Importance of extensional rheological properties on fiber-enriched corn extrudates

Journal of Cereal Science 50 (2009) 227–234

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Importance of extensional rheological properties on fiber-enriched corn extrudates Dhananjay A. Pai a, c, Orane A. Blake a, c, Bruce R. Hamaker a, c, Osvaldo H. Campanella b, c, * a

Department of Food Science, Purdue University, West Lafayette, IN 47907, USA Department of Agricultural and Biological Engineering, Purdue University, West Lafayette, IN 47907, USA c Whistler Carbohydrate Research Center, Purdue University, West Lafayette, IN 47907, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 April 2008 Received in revised form 17 May 2009 Accepted 30 May 2009

Incorporation of insoluble fibers into extruded cereals severely limits their expansion and reduces crispness. The specific objective of this work was to investigate how corn bran and its fractions mixed with cornmeal affect rheology and extrudate expansion. Alkali-treated bran (ATB) and alkaline-soluble bran (ASB) were prepared from unmodified corn bran (UMB). Mixtures of the different corn bran fractions and degermed cornmeal having a 26% (w/w) of total dietary fibers were extruded in a twin screw extruder and expansion of the extrudates was determined. Melt shear rheology of mixtures of cornmeal and different corn bran fractions having a 20% total dietary fiber was determined using a capillary rheometer at 120  C while the extensional rheology was determined using lubricated squeezing flow. The expansion of extrudates containing ATB was larger than those containing UMB, while extrudates containing ASB showed a greater expansion that was close to that of the control. Addition of UMB to cornmeal increased shear and extensional viscosity significantly as compared to the control. ATB addition increased the shear viscosity of the mixture to a small extent but showed the highest extensional viscosity amongst the samples. Addition of ASB resulted in mixtures having lower shear and extensional viscosities than the control. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Corn bran Extrusion Shear rheology Extensional rheology

1. Introduction Dietary fiber has a number of health benefits. However, its incorporation into extruded puffed snack foods and breakfast cereals limits puffing, reduces crispness and, in the latter case, decreases bowl life. Almost invariably, it has been found that increasing fiber concentration in the extrudate formulations reduces the expansion volume of extruded foods. Mendonca et al. (2000) reported that co-extrusion of corn bran and cornmeal resulted in less radial expansion and undesirable product textural characteristics with increased bran content. Ahmed (1999), Chinnaswamy and Hanna (1991), Hseih et al. (1991), Hu et al. (1993), Jin et al. (1994, 1995), Lue et al. (1991), Ozboy and Koksel (2000) and Onwulata et al. (2000) have reported similar results on extrusion of cornmeal, corn flour or corn grits mixed with various

Abbreviations: ASB, Alkali soluble bran; ATB, Alkali-treated bran; IDF, Insoluble dietary fiber; SDF, Soluble dietary fiber; SEI, Sectional expansion index; TDF, Total dietary fiber; UMB, Unmodified bran. * Corresponding author at: Department of Agricultural and Biological Engineering, Purdue University, 745 Agriculture Mall Drive, West Lafayette, IN 47907, USA. Tel.: þ1 765 496 6330; fax: þ1 765 496 1115. E-mail address: [email protected] (O.H. Campanella). 0733-5210/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcs.2009.05.007

types of fiber. In general, it has been observed that reduction in expansion during extrusion results in products that are dense, tough, and non-crispy (Lue et al., 1991). There is abundant literature describing the effects of fiber incorporation on extruded product microstructure and physicochemical properties. Chinnaswamy and Hanna (1991) reported that for extrudates produced by co-extrusion of corn starch with cellulose fibers, water solubility of the extrudates, degradation level of amylopectin, and iodine binding abilities of the gel permeation chromatography fractions changed significantly when compared with extrudates solely based on corn starch. Lue et al. (1991) reported that decreasing the particle size of sugar beet fiber improved both radial and longitudinal expansion. Jin et al. (1995), using scanning electron microscopy, found that increasing fiber content resulted in less expanded extrudates with smaller air cells and thicker cell walls. In a number of studies, addition of fiber made the product darker and the darkness increased with fiber content (Ahmed, 1999; Lin et al., 2002) and fiber particle size (Lue et al., 1994). Moraru and Kokini (2003) hypothesized that, above a critical concentration, fibers may disrupt the continuous structure of the melt, impeding its elastic deformation during expansion. However, no literature has been found showing studies that investigate mechanisms by which fiber polymers affect expansion during extrusion.

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Feeds used for the extrusion of foods consist of biopolymers such as starch and proteins and some minor constituents (e.g. salt, sugars, lipids), each having a specific functionality. Researchers who have studied the mechanism for extrudate expansion have noted that expansion is produced primarily by two effects, one is known as the die swell phenomenon, and the other is produced by the sudden vaporization of water in the food melt (Fan et al., 1994; Padmanabhan and Bhattacharya, 1989). The latter phenomenon is more predominant in foods and it is considered as the main factor affecting the structure of extruded foods (Guy, 2001). Expansion phenomenon has been related to the shear viscosity of the melt by a few researchers (Chinnaswamy and Hanna, 1991; Della Valle et al., 1997; Fan et al., 1994; Kokini et al., 1992; Mercier and Feillet, 1975), though actual experimental data relating rheology to extrusion has been limited to very few studies such as the ones conducted by Della Valle et al. (1997), Kokini et al. (1992), Lai and Kokini (1990) and Li et al. (2004). The expansion of a melt as it emerges from the extruder die can be expressed as a function of many of its physicochemical properties including properties like glass transition temperature and more specifically rheological properties under shear and extensional flow. In order to facilitate the bubble expansion process as the superheated water is quickly vaporized at atmospheric pressure, the melt film must flow easily in the bubble walls (Guy, 2001). In rheological terms, this flow is closely related to the shear viscosity of the melt. Extensional properties of the expanding melt are important at later stages of the expansion process, i.e. when the bubble growth is resisted by the elasticity of thin films separating the extrudate bubbles (Guy, 2001; Steffe, 1996). After expansion, the temperature of the product falls rapidly due to evaporative cooling and to a lesser extent, convective cooling. As a consequence of this rapid decrease of temperature, and also moisture loss, there is a large increase in the extrudate viscosity. The temperature at which transformations occur is very close to the glass transition temperature of the extrudate. For temperatures below the glass transition, i.e. when the material is in the glassy state, the material becomes rigid and forms a solid foam having an open structure. Starches are biopolymers that favor the expansion in an extrusion process and, in general, good and well-expanded cellular structures can be produced from starches of different sources. Increasing fiber content in the form of bran, results in premature rupture of gas cells, which reduces overall expansion (Mendonca et al., 2000; Moore, 1994). In microscopic sections of fiber-enriched extrudates, it is easy to observe a continuous phase formed by starch polymers as well as the presence of a dispersed fiber phase (Guy, 2001). It has been shown that fibers are not much affected by the harsh environment existing inside the extruder and largely retain their macrostructure (Guy, 2001). Thus, their physical presence will reduce the expansion potential of the air cells forming in the extrudate matrix due to disruption of the continuous starch phase. This effect is observed during the extrusion of whole meal wheat flour with bran concentrations of approximately 8–9% (Guy, 2001). The complex extrudate expansion process takes place under high temperature and low moisture conditions. As discussed, it encompasses various structural changes and phase transitions, such as extrudate swell, bubble growth, and bubble collapse contributing to the expansion (Chang, 1992). A comprehensive review was carried out by Moraru and Kokini (2003) on the nucleation and expansion of cereals by direct extrusion and expansion of extruded pellets in microwaves. Most of the reviewed works, along with model development, like for example those described by Alavi et al. (2003), Fan et al. (1994), Schwartzberg et al. (1995), Wang et al. (2005), have indicated that an understanding of the rheology of the extrudate melt during extrusion is of

paramount importance, mainly because of its influence on the expansion process and the quality of the final product. Initial extrusion trials conducted in our laboratory, which consisted of co-extrusion of cornmeal with different corn fiber fractions, showed vast differences in extrusion expansion. Unmodified corn bran at 26% total dietary fiber significantly reduced the expansion of extrudates, but modification of its molecular structure using an alkaline treatment enhanced the sectional expansion index of the extrudates. Incorporation of the alkaline-soluble fraction of corn bran at 26% into the extrudate formulation facilitated significant cross-sectional expansion of extrudates, such that their degree of expansion was similar to a control sample containing no added fiber. These extrusion experiments clearly suggested that the disparity in the expansion behavior was due to the effect of physicochemical characteristics of the fiber on the extrudate expansion process. An understanding at a mechanistic level of how fiber polymers affect melt rheology leading to extrudate expansion is unavailable in the literature. Acknowledging the importance of rheology of the melt under two types of flow, which are prevalent in the expansion process was one of the aims of this research. Specifically, the objective of this work was to investigate how corn bran and its various fractions, mixed with cornmeal, affect melt rheology, characterized by shear and extensional flow, and extrudate expansion. 2. Materials and methods 2.1. Materials Corn bran (particle size < 50 mm) and degermed cornmeal were obtained from Bunge Ltd. (Danville, IL). Sodium hydroxide pellets and concentrated hydrochloric acid were purchased from Mallinckrodt Baker Ltd. (Philipsburg, NJ). Absolute ethanol was obtained from Aaper Alcohol and Chemical Products (Shelbyville, KY). Dow CorningÒ 4 compound silicone oil from Dow CorningÒ (Midland, MI) and vital wheat gluten (HodgsonÒ Mill Brand) were obtained for the lubricated squeezing flow experiments. 2.2. Methods 2.2.1. Fractionation of corn bran Corn bran was mixed with 5% NaOH in a 1:10 ratio for 2 h at room temperature to hydrolyze ferulic acid ester cross-linkages (Doner et al., 2000). The mixture was neutralized with hydrochloric acid. 95% ethanol was then added to the neutralized extract in a ratio of 2:1. The precipitate, termed ‘‘alkali-treated bran’’ (ATB), was dried at 50  C and milled using a CyclotecÔ cyclone mill equipped with a screen of mesh size 40. Separation of the alkaline-soluble hemicellulosic fraction from the alkaline-insoluble fraction was done by centrifuging the alkaline mixture (before the neutralization step to produce alkalitreated bran) at 11,159g for 8 min. The supernatant was neutralized with HCl and 80% ethanol was added to the supernatant in a ratio of 2:1 to obtain the alkaline-soluble bran (ASB). The precipitate was dried at 50  C and milled using the cyclone mill equipped with a screen of mesh size 40. Total dietary fiber content (soluble and insoluble) of the above samples was determined using AOAC Method 991.43 (AOAC, 1995). 2.2.2. Twin screw extrusion Each fiber type [i.e. ATB, ASB, and unmodified bran (UMB)] was mixed with cornmeal in a suitable ratio in order to produce a mixture containing 26% total dietary fiber. A total dietary fiber content of 26% was selected because it would enable manufacturers to claim an ‘excellent source of fiber’ in their final product. The

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mixture was subsequently extruded using a Krupp Werner and Pfleiderer ZSK-25 (WP-25) twin screw extruder (Ramsey, NJ), 25 mm screw diameter, L/D equal to 25 and a standard geometry to produce snacks consisting of two sets of mixing disks and one set of reverse screw elements located close to the die. All the other elements were forward screws. The die head consisted of two circular dies of 4 mm diameter. Extrusion was done under the following conditions: water input flow rate of 9.3 g/min, feed input flow rate of 200 g/min, zone temperatures of 25, 75, 75, 100, and 125  C, and at 250 rpm. Based on the moisture content of the ingredients and the amount of water injected into the extruder, the moisture content of the extruded material ranged between 18 and 20%. Sectional expansion index (SEI) was measured where SEI ¼ (De/Dd)2 [Dd and De are the diameters (m) of the die insert and extrudate, respectively] (Alvarez-Martinez et al., 1988). LEI was determined as described by Alvarez-Martinez et al. (1988), i.e. as LEI ¼ SEI  VEI where VEI is the volumetric expansion index defined as VEI ¼ rsolid density =rbulk . rbulk and rsolid density are the bulk and solid densities of the extrudate, respectively. Bulk density was calculated by the equation given by Ali et al. (1996), as rbulk ¼ 4=pD2 L where L is the length per gram of extrudate (cm/g). The solid density of the extrudate, rsolid density , was calculated by dividing the mass of the sample by the true solid volume (i.e. the volume of the sample that does not include its pores), which was measured by a stereopycnometer (Quantachrome Instruments, Boynton Beach, Fl). SEI and LEI provide expansion of the sample in two different directions, the first is perpendicular to the flow whereas the second is in the flow direction. The expansion patterns defined by these two parameters are strongly influenced by the product formulation (Guy, 2001) but, in most extrusion expanded products, an anisotropic expansion is observed. Although the mechanisms of the expansion phenomenon are not totally elucidated it is believed that the observed anisotropy is due to the elasticity of the melt (Guy, 2001). 2.2.3. Capillary rheometry (shear flow) A twin-bore capillary rheometer (Rosand RH 2000 Capillary Rheometer, Malvern Instruments, Southborough, MA) was used to measure the shear viscosity of the melt. This instrument offers the surest way to prevent evaporation and is one of the simplest extrusion and die flow simulators (Macosko, 1994). 100 g of sample was prepared by mixing cornmeal, fiber fractions and water in proportions such that each sample contained approximately 20% of total dietary fiber and a total moisture content of 25%. Due to limitations in the capillary viscometer pressure transducer, the total amount of dietary fiber had to be reduced to 20% and the water content increased to 25% as compared to 26% total dietary fiber and 18–20% moisture content during the extrusion process. Moisture content of all samples was determined as per standard AACC Approved Methods (1991). The ingredients were mixed in a Kitchen AidÔ mixer (Troy, Ohio, USA) for 4 min. The samples thus obtained were then milled using the cyclone mill equipped with a screen of mesh size 40. Cornmeal with 25% moisture and no added fiber served as the control. Steady shear experiments were performed at 120  C because this temperature was the highest that could be attainable in the viscometer without scorching the samples during the test, mainly due to the longer residence of the sample in the capillary viscometer as compared to the residence time in the melting zone of the extruder. Samples (100 g) were loaded in one of the capillary bores. A die of L/D equal to 8 (length ¼ 32 mm, diameter ¼ 4 mm) was attached to the end of the measurement bore. Use of long capillary dies minimizes the contribution of the non-linear portion of the pressure drop (Byler and Kwei, 1971). Steady shear viscosities were measured at shear rates of 100, 150, 175 and 200 s1 which are

229

typical during an extrusion process. The speed of the moving piston was varied by the software program resulting in different calculated shear rates. No end correction was applied due to the high sample requirement. The software, FlowmasterÔ, was programmed to measure the viscosity readings only at steady pressures. All experiments were performed in triplicate. 2.2.4. Lubricated squeezing flow rheometry (extensional flow) Converging flow into the die of a twin screw extruder involves a combination of shear and extensional flow and bubble growth arising from the evaporation of water from the extrudate matrix (Steffe, 1996). Characterization of the melt extensional viscosity requires sophisticated equipment. Entrance pressure drop measured from flow into a converging die has been used to evaluate the extensional viscosity of cornmeal dough (Bhattacharya et al., 1994; Padmanabhan and Bhattacharya, 1993; Seethamraju and Bhattacharya, 1994). Although this may be the best method for approximating extrusion conditions, the amount of sample needed for each trial is very high. Hence, lubricated squeezing flow between two plates was chosen to characterize the extensional viscosity of the mixed fiber samples. Apart from the simplicity of this method, it offers a practical way to avoid the extensive structural disruption that occurs at the narrow space of the die. By using lubricated squeezing flow, this damage can be almost completely eliminated and the specimen can be tested as practically intact by carefully placing the material on the lower plate with a wide spatula or spoon (Corradini et al., 2000; Suwonsichon and Peleg, 1999). Huang and Kokini (1993) and Wikstro¨m and Bohlin (1999) have used this technique for characterizing extensional viscosity of wheat doughs. A dough, or at least a cohesive mass of the sample, is required to obtain good data from the lubricated squeezing flow experiments. However, apart from cornmeal and sample of cornmeal with ASB, it was found that forming a cohesive mass with UMB or ATB was not possible at the testing temperature (room temperature). Hence, gluten had to be added at an 8.3% level to produce cohesive doughs. The possibility that gluten may affect the rheology of the dough was considered since van Vliet et al. (1992) reported strain hardening of wheat doughs possibly due to the presence of gluten. In our work the mixture of cornmeal and gluten served as a base to observe the effect of the different fiber components on the extensional rheology of the dough. The samples thus prepared consisted of cornmeal mixed with gluten and each of the fiber components ATB, ASB and UMB. Water was added to each of the samples and mixed well so as to form a cohesive dough. 1 g of sample was prepared with resultant composition of approximately 20% total dietary fiber, 25% cornmeal, 8.3% gluten and the rest, water. Accounting for the presence of water in all components of the dough, the sample total moisture content varied between 46 and 47%. Cornmeal with added gluten and no added fiber at the same moisture content served as the control. Although the moisture content was much higher than typical moisture contents encountered in extrusion, the objective of performing these experiments was to observe the effects of various fibers on extensional viscosity during biaxial extensional flow in order to relate the results to the expansion during extrusion. Lubricated squeezing flow experiments were conducted with a Sintech 1/G Universal Testing Machine (MTSÒ, Eden Prairie, MN). Teflon plates, 25.4 mm in diameter, were used for the tests. The plates were lubricated with silicone oil and 7 g of each sample was initially compressed by moving the crosshead at extremely low speeds until the sample reached an initial height of 10 mm. All experiments were conducted with the constant area method in which the initial sample diameter was equal to the diameter of the plates. The crosshead was then moved at constant downward

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velocities of 10 and 20 mm/min until the point where the force reached a value of 100 lbf. A load cell of 100 lbf was used to measure the force exerted on the sample. Experiments were conducted in duplicate. To ensure measurements under true equilibrium conditions, a constant strain rate has to be maintained. The use of a constant deformation speed does not imply the presence of constant strain rate, an aspect that could be challenged in this test. However tests were performed at different deformation velocities and the trends observed were similar. In other words, sufficiently low deformation speeds were used to assure that the extensional viscosities were determined at conditions close to equilibrium.

Table 2 SEI, LEI and bulk density of extrudates obtained by mixing 26% of various fibers with cornmeal. Key: alkali-treated bran (ATB), alkali soluble bran (ASB), unmodified bran (UMB). Sample

SEI

Control (no added fiber) 27a Cornmeal þ ASB 27a Cornmeal þ ATB 21b Cornmeal þ UMB 11d

Bulk density (g/cc)  0.01 K (Pa sn) n

LEI

0.41de 9.3c 0.41e 5.4a 0.49cd 7.2b 0.79a 14.6d

89,119 37,138 106,182 150,178

0.223 0.148 0.227 0.214

Values with the same letter are not significantly different (P < 0.05).

3. Results

in the selected shear rate range. The power law model is described by the following equation:

3.1. Corn bran fraction

h ¼ K g_ n1

The results of analysis of the different fiber components obtained from the fractionation procedure are shown in Table 1. After alkali treatment, there was a significant reduction in insoluble fiber from 79 to 52%, and an increase in soluble fiber from 1.6 to 30%. The water soluble fiber component of the alkali-treated bran is termed hemicellulose B or corn fiber gum (Whistler, 1993). The main components of the water insoluble portion of alkali-treated bran consist of hemicellulose A and cross-linked insoluble hemicellulose, such as hemicellulose C (15% w/w of bran), which is a linear xylan polysaccharide with a few arabinose branches, and cellulose (16% w/w) (Sugawara et al., 1994). The main dietary fiber component of the alkaline-soluble bran fraction (ASB) contains 64% of soluble dietary fiber, which is the branched hemicellulose B. Results given in Table 1 illustrate that alkali treatment of corn bran disrupts its supramolecular structure, producing dietary fiber that can be separated into various components, which are hemicelluloses A, B, and an alkali-insoluble fraction that consists of cellulose and cross-linked hemicellulose (hemicellulose C), and starch.

Or in terms of the natural logarithms as:

ln h ¼ ln K þ ðn  1Þ ln g_

3.4. Extensional rheometry using lubricated squeezing flow The values of stress growth function measured by extensional flow were plotted against the biaxial extensional strain as shown in Fig. 2. The extensional strain can be calculated as shown in Eq. (3).

3.3. Capillary viscometry Fig. 1 shows apparent viscosities obtained from steady shear experiments at different shear rates plotted against the apparent wall shear rate. As shown by the linearity of the plot in logarithmic coordinates, control and fiber-containing (UMB, ATB, ASB) samples showed a pseudoplastic behavior and obeyed the power law model

Table 1 Dietary fiber compositions of corn bran preparations. Key: alkali-treated bran (ATB), alkali soluble bran (ASB), unmodified bran (UMB). Sample

IDF, %

SDF, %

TDF, %

ATB ASB UMB

52  1 9.4  0.3 79  5

30  1 64  0 1.6  0.3

82  1 73  0 83  5

  1 V t 2 HðtÞ

(3)

The extensional viscosity in lubricated squeezing flow is given by the following equation (Campanella and Peleg, 2002).

mb ¼

2FðtÞHðtÞ pR2 V

(4)

8000 6000

Apparent Viscosity (Pa.s)

The sectional expansion index (SEI) of extrudates containing 26% ASB (SEI ¼ 27) was equivalent to the SEI of the control (no fiber added, SEI ¼ 27), and both ASB and ATB have SEIs larger than UMB (Table 2). LEI of the control as well as of the mixture containing alkaline-soluble fiber (ASF) was similar whereas a slight increase was observed for mixtures containing alkali-treated fiber (ATB). Conversely samples containing unmodified bran (UMB) exhibited large values of LEI, that translated into dense and elongated extrudates (Table 2).

(2)

where, h is the apparent viscosity and g_ is the shear rate in the die. K and n are parameters of the power law model known as consistency and flow indexes, respectively. Fig. 1 shows differences in melt shear rheology based on addition of fibers. With the addition of UMB to the cornmeal at the 20% level, the shear viscosity increased substantially compared to the control. However, addition of ATB resulted in a significant increase in shear viscosity while a decrease was noted with the addition of ASB.

3_ ¼

3.2. Extrusion trials with 26% fiber components

(1)

4000 2000 1000 800 600 400 200 100 80

Corn Meal Corn Meal + UMB Corn Meal + ATB Corn Meal + ASB 100

120

140

160

180 200 220 240

Shear Rate (1/s) Fig. 1. Results of capillary rheometry of cornmeal with various fibers. Apparent shear viscosity as a function of shear rate. Key: alkali-treated bran (ATB), alkali soluble bran (ASB), unmodified bran (UMB).

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4. Discussion

Extensional Viscosity (Pa.s)

Corn Meal + ATB

4.1. Effect of rheology on expansion

1000000

Cornmeal with ASB extrudate exhibited a much lower shear viscosity and equal SEI and LEI compared to the control cornmeal extrudate with no added fiber. Fan et al. (1994) suggested that, by neglecting the effect of surface tension, the rate of bubble growth during extrusion expansion can be estimated as:

Corn Meal + UMB

Corn Meal Corn Meal + ASB

100000

DP dR=dt ¼ ma R

10000

1

2

3

Strain Fig. 2. Results of lubricated squeezing flow rheometry of cornmeal with various fibers. Extensional viscosity (stress growth function) as a function of stain. Key: alkali-treated bran (ATB), alkali soluble bran (ASB), unmodified bran (UMB).

where mb is the extensional viscosity, F(t) is the force at time t after commencement of the experiment, H(t) is the momentary sample height, R is the radius of the sample, which is the same as the radius of the plate (constant area method) and V is the crosshead velocity or velocity of deformation. All samples showed strain thinning behavior which is different to the strain hardening observed in doughs prepared with wheat flour. As a consequence, the effect of the addition of gluten needs to be discussed. Stading et al. (2006) showed that the extensional viscosity of wheat flour dough having protein contents of 9.5–12% increased with increases in protein (gluten). In contrast, sorghum dough, which did not have gluten, exhibited a much lower extensional viscosity as compared to wheat dough and the strain hardening effect was not observed. The authors concluded that the presence of gluten was responsible for the higher extensional viscosities and the strain hardening effect observed in wheat dough. Although no research studying the effects of gluten addition on the extensional viscosity of cornmeal was found, our experiments showed that the absence of gluten in cornmeal dough resulted in doughs having lower extensional viscosities (data not shown) but no strain hardening was observed. Similar trends were observed for mixtures containing gluten in quantities up to 10%. The addition of gluten to cornmeal in small quantities (8.3%) increased the extensional viscosity of the mixtures and was able to form a cohesive mass that allowed for squeezing flow testing, but it did not change the rheological trends observed in samples without gluten. Thus, the addition of gluten served to improve the squeezing flow testing and its reproducibility without qualitatively changing the behavior of the samples. Differences in extensional rheology were observed with the type of fiber used, cornmeal with ATB composite showing the highest extensional viscosity. Incorporation of UMB in the cornmeal resulted in an extensional viscosity higher than that for cornmeal with no added fiber, and ASB had an extensional viscosity slightly lower than the control cornmeal. However, no strain hardening phenomena was observed. Samples of ASB with cornmeal and no gluten showed lower extensional viscosity (not shown here). Hence, gluten addition probably exerted its effects through increased extensional viscosity of the system, while also forming a dough-like structure that did not show the strain hardening effect shown in wheat flour dough.

(5)

where, dR/dt is the rate of increase in the radius, R is the radius of the bubble at any given time t, DP is the pressure difference, and ma is the apparent shear viscosity of the melt. A direct inference of this model is that a high melt viscosity may reduce bubble growth and expansion. The model was tested by others (Lai and Kokini, 1990) during extrusion of single starches and with values of viscosity measured by an online viscometer attached to the extruder die. Likewise, Della Valle et al. (1997) showed that, at a given moisture content and temperature, volumetric expansion increased as the melt viscosity decreases. However, an excessively low viscosity resulted in a decrease in volumetric expansion due to shrinkage and collapse after maximum expansion (Della Valle et al., 1997; Kokini et al., 1992). With an appropriate or optimum low shear viscosity, presumably the rate of bubble growth is high and bubbles can grow to larger sizes. Fan et al. (1994) have reported increased bubble diameter as well as increasing collapse with higher moisture contents but in this case both the control and cornmeal with ASB were adjusted to the same moisture content. It could be possible that the lower shear viscosity might have led to a higher degree of collapse in the sample with ASB. However, cross sections of extrudates prepared with cornmeal and ASB did not show any collapse but a very uniform bubble size distribution as opposed to cornmeal alone, which showed collapse and bubble coalescence. Both samples showed a similar radial and longitudinal expansion, suggesting that the shear viscosity could not be the sole rheological parameter governing the extrudate expansion and that extensional viscosity probably has a role to play in expansion. As the material exits the die, bubble growth is initiated. The expansion mechanism in food extrusion is as schematically shown in Figs. 3 and 4. The material expands due to a combination of die swell, mainly contributed by the elasticity of the melt, and bubble expansion driven by the pressure difference between the melt exiting the extruder and the atmospheric pressure. Chang (1992) worked on the effects on dough elasticity of the die swell. However, in foods, due to the presence of a relatively large amount of water, bubble growth driven by vapor pressure differences contributes to

Bubble Moisture Steam

Die

Puff

Air

Collapse

Set

Fig. 3. Mechanism of extrusion expansion.

Heat

Harden

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Fig. 4. Cross-sectional image of extrudates containing modified fibers. Extrudates from left to right contain no added fiber (CTRL), alkali soluble bran (ASB), alkali-treated bran (ATB), unmodified bran (UMB).

expansion to a larger extent than die swell. Hence in this discussion, our focus is on both shear and extensional melt rheology affecting the growth of the bubbles, and neglecting the contribution of elasticity to die swell. The elasticity of the sample, as suggested, could affect the longitudinal expansion measured by LEI. It can be considered that both shear and extensional viscosities are of importance in exerting their influence on extrudate expansion. Following the schematic expansion mechanism illustrated in Fig. 3, once bubble growth is initiated, the bubbles have to overcome the resistance created by the shear viscosity in order to grow. After the bubbles have grown sufficiently, the film of melt material between the bubbles undergoes biaxial extension (Gendron and Daigneault, 2000; Steffe, 1996). Hence, consideration of extensional viscosity becomes an essential part in the prediction of extrusion expansion from the melt rheology. The bubble expansion created by moisture vaporization increases and causes the melt film between the bubbles to get thinner. Once a certain normal stress in the melt film has been exceeded due to further expansion, the melt film ruptures causing bubbles to collapse. For the bubbles to remain with the same expanded size, the melt should have an extensional viscosity high enough to withstand the extensional stress caused by bubble expansion. Guy (2001) has reported that bran particles may act as nucleating agents. In the present study, extrudates with bran had a larger number of bubbles compared to the cornmeal control. Addition of the ASB fraction to cornmeal resulted in the highest expansion which was contributed by a large number of bubbles that were smaller than for the cornmeal alone, but larger than those from the other treatments. Thus, a number of factors can be related to the product characteristics. First, ASB incorporation showed a significantly lower shear viscosity than the control cornmeal. Although low shear viscosity tends to favor bubble growth, it may also cause a faster water loss on exiting the die due to higher diffusion of water vapor. This phenomenon has been investigated by Sun (2004) and Stange and Munstedt (2006) who showed that higher shear viscosity causes a slower diffusion of the gas in polymers resulting in better foam expansion. Second, although the extensional viscosity was lower with the ASB treatment than for the cornmeal control, the reason for similar expansion probably cannot be explained by the reduced extensional viscosity alone. The presence of high amount of bubbles of large sizes may have increased the heat transfer area of the extrudate containing ASB as compared to cornmeal. This would cause a more rapid heat and moisture loss, which promotes the attainment of the glass transition sooner than the control sample, thus preventing further bubble growth. The combination of reasons presented above, i.e. higher water vapor diffusion due to low shear viscosity and enhanced heat and mass transfer area, both leading to faster heat and water loss, may explain why cornmeal with ASB has the same

expansion and more bubbles of smaller size compared to the control despite showing a much lower shear and slightly lower extensional viscosity. Excluding the sample of cornmeal with ASB, shear viscosity increased in the order of cornmeal, cornmeal with ATB, and cornmeal with UMB whereas extensional viscosity increased in the order of cornmeal, cornmeal with UMB, and cornmeal with ATB. Comparing the rheology of cornmeal with UMB to that of cornmeal with ATB and results of the corresponding extrusion treatments, it can be concluded that the noted increase in extensional viscosity and decrease in shear viscosity is concomitant with increased extrusion expansion. This trend suggests that a high relative extensional viscosity and a low shear viscosity are important for good expansion. On the other hand, addition of both UMB and ATB to cornmeal increased shear and extensional viscosities when compared to cornmeal alone, thus increasing overall resistance to expansion, and causing lower expansion. Gendron and Daigneault (2000) have reported that for homogeneous synthetic polymers, the initial growth of bubbles requires a low extensional viscosity. Subsequently, a high extensional viscosity is advantageous such that the cell walls – the melt membranes between the bubbles – may withstand the stretching force experienced during the later stages of bubble growth. This non-linear increase in the extensional viscosities with increasing strain is known as a strain hardening phenomenon (Gendron and Daigneault, 2000). Although no strain hardening phenomena has been found in our study, ATB and UMB exhibited high extensional viscosities at high strains, but also had high extensional viscosities at low strains which may impede expansion due to an increased initial resistance. When looking at the overall picture involving the tested fibers, that exhibiting the lowest shear and extensional viscosities, i.e. cornmeal with ASB, gave the best expansion suggesting that low viscosities create a favorable rheology for expansion. 4.2. Effect of structure of fibers on rheology Solubilization, or in this case molecular dispersal, of arabinoxylan in the ATB fiber caused a decrease in shear viscosity and an increase in extensional viscosity. On the other hand, ASB with the highest soluble fiber content exhibited the lowest shear viscosity and the lowest extensional viscosity as well. This finding suggests that the interaction of ASB with the starch in the cornmeal is different from the interactions of UMB or ATB with the starch. The major component of ASB is hemicellulose B which is a branched arabinoxylan (Whistler, 1993). It is probable that due to its high degree of branching, ASB is able to interact with the starch in a better manner than the other two fibers. Based on these results, it is postulated that ASB is more molecularly dispersed in the starch. This interaction would result in a behavior of the fiber in the melt

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similar to starch, without disrupting the melt structure and resulting in a melt rheology which favors bubble growth and expansion similar to cornmeal. The main components of the water insoluble portion of the ATB fiber consist of cross-linked insoluble hemicellulose, such as hemicellulose C (15% w/w of bran, linear xylan polysaccharide with few arabinose branches), and cellulose (16% w/w) (Sugawara et al., 1994). UMB had 79% insoluble dietary fiber as compared to ATB which had 52%. The high proportions of cross-linked and linear polymers that compose UMB are likely to break the continuity of the melt forming a multiphase melt. Thus, ATB forms a melt that is less discontinuous than the melt formed by UMB. In addition, ASB would be well dispersed in the melt forming a single-phase, continuous melt which has a behavior rheologically similar to starch. A similar phenomenon is found during the extrusion of synthetic polymer, where the use of highly branched polymers such as LDPE exhibits a much superior foaming and expansion behavior than the linear HDPE (Gendron and Daigneault, 2000). Similarly, Moraru and Kokini (2003) have speculated that amylopectin-rich starches expand more than amylose based starches since amylose chains align themselves in the shear field and thus are difficult to pull apart during expansion. The linear structure of amylose allows formation of entanglements and thus increased viscosity (Moraru and Kokini, 2003). Li et al. (2004) measured the rheological properties of high amylose and amylopectin starches and their mixtures during extrusion in a modified slit viscometer and found similar results. These observations suggest that a melt containing highly branched polymers is favorable for expansion. Thus, the branched arabinoxylan fraction of hemicellulose B, which is present most predominately in the ASB fiber, would lead to higher expansion. In conclusion, the results presented support the view that, to obtain good extrudate expansion, the melt should have a shear viscosity which is low enough to promote bubble growth and expansion, but high enough to prevent bubble collapse. Moreover, the melt should exhibit an extensional viscosity high enough to withstand the stretching forces during expansion, but low enough such that the melt can be stretched easily. Conditions where shear and extensional viscosities are either too low to maintain bubble integrity or too high to make bubbles grow, and resulting in difficulty in melt stretching, can lead to a lower expansion. Thus, extrudate expansion depends on the interplay of shear and extensional rheologies. Among the fibers tested, the molecularly dispersed and relatively highly branched ASB fiber likely best interacted with the starch in cornmeal, resulting in a melt with appropriate shear and extensional viscosities similar to cornmeal and resulting in a low resistance to bubble growth and expansion. Both ATB and UMB fibers performed poorly when compared to ASB due to the presence of relatively high amounts of non-dispersed cross-linked molecules, particularly in the UMB. These results indicate that the molecular dispersion of fibers results in a homogenous melt that is likely to enhance expansion of fiber with cornmeal. Shear and extensional rheological properties of the dough produced with these components are both important to predict the expansion of fiber-enriched puffed, extruded snack and breakfast products.

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