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Rheology and Expansion of Starch-Water-CO2 Mixtures with ControlledGelatinization by Supercritical FluidExtrusionK.H. Chen a & S.S.H. Rizvi aa Institute of Food Science, Cornell University , Ithaca, New York,USAPublished online: 18 Apr 2007.

To cite this article: K.H. Chen & S.S.H. Rizvi (2006) Rheology and Expansion of Starch-Water-CO2

Mixtures with Controlled Gelatinization by Supercritical Fluid Extrusion, International Journal of FoodProperties, 9:4, 863-876, DOI: 10.1080/10942910600714414

To link to this article: http://dx.doi.org/10.1080/10942910600714414

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International Journal of Food Properties, 9: 863–876, 2006Copyright © Taylor & Francis Group, LLCISSN: 1094-2912 print / 1532-2386 onlineDOI: 10.1080/10942910600714414

863

RHEOLOGY AND EXPANSION OF STARCH-WATER-CO2 MIXTURES WITH CONTROLLED GELATINIZATION BY SUPERCRITICAL FLUID EXTRUSION

K.H. Chen and S.S.H. RizviInstitute of Food Science, Cornell University, Ithaca, New York, USA

A slit die rheometer fitted to a twin-screw extruder was used to measure the viscosity andexpansion characteristics of starch-based melts plasticized with water and supercriticalCO2. Experiments were conducted for four intermediate moisture starch-water mixtureswith varying degree of starch gelatinization (DG), achieved by blends of pregelatinized andnative common corn starch. SC-CO2 was shown to be an effective plasticizer for starch-water mixture at 0.45 g SC-CO2/100 g sample, lowering the viscosity of the melt by an aver-age of 14%. Apparent viscosity was measured for shear rate ranging from 100–200 s−1. Theviscosity reduction factors based on DG and SC-CO2 indicated that the free volume addedto starch-water mixtures with SC-CO2 is an effective mechanism for viscosity reduction. Inaddition, the higher water affinity of pregelatinized starch increased the water inducedplasticization effect in the mixtures. SC-CO2 was a good blowing agent producing distinctexpanded microcellular morphology for the starch-water mixtures with DG at or above80%. The expansion and cellular characterization of these starch-based extrudates pro-duced by SC-CO2 injection may be governed by the viscosity-dependent parameters includ-ing gas retention capability, CO2 diffusivity and the pressure drop rate.

Keywords: Extrusion, Expansion, Viscosity, Starch melt, Supercritical CO2, Gelatinization.

INTRODUCTION

Generation of extrusion-based foams from starch melt has received considerableattention in recent years for both food and non-food applications. It typically involveslow-moisture (13–20 g water/100 g sample) and high-temperature (130–170°C), short-time (HTST) cooking of starch to produce steam-induced expanded products.[1–3] Theability of starch polymer to form thermoplastic melt upon water plasticization as well asits abundant supply, cheap cost, and high biodegradability makes it a competitive alterna-tive to polystyrene (PS) and other popular foaming polymers. Steam expanded starchfoams of conventional process, however, have its limitations in that they have a coarse andvery non-uniform cellular structure with cell sizes in the range of 1–3 mm.[4] Moreover,there is little or no control over cell size, density, and therefore the resultant mechanicalproperties of the end product.

Received 30 July 2005; accepted 24 March 2006.Address correspondence to S.S.H. Rizvi, Institute of Food Science, Cornell University, Ithaca, New York

14853, USA. E-mail: ssr3@cornell.edu

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864 CHEN AND RIZVI

Conventional plastic foam processing by extrusion involves melting of polymer resinsand incorporation of blowing agents ranging from hydrocarbon, chlorofluorocarbon to envi-ronmental friendly nitrogen and carbon dioxide. Recognizing the potential dissolution andexpansion capability of supercritical carbon dioxide (SC-CO2) in starch melt, Rizvi andMulvaney[5] proposed a novel patented extrusion technology combined with injection ofSC-CO2 as a blowing agent and/or an active carrier for production of highly expandedstarch foams.[6,7] In this high pressure (> 7.38 MPa), low temperature (< 100°C) processtermed supercritical fluid extrusion (SCFX), the conventional roles of water as a plasticizeras well as a blowing agent are decoupled. It obviates the need for steam expansion and highmechanical energy inputs by increasing the moisture content of the feed to reduce the vis-cous dissipation of heat. Hence, low product temperature becomes attainable, which offersmajor advantages over steam puffing such as incorporation of heat sensitive ingredients andactives.[8] A key criterion to achieve the good cellular expansion during the SCFX process-ing of starch-water-CO2 mixtures is to maintain both low temperature (< 100°C) and highmoisture content of the mixtures (35–45 g water/100 g sample), which are conditions neces-sary to attain high CO2 solubility limit in the starch-water mixtures as the main driving forceof expansion. In addition, the highly gelatinized matrix is required to achieve good gasretention capacity. It was found from our previous study,[9] by differential scanning calorim-eter (DSC), that complete starch gelatinization was not needed to achieve gas retentioncapacity for cellular expansion during SCFX processing. This has brought forth the hypoth-esis that there exists a minimum required degree of gelatinization in the starch-based mix-tures to produce good microcellular expansion. Such design of partial starch gelatinizationnot only has the ability to achieve various final product textures, but it also has its applica-tion in resolving product texture loss problem typically among the quick-cooking andmicrowavable food novelty such as noodles, pastas, snacks, and bread products.[10]

Numerous studies on the rheology of polymer/CO2 systems have been previouslyevaluated. Gerhardt et al.[11] investigated the viscosity-shear rate relationships for poly-dimethylsiloxane (PDMS)/CO2 mixtures up to 20 g CO2/100 g sample at 50 and 80°C.Kwag et al.[12] studied the viscosity of PS melts containing CO2 from 0 to 5 g/100 g sam-ple up to 16 MPa and 150–175°C. Areerat et al.[13] studied the melt viscosity of low den-sity polyethylene (LDPE)/CO2 solutions up to 5 g CO2/100 g sample and temperature inthe range of 150–175°C. Maio et al.[14] examined the viscosity of polycaprolactone(PCL)/CO2 system up to 10 g CO2/100 g sample in the range of 70–85°C. In all, the rheologyresults concluded that the free volume added to the melt by dissolved CO2 is the principalmechanism for viscosity reduction. Furthermore, the rheology of CO2/polymer melt sys-tem has been modeled successfully in terms of classical viscoelastic scaling theory using ashift factor approach, reflecting both dilution effect and free volume increase effect con-tributed by the dissolved CO2. The extent of viscosity reduction has been attributed to thedegree of molecular mobility influenced by the additional free volume, reflected upon theproximity of melt temperature from the glass transition of the CO2/polymer system atoperating pressure and temperature condition.[11]

Many SCFX applications ranging from bread dough[15] to masa-based chips,[9] andprotein-fortified snacks[6] have been investigated. In this article, we used blends of pregelati-nized and native corn starches as the model system to determine the minimum degree ofgelatinization (DG) required for good cellular expansion, and to characterize the rheology ofstarch-water-CO2 mixture at SCFX operating pressure and temperature using an online slit-die rheometer system. The expansion characteristics were measured and correlated with themelt viscosity of varying concentrations of SC-CO2 and the DG in the starch-water mixtures.

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RHEOLOGY AND EXPANSION OF STARCH BY SUPERCRITICAL FLUID EXTRUSION 865

MATERIALS AND METHODS

Preparation of Materials

Native (C*Gel) and pregelatinized (C*Instant) corn starch (∼23% amylose) wereobtained from Cerestar (Hammond, IN). Four starch-water mixtures containing a 60:40,80:20, 90:10, and 100:0 ratio of pregelatinized to native starch were prepared by dry blend-ing. Water was added during SCFX processing to achieve 42 g water/100 g extrudate. Highpurity (∼ 99%) CO2 from Air Gas (Ithaca, New York) was used in all SCFX trials.

Extrusion and Rheometer System

The viscosities of the SC-CO2/starch-water mixtures were measured with an onlineslit die rheometer attached to a co-rotating twin screw extruder (Wenger TX-57 Magnum,Sabetha, KS). This extruder with a L/D ratio of 28.5 was specially configured for theSCFX process and SC-CO2 was injected into the barrel at L/D of 24. The screw configura-tion, pressure profile and temperature control zones are shown in Fig. 1. The slit die rhe-ometer used in this study had length (L) = 325 mm, width (W) = 50.8 mm, and height(H) = 3.175 mm as illustrated in Fig. 2. This die dimension with (W/H) ratio of 16 and(L/H) ratio of 102 had the ability to produce fully developed flow and to minimize thewall slippage and entrance effects. Two precalibrated pressure transducers were insertedat two ports along the slit die rheometer at a distance of 170 and 300 mm from the dieentrance. Immediately downstream 38 mm from the die entrance, a bypass line with avalve was included to vary the rate of main flow traveling across the slit die rheometer.

A SC-CO2 generation and metering system with a PID control loop was used to sta-bilize and maintain the mass flow to the set point at high operating pressure range between10 and 20 MPa and target temperature. By activating a back-pressure type regulator, thissystem can rapidly and smoothly correct for any changes in the extruder conditions with-out becoming unstable. Hence, the concentration of injected SC-CO2 into the starch-watermixture can be ensured during viscosity measurement.

For each set of extrusion treatments, three to four different flow rates of extrudatewere used by manipulating the bypass valve. Flow rates of the extrudate were measuredby weighing material outflow during a 30 second time interval at steady extrusion condi-tions. Moisture of the extrudates was determined by oven drying method. Simultaneously,corresponding pressure drop data from the pressure transducers were recorded automati-cally every 10 seconds.

Experimental Procedures

Starch melt viscosity was measured for the starch-water mixtures with four differ-ent blends of pregelatinized and native corn starches with and without injected SC-CO2.The temperature of the extruder zone was set respectively at 35, 45, 45, 30, and 25°Cfrom zone 1 to 5 (Fig. 1). The product temperature at the exit was at 55°C. Screw speedwas set at 180 rpm. Dry feed rate was set at 9.7 × 10−3 kg/s. The moisture contents of theextrudate melt were adjusted to 42% by adding water to the barrel at 5.82 × 10−3 kg/s(60% dry basis). Specific mechanical energy (SME) input of the extruder was 240 kJ/kg.SC-CO2 injection was set at 6.79 × 10−5 kg/s (0.7% dry basis) for expanded samples. Atthis injection rate, the SC-CO2 concentration is at 0.45 g SC-CO2/100 g CO2-starch-watermixture.

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866 CHEN AND RIZVI

Experiments started with maximum flow rate of the extrudate while keeping thebypass flow rate to minimum. Next, the flow rate of extrudate was decreased stepwise byadjustment to the bypass valve. Total of 4 flow rates through the die were obtained foreach starch blend formulation tested. The raw data of pressure readings and measuredmass flow rates were collected in triplicates and used to calculate shear stress and shearrate. Data reported were the results of extrusion runs for these 4 starch blends with andwithout SC-CO2 injection.

Extrudate Expansion and Microstructure Analysis

The expansion ratio (ER) was calculated as the cross-section area of the extrudatedivided by the cross-section area of the circular shaping orifice attached at the end of theslit die. From each formulation, extrudate samples were dried at 75°C convection ovenfor 8 hours. Each ER value was an average of 30 measurements. A scanning electron

Figure 1 Typical screw configuration and corresponding pressure profile developed along the extruder barrelduring SCFX processing.

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RHEOLOGY AND EXPANSION OF STARCH BY SUPERCRITICAL FLUID EXTRUSION 867

microscope (Leica 440 SEM) was used for examining the cellular structure of extrudatesand measuring the average cell size. Samples were cut into 5mm-long cylinders andmounted on aluminum stubs with double side conductive carbon tape, and sputter coatedwith palladium gold before mounting in the vacuum chamber of SEM for imaging. Imageprocessing software was used to measure the average cell diameters from the SEM analy-sis. The cell density for each sample was calculated by:

where Nb = number of cells/cm3 of expanded extrudate; ρs = piece density of unexpandedextrudate (g/cm3); ρb = piece density of expanded extrudate (g/cm3); and, Db = averagecell diameter (cm). Polydispersity index (PDI) is a measure of the uniformity of the cellsize distribution and was calculated by:

where is the weight average of cell sizes, and is the number average of cell sizes.

Figure 2 Schematic of the slit die rheometer system attached to extruder outlet.

ND

bs b

b

=−( / )

/

ρ ρπ

1

63(1)

PDID

D

D nn

D nD n

n

w

i i

i

i i

i i

= =

∑∑

∑∑

( )( )

( )( )

2

(2)

Dw Dn

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868 CHEN AND RIZVI

Apparent Viscosity Calculations

The shear stress at the walls of the slit die was determined from the slit height(H) and the pressure drop (ΔP) data across the first and second transducers separated by adistance of ΔL:

The apparent shear rate at the walls was calculated from the measured mass flow rate (kg/s) and density r (kg/m3) of the mixture estimated by their weight fraction contribu-

tion from the density of starch-water mixtures and of CO2 at measured pressure range and55°C using the following equation:

where W is the slit width. The rheological parameters, K’(consistency index) and n(flow behavior index) were calculated from linear regression of the log-log plot ofshear stress versus apparent shear rate using the power law equation to obtain the bestfitting values:

The actual shear rate at the wall was then calculated using n as follows:

Apparent viscosity of the mixture (ηapp) was calculated as a function of the wallshear rate:

where the consistency index, K (Pa-s) was given as:

Viscosity Reduction Factors Calculation

To quantitatively determine the effects of added SC-CO2 and native starch on theviscosity of a molten polymer-like starch-water mixture, a fundamental understanding ofthe plasticization is required. Several research groups have attempted to address the rela-tionship of free volume and plasticization on the molten polymer rheology. The SC-CO2/polymer mixtures are generally regarded as polymer diluted by a solvent, where entangled

( )τw

τwPH

L=

ΔΔ2

(3)

( )&γ w&m

&&

γρapp

m

WH=

62 (4)

τ γw appnK= ( )& (5)

& &γ γw appn

n=

+⎛⎝⎜

⎞⎠⎟

2 1

3(6)

η γapp wnK= −& 1 (7)

K Kn

n

n

=+

⎛⎝⎜

⎞⎠⎟

3

2 1(8)

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RHEOLOGY AND EXPANSION OF STARCH BY SUPERCRITICAL FLUID EXTRUSION 869

polymer chain dynamics prevail and where the polymer contribution to viscosity predom-inates over solvent contribution. Thus, the viscosities of the SC-CO2/polymer mixtures areexpected to be reduced, relative to pure molten polymer, by the dilution of the concentra-tion of entangled polymer chains and by the increase in mixture free volume due to dis-solved SC-CO2. Han and Ma[16,17] reported that the viscosity curves for mixtures ofpolystyrene and polyethylene with fluorocarbon blowing agents can be reduced to a mas-ter curve by applying a shift factor to only the viscosity values, defined as the viscosityreduction factor (VRF):

Their data were taken in the shear-thinning power-law portions of the viscosity curves.For this work, two viscosity reduction factors: VRFc and VRFg to describe for the viscositychange due to SC-CO2 concentration and DG were determined respectively. Hence, theviscosity reduction contributions would facilitate us in better understanding the highpressure rheology behavior of SC-CO2 plasticized starch-water mixtures during SCFXprocess.

RESULTS AND DISCUSSION

Rheological Parameters

Table 1 shows the consistency index (K), and the flow behavior index (n) of starch-water mixtures determined in the shear rate range of 100 to 200 s−1 using Eqs. (5) and (8).The data for all starch-water and starch-water-CO2 mixtures show the consistency index,K, increased as DG increased from 60 to 100% level. In addition, the K values of starch-water mixtures were always higher than their respective SC-CO2 plasticized mixtures.

This trend of K value is well supported by the work of Willet et al.,[18] in which theK values of thermoplastic starch melts were shown to decrease with increasing moisturecontent and addition of other plasticizer-like low molecular weight additives. The flowbehavior index, n, increased with the addition of SC-CO2 but decreased as DG increasedfrom 60 to 100% level. For the starch-water mixtures, n decreases from 0.46 to 0.21, andfor starch-water-CO2 mixtures from 0.71 to 0.31. These results correspond to the works ofSingh and Smith[19] and of Willet et al.,[20] in which the flow behavior index of thermoplastic

VRF =η

η(

(

mixture of molten polymer and blowing agent)

molten polymer)(9)

Table 1 Consistency index and flow behavior index as a function of degree of gelatinization and SC-CO2

addition for starch-water mixtures with 42% water content.

Degree of gelatinization

Consistency index, K(kPa.sn) Flow behavior index, n R2

0 % 0.45% SC-CO2 0 % 0.45% SC-CO2 0 % 0.45% SC-CO2

100% 6383 3347 0.21 0.31 0.93 0.9490% 2409 1809 0.37 0.39 0.80 0.8180% 2191 711 0.37 0.57 0.78 0.9560% 961 250 0.46 0.71 0.82 0.78

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870 CHEN AND RIZVI

wheat starch and pre-extruded corn starch increased with moisture content, temperature,and with the addition of plasticizer such as wheat germ oil.

These results are in line with the relationship between the flow behavior index andthe polymer entanglement density in the mixture, in which a smaller n value reflects anincreased polymer entanglement density.[19,21] In their work, Della Valle et al.[22] observeda decrease in the power law index with increasing amylose content and came to concludethat linear amylose would more likely contribute to entanglements whereas the highlybranched amylopectin would not form much entanglements. In our study, the lowest nvalue was observed for the starch-water mixtures with 100% DG with no CO2 added.Such result would suggest the mixtures with 100% DG to have the highest polymer entan-glement density due to the least dilution effect contributed by either CO2 molecules ornative starch concentration in the mixtures.

Apparent Viscosity of Starch-Water Mixtures

Apparent viscosity of the starch-water mixture was measured with K and n valuesrespective of each starch-water mixture and the corresponding shear rate using Eq. (7).The viscosity versus shear rate relationship for starch-water mixtures only and with 0.45 gSC-CO2 /100 g sample added at 55°C are shown respectively, in Figs. 3 and 4. For thestarch-water only mixtures, the viscosity shows a decrease as DG lowered from 100 to60%. Such viscosity dilution induced by the lowering of DG can be attributed to thehigher water affinity of pregelatinized starch over native starch fraction. At the same watercontent, the starch-water mixture with lower DG or lower concentration of pregelatinizedstarch would take more water away from the native starch, inducing a stronger water

Figure 3 Viscosity of starch-water mixtures with starch of various degrees of gelatinization as a function ofshear rate at 55°C.

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RHEOLOGY AND EXPANSION OF STARCH BY SUPERCRITICAL FLUID EXTRUSION 871

plasticization effect to produce a lower viscosity. Moreover, the viscosity of starch-watermixture in Fig. 4 shows to decrease consistently with addition of 0.45 g SC-CO2/100 gsample, suggesting that the free volume added to starch-water mixtures with SC-CO2 is aneffective mechanism for viscosity reduction.

From the viscosity data measured between 100–200 s−1 in Figs. 3 and 4, the gelatini-zation and concentration dependent viscosity reduction factors, VRFg and VRFc, were cal-culated using Eq (9) and determined for the various combinations of starch blends and SC-CO2 concentration at 55°C. VRF's showed to be slightly dependent on shear rate, yieldingabout 10% more reduction from 100 s−1 to 200 s−1.

Fig. 5 illustrates the VRF results with respect to DG levels and SC-CO2 addition atthe shear rate of 150 s−1. For the starch-water mixtures with 0.45 g SC-CO2 /100 g sampleaddition, the VRFc has a mean value of 0.86, indicating that mixture with 0.45 g SC-CO2/100 g sample added consistently achieves a 14% viscosity reduction. Moreover, the VRFgwithout the effect of SC-CO2 addition yields 17%, 24%, and 46% viscosity reduction for90%, 80%, and 60% DG mixtures, respectively.

In comparison of the two diluents, SC-CO2 appears to be a more effective diluenton a weight basis to reduce melt viscosity. In their rheology evaluation of starch-watermixtures containing native and pregelatinized wheat starch blends, Alavi et al.[23] indi-cated that native starch granules act as an inert diluent by increasing the free volume ofthe starch-water mixtures and decreasing the volume fraction of starch polymers in thestarch-water system. Moreover, the higher water affinity of pregelatinized starchincreased the water induced plasticization effect in the mixtures with higher native starchconcentration. It is important to note that the mixture viscosity would play a key process

Figure 4 Viscosity of starch-water-CO2 mixtures with 0.45% wt CO2 and starch with various degrees of gelati-nization as a function of shear rate at 55°C.

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872 CHEN AND RIZVI

control for the extrudate expansion and microstructure development during SCFXprocessing.

Effect of Viscosity on Extrudate Expansion

Fig. 6 illustrated the expansion ratio (ER) for the starch-water mixtures with respectto their DG level and SC-CO2 injection. With 0.45 g SC-CO2/100 g sample injection, ERof the extrudates increased with DG from 2.7 to 7.1 times the cross sectional area of thecircular shaped die. The extrudates without SC-CO2 injection only produced ER of 1.1 to1.7 times, reflecting the sole contribution effect of die swelling based on the materialproperty of the starch-water mixtures with 60 to 100% DG levels. Based on the ER resultsand expansion observation, the starch-water mixture with the lowest DG of 60% did notpossess sufficient gelatinized gas holding capability and could not produce distinctexpanded cellular morphology. Since all tests with SC-CO2 addition had 0.45 g SC-CO2/100 g sample, it would indicate that considerable amount of CO2 had escaped from theextrudate for the starch-water mixtures with lower viscosity corresponding to the 60% DGsamples. Moreover, the lower viscosity produced by the starch-water mixtures with 60%DG would also more likely promote product collapse due to its weaker structural integrity.

In general, SC-CO2 showed to be a good blowing agent at 0.45 g/100 g sample levelwhere the starch-water mixtures with highest viscosity corresponding to 100% DG sam-ples produced the maximum expansion. The extent of extrudate expansion is likely gov-erned by the gas retention capability which correlates well with the measured apparentviscosity in the DG range examined. As expected, the higher pressure observed in the slitdie rheometer was obtained by more viscous starch-water mixtures with higher DG levels.The formation of a less expanded product by the less viscous mixture was likely due tolarger loss of CO2 gas since the pressure near die may fall below the saturated solubility ofinjected SC-CO2 to prematurely nucleate and more likely to escape from the extrudate

Figure 5 Variation of viscosity reduction factors with respect to SC-CO2 and DG levels for starch-water mix-tures at the shear rate of 150/s.

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RHEOLOGY AND EXPANSION OF STARCH BY SUPERCRITICAL FLUID EXTRUSION 873

surface. Hence, with prudent manipulation of formulation, SC-CO2 injection rate, andother processing conditions, we can achieve the target expansion and desired texture fornovel food product applications using SCFX process.

Effect of Viscosity on Extrudate Microstructure

Table 2 showed the effect of DG and SC-CO2 injection on the expanded microstruc-ture in terms of their average cell size, average cell density, and cell size distribution forthe various starch-water mixtures with 60–100% DG levels. Based on the scanning elec-tron microscopy (SEM) at 32 times magnification, the average cell size decreased from410 to 310 μm as DG increased from 80 to 100% while average cell density increased anorder of magnitude to 1.4 × 105 cells/cm3. These SCFX extrudates demonstrated its uni-formity in cell size distribution with the polydispersity index (PDI) values calculatedbetween 0.81–0.84, much higher compared to the PDI value of 0.30 of typical steampuffed extrudates.[4]

Fig. 7 showed the expanded starch-based extrudate with 100% DG to producemicrostructure with more smaller bubbles and higher cell density. This result was attrib-uted to the lower gas diffusivity found in the more viscous starch-water mixtures corre-sponding to the higher DG level.[24] In addition, cell coalescence and ruptures were more

Figure 6 Expansion of starch-water mixtures with 0.45 wt% SC-CO2 injection using SCFX system with diediameter of 4.50 mm.

Table 2 Microstructure characteristics of starch-water mixtures with 0.45 wt% SC-CO2 injection.

Degree of gelatinization (DG) Average cell size (μm)

Average cell density (cells/cm3)

Polydispersity index (PDI)

100% 310 1.40E + 05 0.8490% 340 6.87E + 04 0.8280% 410 2.93E + 04 0.8160% NA NA NA

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874 CHEN AND RIZVI

frequently observed for the starch-water mixtures with lower DG levels, indicating thatthe lower mixture viscosity generated a weaker structural integrity unable to withstand thedynamics of adjacent growing nuclei. As expected, the smaller pressure drop rateobserved by the starch-water mixtures with lower DG would experience a smaller thermo-dynamic instability to induce fewer nuclei formation. In all, the final expanded cellularmicrostructure would be governed by the viscosity-dependent CO2 diffusivity into thenuclei and the competition between nuclei formation and bubble growth affected by thepressure drop rate experienced at the die. CO2/polymer system with higher pressure droprate and lower diffusion rate would favor more nucleation activity, while CO2/polymersystem with lower pressure drop rate and higher diffusion rate would favor more bubblegrowth activity. These interpretations are similar to those proposed by Goel andBeckman[25] about CO2-induced microcellular formation.

CONCLUSIONS

Apparent viscosity and expansion characteristics of starch-based melts plasticizedwith water and supercritical CO2 were evaluated using a slit die rheometer fitted to a twin-screw extruder for four intermediate moisture starch-water mixtures with varying DG lev-els, achieved by blends of pregelatinized and native common corn starch. Apparent vis-cosity of the starch-water mixtures measured between 100–200 s−1 was found to reduce byan average of 14% with the addition of 0.45 g SC-CO2/100 g sample independent of theDG level in the mixtures. The viscosity reduction factors (VRF) determined based on SC-CO2 and DG levels suggested that the free volume added to starch-water mixtures withSC-CO2 effectively reduced viscosity, and that the higher water affinity of pregelatinizedstarch increased the water induced plasticization effect in the mixtures. Observing fromthe extrudate expansion, the results indicated that the starch-water mixtures would requireDG at or above 80% to achieve good gas-holding rheology to produce nonporous skin anddistinct cellular interior morphology. Moreover, expansion and cellular characterization ofthese starch-based extrudates can be well justified by the viscosity-dependent gas reten-tion capability, CO2 gas diffusivity in the starch-water mixtures, and the pressure drop rateexperienced at the die. Using this knowledge of high pressure CO2-plasticized rheology of

Figure 7 Characteristic SEM of expanded starch-based extrudates with 0.45 wt% SC-CO2 injection using SCFXsystem with die diameter of 4.50mm. SEM were taken at 32× magnification.

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RHEOLOGY AND EXPANSION OF STARCH BY SUPERCRITICAL FLUID EXTRUSION 875

the starch-water mixtures, we may then achieve better controlled expansion and greaterflexibility of innovation by supercritical fluid extrusion (SCFX).

ACKNOWLEDGMENTS

The authors would like to acknowledge the technical assistance support from Esref Dogan, and thepilot scale extruder system provided by Wenger Manufacturing, Inc.

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