Journal of Food Engineering 66 (2005) 283–289 www.elsevier.com/locate/jfoodeng
The effect of extrusion conditions on the physicochemical properties and sensory characteristics of rice-based expanded snacks Qing-Bo Ding a, Paul Ainsworth b
a,*
, Gregory Tucker b, Hayley Marson
b
a Hollings Faculty, The Manchester Metropolitan University, Old Hall Lane, Manchester M14 6HR, UK School of Biological Sciences, The University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire LE12 5RD, UK
Received 12 December 2003; accepted 14 March 2004
Abstract The effect of extrusion conditions, including feed rate (20–32%), feed moisture content (14–22%), screw speed (180–320 rpm), and barrel temperature (100–140 C) on the physicochemical properties (density, expansion, water absorption index––WAI), and water solubility index (WSI) and sensory characteristics (hardness and crispness) of an expanded rice snack was investigated. Increasing feed rate results in extrudates with a higher expansion, lower WSI, and higher hardness. Increasing feed moisture content results in extrudates with a higher density, lower expansion, higher WAI, lower WSI, higher hardness and lower crispness. Higher barrel temperature increased the extrudate expansion but reduced density, increased the WSI and crispness of extrudate. Screw speed had no significant effect on the physicochemical properties and sensory characteristics of the extrudate. 2004 Elsevier Ltd. All rights reserved. Keywords: Twin-screw extrusion; Rice; Physicochemical; Sensory
1. Introduction Extrusion cooking as a continuous cooking, mixing, and forming process, is a versatile, low cost, and very efficient technology in food processing. During extrusion cooking, the raw materials undergo many chemical and structural transformations, such as starch gelatinization, protein denaturation, complex formation between amylose and lipids, and degradation reactions of vitamins, pigments, etc. (Ilo & Berghofer, 1999). Extrusion cooking has been used increasingly in the production of food and food ingredients such as breakfast cereals, baby foods, flat breads, snacks, meat and cheese analogues, and modified starches, etc. (Anderson, Conway, Pfeifer, & Griffin, 1969; Meuser & van Lengerich, 1992). Despite increased use of extrusion processing, extrusion is still a complicated process that has yet to be mastered. Small variations in processing conditions
*
Corresponding author. Fax: +44-161-247-6331. E-mail address: p.ainsworth@mmu.ac.uk (P. Ainsworth).
0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.03.019
affect process variables as well as product quality (Desrumaux, Bouvier, & Burri, 1999). Product quality can vary considerably depending on the extruder type, screw configuration, feed moisture, and temperature profile in the barrel session, screw speed and feed rate. Rice flour has become an attractive ingredient in the extrusion industry due to its unique attributes such as bland taste, attractive white colour, hypoallergenicity and ease of digestion (Kadan, Bryant, & Pepperman, 2003). The basic investigation of extrusion variables on properties of rice extrudate is still in need, though there have been some reports on rice flour mixed with other ingredients (Ilo, Liu, & Berghofer, 1999; Mouquet, Salvignol, Van Hoan, Monvois, & Treche, 2003). With the increased use of twin-screw extruders in the manufacture of starchy products and starch-based food ingredients, the practice of empirical modification of operating conditions, which yield neither optimum products nor lower cost, cannot be continued. The purpose of this research was to investigate the effects of feed rate, feed moisture, screw speed, and temperature on the physicochemical and textural properties of an extruded rice snack product with a co-rotating twinscrew extruder.
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which five were for the centre point, and sixteen were for non-centre point (Montgomery, 1984). A second order polynomial model for the dependent variables
2. Materials and methods 2.1. Materials Rice flour was supplied by Smiths Flour Mills (Worksop, UK). The typical composition of the rice flour is shown in Table 1.
y ¼ B0 þ B 1 X 1 þ B 2 X2 þ B 3 X3 þ B 4 X4 þ B 5 X 1 X2 þ B6 X1 X 3 þ B 7 X1 X4 þ B8 X2 X 3 þ B 9 X2 X4 þ B10 X3 X4 þ B11 X12 þ B12 X22 þ B13 X32 þ B14 X42
2.2. Extrusion A Werner & Pflederer Continua 37 co-rotating twinscrew extruder (Stuttgart, Germany) with L=D ratio of 27:1 and screw diameter of 37.4 mm was used. The extruder was equipped with a pre-calibrated Rospen twinscrew volumetric feeder (Gloucestershire, UK) and a Watson-Marlow 505 DI pump (Cornwall, UK) which were used to control the solid feed and water inputs respectively. A Sheik MK II model heating unit (Dorset, UK) using circulating oil was also connected to the extruder to control the barrel temperature. Two circular dies with 4 mm diameter were used. During extrusion, the barrel temperature, die pressure, feed rate, screw speed, screw torque, and product temperature were recorded when stable. The extrudates were cooled to room temperature and sealed in polyethylene bags until measurements were taken. 2.3. Experiment design A centre composite RSM design (Table 2) was used to show interactions of feed rate, feed moisture, screw speed and temperature on the extrudate in 21 runs, of
Table 1 Typical composition of rice flour Composition
g/100 g
Protein Moisture Fat Carbohydrate Sugars Starch Fibre
7.6 12.3 1.2 77.4 0.4 76.9 0.7
was established to fit the experimental data. An ANOVA test was carried out using Design Expert 6.0 (State-Ease Inc., Minneapolis, USA) to determine the significance at different levels (0.1%, 1%, and 5%). 2.4. Physicochemical analysis 2.4.1. Density Extrudate density was calculated as (Ali, Hanna, & Chinnaswamy, 1996; Alvarez-Martinez, Kondury, & Harper, 1988): 4 m ð2Þ p D2 L where m is the mass of a length L of cooled extrudate with diameter D. Six replicates of extrudate were randomly selected and an average taken. Density ¼
2.4.2. Expansion Sectional expansion, the ratio of diameter of extrudate and the diameter of die, was used to express the expansion of extrudate (Alvarez-Martinez et al., 1988; Fan, Mitchell, & Blanshard, 1996). Six samples were used for each extrudate to calculate the average. 2.4.3. Water absorption index (WAI) and water solubility index (WSI) The WAI and WSI were measured using a technique developed for cereals (Anderson et al., 1969). The ground extrudate was suspended in water at room temperature for 30 min, gently stirred during this period, and then centrifuged at 3000 · g for 15 min. The supernatant was decanted into an evaporating dish of known weight. The WSI is the weight of dry solids in the supernatant expressed as a percentage of the original weight of sample. The WAI is the weight of gel obtained after removal of the supernatant per unit weight of
Table 2 Coded levels for the response surface design Variables
Feed rate (kg h 1 ), X1 Feed moisture (%), X2 Screw speed (rpm), X3 Temperature ( C), X4 a
a ¼ 1:682.
ð1Þ
Levels aa
)1
0
1
a
15.9 11.27 132.3 86.4
20 14 180 100
26 18 250 120
32 22 320 140
36.1 24.73 367.7 153.6
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original dry solids. Determinations were made in triplicate.
285
3. Results and discussions The coefficients of Eq. (1) (Table 3) were obtained by fitting the response data in Design Expert by employing a backward elimination procedure to find the best-fit model.
2.5. Textural measurement The texture characteristics of extrudate were measured using a Stable Micro System TA-XT2 texture analyser (Surrey, UK) fitted with a 2 mm cylinder probe. A force–time curve (Fig. 1) was recorded and analysed by Texture Exponent 32 (Surrey, UK) to calculate the peak force and area. The peak force, i.e. the resistance of extrudate, and the area under the curve were chosen to represent the textural properties of extrudate. Ten randomly collected samples of each extrudate were measured and an average taken.
3.1. Density and expansion
Density (g cm-3)
The effect of extrusion conditions on extrudate density can also be found in the 3-D surface plot (Fig. 2). The extrudate density was found to be most dependent on feed moisture and temperature.
0.43 0.35 0.27 0.19 0.10
140.0 22.0
130.0
20.0
120.0
Temperature (ºC)
18.0 110.0 16.0 Feed Moisture (%) 100.0 14.0
Fig. 2. Density of extrudate as a function of feed moisture and temperature at a feed rate of 26 kg h 1 and a screw speed of 250 rpm.
Fig. 1. A typical force–time curve of extrudate textural measurement.
Table 3 Significant coefficients of regression equation (1)a for the responses Density (g cm 3 ) B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 R2 a
0.25 0.002 0.021 0.0004 )0.0029
Expansion
5.04 0.029 )0.15 )0.0006 0.0026
WAI (g g 1 ) 38.07 )0.73 0.09 )0.005 )0.35
0.0061
WSI (%)
1.88 )2.06 )3.38 0.27 0.46 0.078 0.011 0.0033
Force (N)
Area (N mm)
)157.06 3.29 13.03
26.2 0.83 )23.9 0.22 0.48
)8.96 0.92 )0.082 0.099
)0.00057
0.91
0.92
X1 : feed rate, X2 : feed moisture, X3 : screw speed, X4 : temperature. Significant at the 5% level. ** Significant at the 1% level. *** Significant at the 0.1% level. *
0.00067 0.97
0.98
0.56 )0.033 0.95
0.92
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Increased feed moisture leads to a sharp increase of extrudate density value at all temperature levels. However, increased barrel temperature caused a slight decrease in the density of extrudate. Feed rate was observed to have no significant effect whereas screw speed was observed to have a slight impact on the density of extrudate in this work. The effects of extrusion conditions on extrudate expansion are shown in Fig. 3. The feed rate and feed moisture were found to have a significant effect on the expansion of extrudate. Increased feed rate significantly increased the extrudate expansion. However, increased feed moisture lead to a sharp decrease in expansion value. Screw speed and temperature both were observed to have no significant effect on the extrudate expansion. Harmann and Harper (1973) postulated two factors in governing expansion: (a) dough viscosity, and (b) elastic force (die swell) in the extrudate. The elastic forces will be dominant at low temperature and low moisture content. The bubble growth, which is driven by the pressure difference between the interior of the growing bubble and atmospheric pressure resisted primarily by the viscosity of the bubble wall, dominate the expansion at high moisture content and high temperature (Panmanabhan & Bhattachayrya, 1989). Increased feed rate would influence the degree of fill and residence time, inducing the degradation of amylopectin networks, and change the melt rheology characteristics, thus leading to greater elastic effect and changes in product density and expansion. Fletcher, Richmond, and Smith (1985) also reported the effect of feed rate on extrudate density and expansion. Feed moisture has been found to be the main factor affecting extrudate density and expansion (Faubion &
Hoseney, 1982; Fletcher et al., 1985; Ilo et al., 1999; Launay & Lisch, 1983), which is consistent with this work. The high dependence of bulk density and expansion on feed moisture would reflect its influence on elasticity characteristics of the starch-based material. Increased feed moisture content during extrusion would change the amylopectin molecular structure of the material reducing the melt elasticity thus decreasing the expansion but increasing the density of extrudate. In twin-screw extrusion, screw speed is generally believed to have little effect on extrudate expansion (Martin-Cabrejas et al., 1999). An increase in the barrel temperature will decrease the melt viscosity, which was confirmed by the report of Mercier and Feillet (1975) that extrudate viscosity decreased with increased temperature. The reduced viscosity effect would favour the bubble growth during extrusion. Moreover, the degree of superheating of water in the extruder would increase at higher temperatures, also leading to greater expansion, which was observed in this work. The reduction of rice extrudate density caused by increased barrel temperature (Fletcher et al., 1985; Ilo et al., 1999) was also observed in this experiment. 3.2. WAI and WSI The effects of extrusion conditions on WAI of extrudate are shown in Fig. 4. The feed moisture and temperature were found to have a significant effect on the WAI of the extrudate. Increasing feed moisture significantly increase the WAI of rice extrudate. However, increase in barrel temperature was observed to cause a significant decrease in extrudate WAI.
7.65
WAI (g g-1)
Expansion
3.87 3.41 2.94 2.48 2.02
32.0 22.0
29.0
20.0
26.0
Feed rate (kg h-1)
23.0 20.0 14.0
18.0 16.00
Feed Moisture (%)
Fig. 3. Expansion of extrudate as a function of feed rate and feed moisture at a screw speed of 250 rpm and a temperature of 120 C.
6.95 6.25 5.56 4.86
140.0 22.0
130.0
20.0
120.0
Temperature (”C)
18.0
110.0 100.0 14.0
16.0
Feed Moisture (%)
Fig. 4. WAI of extrudate as a function of feed moisture and temperature at feed rate of 26 kg h 1 and screw speed of 250 rpm.
Q.-B. Ding et al. / Journal of Food Engineering 66 (2005) 283–289
WSI (%)
The effects of extrusion conditions on WSI of extrudate are shown in Figs. 5 and 6. Feed rate, feed moisture and temperature were found to have a significant effect on the WSI of extrudate. Increasing feed rate at low feed moisture significantly decreased the WSI of extrudate, becoming insignificant at high feed moisture. Increased feed moisture was also observed to result in a significant decrease in the WSI of extrudate, particularly at low feed rate. However, increasing barrel temperature significantly increased the WSI of extrudate. These observations are consistent with previous studies (Anderson et al., 1969; Harper, 1979; Mercier & Feillet, 1975).
32.70 29.91 27.11 24.32 21.52 32.0 29.0 22.0
26.0
20.0
-1 23.0 Feed rate(Kg h )
18.0
Feed Moisture (%)
16.0 14.0 20.0
Fig. 5. WSI of extrudate as a function of feed rate and feed moisture at a screw speed of 250 rpm and a temperature of 120 C.
35.46
WSI (%)
30.11
24.76
287
The WAI measures the volume occupied by the starch after swelling in excess water, which maintains the integrity of starch in aqueous dispersion (Mason & Hoseney, 1986). WSI, often used as an indicator of degradation of molecular components (Kirby, Ollett, Parker, & Smith, 1988), measures the degree of starch conversion during extrusion which is the amount of soluble polysaccharide released from the starch component after extrusion. Gelatinisation, the conversion of raw starch to a cooked and digestible material by the application of water and heat, is one of the important effects that extrusion has on the starch component of foods. Water is absorbed and bound to the starch molecule with a resulting change in the starch granule structure. Barrel temperature and feed moisture are found to exert the greatest effect on gelatinisation. The maximum gelatinisation occurs at high moisture and low temperature or vice versa (Lawton, Henderson, & Derlatka, 1972). Mercier and Feillet (1975) also found that soluble starch increased with increasing extrusion temperature and decreasing feed moisture. They found that as extrusion temperature increased at feed moisture of 18.2%, WSI increased; WAI achieved a maximum value at extrusion temperatures of 180–200 C. Ilo, Tomschik, Berghofer, and Mundigler (1996) reported that the degree of gelatinisation decreased with increasing feed moisture, and increased with increasing feed rate and product temperature, similar to the results found in this study. The higher amylopectin content of rice flour (Tom as, Oliveria, & McCarthy, 1997) would explain why the WSI value was lower in this experiment compared with other results (Anderson et al., 1969; Ding, Ainsworth, Fuller, Tucker, & Marson, submitted for publication; Ilo et al., 1996; Mercier & Feillet, 1975). The specific mechanical energy (SME) has been used by Meuser and van Lengerich (1992) as a system parameter to model the extrudate properties. The starch gelatinisation of extrudate increased with increasing the SME during extrusion (Ilo et al., 1996) through the shear effects. The SME in this study was 450–1000 W h kg 1 and was found to be dependent on feed moisture, feed rate, screw speed and temperature, and among them feed moisture was found to be the most significant factor (unreported results). 3.3. Force (hardness) and area (crispness)
19.41
14.06 100.0
110.0
120.0
130.0
140.0
Temperature (”C) Fig. 6. WSI of extrudate as a function of temperature at feed rate of 26 kg h 1 , feed moisture of 18% and screw speed of 250 rpm.
The effect of extrusion conditions on hardness and crispness of extrudate are shown in Figs. 7 and 8, respectively. Feed moisture was found to have the most significant effect on extrudate hardness. Feed rate was also found to have a significant effect. Increasing feed moisture content significantly increased the hardness of the extrudate, particularly at low
Q.-B. Ding et al. / Journal of Food Engineering 66 (2005) 283–289
Force (N)
288
77.29 60.61 43.94 27.26 10.58
22.0 32.0
20.0
29.0
18.0
Feed moisture (%)
26.0
16.0 14.0
23.0 20.0
Feed rate(kg h-1)
Area (N.mm)
Fig. 7. The hardness (force) of extrudate as a function of feed rate and feed moisture at screw speed of 250 rpm and temperature of 120 C.
267.45 230.92 194.39 157.85 121.32
140.0
22.0
130.0 120.0
20.0
Temperature (ºC) 110.0 100.0
18.0 16.0 Feed Moisture (%) 14.0
Fig. 8. The crispness (area) of extrudate as a function of feed moisture and temperature at feed rate of 26 kg h 1 and screw speed of 250 rpm.
feed rate level. Increasing feed rate at lower feed moisture also significantly increased the extrudate hardness, becoming less significant at higher feed moisture. Feed moisture was found to have the most significant effect on extrudate crispness. Barrel temperature was also found to have a significant effect. Increasing feed moisture content significantly decreased the crispness of rice extrudate. However, increasing temperature slightly increased the crispness of the extrudate. Attempts to establish the correlation between texture measurement and sensory characteristics of extruded products have been ongoing and some positive results have been achieved (Bouvier, Bonneville, & Goullieux, 1997; Desrumaux et al., 1999; Van Hecke, Allaf, & Bouvier, 1998). The hardness and crispness of expanded extrudate is a perception of the human being and is
associated with the expansion and cell structure of the product. The hardness is the average force required for a probe to penetrate the extrudate. Previous studies also reported that the hardness of extrudate increased as the feed moisture content increased (Badrie & Mellowes, 1991; Liu, Hsieh, Heymann, & Huff, 2000). It might due to the reduced expansion caused by the increase in moisture content (Liu et al., 2000). During the extrusion process, the elastic swell effect and bubble growth effect both contribute to the structure change of starch (Panmanabhan & Bhattachayrya, 1989). The degree of starch gelatinisation and extrudate expansion was found to be negatively correlated to the feed moisture in this work. The water acts as a plasticizer to the starch-based material reducing its viscosity and the mechanical energy dissipation in the extruder and thus the product becomes dense and bubble growth is compressed. This was confirmed in this study as the recorded pressure was found to be negatively correlated with the feed moisture (unreported). The reduced starch conversion and compressed bubble growth would result in a dense product and reduced crispness of extrudate, as observed in this work. It is expected that increasing temperature would decreased melt viscosity, which favours the bubble growth and produce low density products with small and thin cells, thus increasing the crispness of extrudate.
4. Conclusion The physicochemical properties and sensory characteristics of rice-based extrudate on twin-screw extrusion process were dependent on process variables. Feed rate, feed moisture and barrel temperature had significant effect on various extrudate properties, with feed moisture having the greatest influence on the properties of the extrudate. The effect of screw speed was not significant.
Acknowledgements This research was supported by DEFRA under the Eating, Food and Health Link programme (EFH/11).
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