© WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001 0038-9056/2001/0202-0090 $17.50+.50/0

90 Starch/Stärke 53 (2001) 90–94

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1 Introduction

Pasting and rheological behaviour of starch dispersionsare dependent on the concentration of starch or the num-ber of starch chains present in the dispersion. Morris [1]reported that the increase in viscosity at higher polysac-charide concentration is due to the overlapping and/or in-terpenetrating of its molecular chains with each other. Thechain overlapping occurs at a certain concentration. Criti-cal concentration is defined as the concentration wherethe plot of viscosity against the concentration shows anabrupt increase in its gradient. In molecular terms thisabrupt transition corresponds to the onset of coil overlap-ping between the polymer chains in the dispersions. Theentanglement starts above this concentration. The onsetof coil overlapping can be determined by the number ofchains present and the volume faction occupied by eachchain. Coil volume is characterized by the intrinsic viscos-ity or limiting viscosity number [η ]. The intrinsic viscosity[η ] is a characteristic of macromolecules that is directlyrelated to their ability to disturb flow and indirectly to thesize and shape of the molecules [2]. The intrinsic viscosi-ty is expressed as:

This value is obtained by measuring specific viscosities atdifferent concentrations at the same shear rate and ex-trapolating the course of specific viscosity to infinite dilu-tion [3]. In dilute solutions the interaction between macro-molecules are negligible [4] and the reduced viscosity ex-pressed as [(η–ηo)/ηoc] becomes independent of concen-

[ ] limηη

=→c c0

sp

tration. McMillan [5] showed that the reduced viscositycan be written in the form of Huggin’s equation

ηsp/c = [η] + k’ [η]2c (1)

where ηsp = (ηr – 1), ηr = (η of solution/η of solvent) and k is the Huggin’s constant. The intrinsic viscosity can alsobe determined by extrapolation to zero concentration using Kramer’s equation [5]

lnηr/c = [η] – k’’ [η]2c (2)

For dilute systems the equation can be shortened by re-taining only the first order term and [η] can be determinedfrom the slope of the plot of c against lnηr. The intrinsicviscosity is generally calculated from the equations usedby Tanglertpaibul and Rao [2] for tomato serum

ηr = 1 + [η]c (3)

ηr = exp[η]c (4)

ηr = 1/(1 – [η]c) (5)

The objective of the present study is to determine the effect of different temperatures and presence of othersolutes (such as sucrose, glucose, sodium chloride etc.)on the onset of coil overlapping of sago starch moleculesand the intrinsic viscosity of the solution. At the sametime, the relationship of the magnitude of intrinsic viscos-ity with starch concentration, solutes concentration andtemperature is also studied.

2 Materials and Methods

Commercial sago (Metroxylon sagu) starch, with an amy-lose content of 27% (w/v) was obtained from a sagostarch factory in Sarawak, Malaysia. Starch at varyingconcentrations ranging from 0.2–4.6 g/100 mL salt (sodi-

Md. Nurul Islam1, Abdul Manan Dos Mohd.2,Mohd. Azemi Bin Mohd.Noor2

1 Department of Food Technology and Rural Industries, BangladeshAgricultural University, Mymensingh, Bangladesh

2 School of Industrial Technology, UniversitiSains Malaysia, Penang,Malaysia

Effect of Temperature and Starch Concentrationon the Intrinsic Viscosity and CriticalConcentration of Sago Starch (Metroxylon sagu)Intrinsic viscosity and critical concentration of sago starch dispersions were studied atdifferent temperatures and presence of solutes (sodium chloride, glucose and su-crose). Ubbelohde capillary viscometer was used to measure relative viscosity. Intrin-sic viscosity decreases with an increase in temperature but the critical concentrationremained fairly constant over the range of temperature studied. Sodium chloride en-hanced the intrinsic viscosity but sugars somehow reduced it. Critical concentration isdefined as the point where the starch molecules start to entangle with each other andabruptly enhance viscosity. Sodium chloride enhanced the molecular entanglementand lowered the critical concentration.

Keywords: Sago starch; Intrinsic viscosity; Critical concentration

Corresponence: Mohd. Azemi Bin Mohd. Noor, School of Indus-trial Technology, Universiti Sains Malaysia, 11800 Penang,Malaysia. Phone: (+) 60-4-6 57 78 88, Fax: (+) 60-4-6 57 36 78.

um chloride) and solutes (glucose and sucrose) solutionwere dispersed. The salt and solutes solution contained0.05–1.0 g sodium chloride and 0.2–2.0 g solutes, respec-tively, per 100 mL distilled water. The starch dispersionswere then heated to gelatinize under constant stirring toavoid settling and agglomeration and then autoclaved at121 °C for 30 min to dissolve and disintegrate the starchgranules completely. The dispersions were then cooled to60 °C and filtered through glass wool using a suctionpump. The filtrate was used to study its flow properties. ACannon Ubbelohde dilution viscometer (Cannon lnst.Co.) was used to measure the flow time at each concen-tration with varying temperature ranging from 60–80 °C.The temperature was established with a constant temper-ature water bath. The time required to flow from one levelindicator to another, known as flow time, was measured.The intrinsic viscosity could be obtained by using threedifferent methods based on the three different equation.

2.1 Determination of intrinsic viscosity

The viscometer was rinsed with solvent and drained forexcess solvent, thereafter, it was placed in a constanttemperature water bath. Exactly 10 mL filtered solventwas transferred to the viscometer. The viscometer washeld for a while to equilibrate temperature, then the liquidlevel in the viscometer was brought above the uppergraduation mark. The liquid was then allowed to flowdown through the capillary and the time required exactlyto pass the meniscus of the lower mark of the viscometerwas recorded. The mean of at minimum three readingswas taken. The procedure was repeated for each variableunder study. The relative viscosity (ηr), was then calculat-ed as follows:

ηr = t / to (6)

where t = efflux time for solution, to = efflux time for thepure solvent.

The intrinsic viscosity was then calculated from the slopeof the plots of the equation of 3, 4, and 5 according to thehypothesis of McMillan [5].

3 Results and Discussion

3.1 Intrinsic viscosity

Equations 3, 4 and 5 were employed to calculate the in-trinsic viscosity of sago starch. Straight line relationshipswith linear regression coefficient of 0.99 were obtained.Determination of the intrinsic viscosity based on slopes ofplots were reported [5] as having higher correlation coef-ficients and lower standard errors than those based on in-tercepts methods. The values of intrinsic viscosities usingthe above approaches varied, but showed similar trends

(Tabs. 1–4). In general the value of the intrinsic viscositycalculated using equation 3 is higher than those obtainedusing equations 4 and 5. This observation holds true forall starches with varying amounts of salts and solutes.The values of intrinsic viscosity gradually decreased withthe increase of temperatures. Sodium chloride enhancedintrinsic viscosity (Tab. 2) as opposed to glucose and su-crose. The result may be explained on the basis that thelong chain molecules do not flow as a whole, but in a seg-ments or in a cluster. The viscosity thus depends on chain

Starch/Stärke 53 (2001) 90–94 Effect of Temperature and Starch Concentration on the Intrinsic Viscosity 91

Tab. 1. Intrinsic viscosity of sago starch as affected bytemperature.

Tempe-rature [°C] [η]1 r1 [η]2 r2 [η]3 r3

60 0.767 0.999 0.563 0.992 0.420 0.972

70 0.763 0.999 0.534 0.992 0.405 0.978

80 0.688 0.999 0.525 0.995 0.380 0.972

Intrinsic viscosity expressed according to Tanglertpaibul and Rao[2]; [η]1 = 1 + [η]c, [η]2 = exp[η]c, [η]3 = 1/(1–[η]c).r = Correlation coefficient.

Tab. 2. Intrinsic viscosity of sago starch solutions as affected by sodium chloride concentration.

Salt (NaCl) [η]1 r1 [η]2 r2 [η]3 r3

[g/100 mLwater]

0.00 0.323 0.999 0.254 0.996 0.202 0.986

0.05 0.388 0.997 0.277 0.995 0.213 0.983

0.10 0.420 0.999 0.311 0.994 0.234 0.979

0.20 0.474 0.999 0.346 0.993 0.254 0.973

0.50 0.480 0.999 0.347 0.994 0.256 0.977

1.00 0.958 0.989 0.557 0.987 0.337 0.957

Intrinsic viscosity expressed according to Tanglertpaibul and Rao[2]; [η]1 = 1 + [η]c, [η]2 = exp[η]c, [η]3 = 1/(1–[η]c).r = Correlation coefficient.

Tab. 3. Intrinsic viscosity of sago starch solutions as affected by glucose concentration.

Glucose [η]1 r1 [η]2 r2 [η]3 r3

[g/100 mLwater]

0.00 0.592 0.999 0.404 0.992 0.283 0.968

0.20 0.398 0.999 0.298 0.995 0.226 0.983

0.50 0.363 0.999 0.276 0.996 0.216 0.985

1.00 0.355 0.999 0.275 0.996 0.213 0.984

1.50 0.332 0.999 0.256 0.996 0.199 0.985

2.00 0.288 0.999 0.233 0.997 0.187 0.989

Intrinsic viscosity expressed according to Tanglertpaibul and Rao[2]; [η]1 = 1 + [η]c, [η]2 = exp[η]c, [η]3 = 1/(1–[η]c).r = Correlation coefficient.

length [6]. When a solution is heated at constant pressureboth factors – thermal energy of the molecules and inter-molecular distance – increase. These combined factorsconsequently reduce its viscosity. Elfak et al. [7] also re-ported that molecular vibration increases with the in-crease in temperature causing thermal instability of thepolymer molecules. Starch molecules thus become insta-ble at higher temperature and their molecular chainsbreak down and consequently the viscosity is reduced. Athigher temperature the hydrogen bonding system instarches or in between starch and water molecules mayhave weakened and/or broken down which may result ina reduction in hydration volume of the molecules whichmay contribute to the reduction in intrinsic viscosity. Therelatively lower viscosity at higher temperature may alsobe due to the tendency of complete release of linearchains surrounding the starch molecules into the solution.

When starch is added to sodium chloride solution, sodiumsalts of starch are formed. Since starch is acidic in nature[8] and is negatively charged, chloride ions are repelledby the starch molecules and sodium ions penetrate intothe starch chains, replacing the H+ ions which migrate tothe water phase. As a result the solution becomes moreacidic. Due to this high acidity extensive expansion andunfolding of starch molecules occurs [9], resulting in high-er hydration volume of the molecules with subsequent in-crease in viscosity.

Sugar alters the properties of the solvent by lowering thedielectric constant [10] and thus decreasing the availabil-ity of water molecules for hydration of the starch polyan-ion. The reduction in dielectric constant would suppressthe dissociation of the starch polyanion and its counterions [11] thus producing a less extended configuration ofstarch molecules and lowering the viscosity. Spies andHoseney [12] reported that sugar may penetrate into theamorphous regions of the starch molecules and form

bridges which may compact the molecules thus loweringthe hydration volume and reduce viscosity. Sugar maydisrupt the organization of starch gel from ordered to dis-ordered structure and as a consequent decreased its vis-cosity [13].

3.2 Critical concentration

Fig. 1 shows that the relative viscosity of sago starch so-lution increases linearly with concentration. The viscosityincreases rapidly above 1.8 g starch per 100 mL solvent(distilled water). The effect of temperature up to this con-centration is negligible. According to Morris [1] this con-centration may be regarded as critical concentration forsago starch. The starch solution is stable at or below thecritical concentration and the viscosity is independent ofconcentration and temperature. Above this critical con-centration the entanglement starts and the viscosityrapidly increases. The gradient of the plot decreased asthe temperature increased implying viscosity reduction at higher temperature. Above the critical concentration in-crease in temperature will disrupt hydrogen bonding andaccordingly the viscosity is decreased. However it wasobserved that starches treated with sodium chloride, glu-cose and sucrose behaved differently (Figs. 2–4). Thecritical concentration increased with increasing amount ofglucose added and decreased as the amount of sodiumchloride increased. Sucrose did not influence the critical

92 Islam et al. Starch/Stärke 53 (2001) 90–94

Tab. 4. Intrinsic viscosity of sago starch solutions as affected by sucrose concentration.

Sucrose [η]1 r1 [η]2 r2 [η]3 r3

[g/100 mLwater]

0.00 2.133 0.999 0.879 0.965 0.403 0.865

0.20 2.071 0.999 0.812 0.967 0.373 0.868

0.50 1.659 0.999 0.770 0.973 0.356 0.895

1.00 1.653 0.999 0.729 0.957 0.355 0.862

1.50 1.576 0.999 0.707 0.975 0.342 0.901

2.00 1.454 0.999 0.690 0.973 0.346 0.894

Intrinsic viscosity expressed according to Tanglertpaibul and Rao[2]; [η]1 = 1 + [η]c, [η]2 = exp[η]c, [η]3 = 1/(1–[η]c).r = Correlation coefficient.

Fig. 1. Relative viscosity versus sago starch concentra-tion.

concentration. Above the critical concentration, sodiumchloride enhanced relative viscosity but the oppositeholds true for glucose and sucrose.

4 Conclusion

The intrinsic viscosity of dilute solutions of sago starch isaffected by the temperature, the presence of sodium chlo-ride and Iow-molecular weight sugars. Sodium chlorideenhances the intrinsic viscosity whereas increase in tem-perature, sucrose and glucose concentration decreasesthe intrinsic viscosity. However, critical concentration issomehow unaffected by temperature but affected by con-tent of sodium chloride and sugars. Critical concentrationdecreased as the amount of sodium chloride increased.Glucose somehow delayed onset of molecular entangle-ment, suggesting the ability of glucose to stabilise the in-trinsic viscosity. Below the critical concentration the intrin-sic viscosity of sago starch remained unaffected by tem-perature and added ingredients.

Acknowledgment

This work was funded by the “Land Custody and Devel-opment Authority (LCDA)” of the Government of Sarawak,Malaysia.

Starch/Stärke 53 (2001) 90–94 Effect of Temperature and Starch Concentration on the Intrinsic Viscosity 93

Fig. 2. Effect of sodium chloride concentration on the crit-ical concentration of sago starch solutions.

Fig. 3. Effect of glucose concentration on the critical con-centration of sago starch solutions.

Fig. 4. Effect of sucrose concentration on critical concen-tration of sago starch solutions.

94 Islam et al. Starch/Stärke 53 (2001) 90–94

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(Received: September 8, 1997)

(Revision received: November 21, 2000)