopy 1992 Meeting held at Chicago, IL during Maay 9–14, 1992.
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