Structural properties of extruded corn starch

Structural properties of extruded corn starch

Journal of Food Engineering 68 (2005) 519–526 www.elsevier.com/locate/jfoodeng Structural properties of extruded corn starch S. Thymi, M.K. Krokida, ...

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Journal of Food Engineering 68 (2005) 519–526 www.elsevier.com/locate/jfoodeng

Structural properties of extruded corn starch S. Thymi, M.K. Krokida, A. Pappa, Z.B. Maroulis

*

Department of Chemical Engineering, National Technical University of Athens, Zografou Campus, 15780 Athens, Greece Received 4 April 2004; accepted 7 July 2004

Abstract Structural properties, such as apparent density, true density, expansion ratio and porosity, of extruded corn grits were measured. Corn grits were extrusion-cooked in a Prism extruder with varied feed rate (1.16–6.44 kg/h), screw speed (150–250 rpm), product temperature (100–260 C) and feed moisture content (12–25 kg/100 kg wet basis). A simple mathematical model was used to correlate the examined properties with the extrusion conditions. Extruded product apparent density, porosity and expansion ratio were found to be dependent on feed moisture content, residence time and temperature while they were not affected by screw speed. More specifically, the apparent density of extruded products had an increasing trend with feed moisture content and residence time, while it decreased with product temperature. Porosity and expansion ratio of extruded products decreased significantly with feed moisture content and residence time, while temperature rise resulted in products of higher porosity and expansion ratio.  2004 Elsevier Ltd. All rights reserved. Keywords: Apparent density; True density; Expansion ratio; Porosity

1. Introduction Extrusion cooking, as an attractive process for continuous food processing, has been developed extensively in recent years. High temperature–short time extrusion cooking is used in the food industry to produce direct expanded products such as snack foods, breakfast cereals and pet foods (Miller, 1990; Moore, 1994; Rahman, 1995; Rokey, 1994). The thermal energy generated by viscous dissipation during extrusion, combined with the shearing effect, cooks quickly the raw mixture so that the properties of the materials are modified by physico-chemical changes of the biopolymers. Residence time, temperature, pressure, and shear history characterize the extrusion cooking of food materials (Meuser & Van Lengerich, 1992). Product quality can vary considerably, depending on the extrusion processing character*

Corresponding author. Fax: +30 210 7723155. E-mail addresses: [email protected] (M.K. Krokida), [email protected] (Z.B. Maroulis). 0260-8774/$ - see front matter  2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.07.002

istics, such as extruder type, screw configuration, feed moisture, and temperature profile in the barrel sections, screw speed and feed rate. The influence of extrusion processing variables on the product quality of starchbased materials has been studied extensively (Barres, Vergnes, Tayeb, & Della Valle, 1990; Bhattacharya & Hanna, 1987; Kirby, Ollet, Parker, & Smith, 1988; Mason & Hoseney, 1986). The degree of expansion determines the extrudate structure and consequently its texture. Several studies have investigated the role of extrusion variables in the expansion of starch based materials (Chinnaswamy & Hanna, 1988; Falcone & Philips, 1988; Fletcher, Richmond, & Smith, 1985; Kim & Maga, 1987; Vainionpaa, 1991). Extrudate expansion has been reported to be the most dependent on material moisture content and extrusion temperature. Several theoretical considerations for extrudate expansion have been published (Alvarez-Martinez, Kondury, & Harper, 1988; Ilo, Tomschik, Berghofer, & Mundigler, 1996; Kokini, Chang, & Lai, 1992; Kumagai & Yano, 1993;

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Nomenclature E e q d L m N n p R S T t V

expansion ratio (–) porosity (–) density (kg/m3) diameter (m) length per gram of extrudate (m/kg) mass (kg) number of experimental points (–) constant (–) number of estimated parameters (–) screw speed (rpm) sum of squares (kg/m3) product temperature (C) residence time (s) volume (m3)

Padmanabhan & Bhattachayrya, 1989). During extrusion cooking of biopolymers the viscoelastic material is forced through a die so that the sudden pressure drop causes part of the water to vaporize, giving an expanded porous structure. The resulting extrudate expansion has extrusion contributions from both elastic swell and bubble growth effects (Padmanabhan & Bhattachayrya, 1989). A general model of extrudate expansion was developed including the radial, longitudinal and volumetric expansion (Alvarez-Martinez et al., 1988). Several studies have reported chemical changes during extrusion cooking and related them to product functional qualities such as expansion volume, water solubility and product color. Most of these studies used radial expansion as a measure of quality for extrudate expansion. However, studies by Launay and Lisch (1983) and Lai, Guetzlaff, and Hoseney (1989) showed that extrudate expansion occurred in both the longitudinal and lateral directions during extrusion cooking. Chinnaswamy and Hanna (1990) noted that normal corn starch produced the most desirable product. Few studies have reported changes in structure of constituents and the chemical reactions that occur during extrusion (Chinnaswamy & Hanna, 1988; Mitchell & Areas, 1992). Addition of lipids at low levels benefits the starch–protein extrusion process by preventing the formation of insoluble materials (Ho & Izzo, 1992). Goedeken (1991) investigated single screw extrusion cooking of corn starch with selected proteins. Dairy proteins showed good results, such as acceptable expansion, but they also indicated a mixed effect on solubility, expansion and shear strength depending on the technique used to isolate the protein. Lacourse and Altieri (1989) extruded corn starches with varying amounts of amylose content (0–700 g/kg) and reported that when the amylose content of corn

X

feed moisture content (kg/100 kg wet basis)

Subscripts b apparent d die 0 reference p true r screw speed s dry solids t residence time T temperature tot total X feed moisture content 1 constant

starch is at least 450 g/kg a low density biodegradable packaging material can be produced. The advantages of agricultural commodities based foams are their degradability and the fact that they are derived from a renewable resource. Furthermore, agricultural commodities, such as starch and protein, can be expanded easily using high temperature, short time extrusion cooking processes, instead of chemical or physical blowing agents. Arambula-Villa, Gonzalez-Hernandez, and Ordorica-Falomir (2001) studied the structural properties of tortillas from extruded instant corn flour supplemented with various types of corn lipids. Each type and concentration of lipids was observed to have a different effect in the quality of masas and tortillas. The objective of this study was to examine the structural property changes of corn starch materials during extrusion cooking as a function of process characteristics and feed moisture content. A simple mathematical model developed by Krokida and Maroulis (1997), was used in order to reveal the significant parameters that influence structural properties.

2. Mathematical modeling A simple mathematical model has been proposed in the literature to predict structural properties of materials during drying as a function of moisture content (Krokida & Maroulis, 1997). Extruded products are assumed to have very low levels of moisture content after extrusion process, even if initial materials have various moisture contents, so their structural properties, namely true density, apparent density and porosity, are similar to the structural properties of dried materials at zero moisture content. The proposed model is summarised

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Table 1 Mathematical model Properties qb qp e E

Apparent density True density Porosity Expansion ratio

(kg/m3) (kg/m3) (–) (–)

Properties equations  nT  nt  nr  nX ms T t R X qb ¼ ¼ n1 T0 t0 R0 X0 V tot ms qp ¼ Vs q e ¼ 1  b qffiffiffiffiffiffiffi qp 4m plqb d tot ¼ E¼ dd dd Extrusion conditions affecting the structural properties • Product temperature, T (C) • Rotation speed, R (rpm) • Resident time, t (s) • Feed moisture content, X (kg/100 kg wb)

(1) (2) (3) (4)

in Table 1. Therefore, the true density of extruded products (qp) is equal to the density of solid materials and it is assumed to be constant. In order to examine the influence of process characteristics on the apparent density (qb), a power model was used, in which, nt, nT, nr and nX are the exponents of residence time, temperature, screw speed and feed moisture content, respectively. It is expected that not all of these parameters will influence to the same degree the apparent density of extruded products and this may be revealed through regression analysis. The porosity of extruded products (e) is a function of the apparent density and true density. Moreover, the expansion ratio (E) of the extruded products is calculated as the extrudate diameter to die diameter ratio. The values of the required parameters can be determined by fitting the proposed model to the experimental data. This can be done by minimising the following residual sum of squares: " S¼

N X

#1=2 2

ðqb  qb Þ =ðN  pÞ

ð1Þ

i¼1

where, qb and qb are the experimental and the predicted values of apparent density respectively, N is the number of experimental points and p is the number of estimated parameters. Following a regression analysis procedure, all the four parameters (n1, nt, nT, nr, nX) can be determined simultaneously. However, it is not likely that all of these parameters affect the residual sum of squares (S) to the same degree. In order to distinguish between the ones that are necessary to accurately predict the shrinkage properties, the following procedure was adopted: Firstly, the minimum change in the sum of squares was evaluated for all the four parameters. Secondly, omitting one parameter at a time, the value of S was

evaluated for all combinations of the remaining three parameters. In this way, the parameter chosen to be eliminated was the one whose elimination produced the minimum sum of squares. Continuing the former procedure, the minimum sum of squares was evaluated for 3, 2 and 1 parameters, respectively. Obviously, the best value of minimum sum of squares is the one that involves all parameters. However, comparing the values of the sum of squares obtained by reducing the number of parameters, there must be an optimum that gives an acceptable accuracy.

3. Materials and methods 3.1. Sample preparation Yellow corn powder was used for sample preparation. Distilled water and sugar were blended with corn in a mixer. The ingredients were mixed 24 h before extrusion so that the mixture had been homogenised before extrusion. After mixing, the samples were stored at 25 C in plastic bottles for 24 h in order to equilibrate. The composition of the samples is shown in Table 2. Table 2 Composition (% wet basis) of the samples Sample

Moisture content

Corn (dry)

Sugar

1 2 3 4 5 6 7 8

13.2 13.6 14.5 15.5 17.8 18.6 23.0 25.0

76.8 76.4 75.5 74.5 72.2 71.4 67.0 65.0

10 10 10 10 10 10 10 10

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3.2. Extrusion cooking

stereopycnometer (model SPV-3) with an accuracy of 0.001 ml.

A Prism Eurolab conical, counter-rotating twin screw extruder, model KX-16HC was used. The general screw geometry was: length 1 m, diameter 16 mm, maximum rotation speed 500 rpm and die diameter 2 mm. The material was fed into the extruder with a twin screw volumetric feeder. The extruder had five temperature control zones. The pressure at the die during extrusion was measured with a pressure transducer. All extrusion variables were displaced on the control panel (barrel, screw die and product temperature, screw speed). The product temperature during extrusion was adjusted by varying the temperature in the barrel screw and die. The independent extrusion variables considered were varied in the following ranges: feed moisture content from 13.2 to 25 g/100 g (wet basis), product temperature from 100 to 260 C, retaining time from 1 to 2.5 s, rotation speed from 150 to 250 rpm. Steady state extrusion conditions were reached after 20 min. The steady-state samples were then collected, dried in air, and stored for further structural property measurement.

4. Measurement of structural properties 4.1. Apparent density The apparent density of extrudate products was determined as the average value of two measurements, using two different experimental methods reported in the literature. In the first method, the apparent density was calculated by measuring the actual dimensions of the extrudates as suggested by Launay and Lisch (1983). The diameter of the extrudates was measured with a Vernier caliper and the length per unit weight (g) of the samples was determined. The bulk density of the extrudates was determined using the equation: qb ¼ 4=pd 2 L

ð2Þ

where d is the diameter (m) and L (m/kg) is the length per kg of the extrudate. In the second method, the bulk density is determined by measuring the volume of the extrudates to an accuracy of 0.05 ml by immersing them in n-heptane and by determining the volume displacement, using the apparatus reported by (Zogzas, Maroulis, & MarinosKouris, 1994). To determine the mass of dry solids, all samples were dried to constant weight in a 5 mm Hg vacuum oven at 70 C for 48 h (Van Arsdel & Copley, 1963). 4.2. True density The sample weight was measured using a Mettler AE-160 electronic balance with an accuracy of 104 g. The true volume was measured using a Quantacrome

4.3. Porosity and expansion ratio The porosity of the extrudates was determined from the bulk and apparent volumes. The expansion ratio was determined as the ratio of the diameters of the extrudate to the die.

5. Results and discussion The regression analysis procedure showed that the optimum number of parameters which produce a reasonable minimum residual sum of squares is four. The use of less parameters increases dramatically the minimum sum of squares. These four parameters were found to be n1, nt, nT, nX. The results of fitting the proposed structural property predictions into the experimental data are shown in Figs. 1–3. The corresponding parameter estimates for all materials are shown in Table 3. Generally, rotation speed does not seem to affect significantly structural properties of extrudate products. The true density of the extrudate products is approximately constant. This should be expected since particle density ranges between the density of water and the dry solid density. Thus, for the extruded products of very low moisture content, the particle density reaches the value of the dry solid density. In Fig. 1(a), the apparent densities are presented as functions of residence time for various moisture contents and product temperatures, while in Fig. 1(b) the apparent densities are presented as functions of temperature and feed moisture content. Solid lines are used to plot the calculated values of apparent density using the mathematical model and the parameters of Table 3. The fitting of the proposed model to the experimental data is considered satisfactory. The apparent density increased slightly as the residence time increased for all the temperature and moisture contents (Fig. 1). An explanation for the observed changes in the apparent density is that the increase of residence time in a counter rotating twin screw extruder caused a degradation of the amylopectin molecular structure of the starch-based material and reduced the radial expansion, resulting in higher density values. An increased feed moisture content during extrusion increased the apparent density values (Fig 1(b)), while a temperature rise seems to have the opposite effect, resulting in a significant decrease of the density. Increasing the moisture content changes the amylopectin molecular structure in the starch-based material reducing the melt elasticity that decreased the radial

S. Thymi et al. / Journal of Food Engineering 68 (2005) 519–526 0.8

523

0.8

o

Xo=14.5kg/100kg wb

T=200 C 25 kg/100kg wb

23 kg/100kg wb 0.6 Apparent density (kg/lt)

Apparent density (kg/lt)

0.6

18.5 kg/100kg wb

17.8kg/100kg wb

o

150 C

0.4

0.4

15.5 kg/100kg wb 14.5 kg/100kg wb

o

170 C

o

200 C

13.6 kg/100kg wb

o

230 C

13.2 kg/100kg wb o

260 C

0.2 1.00

(a)

0.2 1.50

2.00

2.50

1.00

1.50

(b)

Residence Time (s)

0.8

2.00

2.50

Residence Time (s)

0.8

o

200 C X=17.8 kg/100kg wb o

o

170 C

0.6

Apparent density (kg/lt)

Apparent density (kg/lt)

100 C

o

150 C

o

230 C

0.4

0.6

X=18.6 kg/100kg wb

X=15.5 kg/100kg wb X=14.5 kg/100kg wb

0.4

X=13.6 kg/100kg wb o

260 C X=13.2 kg/100kg wb

0.2

0.2 10

(c)

15

20

Feed Moisture Content (kg/kg wb)

25

0

(d)

100

200

o Temperature C

300

Fig. 1. Apparent density of extrudate corn starch.

expansion ratio and increased the apparent density. A temperature increase leads to higher expansion ratio values, resulting in lower apparent densities. In Fig. 2(a), the porosity is presented as a function of residence time for various moisture contents and product temperatures, while in Fig. 2(b), the porosity is presented as a function of temperature and feed moisture content. The porosity decreased slightly as the residence time increased for all temperatures and moisture con-

tents (Fig. 2(b)), showing the opposite effect than the apparent density. Increased feed moisture content during extrusion decreased the porosity values (Fig. 2(b)), while a temperature increase seems to have the opposite effect, resulting in a significant porosity increase. The radial expansion ratio of extrudate products is presented in Fig. 3 as a function of extrusion characteristics. The expansion ratio was found to depend mostly

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Fig. 2. Porosity of extrudate corn starch.

on feed moisture content, product temperature and residence time. The radial expansion ratio of corn extruded products ranged from 1.4 to 4.0. Feed moisture content had a highly significant effect on the radial expansion ratio. The radial expansion decreased with an increased feed moisture content. Launay and Lisch (1983) suggested that radial expansion was most dependent on the melt elasticity. The stored energy was released in the expansion process, increasing the radial expansion

ratio. An increased feed moisture content during extrusion would change the amylopectin molecular structure of the starch-based material, reducing the melt elasticity that decreases the radial expansion ratio. The increase of residence time results in a degradation of amylopectin networks in the material that changes the radial expansion. The increase of melt temperature increased significantly the expansion ratio values for all the examined feed moisture contents.

S. Thymi et al. / Journal of Food Engineering 68 (2005) 519–526 4

4

o

525

Xo=14.5kg/100kg wb

T=200 C

o

260 C o

o

3

170 C

o

150 C

Expansion ratio

Expansion ratio

3 13.6 kg/100 kg wb

14.5 kg/100 kg wb

230 C o 200 C

2

2 18.5 kg/100 kg wb

1 1.0

1 1.0

1.5

(a)

2.0

2.5

(b)

Residence Time (s)

1.5

2.0

2.5

Residence Time (s) 4

4

170

o

X=13.2 kg 100 kg wb

o

260 C X=14.5 kg/100kg wb

3

Expansion ratio

Expansion ratio

3 o

150 C o

230 C o

200 C

X=17.8 kg/100kg wb X=18.6 kg/100kg wb

2

2

1

1 10

15

(c)

20

25

Feed Moisture Content (kg/kg wb)

0

100

(d)

200

300

Temperature (oC)

Fig. 3. Expansion ratio of extrudate corn starch.

6. Conclusions

Table 3 Results of parameter estimation qp

1500

(kg/m3)

n1 nT nt nx S

0.14 0.40 0.10 1.45 0.15

(–) (–) (–) (–) (kg/m3)

The effect of extrusion conditions (temperature, feed moisture content, residence time and rotation speed) on the structural properties of extruded corn starch was investigated. The apparent density increased slightly as the residence time increased for all temperature and moisture

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contents, while the porosity and the expansion ratio of extruded products decreased with the residence time. Higher feed moisture contents decreased the radial expansion ratio of the extrudates, resulting in a higher apparent density and lower porosity values. The temperature increase seems to have the opposite effect, resulting in a significant density decrease and higher porosity.

References Alvarez-Martinez, L., Kondury, K. P., & Harper, J. M. (1988). A general model for expansion of extruded products. Journal of Food Science, 53, 609–615. Arambula-Villa, G., Gonzalez-Hernandez, J., & Ordorica-Falomir, C. A. (2001). Physicochemical, structural and textural properties of tortillas from extruded instant corn flour supplemented with various types of corn lipids. Journal of Cereal Science, 33(3), 245–252. Barres, C., Vergnes, B., Tayeb, J., & Della Valle, G. (1990). Transformation of wheat flour by extrusion cooking: influence of screw configuration and operating conditions. Cereal Chemistry, 67, 427–433. Bhattacharya, M., & Hanna, M. A. (1987). Textural properties of extrusion cooked corn starch. Lebensmittel-Wissenschaft und Technologie, 20, 195–201. Chinnaswamy, R., & Hanna, M. A. (1988). Relationship between amylose content and extrusion expansion properties of corn starches. Cereal Chemistry, 65, 138–143. Chinnaswamy, R., & Hanna, M. A. (1990). Macromolecular and functional properties of native and extrusion cooked corn starches. Cereal Chemistry, 67, 490–499. Falcone, R. G., & Philips, R. D. (1988). Effects of composition feed moisture and barrel temperature on the physical and rheological properties of snack like products prepared from cowpea and sorghum flours by extrusion. Journal of Food Science, 53, 1464–1469. Fletcher, S. I., Richmond, P., & Smith, A. C. (1985). An experimental study of twin screw extrusion cooking of maize grits. Journal of Food Engineering, 4, 291–312. Goedeken, D. (1991). Single screw extrusion cooking of corn with selected proteins, M.S. thesis, University of Nebraska, Lincoln, USA. Ho, C. T., & Izzo, M. T. (1992). Lipid protein and lipid carbohydrate interactions during extrusion. In J. L. Kokini, C. Ho, & M. V. Karwe (Eds.), Food extrusion science and technology (pp. 415–425). New York: Marcel Dekker. Ilo, S., Tomschik, E., Berghofer, E., & Mundigler, N. (1996). The effect of extrusion operating conditions on the apparent viscosity and the properties of extrudates in twin-screw extrusion cooking of maize grits. Lebensmittel-Wissenschaft und Technologie, 29, 593–598. Kim, C. H., & Maga, J. A. (1987). Properties of extruded protein concentrate and cereal flour blends. Lebensmittel-Wissenschaft und Technologie, 20, 311–318.

Kirby, A. R., Ollet, A. L., Parker, R., & Smith, A. C. (1988). An experimental study of screw configuration effects in the twin screw extrusion cooking of maize grits. Journal of Food Engineering, 8, 247–272. Kokini, J. L., Chang, C. N., & Lai, L. S. (1992). The role of rheological properties of extrudate expansion. In J. L. Kokini, C. Ho, & M. V. Karwe (Eds.), Food extrusion science and technology (pp. 631–652). New York: Marcel Dekker. Krokida, M. K., & Maroulis, Z. B. (1997). Effect of drying method on shrinkage and porosity. Drying Technology, 10(15), 1145–1155. Kumagai, H., & Yano, T. (1993). Critical bubble radius for expansion in extrusion cooking. Journal of Food Engineering, 20, 325–338. Lacourse, N. L., & Altieri, P. A. (1989). Biodegradable packaging material and the method of preparation of thereof. US Patent 4,863,655, September 5. Lai, C. S., Guetzlaff, J., & Hoseney, R. C. (1989). Role of sodium bicarbonate and trapped air in extrusion. Cereal Chemistry, 66, 69–73. Launay, B., & Lisch, L. M. (1983). Twin screw extrusion cooking of starches: flow behaviour of starch pastes, expansion and mechanical properties of extrudates. Journal of Food Engineering, 2, 259–280. Mason, W. R., & Hoseney, R. C. (1986). Factors affecting the viscosity of extrusion cooked wheat starch. Cereal Chemistry, 63, 436–451. Meuser, F., & Van Lengerich, B. (1992). System analytical model for the extrusion of starches. In J. L. Kokini, C. Ho, & M. V. Larwe (Eds.), Food extrusion science and technology (pp. 619–630). New York: Marcel Dekker. Miller, R. C. (1990). Unit operations and equipment IV. Extrusion and extruders. In R. B. Fast & E. F. Galdwell (Eds.), Breakfast cereals and how they are made (pp. 135–193). St. Paul, MN: American Association of Cereal Chemists. Mitchell, J. R., & Areas, J. A. G. (1992). Structural changes in biopolymers during extrusion. In J. L. Kokini, C. Ho, & M. V. Karwe (Eds.), Food extrusion science and technology (pp. 45–359). New York: Marcel Dekker. Moore, G. (1994). Snack food extrusion. In N. D. Frame (Ed.), The technology of extrusion cooking (pp. 111–143). St Paul, MN: American Association of Cereal Chemists. Padmanabhan, M., & Bhattachayrya, M. (1989). Extrudate expansion during extrusion cooking of foods. Cereal Food World, 34, 945–949. Rahman, S. (1995). Food properties handbook. New York: CRC Press. Rokey, G. J. (1994). Petfood and fishfood extrusion. In N. D. Frame (Ed.), The technology of extrusion cooking (pp. 144–189). St. Paul, MN: American Association of Cereal Chemists. Vainionpaa, J. (1991). Modelling of extrusion cooking of cereals using response surface methodology. Journal of food Engineering, 13, 1–26. Van Arsdel, W. B., & Copley, M. J. (1963). Food dehydration (Vol. 1). Westport, Conn.: AVI Publishing. Zogzas, N. P., Maroulis, Z. B., & Marinos-Kouris, D. (1994). Densities, shrinkage and porosity of some vegetables during air drying. Drying Technology, 12(7), 1653–1666.