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Food Hydrocolloids 23 (2009) 1570–1578

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Food Hydrocolloids

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Characterization of the conformation and comparison of shear and extensionalproperties of curdlan in DMSO

Hongbin Zhang a,*, Katusyoshi Nishinari b

a Department of Polymer Science and Technology, School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR Chinab Department of Food & Nutrition, Faculty of Human Life Science, Osaka City University, Sumiyoshi, Osaka, 558-5858, Japan

a r t i c l e i n f o

Article history:Received 31 August 2008Accepted 4 November 2008

Keywords:CurdlanDMSOConformationMolecular parameterViscometryRheology

* Corresponding author. Tel.: þ86 21 5474 5005; faE-mail address: hbzhang@sjtu.edu.cn (H. Zhang).

0268-005X/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.foodhyd.2008.11.001

a b s t r a c t

Five samples of curdlan, a kind of bioactivated polysaccharide of 1,3-beta-D-glucan, ranging in molecularweight from 5.1� 104 to 192� 104, were used to determine their conformational parameters andmolecular dimensions in DMSO at 25 �C. Mark–Houwink equation was established as½h� ¼ 4:8� 10�4M0:65

w . On the basis of unperturbed worm-like chain models, the molecular parameterswere estimated as follows: molecular mass per unit contour length ML¼ 694 nm�1, persistence lengthq¼ 5.81 nm, molecular diameter d¼ 0.99 nm, and characteristic ratio CN ¼ 9:58, these suggesting thatthis polysaccharide exists as a relatively extended flexible chain in DMSO. The molecular association andrheological properties of curdlan in DMSO were also investigated. Curdlan in DMSO is capable of formingthree-dimensional weak network involving DMSO above a critical curdlan concentration due to theformation of polymer–DMSO polymer and polymer–polymer hydrogen bonds. The extensional viscositycharacteristics of curdlan in DMSO in semi-dilute regime were determined and compared to the shearproperties of the solutions.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Many polysaccharides have been widely used as industrialmaterials in food, biology and medicine because of existing andcommercial importance as a functional food and a source of newdrugs (Nishinari, 2000; Stephen & Churms, 2006). Curdlan isa neutral, bacterial polysaccharide without branched chains,composed entirely of 1, 3-b-D-glycosidic linkages. It is insoluble inwater, alcohol but soluble in alkaline solution and dimethyl sulfoxide(DMSO). This glucan has been used as a food additive in its uniqueability to form either thermo-reversible or thermo-irreversiblehydrogels depending on heating temperatures alone (Harada,Terasaki, & Harada, 1993; Nishinari & Zhang, 2000, Zhang, Nishinari,Williams, Foster, & Norton, 2002).

In solid state, curdlan may exist in triple helices (Deslandes,Marchessault, & Sarko, 1980). In solutions, however, the conforma-tion of curdlan is strongly related to the solvents. The conformationof curdlan in NaOH solutions varies from a triple helix to a randomcoil as the concentration of NaOH used increases (Nakata,Kawaguchi, Kodama, & Konno, 1998). Nakata et al. (1998) reportedthe relationship between Mw and [h] as ½h� ¼ 7:9� 10�3M0:78

w forcurdlan in a 0.3 M NaOH solution and concluded that curdlan chains

x: þ86 21 5474 1297.

All rights reserved.

were relatively flexible. While curdlan in alkaline solutions has beenstudied extensively, relatively little molecular information is avail-able about curdlan in DMSO, especially its conformation, deforma-tion of chains under flow and rheological properties, though DMSOis commonly used as a solvent for curdlan. DMSO is a highly polarsolvent. Its aprotic nature makes it a proton acceptor, differing fromwater that is both a proton donor and a proton acceptor. For poly-saccharide–DMSO and polysaccharide–water systems, intermolec-ular interactions may be stronger for polymer in DMSO than inwater. This is because DMSO blocks some donor hydrogen-bondingcapacity while water partly locks both acceptor and donor potential(Scott, Heatley, & Wood, 1995) and in DMSO hydroxyl groups mayform strong intra- and intermolecular hydrogen bonds (Bernet &Vasella, 2000). As for the conformation of curdlan in DMSO, the lackof detailed information may be due to the high refractive index ofDMSO so that this solvent may be not good for light scatteringmeasurements. Curdlan might be a flexible disordered coil in DMSOaccording to a previous study by Ogawa, Tsurugi, Watanabe, and Ono(1972). Futatsuyana, Yui, and Ogawa (1999) reported an expressionof ½h� ¼ 1:6� 10�4M0:74

w for curdlan in DMSO, where a¼ 0.74,similar to that in 0.3 M NaOH. Recently by tapping mode atomicforce microscopy (AFM), we observed that curdlan in dilute DMSOsolution adopted flexible single chains, showing a disorderedconformation and that when the concentration of curdlan increased,the chains became more rigid and aggregated to further formnetwork structures (Jin, Zhang, Yin, & Nishinari, 2006).

H. Zhang, K. Nishinari / Food Hydrocolloids 23 (2009) 1570–1578 1571

The deformation of molecular chains affecting the mechanicalproperties of the solutions is characterized generally by rheology.The rheological properties of materials are usually measured inshear flow. Extensional flow, however, is usually regarded asa strong flow as compared with shear flow. For a polymer solutionwhere polymer chains are coiled, extensional flow is strong enoughto extend the polymer chains from a relatively compact confor-mation to a more extended conformation if the deformation rateexceeds the relaxation time of the polymer chain. Therefore, themechanical properties of the polymer solution in an extensionalflow field may be notably different from those in a shear flow field.Recently, quite a few works on extensional flow behavior forpolymer and polysaccharide solutions have been investigated witha Rheometrics RFX fluid analyzer using the opposed-jet device(Al-Assaf, Meadows, Phillips, & Williams, 1996; Clark, 1992; Cooper-White, Fagan, Tirtaatmadja, Lester, & Boger, 2002; Hermansky &Boger, 1995; Meadows, Williams, & Kennedy, 1995; Viebke,Meadows, Kennedy, & Williams, 1998; Yu, Li, & Kawaguchi, 2004). Itseems that RFX fluid analyzer is a reliable means of indexing theextensional properties of low-viscosity fluids though absoluteextensional information is difficult to obtain due to the complexflow achieved in the RFX. This kind of inherent problem may also beencountered when another type of rheometry, capillary breakupextensional rheometry, is adopted (Rodd, Scott, Cooper-White, &McKinley, 2005; Sujatha, Matallah, Banaai, & Webster, 2008). Hereit should be noted that the flow achieved in RFX should be complex,containing shear component, which is unknown and may lead topossible deviation from the really experimental results. Forexample, it was reported that the departure from Trouton ratio of 3was most likely due to a mixed shear and extensional flow(Spiegelberg & McKinley, 1996). Form this viewpoint, the capillarybreakup rheometry may provide a true extensional flow field.However, for low-viscosity non-Newtonian fluids such as dilutepolymer solutions, measurements by capillary breakup rheometrymay also be limited under some conditions such as the effects offluid inertia, which give rise to a complicated filament thinningprocess (Rodd et al., 2005). It is known that the extensional prop-erties of a material cannot be deduced and predicted from shearproperties. In addition to this, the actual flow in polymer or foodprocessing involves extensional flow, and in most cases, the flowintroduced in industrial processing equipments may be mainlyextensional flow (Dontula, Pasquali, Scriven, & Macosko, 1997;Padamanabhan, 1995). Therefore, the investigation of the exten-sional properties of the materials is necessary and important tounderstand not only the deformation of molecular chains but alsotheir full-scale mechanical properties along with shear properties.

In this work, the conformation of curdlan in DMSO and rheo-logical properties of curdlan DMSO solutions were studied bymeans of rheometry and a comparison of the extensional and shearflow behavior of DMSO solutions of curdlan at various concentra-tions in semi-dilute regime was made and discussed.

2. Experimental section

2.1. Materials

Original samples of curdlan were supplied in the form of kibbled,air-blast dried flock with different molecular weights (CUD-1, CUD-2, CUD-3, CUD-4 and CUD-5) from Takeda Chemical Industries Ltd.,Osaka, Japan. The molecular weights of CUD-1, CUD-2, CUD-3, CUD-4 and CUD-5 determined by static light scattering in an alkalinesolution are 192, 101, 50, 21, and 5.1�104, respectively (Zhang et al.,2002). The polydispersity indices ðMw=MnÞ of these samples areabout 1.3. No further purification was carried out for samples.

Curdlan was dried in vacuum at room temperature beforepreparing the DMSO solutions. DMSO solutions of curdlan were

prepared by slowly adding the polymer powders to the solventunder stirring. Two relatively concentrated solutions of 2% and 5%were prepared by standing for two weeks after stirring. A series ofsolutions with different desired concentrations were obtained bya successive dilution of the 2% solution. Solutions were continu-ously stirred for 24 h to homogenize completely.

2.2. Viscosity measurements

Intrinsic viscosities [h] of curdlan were measured in DMSOsolution at 25 �C using an Ubbelohde-type viscometer. The exper-imental temperature was controlled to �0.01 �C. For the presentsamples, the shear rate dependence of [h] was also estimated, andthe effect of shear rate and the kinetic energy correction were foundto be negligible. Huggins and Kraemer equations were used toestimate the intrinsic viscosity [h] by extrapolation to infinitedilution as follows:

hsp=C ¼ ½h� þ k0½h�2C (1)

ðlnhrÞ=C ¼ ½h� þ k00½h�2C (2)

where k0 and k00 are constants for a given polymer at giventemperature in a given solvent; hr (relative viscosity), the ratio ofsolution viscosity (h) to that of the solvent (hs); hsp=c, the reducedviscosity; ðlnhrÞ=c, inherent viscosity. Extrapolation to infinitedilution was done by a combined Huggins (Eq. (1)) and Kraemer(Eq. (2)) method, from which the Huggins’ and Kraemer’sconstants, k0 and k00 , were calculated.

2.3. Rheological measurements

Shear viscosity and dynamic viscoelastic measurements werecarried out using a strain-controlled Fluids Spectrometer RFSII(Rheometrics Co. Ltd., NJ, USA) with cone-and-plate geometry witha small angle cone of 0.04 rad and diameter of 50 mm, and a gap of50 mm between the cone and plate. Flow curves and frequencydependence of storage shear modulus G0 and loss shear modulus G00

of DMSO solutions of curdlan at different concentrations weremeasured.

Extensional viscosity measurements were made with a Rheo-metrics RFX fluid analyzer (Rheometrics Co. Ltd., NJ, USA) using theopposed-jet with five pairs of 5, 4, 3, 2 and 1 mm in diameter. Theextensional viscosities were calculated by

hE¼ s=_3 (3)

where the extensional stress s ¼ 4F=pD2 (F the force at the jet, Dthe diameter of the jet, and pD2=4 the cross-sectional area of thejet), and strain rate _3 ¼ 8Q=pGD2 (Q the volumetric flow rate intoone jet and G the gap between jets).

The Reynolds number was calculated by

Re ¼ r _3GD=4hE (4)

where r is the density of the fluid. This expression of Reynoldsnumber is the same as that used by Dontula et al. (1997) and Tan,Tam, Tirtaatmadja, Jenkins, and Bassett (2000) while twice valueswill be obtained if the expression by Meadows et al. (1995) is used.

The Trouton ratio is taken as a quantified parameter character-izing the difference between shear and extensional behavior offluids. The deviation of Trouton ratio from the Newtonian value of 3may reflect the viscoelasticity of the fluid. The Trouton ratio in thepresent work is that of measured apparent extensional viscosity toshear viscosity, denoted by

Trouton ratio ¼ h�_3�=h�

_g�: (5)

0.0 0.1 0.2 0.3 0.4 0.50

2

4

6

8

sp/C

(d

l g

-1)

C (g dl-1

)

CUD-1CUD-2CUD-3CUD-4CUD-5

Fig. 1. Plot of reduced viscosity, hsp=c, vs. curdlan concentration, c, in DMSO at 25 �C.

Table 1The intrinsic viscosity [h], Huggins’ constant k0 and Kraemer’s constant k00 forcurdlan in DMSO at 25 �C.

Sample Mw� 10�4 [h]/dl g�1 k0 k00 k0 þ k00

CUD-1 192 6.57 0.24 0.21 0.45CUD-2 101 3.80 0.31 0.18 0.49CUD-3 50 2.67 0.31 0.18 0.49CUD-4 21 1.36 0.23 0.23 0.46CUD-5 5.1 0.60 0.22 0.24 0.46

H. Zhang, K. Nishinari / Food Hydrocolloids 23 (2009) 1570–15781572

Two types of flow are available to measure the extensionalviscosity, one being uniaxial extensional flow, another uniaxialcompressive flow (biaxial extensional flow). The extensionalviscosities obtained by either the uniaxial and biaxial extensionalflow are the same for Newtonian fluids (Fuller, Cathey, Hubbard, &Zebrowski, 1987). All the measurements were performed at25� 0.5 �C. It has been noted that measurements by using 0.5-mmjets are very sensitive to alignment of the nozzles (Dontula et al.,1997). Thus, the smallest pair of jets of 0.5 mm in diameter is notused in the present work. When measurements are made at higherstrain rates on some viscous samples, air leaks and cavitation mayoccur in the syringes and joints. Air leaks is easy to overcome justby confirmation of suitably fixing the connectors and the syringes,but cavitation has to be checked only by choosing a suitable upperlimit of strain rate.

Pure water and pure glycerol were chosen as Newtonian fluids inthe present work to check the reliability of the RFX and the experi-mental technique. While shear viscosity of water can be measured byRFX, no believable data of extensional viscosity can be obtained dueto its low viscosity (w0.01poise, i.e., 1 mPA s). The value of Troutonratio for a Newtonian fluid should be 3. But in the present work itexceeds 3, and decreases as the ratio of gap to the jet inside diameterG/D increases. Glycerol is frequently used as a model sample ofNewtonian fluid but it has been found that the pure glycerol is easilyhygroscopic. A very small amount of water will lower its viscositystrongly. When pure glycerol is adopted, this fact should be takeninto account. In the present work, we found the Trouton ratio forpure glycerol is 4.5. Previously, values of 3.6 (Meadows et al., 1995)and 5.5 (Dontula et al.,1997) were reported. A choice of gap¼ jet sizeis recommended and used in the present work.

It is known that the steady extensional viscosity is not availablefor experimental measurement. The measured extensional data inthe present work by RFX are essentially regarded as values ofapparent extensional viscosities. The flow in the opposed-jetsconfigurations may be partially extensional because of the presenceof shear and the lack of uniformity of the deformation.

3. Results and discussion

3.1. Intrinsic viscosity measurements

It was found that according to the dependence of the intrinsicviscosity [h] of curdlan in DMSO on the storage time, the [h]maintained a constant value during storage for a few days, sug-gesting that curdlan may not undergo degradation or association inDMSO during experiments. Good linearity of reduced viscosity,hsp=c, against curdlan concentration, c, in DMSO solution wasobtained as seen in Fig. 1, indicating that curdlan was sufficientlysolubilized in DMSO. Based on the plots of reduced viscosity, hsp=c,and relative viscosity, ðlnhrÞ=c vs. curdlan concentration at 25 �C,the intrinsic viscosity, [h], and Huggins’ and Kraemer’s constants, k0

and k00 , are summed in Table 1, in which the experimental dataindicate that DMSO is a good solvent for curdlan.

It is known that in the Mark–Houwink equation, ½h� ¼ kMaw, the

parameters a and K are both related to the stiffness of the polymer:a¼ 0 for hard spheres, a¼ 1 for semicoils, a¼ 2 for rigid rods, anda usually lies in the range 0.5–0.8 for a flexible coil; K is dependentpredominantly on the geometry of the interresidue linkages withinthe polymer chain (Lapasin & Pricl, 1999; Sperling, 1992). Fig. 2illustrated the double-logarithmic plot of [h] against Mw for curdlanin DMSO at 25 �C, in which the relation of log[h] vs. logMw showeda good linearity in the whole range of molecular weight. The Mark–Houwink equation for curdlan in DMSO was represented by½h� ¼ 4:8� 10�4M0:65

w . This exponent value of 0.65 for curdlan inDMSO, almost the same as that for curdlan in cadoxen (Hirano,Einaga, & Fujita, 1979), suggests that curdlan takes up a flexible

disordered coil in DMSO as does in cadoxen. Futatsuyana et al.(1999) reported an expression of ½h� ¼ 1:6� 10�4M0:74

w for curdlanin DMSO at 25 �C, where a¼ 0.74, similar to that in 0.3 N NaOHð½h� ¼ 7:9� 10�3M0:78

w Þ (Nakata et al., 1998). The differencebetween the two exponent values does not appear to be significantand might originate from the dissimilarity in the sample measured.

3.2. Conformational characteristics

Based on above viscometry analysis, the worm-like cylindermodel can be used for conformational characteristic of curdlan. Thismodel is typically used for semi-flexible polymers including poly-saccharides and DNA. Molecular parameters of the polysaccharides,such as gellan gum (Takahashi et al., 2004), schizophyllan (Yanaki,Norisuye, & Fujita, 1980), and xanthan gum (Sato, Norisuye, & Fujita,1984) have been successfully evaluated by using this method.Bohdanecky (1983) and Bushin, Tsvetkov, and Lysenko (1981)independently showed that Yamakawa–Fujii–Yoshizaki (YFY)theory (Yamakawa & Yoshizaki, 1980) for [h] of unperturbed worm-like cylinder can be replaced approximately by

�M2

w=½h��1=3

¼ Ah þ BhM1=2w (6)

with

Ah ¼ F�1=30;N A0ML

�g1=3 cm�1

�(7)

and

Bh ¼ F�1=30;N B0ð2q=MLÞ�1=2

�g1=3 cm�1

�(8)

where q and ML are the persistence length and the molecular massper unit contour length, respectively. A0 and B0 are given as func-tions of the reduced hydrodynamic diameter dr but B0 is nearlyconstant, equal to 1.05. F0;N is independent of dr and equal to2.86�1023. According to Eq. (6), Ah and Bh can be evaluated fromthe intercept and slope of the plot of ðM2

w=½h�1=3Þ vs. M1=2

w , respec-tively. To determine the molecular parameters, q and ML, anotherrelation is necessary in addition to Eqs. (7) and (8). According to

5 61.5

1.8

2.1

2.4

2.7

3.0y=0.65x-1.14

r=0.998

lo

g[ ]

logMw

Fig. 2. Log–log plot of intrinsic viscosity, [h], and molecular weight in DMSO solutionat 25 �C.

102

H. Zhang, K. Nishinari / Food Hydrocolloids 23 (2009) 1570–1578 1573

Bohdanecky (1983), the hydrodynamic volume occupied by 1 g ofthe worm-like cylinder is equal to the partial specific volume, v, ofthe polymer molecules, so that

v ¼ ðpNA=4Þ�d2=ML

�(9)

where NA is the Avogadro’s constant. Combining Eqs. (7)–(9), thereduced hydrodynamic diameter dr ¼ d=2q can be expressed as

d2r =A0 ¼

�4F0;N=1:215pNA

��v=Ah

�B4

h (10)

Thus, having determined ðv=AhÞB4h , we can evaluate dr via the

dependence of d2r =A0 on dr, which can be described by the

approximate empirical formula:

log�

d2r =A0

�¼ 0:173þ 2:158 logdr ðdr � 0:1Þ (11)

Here, v was assumed to be a reciprocal of the density 1.50 g/cm3,calculated from X-ray data for hydrated curdlan (Nakata et al.,1998).

Fig. 3 showed the plot according to Eq. (6) for the present data.According to the solid line fitted to the data points, we obtainedAh¼ 104.18 and Bh¼ 1.23. And from Eqs. (10) and (11), dr and A0

were calculated to be 0.085 and 0.99, respectively.Thus, we determined the molecular parameters of curdlan in

DMSO as follows: ML¼ 694 nm�1, q¼ 5.81 nm, and the moleculardiameter: d¼ dr� 2q¼ 0.99 nm. Here, the values of ML and q areslightly bigger than those of common random coil and less thanthose of common stiff chain. Generally speaking, as the values of ML

and q increase, the macromolecules become stiffer, more extended,indicating a stretching semi-flexible chain.

Furthermore, the characteristic ratio, CN which represents howmuch the chain is extended by steric hindrance, can also be

600 9000 300 1200 1500

300

600

900

1200

1500

1800 (Mw

2/[ ])

1/3=104.18+1.23M

w

1/2

r=0.998

(M

w

2/[ ])1/3 (g

2/3 cm

-1 m

ol-1/3)

Mw

1/2 (g

1/2 mol

-1/2)

Fig. 3. The plot of ðM2w=½h�

1=3Þ against M1=2w for the curdlan in DMSO at 25 �C.

calculated from this worm-like model description. The CN isdefined as follows (Zhang et al., 2001):

CN ¼ M0=�

lMLb2�

(12)

where M0 is the average molar mass of a glucose residue in repeatunit. l�1 is the Kuhn’s segment length (l�1¼2q) and the virtualbond length b¼ 0.485 nm, so combining M0, together withM0¼162, l�1¼11.62 nm, ML¼ 694 nm�1, CN of curdlan in DMSOwas calculated to be 9.58. This value is much higher than that ofpullulan ðCN ¼ 4:3Þ known as a random coil (Kato, Okamoto,Tokuya, & Takahashi, 1982) but a little lower than that of curdlan in3 M NaOH aqueous solution ðCN ¼ 10:5Þ (Nakata et al., 1998),suggesting that curdlan chain in DMSO is more extended in itsconformation than pullulan but a little more flexible than that in3 M NaOH aqueous solution.

Thus the values of the resulting conformational parametersðML ¼ 694 nm�1; q ¼ 5:81 nm; d ¼ 0:99 nm;CN ¼ 9:58Þ indi-cate that curdlan exists as a flexible chain having relatively lowflexibility in DMSO, somewhat similar to conformation of anotherwater-insoluble (1 / 3)-a-D-glucans isolated from the Poria Cocosmycelia in a 0.25 M LiCl/DMSO solution (Jin et al., 2004). The abovetheoretical calculation is in agreement with our previous AFMobservations for curdlan in dilute DMSO solution, in which curdlanshows an image of flexible single chains (Jin et al., 2006).

3.3. Shear properties

Fig. 4 shows the shear rate dependence of the steady shearviscosity of DMSO solutions of curdlan with a molecular weight of192�104 at various concentrations. DMSO solutions of curdlanshow a strong pseudoplastic character. The steady shear viscosityincreases and the shear-thinning behavior intensifies with curdlanconcentration. This shear flow behavior suggests that curdlan chainin DMSO may not be so flexible as a random coil, consistent withthe results of viscometry mentioned above and our previousobservation by AFM (Jin et al., 2006). The conformation of curdlanmolecules may be semi-flexible, somewhat similar to that of xan-than in water. It is found that the mechanical disruption of inter-molecular interactions, and the deformation and orientation ofcoils are sensitive to shear for 0.44% and 0.29% solutions and noNewtonian region could be observed at accessible shear rates. Theresults in Fig. 4 indicate that the solutions above 0.22% behave likean entangled polymer solution.

The frequency dependence of storage shear modulus G0 and lossshear modulus G00 of 5%, 2%, 0.44% 0.29% and 0.22% solutions isshown in Fig. 5. At a concentration of 0.22%, G0 is just comparable to

10-2

10-1

100

101

DMSO

C=0.05%

0.11%

0.15%

0.22%

0.29%

0.44%

/P

oise

/s-1·

10-2 10-1 100 101 103102

Fig. 4. Steady shear viscosities of DMSO solutions of curdlan (Mw¼ 192�104) atdifferent concentrations.

O

OH

O SCH3

CH3

OOO

HO

CH3

CH3

O SSO

CH3CH3

C < C*

OOH

OH

O

SCH3 CH3

OOH

O

H

Bridging association Self-association

C > C*

Fig. 6. Proposed association of curdlan in DMSO at a concentration of below or abovethe critical concentration C*.

10-2 10-1 100 101 10210-1

100

101

102

103

5%G' G''

2%G' G''

0.44%G' G''

0.29%G' G''

0.22%G' G''

G',G

''/P

a

/rad.s-1

Fig. 5. Mechanic spectra of DMSO Solutions of curdlan at different concentrations.

H. Zhang, K. Nishinari / Food Hydrocolloids 23 (2009) 1570–15781574

G00 in the higher frequency range. Moduli of solutions below thisconcentration could not be detected due to transducer sensitivity.The fact that G0 predominates over G00 indicates a gel-like behavior.It strongly implies that curdlan in DMSO above 0.22% is not onlyentangled but also mostly likely associated and that subsequentlythis may result in the formation of weak network in the solutiondue to strong interactions between curdlan molecules. It has beenreported that curdlan dissolves in NaOH aqueous solutions as lowas 0.01 M (Nakata et al., 1998) and both moduli of G0 and G00 increasewith decreasing NaOH concentration (Tada, Matsumoto, & Masuda,1997). However, the magnitude of G0 and G00 for curdlan in DMSO, atcomparable concentrations and molecular weight of curdlan usedin the study, is much higher than that for curdlan in 0.01 M NaOHsolutions at 30 �C (Tada et al., 1997), comparable to that at 60 �C(Tada et al., 1997), where gelation of curdlan takes place. Thisindicates that in such cases polymer–polymer interactions arestronger in DMSO than in NaOH aqueous solutions. It has been alsoreported that curdlan forms a gel by dialyzing or by adding waterinto the curdlan–DMSO system (Futatsuyana et al., 1999; Kanzawa,Harada, Koreeda, & Harada, 1987). Such gelling phenomena are notsurprising since changing the solvent quality from good to poorusually induces aggregation of the polymer, furthermore, leads tothe gelation. DMSO is a good solvent for curdlan. However, asconcentration of curdlan increases, aggregation of curdlan may alsobe expected because this polysaccharide contains a large amount ofhydroxyl groups that may be associated by hydrogen bonds. Inaddition to the hydrogen bonding between hydroxyl groups,hydroxyl groups may be bonded through DMSO. It is known thatOH groups are able to be bonded to DMSO molecules by hydrogenbonding. Based on quantum chemical calculation, Varnali (1996)reported that DMSO can form a more highly stable DMSO/DMSO/H2O complex over the H2O/DMSO/H2O complex, andWendt, Meiler, Weinhold, and Farrar (1998) proposed that in dilutesolutions MeOH/DMSO is favoured over the MeOH/DMSO/MeOH complex. These imply that in dilute solutions, one DMSOmolecule interacts with one OH group, whereas above a criticalconcentration of solute, DMSO prefers to be bonded to two OHgroups. For curdlan–DMSO systems, OH groups come only fromcurdlan chains. It is likely that, apart from self-association of cur-dlan, the bridging-association of intra- and intermacromoleculesinvolving DMSO, as well as the entanglement of curdlan molecules,renders the formation of the three-dimensional network. Theseresults are consistent with our previous AFM observations, in whichdense three-dimensional network structures of curdlan were foundfor curdlan DMSO solutions with high concentration (Jin et al.,2006). The critical concentration of curdlan with a molecularweight of 192�104 is about 0.2% in the present work. In addition, itis found that in the present work the network is shear-reversible,

and that heat treatment can disrupt the network structure. Thisindicates that the interactions between curdlan molecules are notpermanent and the formed network is not a true gel. The rheologicalcharacteristics of network of curdlan in DMSO may be similar to thatof xanthan in water (Frangou, Morris, Rees, Richardson, & Ross-Murphy, 1982; Richardson & Ross-Murphy, 1987; Ross-Murphy,Morris, & Morris,1983; Southwick, Lee, Jamieson, & Blackwell,1980).A summary of association of curdlan in DMSO below and above thecritical concentration C* is given in Fig. 6.

3.4. Extensional properties

Fig. 7 shows the extensional viscosity hE of solutions of 0.11%,0.15%, 0.22%, 0.29% and 0.44% as a function of strain rate as well asthe Reynolds number. For the 0.44% solution with strong interac-tions between polymers, hE decreases initially and levels off, andthen increases with strain rate. The first drop in hE, strain-thinning,is due to the mechanical disruption of intermolecular association/entanglement as shown in steady shear flow (Fig. 4). The strainthickening may be attributed to stretching and orientation of thecoils, extended coils resulting in a large drag. The plateau regioncorresponds to a balance of combined effects of dissociation/disentanglement and the stretching of the coils. The Reynoldsnumber is the ratio of the inertial and viscous forces of a fluid underflow. Since the Re is quite small (<0.3) in such a case, the increase inhE is not considered to be due to the inertial effect. Similar exten-sional characteristics can be seen for a less associated and entan-gled solution of 0.29% or 0.22%. But the degree of strain thickeningis higher than that of the solution of 0.44%, and the onset of strainthickening takes place at a higher extension rate. At concentrationsabove 0.22%, dissociation and disentanglement occurs in the overallstrain rate range. In the strain-thickening region, dissociation anddisentanglement for the 0.44% solution have more contribution todecrease in viscosity than that for 0.29% and 0.22% solutions;thereby the degree of the apparent strain thickening for 0.44%solution is lower. For non-entangled solutions of 0.11% and 0.15%,no strain-thinning is observed and the strain-thickening behaviormay mainly reflect the material properties other than the fluidmechanic effects. In all the cases, the results obtained by usingdifferent sizes of jets coincide well with each other even though theRe of a few data exceeds unity, indicating that the inertial effect maybe ignored completelyat Re< 5 in the present work. It is worth notingthat it is necessary to try different extension rates by varying the jetsize and look for overlapping data in the extensional measurements.

C=0.15%

C=0.11%

10-1

100

101

102

10-1

100

101

102

10-1

100

101

102

10-1

100

101

102

10-1 100 101 102 103 104 10-2 10-1 100 101

10-1 100 101 104103102 10-2 10-1 100 101

10-1 100 101 102 103 10410-1

100

101

102

10-1

100

101

102

10-1

100

101

102

10-1

100

101

102

10-1

100

101

102

10-1

100

101

102

C=0.44%

Jet Size5mm4mm3mm

2mm1mm1mme

(s-1

)

(s-1

)

(s-1

)

(s-1

)

(s-1

)

10-1 100 101 102 103 104

10-1 100 101 102 103 104

10-5 10-4 10-3 10-2 10-1 100 101

10-2 10-1 100 101

10-2 10-1 100 101

Re

C=0.29%

Re

C=0.22%

(P

oise)

(P

oise)

(P

oise)

(P

oise)

(P

oise)

(P

oise)

(P

oise)

(P

oise)

(P

oise)

(P

oise)

Re

Re

Re

Fig. 7. Apparent extensional viscosities of various curdlan DMSO solutions vs. strain rate and Reynolds number. Jets of different sizes were used. The solid circle symbols correspondto uniaxial compression while the others to uniaxial extension (Mw ¼ 192 � 104).

H. Zhang, K. Nishinari / Food Hydrocolloids 23 (2009) 1570–1578 1575

H. Zhang, K. Nishinari / Food Hydrocolloids 23 (2009) 1570–15781576

An interesting phenomenon seen in Fig. 7 is that at concentra-tions above 0.2% the compressive viscosity is much lower thanextensional viscosity, whereas for Newtonian fluids they should beidentical. The compressive viscosity in Fig. 7 first remains constantto some strain rate and then increases as the strain rate increasesbut it is not measurable at lower strain rates. This phenomenon isattributed to the difference in flow history and deformation model.Under the condition of extensional flow, the sample is brought intothe stagnation point from a region of no flow. In the case ofcompressive flow, the sample is first subjected to shear flow in thejets, in which shear-thinning has taken place due to shear-inducedmolecular dissociation, disentanglement and orientation as seen inshear viscosity measurements in Fig. 4. The plateau region widensand the onset of strain-thickening is delayed significantly in thecompressive flow. For non-entangled solutions of 0.11% and 0.15%,however, the compressive viscosity is comparable to the

0

20

40

60

80

100C=0.11%

extensioncompression

C=0.15%extensioncompression

Tro

uto

n ratio

Tro

uto

n ratio

0

20

40

60

80

100

10-1 100 101 102 103

(s-1

)

(s-1

)

10-1 100 101 102 103

C=0.44%extensioncompression

Tro

uto

n ratio

0

20

40

60

80

100

(

10-1 100

Fig. 8. The Trouton ratio as a function of strain rate for DMSO solutions of curdlan at vario(Mw ¼ 192 � 104).

extensional viscosity initially but becomes much higher thanextensional viscosity at higher strain rates. At such concentrations,the effect of flow history of shear may be ignored. The Reynoldsnumber is about unity. It seems that uniaxial compressive flow ismore efficient than uniaxial extensional flow to stretch the coils innon-entangled solutions.

While extensional viscosity decreases with decreasing concen-tration, there is no marked difference in the magnitude ofcompressive viscosity for all the solutions in the strain-thickeningregion. In particular, almost identical relation between compressiveviscosity and strain rate was observed for solutions of 0.11% and0.15%.

Fig. 8 shows the Trouton ratio as a function of strain rate forcurdlan DMSO solutions at various concentrations under eitheruniaxial extensional flow or uniaxial compressive flow. It is notsurprising to see a decrease in Trouton ratio at lower strain rates for

C=0.22%extensioncompression

C=0.29%extensioncompression

Tro

uto

n ratio

Tro

uto

n ratio

0

20

40

60

80

100

0

20

40

60

80

100

10-1 100 101 102 103

(s-1

)

(s-1

)

10-1 100 101 102 103

s-1

)

101 102 103

us concentrations under either uniaxial extensional flow or uniaxial compressive flow

H. Zhang, K. Nishinari / Food Hydrocolloids 23 (2009) 1570–1578 1577

the 0.44% solution. The initial marked shear-thinning and strain-thinning behavior has already been observed in Fig. 4 and Fig. 7. Thedecrease in Trouton ratio reflects that the mechanical disruption ofintermolecular association and entanglement may be induced byeither shear or extension, and extension may be more efficient(Fig. 7), resulting in the difference between the shear and exten-sional behavior of the solution. At mid-strain rates, Trouton ratioremains a constant of ca. 2, which cannot be attributed toapproximate Newtonian behavior but a balance state in whichdissociation and disentanglement results in a decrease in viscositywhile coil stretching leads to an increase in viscosity as describedabove. In higher strain rate regime, Trouton ratio climbs with strainrate, clearly showing the difference in the deformation of coilsunder the extensional and shear flow fields. Similar behavior can beseen for 0.22% and 0.29% solutions though Trouton ratio is notavailable at lower strain rates. For 0.11% and 0.15% solutions, nodisentanglement would be expected, the increase in Trouton ratiobeing directly attributed to stretching and orientation of coils underextension while only orientation of coils without significantdeformation under shear. Curdlan in DMSO shows a similar rheo-logical behavior to xanthan in water, but it seems that curdlan coilsare more flexible, readily stretching under extension, than xanthancoils (Clark, 1992; Fuller et al., 1987).

4. Concluding remarks

The dependence of intrinsic viscosity [h] on molecular mass Mw

for curdlan in DMSO at 25 �C was found to be ½h� ¼ 4:8� 10�4M0:65w

in the range of Mw from 5.1�104 to 192�104. Analysis of experi-mental data in terms of the theories for worm-like chain gave theconformational parameters of curdlan in DMSO to be 694 nm�1 forML, 5.81 nm for q, 0.99 nm for d and 9.58 for CN, indicating thatcurdlan at low concentration exists as a relatively extended flexiblechain in DMSO. At high concentrations, the association of intra- andintercurdlan macromolecules involving DMSO, as well as theentanglement of curdlan chains, results in the formation of theaggregations and the three-dimensional network.

For curdlan of molecular weight 192�104, a critical concen-tration of about 0.2% is found. DMSO solutions of curdlan showa shear-thinning behavior in shear flow, whereas they exhibitstrain-thinning following strain-thickening at concentrationsabove 0.2% but only strain-thickening below 0.2%. Below the criticalconcentration, curdlan chains are non-entangled while above it thechains are both entangled and associated.

Acknowledgements

H Zhang thanks for the support from Shanghai LeadingAcademic Discipline Project (No. B202).

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