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amylopectin ratios
Journal of Cereal Science 49 (2009) 371–377
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Rheological properties of starches with different amylose/amylopectin ratios Fengwei Xie a, Long Yu a, b, *, Bing Su a, Peng Liu a, Jun Wang a, Hongshen Liu a, b, Ling Chen a a b
Centre for Polymers from Renewable Resources, ERCPSP, School of Light Industry and Food Science, SCUT, Guangzhou, China CSIRO, Materials Science and Engineering, Melbourne, Vic 3168, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 October 2008 Received in revised form 3 January 2009 Accepted 5 January 2009
The rheological properties of corn starches with different amylose/amylopectin ratios (80/20, 50/50, 23/ 77, and 0/100) were systematically studied by Haake rheometry. The starches were initially pre-compounded with water to designated moisture content levels using a twin-screw extruder. A single-screw extruder with a slit capillary die was then used to characterize the shear stress and melt viscosity characteristics of sample pellets, as a function of both moisture content (19–27%) and extrusion temperature (110–140 C). The melts exhibited shear thinning behavior under all conditions, with the power law index (0 < n < 1) increasing with increasing temperature and moisture content in the majority of cases. The higher the amylose content, the higher is the viscosity (for example, h increases from 277 Pa s to 1254 Pa s when amylose content increases from 0% to 80% under a certain condition), which is opposite to the sequence of molecular weight; amylopectin-rich starches exhibited increased Newtonian behavior. These rheological behaviors are attributed to the higher gelatinization temperature of amyloserich starches, and in particular the multiphase transitions that occur in these starches at higher temperatures, and the gel-ball structure of gelatinized amylopectin. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Starch Rheometry Rheological Amylose/amylopectin Extrusion
1. Introduction Extrusion cooking has been practiced for more than 50 years, with early developments focused on the preparation of ready-toeat cereals (Harper, 1989). Some researchers have investigated the effects of amylose content in starch or dough on the physical and functional properties of their extruded products (Guha et al., 2003; Matthey and Hanna, 1997). Others have analyzed viscoelastic characterization of different biopolymers and their mixtures with additives (Bhattacharya and Padmanabhan, 1992; Bhattacharya et al., 1988; Seethamaju and Bhattacharya, 1994). More recently, Chanvrier et al. (2007) investigated the effects of starch and protein on the rheological properties of wheat flour dough during processing at low hydration. Different viscosity models have been proposed to describe the rheological properties of dough based on the traditional power law function. For example, Jao et al. (1978) derived a regression model for the die viscosity of soy dough by considering the effects of temperature, shear rate and moisture content; Bhattacharya and Hanna (1986, 1987) and Harper (1981)
* Corresponding author. Commonwealth Scientific and Industrial Research Organization, Materials Science and Engineering, Gate 4, Normanby Rd, Clayton South, Melbourne, Vic 3168, Australia. Tel.: þ61 3 9545 2777; fax: þ61 3 9544 1128. E-mail address: [email protected] (L. Yu). 0733-5210/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcs.2009.01.002
presented examples of viscosity models applied to cooked dough; and Toma´s et al. (1997) derived a regression viscosity model for extruding rice flour using a stepwise model starting from a power law equation. Due to environmental considerations and the shortage of oil, starches are now attracting increased attention as raw materials in the production of biodegradable plastics (Arvanitoyannis, 1999; Signori et al., 2005; So¨derqvist Lindblad et al., 2005). In addition, the water requirements during the thermal processing of these materials are normally lower than for conventional additives. There are many reported investigations of the effects of starch type or amylose/amylopectin ratio on the final properties of starch-based materials, including: Foams (Babin et al., 2007; Chinnaswamy and Hanna, 1988; Della Valle et al., 1997; Fang and Hanna, 2001a,b; Guan et al., 2005; Suknark et al., 1997). Sheets or films (Fishman et al., 2006; Yu and Christie, 2005; Yu et al., 2006). Lightweight concrete (Glenn et al., 1999). Injection-molded products (Funke et al., 1998; de Graaf et al., 2003). The melt rheology of starch-based materials has both scientific and industrial importance, and thus it has been widely
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investigated. For example, Llo et al. (1996) studied the effects of extrusion conditions on the apparent viscosity of maize grits, including the effects of melt temperature, shear rate, material composition, and extrusion processing history. Willett et al. (1997) reported that waxy corn starch exhibited shear thinning behavior, becoming more Newtonian as either temperature or moisture content was increased. Gonza´lez et al. (2006) investigated the effects of several factors including screw speed, die restriction (l/r) and moisture content, together with corn endosperm hardness and rice amylose content, on apparent melt viscosity using a Brabender single-screw extruder. Recently, Thuwall et al. (2006) studied the effects of amylose content, moisture content and starch/glycerol ratio on the apparent viscosity of potato starch. The effects of amylose content on reactive extrusion have also been studied (Wing and Willett, 1997). Corn starches have attracted particular scientific interests, because different amylose/amylopectin content materials can be obtained from natural, renewable resources, and they exhibit multiphase transitions during thermal processing (Chen et al., 2006, 2007; Liu et al., 2006). Previous studies have shown that higher amylose-content starches exhibit superior strength and toughness in the preparation of starch-based materials and in producing modified starches by reactive extrusion (Cha et al., 2001; Dean et al., 2007; Guan and Hanna, 2004; Guan and Hanna, 2006; Miladinov and Hanna, 2000, 2001; Nabar et al., 2006; van Soest and Borger, 1997; Yu and Christie, 2005; Yu et al., 2006; Zhou and Hanna, 2004). However the extrusion of high-amylose starches is more difficult than that of normal starches, partly due to the higher die pressure and torque requirements due to the higher melting temperature and viscosity of these starches (Liu et al., 2006; Shogren, 1992; Shogren and Jasberg, 1994). In this work, the rheological properties of corn starches with different amylose/amylopectin ratios (80/20, 50/50, 23/77 and 0/100) were systematically studied as functions of moisture content and extrusion temperature using a Haake rheometer. The effects of different amylose/amylopectin ratios on the rheological behaviors will be discussed based on their microstructures and gelatinization behaviors. 2. Experimental work 2.1. Materials Four commercially available corn starches with different amylose/amylopectin ratios were used in this experimental work: Gelose 80 (G80, 80/20), supplied by Penford (Australia). Gelose 50 (G50, 50/50), supplied by Penford (Australia). A normal corn starch (NC, 23/77), supplied by Huanglong Food Industry Co. Ltd (P.R. China). A waxy corn starch (WC, 0/100), supplied by Shanxi Jinli Industry Group Co. Ltd (P.R. China). An infra-red heating balance (Model DHS-20) was used to measure the moisture contents of the original starches during heating to 110 C for 20 min. All formulations were prepared on a dry weight basis. 2.2. Sample preparation Starches were firstly premixed with water to achieve designated moisture contents. A Haake parallel, co-rotating twin-screw extruder (Rheomex PTW 24/40p, Ø 30) with a rod die (nozzle L/ D ¼ 3, diameter ¼ 3 mm) was used to produce gelatinized starch pellets at a screw speed of 90 rpm. The extruder heating barrel comprises 10 sections. The highest temperature used during
compounding was 160 C, and the die temperature was kept at 105 C to avoid foaming. A gravimetric feeder was used to deliver the materials into the extruder. The moisture contents in the compounded pellets were measured after heating the samples overnight at 130 C in a vacuum oven. 2.3. Rheological measurements Compounded pellets were used to study the rheological properties of various samples. A Haake Rheocord Polylab RC500p incorporating a single-screw extruder (Rheomex 252p, Ø 19, screw 2:1, L/D 25) with a slit capillary die (20 1 mm) was used to measure the shear stress and apparent viscosity of samples under different shear rates at specific temperatures. The screw speed was varied between 30 and 180 min1, and normally six points were recorded for each sample using the cutting and manual entry measurement mode. Apparent shear rates were calculated by:
g¼
6Q WH 2
(1)
where g is the shear rate, Q is the volumetric flow rate (in cm3/s), W is the slit width, and H is the slit height. Shear stress values were calculated using the following equation:
s¼
H$DP 2L
(2)
in which s is the shear stress, DP is the pressure drop over the capillary, and L is the slit length. 2.4. Differential scanning calorimetry A Perkin–Elmer Diamond-I differential scanning calorimeter (DSC) with an internal coolant (Intercooler 1P), nitrogen purge gas, and stainless steel sample pans, was used to study the thermal properties of samples with high moisture content over a high temperature range (up to 350 C). The melting point and enthalpy of indium were used for temperature and heat capacity calibrations, respectively. The detailed methodology and a discussion of some critical issues are presented elsewhere (Liu et al., 2006; Yu and Christie, 2001). 3. Results and discussions 3.1. Effects of moisture content Fig. 1 shows the results for shear stress and melt viscosity as a function of shear rate for the various corn starches (G80, G50, NC, and WC) with different moisture contents at an extrusion temperature of 130 C. It can be seen that, for all moisture contents, the apparent viscosity decreased with increasing shear rate, and that, in general, it also decreased with increasing moisture content in the range studied (19–27%). Similar patterns have been observed at different extrusion temperatures. A strong power law dependence of apparent viscosity on shear rate was observed for all the starch samples and measurement conditions studied, and this dependence was linear on doublelogarithmic plots, indicating that the power law model of Bird et al. (1960) could be used to describe the rheological behaviors of starch-based materials:
h ¼ K gn1
(3)
F. Xie et al. / Journal of Cereal Science 49 (2009) 371–377
10000
τ (27% mc) η (19% mc)
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η (23% mc) η (27% mc)
G80
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τ (23% mc) τ (27% mc) η (19% mc)
100000
η (27% mc) G50
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η (23% mc) η (27% mc)
NC 10
100
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Shear stress (Pa)
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τ (19% mc) τ (23% mc) τ (27% mc) η (19% mc)
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η (23% mc) η (27% mc)
WC
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1000
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τ (23% mc)
1000000
Shear stress (Pa)
τ (19% mc)
Viscosity (Pa·s)
Shear stress (Pa)
1000000
373
100
Shear rate (-s)
Fig. 1. Effect of moisture content on shear stress and melt viscosity of various starches (extrusion temperature 130 C).
where h is the melt viscosity, K is the consistency, g is the shear rate, and n is the pseudoplastic index. The corresponding consistency and pseudoplastic index can be determined individually from the intercept and the slope of each single straight line in the double-log plots. For starch-based materials in this study, values of n are between 0 and 1, and when n < 1, the apparent viscosity will decrease with increasing shear rate and the materials will undergo shear thinning (Tanner, 2000).
a function of shear rate. It can be seen that the apparent viscosity generally decreased with increasing temperature from 110 to 140 C. A strong power law dependence of apparent viscosity on shear rate is observed. For all the starch samples and measurement conditions studied, the dependence of apparent viscosity on the shear rate was linear on double-logarithmic plots, again indicating that the power law model could describe the rheological behaviors of the molten starches.
3.2. Effects of extrusion temperature
3.3. Effects of amylose content
Fig. 2 shows the effect of temperature on the rheological properties of the different starches at 23% moisture content, as
Fig. 3 shows the effect of amylose content on the shear stress and melt viscosity of various samples at a moisture content of 23%
1000
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G80
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Shear stress (Pa)
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1000
WC
100
Shear rate (-s)
100 1000
10000
10
100
Shear rate (-s)
Fig. 2. Effect of temperature on shear stress and melt viscosity of various starches (moisture content 23%).
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τ 110°C τ 120 °C τ 130 °C τ 140 °C η 110°C η 120 °C η 130 °C η 140 °C
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10000 τ G80 τ G50 τ NC τ WS η G80 η G50 η NC η WS
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10
Viscosity (Pa·s)
Shear stress (Pa)
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100 1000
100
Shear rate (-s) Fig. 3. Effect of amylose content on shear stress and melt viscosity of various starches (moisture content 23%; extrusion temperature 130 C).
and extrusion temperature of 130 C, as a function of shear rate. It was found that higher amylose content resulted in a higher apparent viscosity over the same shear rate range. For example, at a shear rate of 100 S1, h is 277, 635, 1008, and 1254 Pa s for WC, NC, G50, and G80 respectively under the condition of 23% moisture content and 130 C extrusion temperature. 3.4. Power law parameters Table 1 lists the detailed parameters of regression power law for the different samples and extrusion conditions investigated. It can be seen that, in most cases, n was higher when the moisture content was higher, which is to be expected, since increased water content in starches could make them more Newtonian. This corresponds with most previous studies (Della Valle et al., 1996a; Gonza´lez et al., 2006; Kokini et al., 1992; Lai and Kokini, 1990), although Willett et al. (1997) reported that the effect of moisture on n was not clear in their study of waxy corn starch. The results in this experimental work show that there are only a few conditions (e.g.
Table 1 Power law parameters of the various starches. Starch
Extrusion conditions
Power law index (n)
K (¼h at 1 s1)
Correlation coefficient (R2)
Temperature ( C)
% MC
110 110 110 120 120 130 130 130 140 140
19 23 27 23 27 19 23 27 23 27
– 0.161 0.197 0.206 0.284 0.031 0.259 0.386 0.316 0.431
– 97,500 65,300 65,500 300 201,000 38,000 14,400 22,800 8510
– 0.9670 0.9800 0.9805 0.9823 0.9950 0.9768 0.9677 0.9898 0.9734
G50
110 110 110 120 120 130 130 130 140 140 140
19 23 27 23 27 19 23 27 19 23 27
– 0.102 0.243 0.217 0.263 0.154 0.266 0.345 0.562 0.322 0.400
– 119,000 28,300 50,200 19,500 59,400 29,600 10,400 3660 17,400 5480
– 0.9897 0.9961 0.9932 0.9956 0.9633 0.9916 0.9959 0.8273 0.9975 0.9990
NC
110 110 110 120 120 120 130 130 130 140 140 140
19 23 27 19 23 27 19 23 27 19 23 27
0.289 0.331 0.351 0.383 0.434 0.475 0.411 0.435 0.589 0.535 0.494 0.571
43,900 25,300 15,900 20,400 10,900 5970 13,800 8560 2390 5570 5590 2260
0.9654 0.9768 0.9938 0.9819 0.9963 0.9590 0.9963 0.9399 0.9599 0.9736 0.9889 0.9856
WC
110 110 110 120 120 120 130 130 130 140 140 140
19 23 27 19 23 27 19 23 27 19 23 27
0.477 0.455 0.458 0.549 0.658 0.492 0.780 0.741 0.504 0.825 0.847 0.680
14,000 8020 7400 5950 2060 4290 1260 913 2800 710 366 835
0.9503 0.9564 0.9715 0.9296 0.9196 0.9930 0.9551 0.9503 0.9479 0.8592 0.9618 0.9701
G80
F. Xie et al. / Journal of Cereal Science 49 (2009) 371–377
Heat Flow Endo Up (mW)
5
4
WC
3
NC
2
G50 1 M2 G
0 50
G80 M
100
150
Temperature (ºC) Fig. 4. Gelatinization endotherms of different starches.
at 140 C) that produce abnormal effects of moisture content on the power law index n. One explanation could relate to the slight expansion at the higher temperature and the enhancement of moisture evaporation during measurement. Variations of volume or mass at higher temperatures will affect the shear rate, which will in turn affect the exponent n (see Eqs. (1) and (3)). Increasing temperature will increase the power law index n and thus make the starch melt less pseudoplastic and more Newtonian. Similar results have been reported for both waxy corn starch (Della Valle et al., 1996b; Willett et al., 1997) and normal corn starch (Willett et al., 1994, 1995). An increase in n with increasing temperature has also been observed with waxy corn starch during first-pass twin-screw extrusion (Della Valle et al., 1996b; Lai and Kokini, 1990). It is important to note that n became higher with decreasing amylose content for the studied corn starches, clearly indicating that lower amylose-content corn starch has a more Newtonian behavior in its melting state. Similar results have been observed in previous studies (Della Valle et al., 1996b; Lai and Kokini, 1990). The decrease in the power law index with increasing amylose content was generally attributed to an increase in entanglements between amylose chains, since the highly branched amylopectin was not expected to form effective entanglements (Willett et al., 1997; Yu and Christie, 2005). Table 1 also shows that the power law consistency value K decreased with increasing moisture content and extrusion temperature. Since the K value has a direct relationship with viscosity (see Eq. (3)), it can be used to represent the viscosity characteristics of a material under certain conditions. The K results are as expected, since both water (as plasticizer) and temperature will normally decrease the viscosity of a polymeric material. By plotting the effects of amylose content and temperature on K
375
(figure not shown here), it can be seen that higher amylose content resulted in a higher K value. For example, K increased from 103 for WC to 105 for G80 at the highest temperature investigated (140 C). However, all starches recorded a lower K value with increasing temperature, although the decrease in K was not linear for starches with different amylose/amylopectin ratios. It should be pointed out that there were a few anomalies for the waxy starch at higher temperature, which could be explained by the expansion of the material during extrusion, as discussed earlier. Gelatinization during thermal processing is one of the unique characteristics of starch-based materials, and Fig. 4 shows the gelatinization endotherms of the various starches as measured by DSC. It can be seen that for the waxy and normal corn starches, a large gelatinization endotherm, G, appeared at about 70 C. A second endotherm, M2, was detected for NC at about 90 C, which was considered to be a phase transition within an amylose–lipid ˜ o´n, 1999; Liu complex (Biliaderis et al., 1985; Jovanovich and An et al., 2006; Raphaelides and Karkalas, 1988). A very broad endotherm was observed in the temperature range of 65–115 C for both high amylose-content starches (G50 and G80), which represents a composite of gelatinization endotherm G and a phase transition within the amylose–lipid complex M2. For G80, a small endotherm, M, was also detected at about 155 C, which was considered to be amylose melting (Liu et al., 2006). The higher temperature detected for the amylose-rich starches can be used to explain their higher viscosity and less Newtonian behavior. It is has been noticed that the higher the amylose content, the higher the viscosity, which is opposite to the sequence of molecular weight (see Fig. 3) (Liu et al., 2006). The unique microstructure and phase transition can be used to explain this phenomenon. Fig. 5 shows a schematic representation of the microstructure and phase transition of starch during gelatinization, in which the double helical, crystalline structure formed by the short branched chains in amylopectin are torn apart. However, these short branched chains remain in a regular pattern by retaining a certain ‘‘memory’’. French (1984) reported that the thickness of crystalline lamellae in native amylopectin and recrystallized amylopectin is the same (about 50 Å), which supports the ‘‘memory’’ theory. In a previous study, Yu and Christie (2005) indicated that these short branched chains form gel-balls that are comprised mainly of chains from the same sub-main chain. In addition, one amylopectin molecule may form a relatively separate super-globe. The molecular entanglements between gel-balls and super-globes are much less than those between linear polymer chains, due to their size and the length of the chains (only 4–6 glucose). These gel-balls require less energy to move than long linear chains, especially when they are lubricated by a plasticizer (water). Because of the highly branched microstructure of starch, and the formation of gel-balls and super-globes after gelatinization, the entanglement of polymer chains in amylopectin-rich starch is much less than that in amylose-rich linear starch. This could explain why amylopectin-rich materials initially have lower
Fig. 5. Schematic representation of microstructure and phase transition of starch during gelatinization.
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modulus and higher elongation, and lower viscosity during extrusion (Yu and Christie, 2005). It could also explain why amylopectinrich starches exhibit increased Newtonian behavior. 4. Conclusions In this work, the rheological properties of various corn starches with different amylose/amylopectin ratios (0/100, 23/77, 50/50, and 80/20) were firstly systematically studied using a Haake rheometer. A single-screw extruder with a slit capillary die was used to characterize shear stress and apparent melt viscosity as functions of moisture content (19–27 C) and extrusion temperature (110– 140 C). The melts exhibited shear thinning behavior under all conditions, with the power law index n (0 < n < 1) increasing with increasing temperature and moisture content in most cases; and it decreased as amylose content increased. The K value decreased with increasing moisture content and extrusion temperature; and it increased with increasing amylose content (for example, K increased from 103 for WC to 105 for G80 at 140 C). The amyloserich corn starches showed higher viscosity and less Newtonian behavior, which can be explained by its higher gelatinization temperature (and in particular multiphase transition at higher temperature), greater molecular entanglements between linear polymer chains, and less gel-balls and super-globes that are much easier to move than long linear chains. The results of this work have confirmed some previously reported results, and have addressed some earlier confusion. Acknowledgement The authors from SCUT, China, would like to acknowledge the research funds NRDPHT (863) (2007AA10Z312, 2007AA100407), GNSF (05200617) and ETRFNK (2006C40038). References Arvanitoyannis, I.S., 1999. Totally-and-partially biodegradable polymer blends based on natural and synthetic macromolecules: preparation and physical properties and potential as food packaging materials. Journal of Macromolecular Science – Reviews in Macromolecular Chemistry and Physics 39, 205–271. Babin, P., Della Valle, G., Dendievel, R., Lourdin, D., Salvo, L., 2007. X-ray tomography study of the cellular structure of extruded starches and its relations with expansion phenomenon and foam mechanical properties. Carbohydrate Polymers 68, 329–340. Bhattacharya, M., Hanna, M.A., 1986. Viscosity modelling of dough in extrusion. Journal of Food Technology 21, 167–174. Bhattacharya, M., Hanna, M.A., 1987. Influence of process and product variables on extrusion energy and pressure requirements. Journal of Food Engineering 6, 153–163. Bhattacharya, M., Hanna, M.A., Kaufman, R.E., 1988. Viscoelastic properties of extrusion-cooked plant protein mixtures. Journal of Food Engineering 7, 5–18. Bhattacharya, M., Padmanabhan, M., 1992. On-line rheological measurements of food dough during extrusion cooking. In: Kokini, J.L., Ho, C., Karwe, M.V. (Eds.), Food Extrusion Science and Technology. Marcel Dekker, New York, pp. 213–231. Biliaderis, C.G., Page, C.M., Slade, L., Sirett, R.R., 1985. Thermal behavior of amylose– lipid complexes. Carbohydrate Polymers 5, 367–389. Bird, R.B., Stewart, W.E., Lightfoot, E.N., 1960. Transport Phenomena. Wiley, New York. Cha, J.Y., Chung, D.S., Seib, P.A., Flores, R.A., Hanna, M.A., 2001. Physical properties of starch-based foams as affected by extrusion temperature and moisture content. Industrial Crops and Products 14, 23–30. Chanvrier, H., Uthayakumaran, S., Lillford, P., 2007. Rheological properties of wheat flour processed at low levels of hydration: influence of starch and gluten. Journal of Cereal Science 45, 263–274. Chen, P., Yu, L., Chen, L., Li, X., 2006. Morphologies and microstructure of cornstarch with different amylose/amylopectin content. Starch/Sta¨rke 58, 611–615. Chen, P., Yu, L., Kealy, T., Chen, L., Li, L., 2007. Phase transition of cornstarch under shearless and shear stress conditions. Carbohydrate Polymers 68, 495–501. Chinnaswamy, R., Hanna, M.A., 1988. Relationship between amylose content and extrusion-expansion properties of corn starches. Cereal Chemistry 65, 138–143. Dean, K., Yu, L., Wu, D.Y., 2007. Preparation and characterization of melt-extruded thermoplastic starch/clay nanocomposites. Composites Science and Technology 67, 413–421.
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Contents lists available at ScienceDirect
Journal of Cereal Science journal homepage: www.elsevier.com/locate/jcs
Rheological properties of starches with different amylose/amylopectin ratios Fengwei Xie a, Long Yu a, b, *, Bing Su a, Peng Liu a, Jun Wang a, Hongshen Liu a, b, Ling Chen a a b
Centre for Polymers from Renewable Resources, ERCPSP, School of Light Industry and Food Science, SCUT, Guangzhou, China CSIRO, Materials Science and Engineering, Melbourne, Vic 3168, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 October 2008 Received in revised form 3 January 2009 Accepted 5 January 2009
The rheological properties of corn starches with different amylose/amylopectin ratios (80/20, 50/50, 23/ 77, and 0/100) were systematically studied by Haake rheometry. The starches were initially pre-compounded with water to designated moisture content levels using a twin-screw extruder. A single-screw extruder with a slit capillary die was then used to characterize the shear stress and melt viscosity characteristics of sample pellets, as a function of both moisture content (19–27%) and extrusion temperature (110–140 C). The melts exhibited shear thinning behavior under all conditions, with the power law index (0 < n < 1) increasing with increasing temperature and moisture content in the majority of cases. The higher the amylose content, the higher is the viscosity (for example, h increases from 277 Pa s to 1254 Pa s when amylose content increases from 0% to 80% under a certain condition), which is opposite to the sequence of molecular weight; amylopectin-rich starches exhibited increased Newtonian behavior. These rheological behaviors are attributed to the higher gelatinization temperature of amyloserich starches, and in particular the multiphase transitions that occur in these starches at higher temperatures, and the gel-ball structure of gelatinized amylopectin. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Starch Rheometry Rheological Amylose/amylopectin Extrusion
1. Introduction Extrusion cooking has been practiced for more than 50 years, with early developments focused on the preparation of ready-toeat cereals (Harper, 1989). Some researchers have investigated the effects of amylose content in starch or dough on the physical and functional properties of their extruded products (Guha et al., 2003; Matthey and Hanna, 1997). Others have analyzed viscoelastic characterization of different biopolymers and their mixtures with additives (Bhattacharya and Padmanabhan, 1992; Bhattacharya et al., 1988; Seethamaju and Bhattacharya, 1994). More recently, Chanvrier et al. (2007) investigated the effects of starch and protein on the rheological properties of wheat flour dough during processing at low hydration. Different viscosity models have been proposed to describe the rheological properties of dough based on the traditional power law function. For example, Jao et al. (1978) derived a regression model for the die viscosity of soy dough by considering the effects of temperature, shear rate and moisture content; Bhattacharya and Hanna (1986, 1987) and Harper (1981)
* Corresponding author. Commonwealth Scientific and Industrial Research Organization, Materials Science and Engineering, Gate 4, Normanby Rd, Clayton South, Melbourne, Vic 3168, Australia. Tel.: þ61 3 9545 2777; fax: þ61 3 9544 1128. E-mail address: [email protected] (L. Yu). 0733-5210/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcs.2009.01.002
presented examples of viscosity models applied to cooked dough; and Toma´s et al. (1997) derived a regression viscosity model for extruding rice flour using a stepwise model starting from a power law equation. Due to environmental considerations and the shortage of oil, starches are now attracting increased attention as raw materials in the production of biodegradable plastics (Arvanitoyannis, 1999; Signori et al., 2005; So¨derqvist Lindblad et al., 2005). In addition, the water requirements during the thermal processing of these materials are normally lower than for conventional additives. There are many reported investigations of the effects of starch type or amylose/amylopectin ratio on the final properties of starch-based materials, including: Foams (Babin et al., 2007; Chinnaswamy and Hanna, 1988; Della Valle et al., 1997; Fang and Hanna, 2001a,b; Guan et al., 2005; Suknark et al., 1997). Sheets or films (Fishman et al., 2006; Yu and Christie, 2005; Yu et al., 2006). Lightweight concrete (Glenn et al., 1999). Injection-molded products (Funke et al., 1998; de Graaf et al., 2003). The melt rheology of starch-based materials has both scientific and industrial importance, and thus it has been widely
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investigated. For example, Llo et al. (1996) studied the effects of extrusion conditions on the apparent viscosity of maize grits, including the effects of melt temperature, shear rate, material composition, and extrusion processing history. Willett et al. (1997) reported that waxy corn starch exhibited shear thinning behavior, becoming more Newtonian as either temperature or moisture content was increased. Gonza´lez et al. (2006) investigated the effects of several factors including screw speed, die restriction (l/r) and moisture content, together with corn endosperm hardness and rice amylose content, on apparent melt viscosity using a Brabender single-screw extruder. Recently, Thuwall et al. (2006) studied the effects of amylose content, moisture content and starch/glycerol ratio on the apparent viscosity of potato starch. The effects of amylose content on reactive extrusion have also been studied (Wing and Willett, 1997). Corn starches have attracted particular scientific interests, because different amylose/amylopectin content materials can be obtained from natural, renewable resources, and they exhibit multiphase transitions during thermal processing (Chen et al., 2006, 2007; Liu et al., 2006). Previous studies have shown that higher amylose-content starches exhibit superior strength and toughness in the preparation of starch-based materials and in producing modified starches by reactive extrusion (Cha et al., 2001; Dean et al., 2007; Guan and Hanna, 2004; Guan and Hanna, 2006; Miladinov and Hanna, 2000, 2001; Nabar et al., 2006; van Soest and Borger, 1997; Yu and Christie, 2005; Yu et al., 2006; Zhou and Hanna, 2004). However the extrusion of high-amylose starches is more difficult than that of normal starches, partly due to the higher die pressure and torque requirements due to the higher melting temperature and viscosity of these starches (Liu et al., 2006; Shogren, 1992; Shogren and Jasberg, 1994). In this work, the rheological properties of corn starches with different amylose/amylopectin ratios (80/20, 50/50, 23/77 and 0/100) were systematically studied as functions of moisture content and extrusion temperature using a Haake rheometer. The effects of different amylose/amylopectin ratios on the rheological behaviors will be discussed based on their microstructures and gelatinization behaviors. 2. Experimental work 2.1. Materials Four commercially available corn starches with different amylose/amylopectin ratios were used in this experimental work: Gelose 80 (G80, 80/20), supplied by Penford (Australia). Gelose 50 (G50, 50/50), supplied by Penford (Australia). A normal corn starch (NC, 23/77), supplied by Huanglong Food Industry Co. Ltd (P.R. China). A waxy corn starch (WC, 0/100), supplied by Shanxi Jinli Industry Group Co. Ltd (P.R. China). An infra-red heating balance (Model DHS-20) was used to measure the moisture contents of the original starches during heating to 110 C for 20 min. All formulations were prepared on a dry weight basis. 2.2. Sample preparation Starches were firstly premixed with water to achieve designated moisture contents. A Haake parallel, co-rotating twin-screw extruder (Rheomex PTW 24/40p, Ø 30) with a rod die (nozzle L/ D ¼ 3, diameter ¼ 3 mm) was used to produce gelatinized starch pellets at a screw speed of 90 rpm. The extruder heating barrel comprises 10 sections. The highest temperature used during
compounding was 160 C, and the die temperature was kept at 105 C to avoid foaming. A gravimetric feeder was used to deliver the materials into the extruder. The moisture contents in the compounded pellets were measured after heating the samples overnight at 130 C in a vacuum oven. 2.3. Rheological measurements Compounded pellets were used to study the rheological properties of various samples. A Haake Rheocord Polylab RC500p incorporating a single-screw extruder (Rheomex 252p, Ø 19, screw 2:1, L/D 25) with a slit capillary die (20 1 mm) was used to measure the shear stress and apparent viscosity of samples under different shear rates at specific temperatures. The screw speed was varied between 30 and 180 min1, and normally six points were recorded for each sample using the cutting and manual entry measurement mode. Apparent shear rates were calculated by:
g¼
6Q WH 2
(1)
where g is the shear rate, Q is the volumetric flow rate (in cm3/s), W is the slit width, and H is the slit height. Shear stress values were calculated using the following equation:
s¼
H$DP 2L
(2)
in which s is the shear stress, DP is the pressure drop over the capillary, and L is the slit length. 2.4. Differential scanning calorimetry A Perkin–Elmer Diamond-I differential scanning calorimeter (DSC) with an internal coolant (Intercooler 1P), nitrogen purge gas, and stainless steel sample pans, was used to study the thermal properties of samples with high moisture content over a high temperature range (up to 350 C). The melting point and enthalpy of indium were used for temperature and heat capacity calibrations, respectively. The detailed methodology and a discussion of some critical issues are presented elsewhere (Liu et al., 2006; Yu and Christie, 2001). 3. Results and discussions 3.1. Effects of moisture content Fig. 1 shows the results for shear stress and melt viscosity as a function of shear rate for the various corn starches (G80, G50, NC, and WC) with different moisture contents at an extrusion temperature of 130 C. It can be seen that, for all moisture contents, the apparent viscosity decreased with increasing shear rate, and that, in general, it also decreased with increasing moisture content in the range studied (19–27%). Similar patterns have been observed at different extrusion temperatures. A strong power law dependence of apparent viscosity on shear rate was observed for all the starch samples and measurement conditions studied, and this dependence was linear on doublelogarithmic plots, indicating that the power law model of Bird et al. (1960) could be used to describe the rheological behaviors of starch-based materials:
h ¼ K gn1
(3)
F. Xie et al. / Journal of Cereal Science 49 (2009) 371–377
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373
100
Shear rate (-s)
Fig. 1. Effect of moisture content on shear stress and melt viscosity of various starches (extrusion temperature 130 C).
where h is the melt viscosity, K is the consistency, g is the shear rate, and n is the pseudoplastic index. The corresponding consistency and pseudoplastic index can be determined individually from the intercept and the slope of each single straight line in the double-log plots. For starch-based materials in this study, values of n are between 0 and 1, and when n < 1, the apparent viscosity will decrease with increasing shear rate and the materials will undergo shear thinning (Tanner, 2000).
a function of shear rate. It can be seen that the apparent viscosity generally decreased with increasing temperature from 110 to 140 C. A strong power law dependence of apparent viscosity on shear rate is observed. For all the starch samples and measurement conditions studied, the dependence of apparent viscosity on the shear rate was linear on double-logarithmic plots, again indicating that the power law model could describe the rheological behaviors of the molten starches.
3.2. Effects of extrusion temperature
3.3. Effects of amylose content
Fig. 2 shows the effect of temperature on the rheological properties of the different starches at 23% moisture content, as
Fig. 3 shows the effect of amylose content on the shear stress and melt viscosity of various samples at a moisture content of 23%
1000
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Fig. 2. Effect of temperature on shear stress and melt viscosity of various starches (moisture content 23%).
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10000 τ G80 τ G50 τ NC τ WS η G80 η G50 η NC η WS
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Shear rate (-s) Fig. 3. Effect of amylose content on shear stress and melt viscosity of various starches (moisture content 23%; extrusion temperature 130 C).
and extrusion temperature of 130 C, as a function of shear rate. It was found that higher amylose content resulted in a higher apparent viscosity over the same shear rate range. For example, at a shear rate of 100 S1, h is 277, 635, 1008, and 1254 Pa s for WC, NC, G50, and G80 respectively under the condition of 23% moisture content and 130 C extrusion temperature. 3.4. Power law parameters Table 1 lists the detailed parameters of regression power law for the different samples and extrusion conditions investigated. It can be seen that, in most cases, n was higher when the moisture content was higher, which is to be expected, since increased water content in starches could make them more Newtonian. This corresponds with most previous studies (Della Valle et al., 1996a; Gonza´lez et al., 2006; Kokini et al., 1992; Lai and Kokini, 1990), although Willett et al. (1997) reported that the effect of moisture on n was not clear in their study of waxy corn starch. The results in this experimental work show that there are only a few conditions (e.g.
Table 1 Power law parameters of the various starches. Starch
Extrusion conditions
Power law index (n)
K (¼h at 1 s1)
Correlation coefficient (R2)
Temperature ( C)
% MC
110 110 110 120 120 130 130 130 140 140
19 23 27 23 27 19 23 27 23 27
– 0.161 0.197 0.206 0.284 0.031 0.259 0.386 0.316 0.431
– 97,500 65,300 65,500 300 201,000 38,000 14,400 22,800 8510
– 0.9670 0.9800 0.9805 0.9823 0.9950 0.9768 0.9677 0.9898 0.9734
G50
110 110 110 120 120 130 130 130 140 140 140
19 23 27 23 27 19 23 27 19 23 27
– 0.102 0.243 0.217 0.263 0.154 0.266 0.345 0.562 0.322 0.400
– 119,000 28,300 50,200 19,500 59,400 29,600 10,400 3660 17,400 5480
– 0.9897 0.9961 0.9932 0.9956 0.9633 0.9916 0.9959 0.8273 0.9975 0.9990
NC
110 110 110 120 120 120 130 130 130 140 140 140
19 23 27 19 23 27 19 23 27 19 23 27
0.289 0.331 0.351 0.383 0.434 0.475 0.411 0.435 0.589 0.535 0.494 0.571
43,900 25,300 15,900 20,400 10,900 5970 13,800 8560 2390 5570 5590 2260
0.9654 0.9768 0.9938 0.9819 0.9963 0.9590 0.9963 0.9399 0.9599 0.9736 0.9889 0.9856
WC
110 110 110 120 120 120 130 130 130 140 140 140
19 23 27 19 23 27 19 23 27 19 23 27
0.477 0.455 0.458 0.549 0.658 0.492 0.780 0.741 0.504 0.825 0.847 0.680
14,000 8020 7400 5950 2060 4290 1260 913 2800 710 366 835
0.9503 0.9564 0.9715 0.9296 0.9196 0.9930 0.9551 0.9503 0.9479 0.8592 0.9618 0.9701
G80
F. Xie et al. / Journal of Cereal Science 49 (2009) 371–377
Heat Flow Endo Up (mW)
5
4
WC
3
NC
2
G50 1 M2 G
0 50
G80 M
100
150
Temperature (ºC) Fig. 4. Gelatinization endotherms of different starches.
at 140 C) that produce abnormal effects of moisture content on the power law index n. One explanation could relate to the slight expansion at the higher temperature and the enhancement of moisture evaporation during measurement. Variations of volume or mass at higher temperatures will affect the shear rate, which will in turn affect the exponent n (see Eqs. (1) and (3)). Increasing temperature will increase the power law index n and thus make the starch melt less pseudoplastic and more Newtonian. Similar results have been reported for both waxy corn starch (Della Valle et al., 1996b; Willett et al., 1997) and normal corn starch (Willett et al., 1994, 1995). An increase in n with increasing temperature has also been observed with waxy corn starch during first-pass twin-screw extrusion (Della Valle et al., 1996b; Lai and Kokini, 1990). It is important to note that n became higher with decreasing amylose content for the studied corn starches, clearly indicating that lower amylose-content corn starch has a more Newtonian behavior in its melting state. Similar results have been observed in previous studies (Della Valle et al., 1996b; Lai and Kokini, 1990). The decrease in the power law index with increasing amylose content was generally attributed to an increase in entanglements between amylose chains, since the highly branched amylopectin was not expected to form effective entanglements (Willett et al., 1997; Yu and Christie, 2005). Table 1 also shows that the power law consistency value K decreased with increasing moisture content and extrusion temperature. Since the K value has a direct relationship with viscosity (see Eq. (3)), it can be used to represent the viscosity characteristics of a material under certain conditions. The K results are as expected, since both water (as plasticizer) and temperature will normally decrease the viscosity of a polymeric material. By plotting the effects of amylose content and temperature on K
375
(figure not shown here), it can be seen that higher amylose content resulted in a higher K value. For example, K increased from 103 for WC to 105 for G80 at the highest temperature investigated (140 C). However, all starches recorded a lower K value with increasing temperature, although the decrease in K was not linear for starches with different amylose/amylopectin ratios. It should be pointed out that there were a few anomalies for the waxy starch at higher temperature, which could be explained by the expansion of the material during extrusion, as discussed earlier. Gelatinization during thermal processing is one of the unique characteristics of starch-based materials, and Fig. 4 shows the gelatinization endotherms of the various starches as measured by DSC. It can be seen that for the waxy and normal corn starches, a large gelatinization endotherm, G, appeared at about 70 C. A second endotherm, M2, was detected for NC at about 90 C, which was considered to be a phase transition within an amylose–lipid ˜ o´n, 1999; Liu complex (Biliaderis et al., 1985; Jovanovich and An et al., 2006; Raphaelides and Karkalas, 1988). A very broad endotherm was observed in the temperature range of 65–115 C for both high amylose-content starches (G50 and G80), which represents a composite of gelatinization endotherm G and a phase transition within the amylose–lipid complex M2. For G80, a small endotherm, M, was also detected at about 155 C, which was considered to be amylose melting (Liu et al., 2006). The higher temperature detected for the amylose-rich starches can be used to explain their higher viscosity and less Newtonian behavior. It is has been noticed that the higher the amylose content, the higher the viscosity, which is opposite to the sequence of molecular weight (see Fig. 3) (Liu et al., 2006). The unique microstructure and phase transition can be used to explain this phenomenon. Fig. 5 shows a schematic representation of the microstructure and phase transition of starch during gelatinization, in which the double helical, crystalline structure formed by the short branched chains in amylopectin are torn apart. However, these short branched chains remain in a regular pattern by retaining a certain ‘‘memory’’. French (1984) reported that the thickness of crystalline lamellae in native amylopectin and recrystallized amylopectin is the same (about 50 Å), which supports the ‘‘memory’’ theory. In a previous study, Yu and Christie (2005) indicated that these short branched chains form gel-balls that are comprised mainly of chains from the same sub-main chain. In addition, one amylopectin molecule may form a relatively separate super-globe. The molecular entanglements between gel-balls and super-globes are much less than those between linear polymer chains, due to their size and the length of the chains (only 4–6 glucose). These gel-balls require less energy to move than long linear chains, especially when they are lubricated by a plasticizer (water). Because of the highly branched microstructure of starch, and the formation of gel-balls and super-globes after gelatinization, the entanglement of polymer chains in amylopectin-rich starch is much less than that in amylose-rich linear starch. This could explain why amylopectin-rich materials initially have lower
Fig. 5. Schematic representation of microstructure and phase transition of starch during gelatinization.
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modulus and higher elongation, and lower viscosity during extrusion (Yu and Christie, 2005). It could also explain why amylopectinrich starches exhibit increased Newtonian behavior. 4. Conclusions In this work, the rheological properties of various corn starches with different amylose/amylopectin ratios (0/100, 23/77, 50/50, and 80/20) were firstly systematically studied using a Haake rheometer. A single-screw extruder with a slit capillary die was used to characterize shear stress and apparent melt viscosity as functions of moisture content (19–27 C) and extrusion temperature (110– 140 C). The melts exhibited shear thinning behavior under all conditions, with the power law index n (0 < n < 1) increasing with increasing temperature and moisture content in most cases; and it decreased as amylose content increased. The K value decreased with increasing moisture content and extrusion temperature; and it increased with increasing amylose content (for example, K increased from 103 for WC to 105 for G80 at 140 C). The amyloserich corn starches showed higher viscosity and less Newtonian behavior, which can be explained by its higher gelatinization temperature (and in particular multiphase transition at higher temperature), greater molecular entanglements between linear polymer chains, and less gel-balls and super-globes that are much easier to move than long linear chains. The results of this work have confirmed some previously reported results, and have addressed some earlier confusion. Acknowledgement The authors from SCUT, China, would like to acknowledge the research funds NRDPHT (863) (2007AA10Z312, 2007AA100407), GNSF (05200617) and ETRFNK (2006C40038). References Arvanitoyannis, I.S., 1999. Totally-and-partially biodegradable polymer blends based on natural and synthetic macromolecules: preparation and physical properties and potential as food packaging materials. Journal of Macromolecular Science – Reviews in Macromolecular Chemistry and Physics 39, 205–271. Babin, P., Della Valle, G., Dendievel, R., Lourdin, D., Salvo, L., 2007. X-ray tomography study of the cellular structure of extruded starches and its relations with expansion phenomenon and foam mechanical properties. Carbohydrate Polymers 68, 329–340. Bhattacharya, M., Hanna, M.A., 1986. Viscosity modelling of dough in extrusion. Journal of Food Technology 21, 167–174. Bhattacharya, M., Hanna, M.A., 1987. Influence of process and product variables on extrusion energy and pressure requirements. Journal of Food Engineering 6, 153–163. Bhattacharya, M., Hanna, M.A., Kaufman, R.E., 1988. Viscoelastic properties of extrusion-cooked plant protein mixtures. Journal of Food Engineering 7, 5–18. Bhattacharya, M., Padmanabhan, M., 1992. On-line rheological measurements of food dough during extrusion cooking. In: Kokini, J.L., Ho, C., Karwe, M.V. (Eds.), Food Extrusion Science and Technology. Marcel Dekker, New York, pp. 213–231. Biliaderis, C.G., Page, C.M., Slade, L., Sirett, R.R., 1985. Thermal behavior of amylose– lipid complexes. Carbohydrate Polymers 5, 367–389. Bird, R.B., Stewart, W.E., Lightfoot, E.N., 1960. Transport Phenomena. Wiley, New York. Cha, J.Y., Chung, D.S., Seib, P.A., Flores, R.A., Hanna, M.A., 2001. Physical properties of starch-based foams as affected by extrusion temperature and moisture content. Industrial Crops and Products 14, 23–30. Chanvrier, H., Uthayakumaran, S., Lillford, P., 2007. Rheological properties of wheat flour processed at low levels of hydration: influence of starch and gluten. Journal of Cereal Science 45, 263–274. Chen, P., Yu, L., Chen, L., Li, X., 2006. Morphologies and microstructure of cornstarch with different amylose/amylopectin content. Starch/Sta¨rke 58, 611–615. Chen, P., Yu, L., Kealy, T., Chen, L., Li, L., 2007. Phase transition of cornstarch under shearless and shear stress conditions. Carbohydrate Polymers 68, 495–501. Chinnaswamy, R., Hanna, M.A., 1988. Relationship between amylose content and extrusion-expansion properties of corn starches. Cereal Chemistry 65, 138–143. Dean, K., Yu, L., Wu, D.Y., 2007. Preparation and characterization of melt-extruded thermoplastic starch/clay nanocomposites. Composites Science and Technology 67, 413–421.
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