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Particle size of milled barley and sorghum and physico-chemical properties of grain following extrusion
Journal of Food Engineering 103 (2011) 464–472
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Particle size of milled barley and sorghum and physico-chemical properties of grain following extrusion Ghaid J. Al-Rabadi a, Peter J. Torley b, Barbara A. Williams a, Wayne L. Bryden a,c, Michael J. Gidley a,⇑ a
The University of Queensland, Centre for Nutrition and Food Sciences, School of Land Crop and Food Sciences, St. Lucia, Brisbane, Qld 4072, Australia National Wine and Grape Industry Centre, School of Agricultural and Wine Sciences, Charles Sturt University, Wagga Wagga NSW 2678, Australia c The University of Queensland, School of Animal Studies, Gatton, Qld 4343, Australia b
a r t i c l e
i n f o
Article history: Received 14 April 2010 Received in revised form 29 October 2010 Accepted 11 November 2010 Available online 20 November 2010 Keywords: Barley Sorghum Particle size Processing parameters Hydration properties Extrusion
a b s t r a c t Milled barley and sorghum grains were separated into three size fractions (fine, <0.5 mm; medium, 0.5– 1.0 mm; coarse, >1.0 mm) and extruded at two maximum temperatures (100 °C; 140 °C). Mechanical resistance and specific mechanical energy during extrusion was significantly higher for fine fractions, and extrusion at high temperature resulted in higher mechanical resistance. Pressure generated during extrusion was higher for the fine fraction in sorghum but lower in barley. Expansion index was highest for the fine fraction for barley, but did not differ significantly between sorghum fractions or with extrusion temperature. For all samples, extrusion at low temperature resulted in a higher final paste consistency and lower water absorption index, but there was no significant effect on water solubility index (WSI). Fraction size showed a significant effect on WSI in sorghum but not in barley. The results are rationalised in terms of differences in grain composition between sorghum and barley. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Extrusion cooking is a high temperature, high pressure, short time treatment in which food or feed material is exposed to mechanical shearing (Baik et al., 2004; Lai and Kokini, 1991). The reactions occurring during extrusion are influenced by a large number of variables that are related both to the machine and to the raw materials (Mathew et al., 1999). The effect of different equipment variables (such as feed rate, water addition rate, barrel temperature, screw configuration and screw speed) on rheological and physiochemical properties of the extrudate have been reviewed (Lai and Kokini, 1991). Changing these independent variables can change one or more dependent variable such as mechanical energy input to the extruder, generated pressure and product temperature. As a consequence, independent variables will affect the physico-chemical properties of the melt (Akdogan, 1996; Lai and Kokini, 1991; Whalen et al., 1997) and the physical characteristics of the extrudate (Ryu and Ng, 2001). Changes in extrusion dependent variables, and thus physicochemical properties of extrudate, can directly and indirectly influence subsequent application value. In animal feeds, extrusion has been shown to influence animal performance as well as affecting feed processing decisions in industry. For example, specific ⇑ Corresponding author. Tel.: +61 733652145; fax: +61 73365117. E-mail address: [email protected] (M.J. Gidley). 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.11.016
mechanical energy (SME) input during extrusion has been linked with palatability acceptance by animals (Tran et al., 2008), the relative ease with which a material can be extruded, and the cost of the extrusion operation (Miladinov and Hanna, 2000). The extent of extrudate expansion is one of the most important properties as it influences the porous structure of the extrudate (Plews et al., 2009). Extruded animal feeds often have coatings, such as heat sensitive enzymes and flavours, applied by vacuum, and the efficiency of this process depends on the porosity of the extrudate (Li et al., 2003). Changes in hydration properties such as water absorption index (WAI) and water solubility index (WSI) have been reported to affect both the ability of feed to mix with digestive enzymes and the general behaviour of feed in the digestive tract of monogastric animals (White et al., 2008). In addition, hydration properties and integrity of extrudate have been reported to affect extrudate stability in water which is an important property in fish and aquaculture feeds (Lim and Cuzon, 1994). Compared with the large body of research on extruder parameters, relatively little research has been conducted into the role of raw material parameters, particularly particle size. The research which has been undertaken into feed material particle size, shows that it influences both extruder operation and extrudate properties (Altan et al., 2009; Carvalho et al., 2010; Garber et al., 1997; Mathew et al., 1999; Onwulata and Konstance, 2006; Zhang and Hoseney, 1998). Different particle sizes have been achieved by changing grain milling screen size (Mathew et al., 1999) to give
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different distributions of particle size (Altan et al., 2009; Garber et al., 1997; Zhang and Hoseney, 1998) or segregation by sieving (Al-Rabadi et al., 2009; Carvalho et al., 2010; Onwulata and Konstance, 2006). Generally, it can be concluded form these studies, that particle size has a significant effect on a wide range of processing variables and hydration properties of extrudates generated. Effects were also dependent on grain fragment pre-conditioning, as well as on particle size and the interaction effect of other extrusion processing variables. Most reported experiments were based on maize as the main starch/grain material. The objective of the present study was to characterise a range of compositional and functional properties, as well as processing parameters, of separated size fractions after extrusion of two grains that are representative of grains used in animal diets, and which can serve as models of wholegrain extrusion for human foods. Barley was examined as an example of a grain that contains appreciable levels of (soluble) dietary fibre and has relatively low starch content compared to maize. For comparison, sorghum, which is a grain that is high in starch and low in soluble dietary fibre, was also examined. The processing dependent variables during extrusion were torque, specific mechanical energy (SME) and die pressure. The functional property characteristics studied were water absorption index, water solubility index, rheological profile on heating in excess water (paste consistency), and expansion index (EI).
Table 1 Fraction yield, average particle size and starch content (±SEM) in ground and segregated grain particles of barley and sorghum. Grain characteristic
dgwa (mm) Sgwb Starch content (%) Ground (control) >1.0 mm 1.0–0.5 mm <0.5 mm Sieve opening (mm) 3.35 2.8 1.7 1.0 0.5 0.25 0.125 0.75 Pan
Grain Barley
Sorghum
0.684 2.11
0.661 1.96
48.23 ± 1.59 63.8 ± 1.48 53.60 ± 0.47c 69.7 ± 1.42 47.7 ± 1.48 64.0 ± 3.04 43.0 ± 0.92 63.5 ± 1.72 Distribution of particles after sieving (g/l00 g of sample) 0d 0 0.12 1.07 6.67 4.46 27.50 22.91 31.57 37.73 14.67 21.27 12.30 9.10 4.81 1.74 1.08 0.22
a
dgw geometric mean diameter. Sgw geometric standard deviation and equal to Log normal standard deviation (ASAE, 2003). c Estimated by subtraction of starch content in unsieved ground from starch content in the fine and medium fraction. d Values are duplicate measurements of 100 g sample retained on top of sieves after 29 min shaking. b
2. Materials and methods 2.1. Materials 2.1.1. Cereals and milling Barley (variety; Buster) and sorghum (variety; Binalong, PBI Narrabri) were obtained from the Department of Primary Industries and Fisheries (Queensland, Australia). Grains were milled using a hammer mill (HM) (Australian Agriculture Machinery Group, Australia) at a speed of 1140 rpm, with samples of milled grain collected at a constant motor load. Equal portions of each grain were milled through three screen sizes (2, 4 and 6 mm) separately to produce ground material with a range of different particle size distributions. Grains milled through different hammer-mill screen sizes were mixed manually, then sieved using a sieve shaker to produce three particle size ranges: coarse (>1.0 mm), medium (1.0–0.5 mm), and fine (<0.5 mm), in addition to the unsieved ground grain (control). Table 1 shows the particle distribution, average particle size (dgw) and geometric standard deviation (sgw) of the ground barley and sorghum grains after being hammer-milled at three hammer-mill screen sizes and after being mixed. The moisture content of each particle size was determined by drying in a hot air oven (135 °C for 3 h). 2.1.2. Extrusion conditions and processing parameters High-temperature short-time (HTST) extrusion cooking was conducted using a co-rotating twin-screw model Prism Eurolab KX16 (Thermo Prism, Staffordshire, UK). The screw profile is shown in Table 2. The barrel diameter was 16 mm with a length/diameter ratio of 40:1. The die had two openings each 2 mm in diameter and 8 mm in length. Melt pressure was measured with a pressure transducer fitted to the die block (Terwin, Nottinghamshire, UK). Motor torque, screw speed, barrel temperatures and melt pressure were monitored with Prism software (Sysmac-SCS version 2.2; Omron Corporation, Milton Keynes, UK). Liquid feed rate and dry feed rate were recorded manually after being calibrated before each run. Dry feed was fed through a single screw volumetric feeder (KX16 Powder feeder; Brabender Technologie, Duisburg, Germany). Water was injected through a port 150 mm from the
Table 2 Extruder screw configuration profile.
*
Screw elements
Number of screw elements and their length in D*
Section length of each type of screw element (mm)
Twin flight feed screw 60° Forward paddle Twin flight feed screw 60° Forward paddle 90° Paddle Twin flight feed screw 60° Forward paddle 90° Paddle Twin flight feed screw Twin flight feed screw Single flight feed screw Total
13 1D 4 0.25D 4 1D 8 0.25D 4 0.25D 6 1D 8 0.25D 4 0.25D 1 0.5D 8 1D 1 1.5D 40
208 16 64 32 16 80 32 16 8 128 24 640
D: the extruder barrel diameter; for the Prism Eurolab KX16 D = 16 mm.
start of the barrel using a peristaltic pump (L/S 7523) with a Tygon Lab tubing 13 (0.8 mm internal diameter, Masterflex; Cole-Parmer Instrument Company, Vernon Hills, IL, USA). The dry feed rate for barley and sorghum was 20 g/min and 25 g/min, respectively, and the amount of water added at the extruder barrel was adjusted to compensate for moisture differences in the samples to have a dough moisture content of 55% for barley and 50% for sorghum (wb). Barley fractions were extruded at lower feed rate and higher moisture content, compared to sorghum, to avoid any possible blockage during extrusion. Two sets of barrel temperature settings (high and low) (Table 3), and constant screw speed of 200 r.p.m. were used. Samples were collected when the extruder was running at steady state (i.e. stable values for both torque and die pressure). The samples were collected over 15–20 min, placed in an aluminium tray, and dried in a hot air oven (50 °C for 24 h). After drying, they were sealed into plastic bags and stored at 18 °C pending further analysis.
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2 min. Each analysis took 13 min, and duplicate analyses were performed on each sample.
Table 3 Low and high temperature profiles used in the extruder barrel. Extruder zone *
Low temperature profile (°C) ***
1 2 3** 4 5 6 7 8 9 10 Die
– 50 50 75 90 100 100 100 100 100 100
High temperature profile (°C)
2.2.3. Scanning electron microscopy Specimens were mounted onto aluminium stubs with carbon tabs and sputter coated with a 10–15 nm layer of platinum using an EIKO IB-5 Platinum Sputter Coater. Samples were viewed in either a JEOL 6300 or JEOL 6400 field emission scanning electron microscope.
–*** 50 50 75 90 140 140 140 105 100 95
*
Dry feed port was located in the first extruder barrel section. Liquid injection port was located in the third extruder barrel section. The first zone in the extruder barrel is cooled with circulating tap water (about 25 °C). **
***
All extrudates were ground using a hammer-mill to pass through a 1 mm sieve for evaluation of physico-chemical properties and for total starch measurements. Ground extrudate samples were sealed in plastic bags and stored at 4 °C. Specific mechanical energy (SME) was calculated using Eq. (1) (Frame, 1994):
SME ¼
mechanical energy input Nact T act K w ¼ mass flow rate Nmax T max Q
ð1Þ
where Nact is the actual screw speed (r.p.m), Nmax the maximum screw speed (r.p.m), Tact the actual torque (N m), Kw the motor power (kW) and Q the output (kg s1). 2.2. Analyses 2.2.1. Water absorption index (WAI) and water solubility index (WSI) The WAI and WSI analysis methods used were adapted, with modifications, from Anderson et al. (1969). To maintain constant temperature and stirring, a 2.5 g sample was suspended in 30 mL distilled water at 30 °C for 30 min under constant stirring (60 rev/min) using a Rapid Visco Analyser (RVA) (Series 4, Newport Scientific, Warriewood, NSW, Australia). The slurry was poured into a tared centrifuge tube of 50 ml, and centrifuged at 3000g for 10 min. The sample used to measure WAI was the same sample as that used to measure WSI. The supernatant was carefully poured into a tared evaporating dish and dried at 135 °C for 3 h for total solid measurements. The weight of remaining gel was measured in the tubes after centrifugation. WAI and WSI analyses were conducted at random and in duplicate, and were calculated using Eqs. (2) and (3), respectively (Dogan and Karwe, 2003), as follows:
g water soluble matter g dry sample g water absorbed WAI ¼ g dry sample ð1-souble fractionÞ
WSI ¼
ð2Þ
2.2.4. Radial expansion index Radial expansion was evaluated by measuring extrudate diameter using digital callipers. Expansion ratio was calculated by dividing extrudate diameter by the diameter of die (Yuliani et al., 2006). A total of twenty readings were recorded for each sample. 2.2.5. Experimental design and statistical analysis For each grain, the experimental design was a split-plot, with particle size as the main plot factor (four levels of particle size), and the two temperatures as the split-plot factors (two temperature profiles). The main plot design was a randomized complete block with two blocks (blocked by day). Data were analysed by the PROC MIXIED procedure using SAS 9.1.2 Software (Cary, NC) to assign main treatment effects and probabilities. Means were determined using the LSMEANS statement and the differences between means (i.e. main effect) by using the ‘‘pdiff’’ option. Significant differences were defined as P < 0.05. 3. Results and discussion The effect of particle size and thermal treatment on extrusion dependent variables (torque, SME and die pressure) and hydrothermal properties (paste consistency, WAI and WSI and expansion index) were determined for both barley (Table 4) and sorghum (Table 5). The influence of the main effects (i.e. particle size fraction and extrusion temperature) in barley (Table 6) and sorghum (Table 7) are discussed in the following sections. 3.1. Effect of particle size and temperature on torque and SME Pairwise comparisons between treatments showed that there were significant differences between the main treatments (particle size and temperature) on the dependent processing variables of extruder torque and specific mechanical energy for both barley and sorghum. Torque and SME are direct indicators of the work performed on the material being processed in the extruder, and are controlled by melt rheology and the shear conditions created by the extruder screw and die (Akdogan, 1996). SME has been reported to play an important role in starch conversion, by rupturing stabilising semi-crystalline structures, and thus contributing to starch granule breakage during heating and shearing stress
ð3Þ
2.2.2. Paste consistency Paste consistency profiles were determined using a Rapid Visco Analyser (RVA) (Series 4, Newport Scientific, Warriewood, NSW, Australia). The paste consistency profile for each grain type and particle size was measured using a grain sample of 2.5 g (db) with distilled water added until a final weight of 25 g ± 0.01 g was reached. Each sample was held at 50 °C for one min and stirred at 960 rev/min for 10 s followed by constant stirring at 160 rev/ min, heating from 50 °C to 95 °C in 3.7 min, holding at 95 °C for 2.5 min, cooling to 50 °C in 3.8 min and holding at 50 °C for
Table 4 Least square means and SEM values for particle size temperature effects on processing parameters and extrudate physico-chemical properties in barley. Parameter
Fraction size Fine
Torque (Nm) SME (kJ/kg) Pressure (bar) WSI (%) WAI (g/g) EI
SEM Medium
Unsieved
Low
High
Low
High
Low
High
2.56 125.5 5.1 8.93 3.56 1.59
2.54 124.4 5.4 8.94 4.16 1.55
1.56 76.5 6.4 6.95 3.69 0.98
2.15 105.4 7.3 6.45 4.21 1.08
1.32 64.6 5.0 8.03 3.71 0.97
1.40 68.2 4.8 6.87 3.50 1.11
0.12 6.0 0.2 0.57 0.08 0.05
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G.J. Al-Rabadi et al. / Journal of Food Engineering 103 (2011) 464–472 Table 5 Least square means and SEM values for particle size temperature effects on processing parameters and extrudate physico-chemical properties in sorghum. Parameter
Fraction size
SEM
Fine
Torque (Nm) SME (kJ/kg) Pressure (bar) WSI (%) WAI (g/g) EI
Medium
Coarse
Unsieved
Low
High
Low
High
Low
High
Low
High
3.60 131.1 12.8 4.31 3.57 1.17
5.41 197.6 16.6 3.97 4.02 1.12
2.26 82.1 10.2 3.14 3.05 1.11
3.69 133.8 11.4 3.36 3.36 1.34
1.98 71.8 9.4 2.67 3.21 1.12
2.71 89.5 9.5 2.59 3.57 1.41
3.11 113.4 9.3 3.49 3.44 1.10
3.38 122.6 9.7 3.43 3.61 1.41
0.23 8.5 1.2 0.11 0.17 0.061
Table 6 Effect of particle size and temperature level on processing variables and hydration properties of extruded barley. Parameter
Torque (Nm) SME (kJ/kg) Pressure (bar) WSI (%) WAI (g/g) EI *,a,b
Fraction size
Temperature level
Fine
Medium
Unsieved
SEM
Low
High
SEM
2.55a* 124.9a 5.3b 8.94a 3.86a,b 1.57a
1.86b 90.9b 6.5a 6.71a 3.95a 1.03b
1.36b 66.4b 4.9b 7.45a 3.61b 1.04b
0.11 5.6 0.1 0.52 0.06 0.03
1.81b 88.9b 5.5a 7.97a 3.65b 1.18a
2.02a 99.1a 5.9a 7.42a 3.96a 1.25a
0.07 3.5 0.1 0.33 0.04 0.03
Values within a single row, within main effect treatment, with different superscripts differ significantly (P < 0.05).
Table 7 Effect of particle size and temperature level on processing variables and hydration properties in extruded sorghum. Parameter
Torque (Nm) SME (kJ/kg) Pressure (bar) WSI (%) WAI (g/g) EI a,b,c
Particle size
Temperature level
Fine
Medium
Coarse
Unsieved
SEM
Low
High
SEM
4.51a 164.3a 14.7a 4.14a 3.79a 1.14a
2.97b,c 108.0b,c 10.8a,b 3.25b 3.20a 1.22a
2.34d,c 85.2d,c 9.4b 2.63c 3.39a 1.26a
3.24b 118.0b 9.5b 3.46b 3.52a 1.25a
0.16 6.0 1.0 0.09 0.16 0.04
2.74b 99.6b 10.5a 3.40a 3.32b 1.12a
3.80a 138.1a 11.8a 3.34a 3.64a 1.32a
0.12 4.3 0.6 0.05 0.05 0.58
Values within a single row, within main effect treatment, with different superscripts differ significantly (P < 0.05).
(Carvalho et al., 2010). Torque and SME are usually controlled by dough (melt) viscosity, if extruder operating conditions (e.g. screw speed) are held constant (Zhang and Hoseney, 1998). In a single set of extrusion conditions, torque and SME are directly related. However, SME is a more meaningful parameter as it determines the amount of energy used per unit mass, making it possible to compare results obtained at different feed rates, screw speeds and in different extruders. In barley, extrusion of the fine fraction gave a higher torque compared to the medium size fraction and the unsieved ground barley (Tables 4 and 6). SME responded in a similar manner to particle size, with a higher SME for the fine fraction, which was almost double that of the unsieved ground barley, while the medium size fraction had an intermediate SME (Tables 4 and 6). Different sorghum size fractions showed a similar trend effect on torque and SME, but with higher values than the corresponding size fractions for barley. This could be attributed to the expected higher dough viscosity as sorghum fractions have both a higher starch content and were extruded under lower moisture conditions (higher grain fragment feed rate and lower moisture feed rate) than barley. Extrusion of the fine fraction gave a higher torque compared to the medium size fraction and the coarse fraction (Tables 5 and 7), while the unsieved ground sorghum had an intermediate SME. SME showed a similar response to torque, with the fine fraction giving the highest SME, and lower SMEs for the medium
and coarse fractions, while the unsieved ground sorghum had an intermediate SME (Tables 5 and 7). Torque during extrusion has been reported to be dependent on the rheological status of the plasticised mass inside the extruder which is positively related to SME (Guhaa et al., 1997). The higher values of torque and SME for extrusion of the fine fractions could be attributed to greater viscosity caused by swelling and presumed gelatinization of starch (Mathew et al., 1999; Zhang and Hoseney, 1998). The lower melt viscosity during the extrusion of the medium and coarse fractions could be related to incomplete water penetration into the larger particles limiting starch swelling, due to lower surface to volume ratio (Hsu, 1983), as well as the presence of more outer grain layers (i.e. grain coat) and more impenetrable endosperm due to intact cell walls and protein matrices in the large particle size. Any starch swelling/gelatinization that occurs within intact cellular structures would also not be expected to alter bulk rheological properties. Water was added directly to the extruder barrel during extrusion, meaning that limited time was available for mass transfer of water. This would be expected to reduce the amount of water penetrating through large grain fragments compared to the finer fraction (Zhang and Hoseney, 1998) For these reasons, mechanical properties of large particles may not be greatly altered during extrusion, and be present in the extrudate in a relatively intact state as illustrated by electron micrographs of both sorghum (Fig. 1c and d) and barley (Fig. 1e) compared to the
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more disrupted structures evident in extruded fine fractions from sorghum and barley (Fig. 1a and b, respectively). Coarse corn grits have also been reported to give a lower extruder motor load and lower melt viscosity than smaller grit particles (Carvalho et al., 2010; Garber et al., 1997; Mathew et al., 1999; Zhang and Hoseney, 1998). In contrast, Altan et al. (2009) reported a higher SME value for extrusion of barley grits, compared with ground barley flour following a pre-conditioning step. The lower SME in the present study for the medium and coarse fractions could be attributed to the absence of pre-conditioning before extrusion. Pre-conditioning of coarse particles before extrusion has been reported to result in equal starch gelatinization to that of fine particles (Mathew et al., 1999), presumably because pre-conditioning allows sufficient time for water penetration into even the largest particles prior to extrusion. In both sorghum and barley, extrusion at a low temperature resulted in a lower torque and SME, compared with extrusion at a high temperature. This may be due to the lower extrusion temperature reducing the water diffusion rate into the grain fragments (Addo et al., 2006), reducing the potential for starch swelling dur-
ing extrusion (Lai and Kokini, 1991; Li et al., 2004), and thus reducing the melt viscosity (Li et al., 2004), which is reported to be the main influence on torque (Garber et al., 1997). The impact of temperature on torque and SME was clearer in sorghum than in barley, with all sorghum values higher than barley for comparable size fractions. This may be due to the higher starch content in sorghum than in barley. However, it could also be due to the greater fibre concentration in barley with, particularly, the (1,3;1,4)-b-glucanrich endosperm cell walls acting as a more effective barrier to starch swelling than the cell walls of sorghum endosperm. 3.2. Die pressure There was a significant effect of particle size on die pressure for both barley and sorghum. In barley, extrusion of the medium size fraction resulted in a higher pressure generated (6.48 bar) at the die, compared with the fine fraction (5.29 bar) or the unsieved ground barley (4.93 bar). Extrusion was conducted at the same feed rate and screw speed, so it was not obvious that the medium fraction should generate higher pressure than the fine fraction, as
Fig. 1. High temperature extrusion treatment results in extensive disruption of grain particles and starch granules in (a) sorghum (fine size fraction), and (b) barley (fine size fraction – high magnification), but not in (c) sorghum (coarse size fraction – high magnification), (d) sorghum (coarse size fraction – low magnification) or (e) barley (medium size fraction) – low magnification).
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the medium fraction had a lower viscosity (as indicated by higher torque and SME outcome values). The lower die pressure for the fine fraction may be due to increased functionality of water-soluble fibre caused by the heat-moisture environment. An increase in the amount of the water-soluble fibre fraction has been reported after extrusion (Dust et al., 2004; Qian and Ding, 1996), and might be expected to be higher for the fine fraction due to more surface area exposure. Fibre in the fine fraction is likely to have a lower swelling potential due to a lower water-holding capacity (Anguita et al., 2006), and more rapid penetration of water into, and diffusion through the particles. These properties may have reduced the amount of material that built up behind the die plate, producing a lower pressure in the fine fraction compared to the medium fraction (Hu et al., 1993). Partial build-up of swollen fibre at the die, in the case of extrusion of the medium particles, could conversely have increased the pressure. In sorghum, extrusion of the fine fraction resulted in a higher generated pressure (14.72 bar) at the die, compared to both the coarse fraction (9.44 bar) and the unsieved ground sorghum (9.54 bar), while it was intermediate for the medium-sized fraction (10.81 bar). The higher pressure generated at the die by the fine fraction, could be attributed to higher viscosity (Hu et al., 1993). An increase in the die pressure due to the effect of fine particles on viscosity has been reported by (Zhang and Hoseney, 1998). Sorghum does not have the same potential to release soluble fibre as barley, so starch and water penetration effects are apparently dominant. For both sorghum and barley, extrusion at low (max temperature setting 100 °C) or high temperature (max temperature setting
469
140 °C) did not have a significant effect on die pressure measurements. 3.3. Extrudate hydrothermal properties and expansion index 3.3.1. WAI The water absorption index is a physico-chemical parameter that has been reported to indicate the hydrolytic breakdown of starch during extrusion (Owusuansah et al., 1983), and the swelling behaviour of the starch component (Doucet et al., 2009; White et al., 2008). The presence of non-starch components, such as fibre, has been also reported to affect WAI (Lopez et al., 1996). In addition, WAI has been reported to be a measure of damaged starch, as well as protein denaturation and macromolecular complex formation (Dogan and Karwe, 2003). The results of this study showed that there was a significant difference in WAI values between different particle size fractions of barley (Table 6), although the differences were quantitatively small. Extrusion of the medium-sized fraction produced the highest WAI (3.95), while the unsieved ground barley gave the lowest WAI (3.61), and the fine fraction gave an intermediate WAI (3.86). The higher WAI value for the medium fraction could be attributed to the physical structure of fibre, although grinding of the extruded samples prior to analysis may have masked to some extent the effect of particle size on WAI. Larger fibre particles have been reported to have a higher water-holding capacity compared to the smaller particles, due to their greater ability to hold water within the cell wall matrix (Cadden, 1987). In sorghum, there was no significant effect of particle size on WAI (Table 7), but
Fig. 2. The effect of temperature on viscosity profile of barley extrudates prepared from the (a) fine size fraction, (b) medium size fraction and (c) unsieved ground grain. Samples were ground through a 1 mm screen before RVA analysis.
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grinding of the extruded samples prior to analysis may have masked any effect of particle size on WAI in sorghum. Extrusion under the higher temperature conditions (Table 3) resulted in a higher WAI for both barley and sorghum (Tables 4 and 5). An increase in WAI has been reported for extruded products from ground oat grains (Gutkoski and El-Dash, 1999), and wheat, corn and rice starches (Mercier and Feillet, 1975). Changes during extrusion such as protein denaturation, starch swelling, and swelling of the crude fibre, could all contribute to the increased WAI (Singh et al., 2007). 3.3.2. WSI WSI has been reported to represent the extent of soluble polysaccharides released from the grain in an excess of water (Doucet et al., 2009; White et al., 2008). In this study, neither of the main effects (particle size and temperature) had a significant effect on WSI in barley, although there was a trend for finer particles to have higher WSI values (Tables 4 and 6). However, there were significant differences between different ranges of size fractions in sorghum (Tables 5 and 7). Extrusion of the fine fraction resulted in the highest WSI (4.14) compared to the coarse fraction (2.36), while it was intermediate for the medium-sized fraction (3.25) and the unsieved ground material (3.46). The increase in WSI with decreasing particle size may be attributed to the greater specific surface area before extrusion, resulting in higher leaching of soluble starch-derived molecules after extrusion that dissolved in water during the WSI assay. In sorghum, extrusion temperature had no significant effect on WSI.
3.3.3. Pasting profiles Paste consistency measurements using RVA have been reported to be a useful method to monitor the degree of starch cooking during extrusion processing (Becker et al., 2001) and to provide a quick analytical tool for measuring extruded products for diagnostic development applications, due to its sensitivity to changes in processing parameters (Whalen et al., 1997). The paste consistency profile of ground extrudate was measured for both barley (Fig. 2) and sorghum (Fig. 3), to investigate the effect of extrusion temperature conditions for the same sized fraction. For both grains, and at all particle size fractions, extrusion at low temperature resulted in a higher final paste consistency. When the paste consistency of ground extrudates was measured, the higher final paste consistency could be linked to a higher leaching of macromolecules (Jacobs et al., 1995), and the formation of a viscous paste or gel (Ozcan and Jackson, 2005). Sopade et al. (2006) reported that the amount of ungelatinised starch (i.e. raw starch) present in the sample is responsible for the final paste consistency that develops during the cooling stage. Other pasting curve features also showed that less starch damage occurred during extrusion at lower temperature, accompanied by lower SME values. In barley extruded at the lower temperature, there was a higher peak paste consistency (fine fraction, Fig. 2a; unsieved ground grain, Fig. 2c), and a delay in paste consistency onset for the medium fraction (Fig. 2b). These characteristics indicate higher thermal stability and a higher potential for swelling after low temperature processing, indicative of less starch damage (Doucet et al., 2009; Whalen et al., 1997) compared to high temperature processing.
Fig. 3. The effect of temperature on viscosity profile of sorghum extrudates prepared from the (a) fine size fraction, (b) medium size fraction, (c) coarse size fraction and (d) unsieved ground grain. Samples were ground through a 1 mm screen before RVA analysis.
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This is consistent with the lower WAI values found after low temperature processing. In sorghum, extrusion at the low temperature, which was associated with a lower SME value, resulted in a lower initial paste consistency profile (low cold swelling profile) and lower peak paste consistency values in the medium fraction (Fig. 3b), the coarse fraction (Fig. 3c) and the unsieved ground material (Fig. 3d). The lower initial increase in paste consistency (nonappearance of cold swelling) could be attributed to lower starch damage after extrusion at the lower temperature (Carvalho and Mitchell, 2000; Doucet et al., 2009; Sopade et al., 2006). Initial paste consistency development (cold swelling) was higher for extrudates processed at high temperature (except for the fine fraction) and was apparent directly after starting paste consistency measurement at 50 °C. Cold swelling has been reported previously to be one of the parameters that is related to more damaged and converted starch (Ozcan and Jackson, 2005; Whalen et al., 1997), and the rate of particle hydration (Becker et al., 2001). The lower peak viscosity values for those size fractions could be attributed to the presence of large intact particles that exited the extruder without thermo-mechanical damage (Fig. 1), and failed to swell when extruded at the low temperature. In extrudates made from the fine fraction, the presence of large particles was not the limiting factor, and the low extrusion temperature may have resulted in less starch conversion, and thus more swelling and higher peak and final viscosities, compared to extrusion at the high temperature (Fig. 3a). 3.3.4. Expansion index (EI) The results of this study show that the fraction size resulted in a significant effect on EI in barley. Extrusion of the fine fraction resulted in a higher EI (1.57), compared with the medium fraction (1.03) or unsieved ground barley (1.04). Extrusion of large particles has been reported to have a lower expansion due to incomplete starch gelatinization (Garber et al., 1997) and a lower melt viscosity (Dogan and Karwe, 2003). A high degree of starch gelatinization and SME has been reported to correlate very well with the expansion index (Dogan and Karwe, 2003). However, in sorghum, the particle size had no effect on the expansion index, although differences in SME and torque were found between size fractions. This may be attributed to the high paste consistency of the fine fraction at the die (indicated by the high torque and SME measurements). Materials with high viscosity have been reported to require high pressure for expansion (Zhang and Hoseney, 1998), which may have been the limiting factor in this case. For both sorghum and barley, temperature did not show any significant effect on the expansion index, although it was numerically higher when extrusion was performed at the higher temperature. The temperature effect may be masked by lower water penetration into large particles in the medium and coarse fraction as reported above. It has been reported that the interaction between temperature and moisture are the most important factors in determining the extent of starch conversion in twin screw (Owusuansah et al., 1983) and single screw (Chiang and Johnson, 1977) extruders. Addition of water in a pre-conditioner before extrusion, to increase the time available for penetration into particles, has been reported to increase the volumetric expansion index (Zhang and Hoseney, 1998). 4. Conclusion Extrusion of different size fractions of milled barley and sorghum grain under two temperature profiles resulted in a significant effect on independent variables (torque, SME and die pressure) and on physico-chemical properties (paste consistency, WAI, WSI, and expansion). Particle size and processing tempera-
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ture were assumed to affect water penetration into particles, and their resultant hydration. Different chemical compositions between the different size fractions can affect both independent variables as well as physico-chemical properties, although regrinding of the extrudate is likely to mask the effect of extrusion variables on physico-chemical properties to some extent. Paste consistency measurement with RVA could be used as a complementary tool, in combination with independent variables such as SME, to examine the effect of processing condition on extrudate properties and the extent of starch damage. The separate size fractions studied here are typical of the range of sizes that are present in non-fractionated ground grains, used in both animal feed and human food. The fact that different size fractions from the same grain performed differently, both within the extruder and subsequently, illustrates the difficulty in delineating the complex extrusion behaviour of grains. This study suggests that insights into the extrusion performance of individual size fractions are needed to appreciate the factors responsible for extrusion performance and extrudate properties of non-fractionated ground grains. Acknowledgements The authors would like to thank Drs. John Black, Ali Youssef, Dagong Zhang, Ashok Shrestha, Honest Madziva, Lesleigh Force, and Deirdre Mikkelsen for their help and technical advice. The authors thank the Queensland Department of Primary Industries and Fisheries (Alexandra Hill, Brisbane, Australia) for supply of grains, and for assistance in milling. Funding was provided by the Co-operative Research Centre for an Internationally Competitive Pork Industry. References Addo, A., Bart-Plange, A., Dzisi, K., 2006. Water absorption characteristics of obatanpa and mamaba maize hybrids (Zea mays). International Journal of Food Engineering, 2(3), article 7. DOI:10.2202/1556-3758.1067. Akdogan, H., 1996. Pressure, torque, and energy responses of a twin screw extruder at high moisture contents. Food Research International 29 (5–6), 423–429. Al-Rabadi, G.J., Gilbert, R.G., Gidley, M.J., 2009. Effect of particle size on kinetics of starch digestion in milled barley and sorghum grains by porcine alpha-amylase. Journal of Cereal Science 50 (2), 198–204. Altan, A., McCarthy, K.L., Maskan, M., 2009. Effect of screw configuration and raw material on some properties of barley extrudates. Journal of Food Engineering 92 (4), 377–382. Anderson, R.A., Conway, H.F., Pfeifer, V.F., Griffin, E.L., 1969. Gelatinization of corn grits by roll and extrusion cooking. Cereal Science Today 14 (1), 4–12. Anguita, M., Gasa, J., Martin-Orue, S.M., Perez, J.F., 2006. Study of the effect of technological processes on starch hydrolysis, non-starch polysaccharides solubilization and physicochemical properties of different ingredients using a two-step in vitro system. Animal Feed Science and Technology 129 (1–2), 99– 115. ASAE, 2003. Methods of determining and expressing fines of feed materials by sieving. Standard No. S319.3. American Society of Agriculture and Biological Engineers, pp. 202–205. Baik, B.K., Powers, J., Nguyen, L.T., 2004. Extrusion of regular and waxy barley flours for production of expanded cereals. Cereal Chemistry 81 (1), 94–99. Becker, A., Hill, S.E., Mitchell, J.R., 2001. Milling - A further parameter affecting the Rapid Visco Analyser (RVA) profile. Cereal Chemistry 78 (2), 166–172. Cadden, A.M., 1987. Comparative effects of particle-size reduction on physical structure and water binding-properties of several plant fibers. Journal of Food Science 52 (6), 1595–1599. Carvalho, C.W.P., Mitchell, J.R., 2000. Effect of sugar on the extrusion of maize grits and wheat flour. International Journal of Food Science and Technology 35 (6), 569–576. Carvalho, C.W.P., Takeiti, C.Y., Onwulata, C.I., Pordesimo, L.O., 2010. Relative effect of particle size on the physical properties of corn meal extrudates: Effect of particle size on the extrusion of corn meal. Journal of Food Engineering 98 (1), 103–109. Chiang, B.Y., Johnson, J.A., 1977. Gelatinization of starch in extruded products. Cereal Chemistry 54 (3), 436–443. Dogan, H., Karwe, M.V., 2003. Physicochemical properties of quinoa extrudates. Food Science and Technology International 9 (2), 101–114. Doucet, F.J., White, A.G., Wulfert, F., Hill, S.E., Wiseman, J., 2009. Predicting in vivo starch digestibility coefficients in newly weaned piglets from in vitro assessment of diets using multivariate analysis. British Journal of Nutrition 21, 1–10.
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Dust, J.M., Gajda, A.M., Flickinger, E.A., Burkhalter, T.M., Merchen, N.R., Fahey, G.C., 2004. Extrusion conditions affect chemical composition and in vitro digestion of select food ingredients. Journal of Agricultural and Food Chemistry 52 (10), 2989–2996. Frame, N.D., 1994. Operational characteristics of the co-rotating twin-screw extruder. In: Frame, N.D. (Ed.), The Technology of Extrusion Cooking. Blackie Academic and Professional, Glasgow, pp. 1–51. Garber, B.W., Hsieh, F., Huff, H.E., 1997. Influence of particle size on the twin-screw extrusion of corn meal. Cereal Chemistry 74 (5), 656–661. Guhaa, M., Alia, S., Bhattacharya, S., 1997. Twin-screw extrusion of rice flour without a die: effect of barrel temperature and screw speed on extrusion and extrudate characteristics. Journal of Food Engineering 32 (3), 251–267. Gutkoski, L.C., El-Dash, A.A., 1999. Effect of extrusion process variables on physical and chemical properties of extruded oat products. Plant Foods for Human Nutrition 54 (4), 315–325. Hsu, K.H., 1983. A diffusion-model with a concentration-dependent diffusioncoefficient for describing water-movement in legumes during soaking. Journal of Food Science 48 (2), 618–622. Hu, L., Hsieh, F., Huff, H.E., 1993. Corn meal extrusion with emulsifier and soybean fiber. Food Science and Technology-Lebensmittel–Wissenschaft and Technologie 26 (6), 544–551. Jacobs, H., Eerlingen, R.C., Clauwaert, W., Delcour, J.A., 1995. Influence of annealing on the pasting properties of starches from varying botanical sources. Cereal Chemistry 72 (5), 480–487. Lai, L.S., Kokini, J.L., 1991. Physicochemical changes and rheological properties of starch during extrusion. Biotechnology Progress 7 (3), 251–266. Li, P.X., Campanella, O.H., Hardacre, A.K., 2004. Using an in-line slit-die viscometer to study the effects of extrusion parameters on corn melt rheology. Cereal Chemistry 81 (1), 70–76. Li, Y., Li, J., Liu, Z., Ruan, R., Mao, Z., 2003. Vacuum coating of heat-sensitive liquid ingredients onto feed pellets. Transactions of the ASAE 46 (2), 383–387. Lim, C., Cuzon, G., 1994. Water stability of shrimp pellet, a review. Asian Fisheries Science 7, 115–127. Lopez, G., Ros, G., Rincon, F., Periago, M.J., Martinez, M.C., Ortuno, J., 1996. Relationship between physical and hydration properties of soluble and insoluble fiber of artichoke. Journal of Agricultural and Food Chemistry 44 (9), 2773–2778. Mathew, J.M., Hoseney, R.C., Faubion, J.M., 1999. Effects of corn sample, mill type, and particle size on corn curl and pet food extrudates. Cereal Chemistry 76 (5), 621–624. Mercier, C., Feillet, P., 1975. Modification of carbohydrate components by extrusioncooking of cereal products. Cereal Chemistry 52 (3), 283–297.
Miladinov, V.D., Hanna, M.A., 2000. Starch esterification by reactive extrusion. Industrial Crops and Products 11 (1), 51–57. Onwulata, C.I., Konstance, R.P., 2006. Extruded corn meal and whey protein concentrate: effect of particle size. Journal of Food Processing and Preservation 30 (4), 475–487. Owusuansah, J., Vandevoort, F.R., Stanley, D.W., 1983. Physicochemical changes in cornstarch as a function of extrusion variables. Cereal Chemistry 60 (4), 319– 324. Ozcan, S., Jackson, D.S., 2005. Functionality behaviour of raw and extruded corn starch mixtures. Cereal Chemistry 82 (2), 223–227. Plews, A.G., Atkinson, A., Trampe, T., McGrane, S., 2009. Control of porosity and expansion in starch extrusion by monitoring pressure at die outlet. Journal of Cellular Plastics 45 (1), 67–82. Qian, J.Y., Ding, X.L., 1996. Effect of twin-screw extrusion on the functional properties of soya fibre. Journal of the Science of Food and Agriculture 71 (1), 64–68. Ryu, G.H., Ng, P.K.W., 2001. Effects of selected process parameters on expansion and mechanical properties of wheat flour and whole cornmeal extrudates. StarchStarke 53 (3–4), 147–154. Singh, B., Sekhon, K.S., Singh, N., 2007. Effects of moisture, temperature and level of pea grits on extrusion behaviour and product characteristics of rice. Food Chemistry 100 (1), 198–202. Sopade, P.A., Hardin, M., Fitzpatrick, P., Desmee, H., Halley, P., 2006. Macromolecular interactions during gelatinisation and retrogradation in starch-whey systems as studied by Rapid Visco-Analyser. International Journal of Food Engineering, 2(4), article 7. DOI: 10.2202/1556-3758.1074. Tran, Q.D., Hendriks, W.H., van der Poell, A.F.B., 2008. Effects of extrusion processing on nutrients in dry pet food. Journal of the Science of Food and Agriculture 88 (9), 1487–1493. Whalen, P.J., Bason, M.L., Booth, R.I., Walker, C.E., Williams, P.J., 1997. Measurement of extrusion effects by viscosity profile using the Rapid Visco Analyser. Cereal Foods World 42 (6), 469–475. White, G.A., Doucet, F.J., Hill, S.E., Wiseman, J., 2008. Physicochemical properties and nutritional quality of raw cereals for newly weaned piglets. Animal 2 (6), 867– 878. Yuliani, S., Torley, P.J., D’Arcy, B., Nicholson, T., Bhandari, B., 2006. Effect of extrusion parameters on flavour retention, functional and physical properties of mixtures of starch and D-limonene encapsulated in milk protein. International Journal of Food Science and Technology 41 (s2), 83–94. Zhang, W., Hoseney, R.C., 1998. Factors affecting expansion of corn meals with poor and good expansion properties. Cereal Chemistry 75 (5), 639–643.
Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Particle size of milled barley and sorghum and physico-chemical properties of grain following extrusion Ghaid J. Al-Rabadi a, Peter J. Torley b, Barbara A. Williams a, Wayne L. Bryden a,c, Michael J. Gidley a,⇑ a
The University of Queensland, Centre for Nutrition and Food Sciences, School of Land Crop and Food Sciences, St. Lucia, Brisbane, Qld 4072, Australia National Wine and Grape Industry Centre, School of Agricultural and Wine Sciences, Charles Sturt University, Wagga Wagga NSW 2678, Australia c The University of Queensland, School of Animal Studies, Gatton, Qld 4343, Australia b
a r t i c l e
i n f o
Article history: Received 14 April 2010 Received in revised form 29 October 2010 Accepted 11 November 2010 Available online 20 November 2010 Keywords: Barley Sorghum Particle size Processing parameters Hydration properties Extrusion
a b s t r a c t Milled barley and sorghum grains were separated into three size fractions (fine, <0.5 mm; medium, 0.5– 1.0 mm; coarse, >1.0 mm) and extruded at two maximum temperatures (100 °C; 140 °C). Mechanical resistance and specific mechanical energy during extrusion was significantly higher for fine fractions, and extrusion at high temperature resulted in higher mechanical resistance. Pressure generated during extrusion was higher for the fine fraction in sorghum but lower in barley. Expansion index was highest for the fine fraction for barley, but did not differ significantly between sorghum fractions or with extrusion temperature. For all samples, extrusion at low temperature resulted in a higher final paste consistency and lower water absorption index, but there was no significant effect on water solubility index (WSI). Fraction size showed a significant effect on WSI in sorghum but not in barley. The results are rationalised in terms of differences in grain composition between sorghum and barley. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Extrusion cooking is a high temperature, high pressure, short time treatment in which food or feed material is exposed to mechanical shearing (Baik et al., 2004; Lai and Kokini, 1991). The reactions occurring during extrusion are influenced by a large number of variables that are related both to the machine and to the raw materials (Mathew et al., 1999). The effect of different equipment variables (such as feed rate, water addition rate, barrel temperature, screw configuration and screw speed) on rheological and physiochemical properties of the extrudate have been reviewed (Lai and Kokini, 1991). Changing these independent variables can change one or more dependent variable such as mechanical energy input to the extruder, generated pressure and product temperature. As a consequence, independent variables will affect the physico-chemical properties of the melt (Akdogan, 1996; Lai and Kokini, 1991; Whalen et al., 1997) and the physical characteristics of the extrudate (Ryu and Ng, 2001). Changes in extrusion dependent variables, and thus physicochemical properties of extrudate, can directly and indirectly influence subsequent application value. In animal feeds, extrusion has been shown to influence animal performance as well as affecting feed processing decisions in industry. For example, specific ⇑ Corresponding author. Tel.: +61 733652145; fax: +61 73365117. E-mail address: [email protected] (M.J. Gidley). 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.11.016
mechanical energy (SME) input during extrusion has been linked with palatability acceptance by animals (Tran et al., 2008), the relative ease with which a material can be extruded, and the cost of the extrusion operation (Miladinov and Hanna, 2000). The extent of extrudate expansion is one of the most important properties as it influences the porous structure of the extrudate (Plews et al., 2009). Extruded animal feeds often have coatings, such as heat sensitive enzymes and flavours, applied by vacuum, and the efficiency of this process depends on the porosity of the extrudate (Li et al., 2003). Changes in hydration properties such as water absorption index (WAI) and water solubility index (WSI) have been reported to affect both the ability of feed to mix with digestive enzymes and the general behaviour of feed in the digestive tract of monogastric animals (White et al., 2008). In addition, hydration properties and integrity of extrudate have been reported to affect extrudate stability in water which is an important property in fish and aquaculture feeds (Lim and Cuzon, 1994). Compared with the large body of research on extruder parameters, relatively little research has been conducted into the role of raw material parameters, particularly particle size. The research which has been undertaken into feed material particle size, shows that it influences both extruder operation and extrudate properties (Altan et al., 2009; Carvalho et al., 2010; Garber et al., 1997; Mathew et al., 1999; Onwulata and Konstance, 2006; Zhang and Hoseney, 1998). Different particle sizes have been achieved by changing grain milling screen size (Mathew et al., 1999) to give
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different distributions of particle size (Altan et al., 2009; Garber et al., 1997; Zhang and Hoseney, 1998) or segregation by sieving (Al-Rabadi et al., 2009; Carvalho et al., 2010; Onwulata and Konstance, 2006). Generally, it can be concluded form these studies, that particle size has a significant effect on a wide range of processing variables and hydration properties of extrudates generated. Effects were also dependent on grain fragment pre-conditioning, as well as on particle size and the interaction effect of other extrusion processing variables. Most reported experiments were based on maize as the main starch/grain material. The objective of the present study was to characterise a range of compositional and functional properties, as well as processing parameters, of separated size fractions after extrusion of two grains that are representative of grains used in animal diets, and which can serve as models of wholegrain extrusion for human foods. Barley was examined as an example of a grain that contains appreciable levels of (soluble) dietary fibre and has relatively low starch content compared to maize. For comparison, sorghum, which is a grain that is high in starch and low in soluble dietary fibre, was also examined. The processing dependent variables during extrusion were torque, specific mechanical energy (SME) and die pressure. The functional property characteristics studied were water absorption index, water solubility index, rheological profile on heating in excess water (paste consistency), and expansion index (EI).
Table 1 Fraction yield, average particle size and starch content (±SEM) in ground and segregated grain particles of barley and sorghum. Grain characteristic
dgwa (mm) Sgwb Starch content (%) Ground (control) >1.0 mm 1.0–0.5 mm <0.5 mm Sieve opening (mm) 3.35 2.8 1.7 1.0 0.5 0.25 0.125 0.75 Pan
Grain Barley
Sorghum
0.684 2.11
0.661 1.96
48.23 ± 1.59 63.8 ± 1.48 53.60 ± 0.47c 69.7 ± 1.42 47.7 ± 1.48 64.0 ± 3.04 43.0 ± 0.92 63.5 ± 1.72 Distribution of particles after sieving (g/l00 g of sample) 0d 0 0.12 1.07 6.67 4.46 27.50 22.91 31.57 37.73 14.67 21.27 12.30 9.10 4.81 1.74 1.08 0.22
a
dgw geometric mean diameter. Sgw geometric standard deviation and equal to Log normal standard deviation (ASAE, 2003). c Estimated by subtraction of starch content in unsieved ground from starch content in the fine and medium fraction. d Values are duplicate measurements of 100 g sample retained on top of sieves after 29 min shaking. b
2. Materials and methods 2.1. Materials 2.1.1. Cereals and milling Barley (variety; Buster) and sorghum (variety; Binalong, PBI Narrabri) were obtained from the Department of Primary Industries and Fisheries (Queensland, Australia). Grains were milled using a hammer mill (HM) (Australian Agriculture Machinery Group, Australia) at a speed of 1140 rpm, with samples of milled grain collected at a constant motor load. Equal portions of each grain were milled through three screen sizes (2, 4 and 6 mm) separately to produce ground material with a range of different particle size distributions. Grains milled through different hammer-mill screen sizes were mixed manually, then sieved using a sieve shaker to produce three particle size ranges: coarse (>1.0 mm), medium (1.0–0.5 mm), and fine (<0.5 mm), in addition to the unsieved ground grain (control). Table 1 shows the particle distribution, average particle size (dgw) and geometric standard deviation (sgw) of the ground barley and sorghum grains after being hammer-milled at three hammer-mill screen sizes and after being mixed. The moisture content of each particle size was determined by drying in a hot air oven (135 °C for 3 h). 2.1.2. Extrusion conditions and processing parameters High-temperature short-time (HTST) extrusion cooking was conducted using a co-rotating twin-screw model Prism Eurolab KX16 (Thermo Prism, Staffordshire, UK). The screw profile is shown in Table 2. The barrel diameter was 16 mm with a length/diameter ratio of 40:1. The die had two openings each 2 mm in diameter and 8 mm in length. Melt pressure was measured with a pressure transducer fitted to the die block (Terwin, Nottinghamshire, UK). Motor torque, screw speed, barrel temperatures and melt pressure were monitored with Prism software (Sysmac-SCS version 2.2; Omron Corporation, Milton Keynes, UK). Liquid feed rate and dry feed rate were recorded manually after being calibrated before each run. Dry feed was fed through a single screw volumetric feeder (KX16 Powder feeder; Brabender Technologie, Duisburg, Germany). Water was injected through a port 150 mm from the
Table 2 Extruder screw configuration profile.
*
Screw elements
Number of screw elements and their length in D*
Section length of each type of screw element (mm)
Twin flight feed screw 60° Forward paddle Twin flight feed screw 60° Forward paddle 90° Paddle Twin flight feed screw 60° Forward paddle 90° Paddle Twin flight feed screw Twin flight feed screw Single flight feed screw Total
13 1D 4 0.25D 4 1D 8 0.25D 4 0.25D 6 1D 8 0.25D 4 0.25D 1 0.5D 8 1D 1 1.5D 40
208 16 64 32 16 80 32 16 8 128 24 640
D: the extruder barrel diameter; for the Prism Eurolab KX16 D = 16 mm.
start of the barrel using a peristaltic pump (L/S 7523) with a Tygon Lab tubing 13 (0.8 mm internal diameter, Masterflex; Cole-Parmer Instrument Company, Vernon Hills, IL, USA). The dry feed rate for barley and sorghum was 20 g/min and 25 g/min, respectively, and the amount of water added at the extruder barrel was adjusted to compensate for moisture differences in the samples to have a dough moisture content of 55% for barley and 50% for sorghum (wb). Barley fractions were extruded at lower feed rate and higher moisture content, compared to sorghum, to avoid any possible blockage during extrusion. Two sets of barrel temperature settings (high and low) (Table 3), and constant screw speed of 200 r.p.m. were used. Samples were collected when the extruder was running at steady state (i.e. stable values for both torque and die pressure). The samples were collected over 15–20 min, placed in an aluminium tray, and dried in a hot air oven (50 °C for 24 h). After drying, they were sealed into plastic bags and stored at 18 °C pending further analysis.
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2 min. Each analysis took 13 min, and duplicate analyses were performed on each sample.
Table 3 Low and high temperature profiles used in the extruder barrel. Extruder zone *
Low temperature profile (°C) ***
1 2 3** 4 5 6 7 8 9 10 Die
– 50 50 75 90 100 100 100 100 100 100
High temperature profile (°C)
2.2.3. Scanning electron microscopy Specimens were mounted onto aluminium stubs with carbon tabs and sputter coated with a 10–15 nm layer of platinum using an EIKO IB-5 Platinum Sputter Coater. Samples were viewed in either a JEOL 6300 or JEOL 6400 field emission scanning electron microscope.
–*** 50 50 75 90 140 140 140 105 100 95
*
Dry feed port was located in the first extruder barrel section. Liquid injection port was located in the third extruder barrel section. The first zone in the extruder barrel is cooled with circulating tap water (about 25 °C). **
***
All extrudates were ground using a hammer-mill to pass through a 1 mm sieve for evaluation of physico-chemical properties and for total starch measurements. Ground extrudate samples were sealed in plastic bags and stored at 4 °C. Specific mechanical energy (SME) was calculated using Eq. (1) (Frame, 1994):
SME ¼
mechanical energy input Nact T act K w ¼ mass flow rate Nmax T max Q
ð1Þ
where Nact is the actual screw speed (r.p.m), Nmax the maximum screw speed (r.p.m), Tact the actual torque (N m), Kw the motor power (kW) and Q the output (kg s1). 2.2. Analyses 2.2.1. Water absorption index (WAI) and water solubility index (WSI) The WAI and WSI analysis methods used were adapted, with modifications, from Anderson et al. (1969). To maintain constant temperature and stirring, a 2.5 g sample was suspended in 30 mL distilled water at 30 °C for 30 min under constant stirring (60 rev/min) using a Rapid Visco Analyser (RVA) (Series 4, Newport Scientific, Warriewood, NSW, Australia). The slurry was poured into a tared centrifuge tube of 50 ml, and centrifuged at 3000g for 10 min. The sample used to measure WAI was the same sample as that used to measure WSI. The supernatant was carefully poured into a tared evaporating dish and dried at 135 °C for 3 h for total solid measurements. The weight of remaining gel was measured in the tubes after centrifugation. WAI and WSI analyses were conducted at random and in duplicate, and were calculated using Eqs. (2) and (3), respectively (Dogan and Karwe, 2003), as follows:
g water soluble matter g dry sample g water absorbed WAI ¼ g dry sample ð1-souble fractionÞ
WSI ¼
ð2Þ
2.2.4. Radial expansion index Radial expansion was evaluated by measuring extrudate diameter using digital callipers. Expansion ratio was calculated by dividing extrudate diameter by the diameter of die (Yuliani et al., 2006). A total of twenty readings were recorded for each sample. 2.2.5. Experimental design and statistical analysis For each grain, the experimental design was a split-plot, with particle size as the main plot factor (four levels of particle size), and the two temperatures as the split-plot factors (two temperature profiles). The main plot design was a randomized complete block with two blocks (blocked by day). Data were analysed by the PROC MIXIED procedure using SAS 9.1.2 Software (Cary, NC) to assign main treatment effects and probabilities. Means were determined using the LSMEANS statement and the differences between means (i.e. main effect) by using the ‘‘pdiff’’ option. Significant differences were defined as P < 0.05. 3. Results and discussion The effect of particle size and thermal treatment on extrusion dependent variables (torque, SME and die pressure) and hydrothermal properties (paste consistency, WAI and WSI and expansion index) were determined for both barley (Table 4) and sorghum (Table 5). The influence of the main effects (i.e. particle size fraction and extrusion temperature) in barley (Table 6) and sorghum (Table 7) are discussed in the following sections. 3.1. Effect of particle size and temperature on torque and SME Pairwise comparisons between treatments showed that there were significant differences between the main treatments (particle size and temperature) on the dependent processing variables of extruder torque and specific mechanical energy for both barley and sorghum. Torque and SME are direct indicators of the work performed on the material being processed in the extruder, and are controlled by melt rheology and the shear conditions created by the extruder screw and die (Akdogan, 1996). SME has been reported to play an important role in starch conversion, by rupturing stabilising semi-crystalline structures, and thus contributing to starch granule breakage during heating and shearing stress
ð3Þ
2.2.2. Paste consistency Paste consistency profiles were determined using a Rapid Visco Analyser (RVA) (Series 4, Newport Scientific, Warriewood, NSW, Australia). The paste consistency profile for each grain type and particle size was measured using a grain sample of 2.5 g (db) with distilled water added until a final weight of 25 g ± 0.01 g was reached. Each sample was held at 50 °C for one min and stirred at 960 rev/min for 10 s followed by constant stirring at 160 rev/ min, heating from 50 °C to 95 °C in 3.7 min, holding at 95 °C for 2.5 min, cooling to 50 °C in 3.8 min and holding at 50 °C for
Table 4 Least square means and SEM values for particle size temperature effects on processing parameters and extrudate physico-chemical properties in barley. Parameter
Fraction size Fine
Torque (Nm) SME (kJ/kg) Pressure (bar) WSI (%) WAI (g/g) EI
SEM Medium
Unsieved
Low
High
Low
High
Low
High
2.56 125.5 5.1 8.93 3.56 1.59
2.54 124.4 5.4 8.94 4.16 1.55
1.56 76.5 6.4 6.95 3.69 0.98
2.15 105.4 7.3 6.45 4.21 1.08
1.32 64.6 5.0 8.03 3.71 0.97
1.40 68.2 4.8 6.87 3.50 1.11
0.12 6.0 0.2 0.57 0.08 0.05
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G.J. Al-Rabadi et al. / Journal of Food Engineering 103 (2011) 464–472 Table 5 Least square means and SEM values for particle size temperature effects on processing parameters and extrudate physico-chemical properties in sorghum. Parameter
Fraction size
SEM
Fine
Torque (Nm) SME (kJ/kg) Pressure (bar) WSI (%) WAI (g/g) EI
Medium
Coarse
Unsieved
Low
High
Low
High
Low
High
Low
High
3.60 131.1 12.8 4.31 3.57 1.17
5.41 197.6 16.6 3.97 4.02 1.12
2.26 82.1 10.2 3.14 3.05 1.11
3.69 133.8 11.4 3.36 3.36 1.34
1.98 71.8 9.4 2.67 3.21 1.12
2.71 89.5 9.5 2.59 3.57 1.41
3.11 113.4 9.3 3.49 3.44 1.10
3.38 122.6 9.7 3.43 3.61 1.41
0.23 8.5 1.2 0.11 0.17 0.061
Table 6 Effect of particle size and temperature level on processing variables and hydration properties of extruded barley. Parameter
Torque (Nm) SME (kJ/kg) Pressure (bar) WSI (%) WAI (g/g) EI *,a,b
Fraction size
Temperature level
Fine
Medium
Unsieved
SEM
Low
High
SEM
2.55a* 124.9a 5.3b 8.94a 3.86a,b 1.57a
1.86b 90.9b 6.5a 6.71a 3.95a 1.03b
1.36b 66.4b 4.9b 7.45a 3.61b 1.04b
0.11 5.6 0.1 0.52 0.06 0.03
1.81b 88.9b 5.5a 7.97a 3.65b 1.18a
2.02a 99.1a 5.9a 7.42a 3.96a 1.25a
0.07 3.5 0.1 0.33 0.04 0.03
Values within a single row, within main effect treatment, with different superscripts differ significantly (P < 0.05).
Table 7 Effect of particle size and temperature level on processing variables and hydration properties in extruded sorghum. Parameter
Torque (Nm) SME (kJ/kg) Pressure (bar) WSI (%) WAI (g/g) EI a,b,c
Particle size
Temperature level
Fine
Medium
Coarse
Unsieved
SEM
Low
High
SEM
4.51a 164.3a 14.7a 4.14a 3.79a 1.14a
2.97b,c 108.0b,c 10.8a,b 3.25b 3.20a 1.22a
2.34d,c 85.2d,c 9.4b 2.63c 3.39a 1.26a
3.24b 118.0b 9.5b 3.46b 3.52a 1.25a
0.16 6.0 1.0 0.09 0.16 0.04
2.74b 99.6b 10.5a 3.40a 3.32b 1.12a
3.80a 138.1a 11.8a 3.34a 3.64a 1.32a
0.12 4.3 0.6 0.05 0.05 0.58
Values within a single row, within main effect treatment, with different superscripts differ significantly (P < 0.05).
(Carvalho et al., 2010). Torque and SME are usually controlled by dough (melt) viscosity, if extruder operating conditions (e.g. screw speed) are held constant (Zhang and Hoseney, 1998). In a single set of extrusion conditions, torque and SME are directly related. However, SME is a more meaningful parameter as it determines the amount of energy used per unit mass, making it possible to compare results obtained at different feed rates, screw speeds and in different extruders. In barley, extrusion of the fine fraction gave a higher torque compared to the medium size fraction and the unsieved ground barley (Tables 4 and 6). SME responded in a similar manner to particle size, with a higher SME for the fine fraction, which was almost double that of the unsieved ground barley, while the medium size fraction had an intermediate SME (Tables 4 and 6). Different sorghum size fractions showed a similar trend effect on torque and SME, but with higher values than the corresponding size fractions for barley. This could be attributed to the expected higher dough viscosity as sorghum fractions have both a higher starch content and were extruded under lower moisture conditions (higher grain fragment feed rate and lower moisture feed rate) than barley. Extrusion of the fine fraction gave a higher torque compared to the medium size fraction and the coarse fraction (Tables 5 and 7), while the unsieved ground sorghum had an intermediate SME. SME showed a similar response to torque, with the fine fraction giving the highest SME, and lower SMEs for the medium
and coarse fractions, while the unsieved ground sorghum had an intermediate SME (Tables 5 and 7). Torque during extrusion has been reported to be dependent on the rheological status of the plasticised mass inside the extruder which is positively related to SME (Guhaa et al., 1997). The higher values of torque and SME for extrusion of the fine fractions could be attributed to greater viscosity caused by swelling and presumed gelatinization of starch (Mathew et al., 1999; Zhang and Hoseney, 1998). The lower melt viscosity during the extrusion of the medium and coarse fractions could be related to incomplete water penetration into the larger particles limiting starch swelling, due to lower surface to volume ratio (Hsu, 1983), as well as the presence of more outer grain layers (i.e. grain coat) and more impenetrable endosperm due to intact cell walls and protein matrices in the large particle size. Any starch swelling/gelatinization that occurs within intact cellular structures would also not be expected to alter bulk rheological properties. Water was added directly to the extruder barrel during extrusion, meaning that limited time was available for mass transfer of water. This would be expected to reduce the amount of water penetrating through large grain fragments compared to the finer fraction (Zhang and Hoseney, 1998) For these reasons, mechanical properties of large particles may not be greatly altered during extrusion, and be present in the extrudate in a relatively intact state as illustrated by electron micrographs of both sorghum (Fig. 1c and d) and barley (Fig. 1e) compared to the
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more disrupted structures evident in extruded fine fractions from sorghum and barley (Fig. 1a and b, respectively). Coarse corn grits have also been reported to give a lower extruder motor load and lower melt viscosity than smaller grit particles (Carvalho et al., 2010; Garber et al., 1997; Mathew et al., 1999; Zhang and Hoseney, 1998). In contrast, Altan et al. (2009) reported a higher SME value for extrusion of barley grits, compared with ground barley flour following a pre-conditioning step. The lower SME in the present study for the medium and coarse fractions could be attributed to the absence of pre-conditioning before extrusion. Pre-conditioning of coarse particles before extrusion has been reported to result in equal starch gelatinization to that of fine particles (Mathew et al., 1999), presumably because pre-conditioning allows sufficient time for water penetration into even the largest particles prior to extrusion. In both sorghum and barley, extrusion at a low temperature resulted in a lower torque and SME, compared with extrusion at a high temperature. This may be due to the lower extrusion temperature reducing the water diffusion rate into the grain fragments (Addo et al., 2006), reducing the potential for starch swelling dur-
ing extrusion (Lai and Kokini, 1991; Li et al., 2004), and thus reducing the melt viscosity (Li et al., 2004), which is reported to be the main influence on torque (Garber et al., 1997). The impact of temperature on torque and SME was clearer in sorghum than in barley, with all sorghum values higher than barley for comparable size fractions. This may be due to the higher starch content in sorghum than in barley. However, it could also be due to the greater fibre concentration in barley with, particularly, the (1,3;1,4)-b-glucanrich endosperm cell walls acting as a more effective barrier to starch swelling than the cell walls of sorghum endosperm. 3.2. Die pressure There was a significant effect of particle size on die pressure for both barley and sorghum. In barley, extrusion of the medium size fraction resulted in a higher pressure generated (6.48 bar) at the die, compared with the fine fraction (5.29 bar) or the unsieved ground barley (4.93 bar). Extrusion was conducted at the same feed rate and screw speed, so it was not obvious that the medium fraction should generate higher pressure than the fine fraction, as
Fig. 1. High temperature extrusion treatment results in extensive disruption of grain particles and starch granules in (a) sorghum (fine size fraction), and (b) barley (fine size fraction – high magnification), but not in (c) sorghum (coarse size fraction – high magnification), (d) sorghum (coarse size fraction – low magnification) or (e) barley (medium size fraction) – low magnification).
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the medium fraction had a lower viscosity (as indicated by higher torque and SME outcome values). The lower die pressure for the fine fraction may be due to increased functionality of water-soluble fibre caused by the heat-moisture environment. An increase in the amount of the water-soluble fibre fraction has been reported after extrusion (Dust et al., 2004; Qian and Ding, 1996), and might be expected to be higher for the fine fraction due to more surface area exposure. Fibre in the fine fraction is likely to have a lower swelling potential due to a lower water-holding capacity (Anguita et al., 2006), and more rapid penetration of water into, and diffusion through the particles. These properties may have reduced the amount of material that built up behind the die plate, producing a lower pressure in the fine fraction compared to the medium fraction (Hu et al., 1993). Partial build-up of swollen fibre at the die, in the case of extrusion of the medium particles, could conversely have increased the pressure. In sorghum, extrusion of the fine fraction resulted in a higher generated pressure (14.72 bar) at the die, compared to both the coarse fraction (9.44 bar) and the unsieved ground sorghum (9.54 bar), while it was intermediate for the medium-sized fraction (10.81 bar). The higher pressure generated at the die by the fine fraction, could be attributed to higher viscosity (Hu et al., 1993). An increase in the die pressure due to the effect of fine particles on viscosity has been reported by (Zhang and Hoseney, 1998). Sorghum does not have the same potential to release soluble fibre as barley, so starch and water penetration effects are apparently dominant. For both sorghum and barley, extrusion at low (max temperature setting 100 °C) or high temperature (max temperature setting
469
140 °C) did not have a significant effect on die pressure measurements. 3.3. Extrudate hydrothermal properties and expansion index 3.3.1. WAI The water absorption index is a physico-chemical parameter that has been reported to indicate the hydrolytic breakdown of starch during extrusion (Owusuansah et al., 1983), and the swelling behaviour of the starch component (Doucet et al., 2009; White et al., 2008). The presence of non-starch components, such as fibre, has been also reported to affect WAI (Lopez et al., 1996). In addition, WAI has been reported to be a measure of damaged starch, as well as protein denaturation and macromolecular complex formation (Dogan and Karwe, 2003). The results of this study showed that there was a significant difference in WAI values between different particle size fractions of barley (Table 6), although the differences were quantitatively small. Extrusion of the medium-sized fraction produced the highest WAI (3.95), while the unsieved ground barley gave the lowest WAI (3.61), and the fine fraction gave an intermediate WAI (3.86). The higher WAI value for the medium fraction could be attributed to the physical structure of fibre, although grinding of the extruded samples prior to analysis may have masked to some extent the effect of particle size on WAI. Larger fibre particles have been reported to have a higher water-holding capacity compared to the smaller particles, due to their greater ability to hold water within the cell wall matrix (Cadden, 1987). In sorghum, there was no significant effect of particle size on WAI (Table 7), but
Fig. 2. The effect of temperature on viscosity profile of barley extrudates prepared from the (a) fine size fraction, (b) medium size fraction and (c) unsieved ground grain. Samples were ground through a 1 mm screen before RVA analysis.
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grinding of the extruded samples prior to analysis may have masked any effect of particle size on WAI in sorghum. Extrusion under the higher temperature conditions (Table 3) resulted in a higher WAI for both barley and sorghum (Tables 4 and 5). An increase in WAI has been reported for extruded products from ground oat grains (Gutkoski and El-Dash, 1999), and wheat, corn and rice starches (Mercier and Feillet, 1975). Changes during extrusion such as protein denaturation, starch swelling, and swelling of the crude fibre, could all contribute to the increased WAI (Singh et al., 2007). 3.3.2. WSI WSI has been reported to represent the extent of soluble polysaccharides released from the grain in an excess of water (Doucet et al., 2009; White et al., 2008). In this study, neither of the main effects (particle size and temperature) had a significant effect on WSI in barley, although there was a trend for finer particles to have higher WSI values (Tables 4 and 6). However, there were significant differences between different ranges of size fractions in sorghum (Tables 5 and 7). Extrusion of the fine fraction resulted in the highest WSI (4.14) compared to the coarse fraction (2.36), while it was intermediate for the medium-sized fraction (3.25) and the unsieved ground material (3.46). The increase in WSI with decreasing particle size may be attributed to the greater specific surface area before extrusion, resulting in higher leaching of soluble starch-derived molecules after extrusion that dissolved in water during the WSI assay. In sorghum, extrusion temperature had no significant effect on WSI.
3.3.3. Pasting profiles Paste consistency measurements using RVA have been reported to be a useful method to monitor the degree of starch cooking during extrusion processing (Becker et al., 2001) and to provide a quick analytical tool for measuring extruded products for diagnostic development applications, due to its sensitivity to changes in processing parameters (Whalen et al., 1997). The paste consistency profile of ground extrudate was measured for both barley (Fig. 2) and sorghum (Fig. 3), to investigate the effect of extrusion temperature conditions for the same sized fraction. For both grains, and at all particle size fractions, extrusion at low temperature resulted in a higher final paste consistency. When the paste consistency of ground extrudates was measured, the higher final paste consistency could be linked to a higher leaching of macromolecules (Jacobs et al., 1995), and the formation of a viscous paste or gel (Ozcan and Jackson, 2005). Sopade et al. (2006) reported that the amount of ungelatinised starch (i.e. raw starch) present in the sample is responsible for the final paste consistency that develops during the cooling stage. Other pasting curve features also showed that less starch damage occurred during extrusion at lower temperature, accompanied by lower SME values. In barley extruded at the lower temperature, there was a higher peak paste consistency (fine fraction, Fig. 2a; unsieved ground grain, Fig. 2c), and a delay in paste consistency onset for the medium fraction (Fig. 2b). These characteristics indicate higher thermal stability and a higher potential for swelling after low temperature processing, indicative of less starch damage (Doucet et al., 2009; Whalen et al., 1997) compared to high temperature processing.
Fig. 3. The effect of temperature on viscosity profile of sorghum extrudates prepared from the (a) fine size fraction, (b) medium size fraction, (c) coarse size fraction and (d) unsieved ground grain. Samples were ground through a 1 mm screen before RVA analysis.
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This is consistent with the lower WAI values found after low temperature processing. In sorghum, extrusion at the low temperature, which was associated with a lower SME value, resulted in a lower initial paste consistency profile (low cold swelling profile) and lower peak paste consistency values in the medium fraction (Fig. 3b), the coarse fraction (Fig. 3c) and the unsieved ground material (Fig. 3d). The lower initial increase in paste consistency (nonappearance of cold swelling) could be attributed to lower starch damage after extrusion at the lower temperature (Carvalho and Mitchell, 2000; Doucet et al., 2009; Sopade et al., 2006). Initial paste consistency development (cold swelling) was higher for extrudates processed at high temperature (except for the fine fraction) and was apparent directly after starting paste consistency measurement at 50 °C. Cold swelling has been reported previously to be one of the parameters that is related to more damaged and converted starch (Ozcan and Jackson, 2005; Whalen et al., 1997), and the rate of particle hydration (Becker et al., 2001). The lower peak viscosity values for those size fractions could be attributed to the presence of large intact particles that exited the extruder without thermo-mechanical damage (Fig. 1), and failed to swell when extruded at the low temperature. In extrudates made from the fine fraction, the presence of large particles was not the limiting factor, and the low extrusion temperature may have resulted in less starch conversion, and thus more swelling and higher peak and final viscosities, compared to extrusion at the high temperature (Fig. 3a). 3.3.4. Expansion index (EI) The results of this study show that the fraction size resulted in a significant effect on EI in barley. Extrusion of the fine fraction resulted in a higher EI (1.57), compared with the medium fraction (1.03) or unsieved ground barley (1.04). Extrusion of large particles has been reported to have a lower expansion due to incomplete starch gelatinization (Garber et al., 1997) and a lower melt viscosity (Dogan and Karwe, 2003). A high degree of starch gelatinization and SME has been reported to correlate very well with the expansion index (Dogan and Karwe, 2003). However, in sorghum, the particle size had no effect on the expansion index, although differences in SME and torque were found between size fractions. This may be attributed to the high paste consistency of the fine fraction at the die (indicated by the high torque and SME measurements). Materials with high viscosity have been reported to require high pressure for expansion (Zhang and Hoseney, 1998), which may have been the limiting factor in this case. For both sorghum and barley, temperature did not show any significant effect on the expansion index, although it was numerically higher when extrusion was performed at the higher temperature. The temperature effect may be masked by lower water penetration into large particles in the medium and coarse fraction as reported above. It has been reported that the interaction between temperature and moisture are the most important factors in determining the extent of starch conversion in twin screw (Owusuansah et al., 1983) and single screw (Chiang and Johnson, 1977) extruders. Addition of water in a pre-conditioner before extrusion, to increase the time available for penetration into particles, has been reported to increase the volumetric expansion index (Zhang and Hoseney, 1998). 4. Conclusion Extrusion of different size fractions of milled barley and sorghum grain under two temperature profiles resulted in a significant effect on independent variables (torque, SME and die pressure) and on physico-chemical properties (paste consistency, WAI, WSI, and expansion). Particle size and processing tempera-
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ture were assumed to affect water penetration into particles, and their resultant hydration. Different chemical compositions between the different size fractions can affect both independent variables as well as physico-chemical properties, although regrinding of the extrudate is likely to mask the effect of extrusion variables on physico-chemical properties to some extent. Paste consistency measurement with RVA could be used as a complementary tool, in combination with independent variables such as SME, to examine the effect of processing condition on extrudate properties and the extent of starch damage. The separate size fractions studied here are typical of the range of sizes that are present in non-fractionated ground grains, used in both animal feed and human food. The fact that different size fractions from the same grain performed differently, both within the extruder and subsequently, illustrates the difficulty in delineating the complex extrusion behaviour of grains. This study suggests that insights into the extrusion performance of individual size fractions are needed to appreciate the factors responsible for extrusion performance and extrudate properties of non-fractionated ground grains. Acknowledgements The authors would like to thank Drs. John Black, Ali Youssef, Dagong Zhang, Ashok Shrestha, Honest Madziva, Lesleigh Force, and Deirdre Mikkelsen for their help and technical advice. The authors thank the Queensland Department of Primary Industries and Fisheries (Alexandra Hill, Brisbane, Australia) for supply of grains, and for assistance in milling. Funding was provided by the Co-operative Research Centre for an Internationally Competitive Pork Industry. References Addo, A., Bart-Plange, A., Dzisi, K., 2006. Water absorption characteristics of obatanpa and mamaba maize hybrids (Zea mays). International Journal of Food Engineering, 2(3), article 7. DOI:10.2202/1556-3758.1067. Akdogan, H., 1996. Pressure, torque, and energy responses of a twin screw extruder at high moisture contents. Food Research International 29 (5–6), 423–429. Al-Rabadi, G.J., Gilbert, R.G., Gidley, M.J., 2009. Effect of particle size on kinetics of starch digestion in milled barley and sorghum grains by porcine alpha-amylase. Journal of Cereal Science 50 (2), 198–204. Altan, A., McCarthy, K.L., Maskan, M., 2009. Effect of screw configuration and raw material on some properties of barley extrudates. Journal of Food Engineering 92 (4), 377–382. Anderson, R.A., Conway, H.F., Pfeifer, V.F., Griffin, E.L., 1969. Gelatinization of corn grits by roll and extrusion cooking. Cereal Science Today 14 (1), 4–12. Anguita, M., Gasa, J., Martin-Orue, S.M., Perez, J.F., 2006. Study of the effect of technological processes on starch hydrolysis, non-starch polysaccharides solubilization and physicochemical properties of different ingredients using a two-step in vitro system. Animal Feed Science and Technology 129 (1–2), 99– 115. ASAE, 2003. Methods of determining and expressing fines of feed materials by sieving. Standard No. S319.3. American Society of Agriculture and Biological Engineers, pp. 202–205. Baik, B.K., Powers, J., Nguyen, L.T., 2004. Extrusion of regular and waxy barley flours for production of expanded cereals. Cereal Chemistry 81 (1), 94–99. Becker, A., Hill, S.E., Mitchell, J.R., 2001. Milling - A further parameter affecting the Rapid Visco Analyser (RVA) profile. Cereal Chemistry 78 (2), 166–172. Cadden, A.M., 1987. Comparative effects of particle-size reduction on physical structure and water binding-properties of several plant fibers. Journal of Food Science 52 (6), 1595–1599. Carvalho, C.W.P., Mitchell, J.R., 2000. Effect of sugar on the extrusion of maize grits and wheat flour. International Journal of Food Science and Technology 35 (6), 569–576. Carvalho, C.W.P., Takeiti, C.Y., Onwulata, C.I., Pordesimo, L.O., 2010. Relative effect of particle size on the physical properties of corn meal extrudates: Effect of particle size on the extrusion of corn meal. Journal of Food Engineering 98 (1), 103–109. Chiang, B.Y., Johnson, J.A., 1977. Gelatinization of starch in extruded products. Cereal Chemistry 54 (3), 436–443. Dogan, H., Karwe, M.V., 2003. Physicochemical properties of quinoa extrudates. Food Science and Technology International 9 (2), 101–114. Doucet, F.J., White, A.G., Wulfert, F., Hill, S.E., Wiseman, J., 2009. Predicting in vivo starch digestibility coefficients in newly weaned piglets from in vitro assessment of diets using multivariate analysis. British Journal of Nutrition 21, 1–10.
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