FORTY-FIFTH CONFERENCE
VISCOSITY INCREASES IN CONCENTRATED SUGAR
SOLUTIONS AND MOLASSES DUE TO DEXTRANS
BY
G . L. GERONIMOS and P. F. GREENFIELD
Department of Chenzical Engineering, Universitj, of Qz~eensland
Summary
The effect of the polymer dextran on the viscosity of sugar and molasses solutions
has been measured and expressions developed to predict the resulting increase in
viscosity. A number of concentrations of sugar and dextran, dextran molecular
weights, temperatures and different molasses were tested. It was found that the relative
effect of dextran on viscosity for all cases could be explained by either of two equations.
Introduction
The presence of dextrans (glucans with a majority of a-(1-+6)-D-glucosidic
linkages) in cane juice has been shown to affect adversely raw sugar processing and
quality. The polymer is produced during the growth of micro-organisms of which the
species Leuconostoc nzesenteroides is most commonly found in cane samples. Dextrans
cause processing problems because of their effect on the viscosity of molasses and
massecuites and sometimes filterability, and quality problems because of their effect on
crystal shape. The effects are widely recognised and with appropriate sampling of the
raw juice, dextran levels can be ascertained early in the processing stage. In recent
years, Fulcher and Inkerman (1976) and Inkerman and Riddell (1977) have shown that
dextran may be removed by using the enzyme, dextranase.
It is difficult to determine just what effect the dextran will have on further
processing because of a lack of quantitative information relating levels of dextran to
the viscosity of sugar solutions, molasses and massecuites. Ruiz et al. (1975) have
determined the viscosity of 60' brix sugar solutions for a limited number of dextran
concentrations. They found a relationship of the following form fitted the data well:
p = ke0.26c
(1)
where p = viscosity, Pa s
c = dextran concentration, gldecilitre
No information is provided concerning the average molecular weight of the dextran.
Atherton (1959) also found an exponential relationship between solution viscosity and
impurity concentration, although no specific reference was made to dextran. Day
(1971) related the viscosity of sugar solutions linearly with dextran concentration over
a very limited range of concentrations.
The viscosities of sugar solutions, molasses and massecuites have been the subject
of considerable research. Whde sugar solutions arc newtonian, molasses and
massecuites have been found to exhibit newtonian, psoudoplastic~thixotropic, or
Bingham plastic behaviour. Smolnik and Delavier (1973) determined that cane sugar
molasses was non-newtonian in nature but that? when the soluble colloids were
removed, the samples reverted to newtonian behaviour. Bhattacharya et al. (1972)
reported increasing pseudoplasticity with increasing degradation. It appears that
dextrans and other polymeric compounds are the main causes of non-newtonian
behaviour in molasses, while in massecuites the problem is complicated by the presence
of sugar crystals (Awang and White, 1976). In this paper, a relationship is reported
which predicts the effect of dextran on thc viscosity of molasses and of sugar solutions.
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FORTY-FIFTH CONFERENCE
lg8
Equipment
The instrument used to measure viscosity was a Contraves Rheomat 15
viscometer. The viscometer operates by shearing a .fluid between two concentric
cylinders, the inner one of which is rotating. A selection of bobs and cups is available to
extend the range of the instrument. The gap between the cup and bob is small so that a
uniform shear field is approximated, and the bobs are designed to minimise end effects.
The torque developed by viscous drag on the inner cylinder for varying cylinder speeds
is read from a calibrated gauge (Middleman, 1968).
Materials
CSR refined sucrose was used to make up the sugar solutions. Molasses samples
were obtained from Qunaba and Racecourse mills in central and north Queensland.
They were tested to ensure that measurable traces of dextran were not present.
Pharmacia dextrans of weight average molecular weights ranging from 104to 2 X 106
were used; these fractions were of relatively narrow distribution. For high molecular
weight fractions, a Sigma product (Cat No. D5884) with a weight average molecular
weight of approximately 2 X 107(viscosity determination) was employed.
Procedure
The moisture content of the different dextrans was determined by drying to
constant weight. This was of the order of 84%.
Sucrose solutions of different concentrations were made up and the various
dextrans dissolved (by immersing in a hot water bath) to yield the required dextran
concentrations on a weightivolume basis. Concentrations of sucrose varied from
100g,'100g water to 350g/100g water covering both the undersaturated and
supersaturated regions. Dextran concentrations varied from 0 ppm to 105 ppm
(solution basis). Although some previous workers have expressed dextran concentration in terms of "ppm on brix" it is the absolute concentration which is of
interest from a rheological viewpoint. Care was taken to ensure that the dextran
dissolved completely.
The sucrose-dextran solution was then poured into the appropriate viscometer
cup and allowed to equilibrate to the water bath temperature. Temperatures ranged
from 30'C to 65'C. Series of shear stress-shear rate data were then obtained. On most
occasions two bob-cup combinations were used to provide data over a wide range of
shear rates. The data were then fitted with a power law expression and values obtained
for the power law index, n, and for the apparent viscosities at the different shear rates.
For molasses a similar procedure was followed except that an amount of water
was evaporated under vacuum and the dextran added to the molasses in the form of a
solution to give the appropriate concentration. This was necessary because of the
difficulty in dissolving the higher molecular weight dextrans in the molasses. In total,
210 different solution-temperature combinations of sucrose were tested along with 62
molasses-temperature combinations excluding replicates. The apparent viscosity at
100 sec-' was used for comparative purposes; a relative viscosity was obtained by
reference to the viscosity of a solution of the same total solids but zero dextran. In this
way comparison with the equivalent sucrose solution may be made and viscosity effects
attributed solely to the polymer.
Effect of Dextrans on Flow Behaviour
The addition of dextrans to sucrose solutions caused mlld pseudoplastic
behaviour to be exhibited, whereas zero levels of dextran gave rise to the expected
newtonian behaviour. A power law expression fitted the data satisfactorily-for all
polymers of welght average molecular welght less than 2 X 106 the exponent, n, lay
between 0.95 and 1.0. A marked lncrease m pseudoplastic behaviour was observed for
dextran of molecular weight 2 X 10' with values of n being in the range 0.67 to 1.0.
There was a concentration dependence of the degree of pseudoplasticity with n tending
to decrease at the higher levels of dextran, t h ~ sphenomenon was temperature
Independent over the range 30 C to 60 C. The non-newtonian effect was more
noticeable when dextran wds added to molasses with the power law ~ndexbeing
apprec~ablyless t h m 1 even for the lower molecular weight dextrans. It is not clear why
1978
FORTY-FIFTH CONFERENCE
121
the dextrans affect the molasses flow behaviour more significantly than that of sucrose
solutions. It is possible that the wider variety of species present in the molasses may
cause polymer association, or more likely. there is interaction between the dextran
molecules and other polymer species present. Certainly, the data suggests as did that of
Smolnik and Delavier (1973) that one of the principal causes of non-newtonian
behaviour in molasses is the presence of high molecular weight polymers.
Effect of Dextrans on Apparent Viscosity
Greenfield and Geronimos (1977) have shown that the theory of dilute polymer
solutions may be applied to the prediction of viscosity increases due to dextrans in
concentrated sugar solutions and molasses. The theory suggests that both the
concentration and the average molecular weight must be taken into account if the
prediction is to be reliable. On the basis of this work the.data were fitted by least
squares regression to a power law expression and a modified exponential expression.
where
of dextran containing solution
viscosity of solution of same total solids but zero dextran
y = weight average degree polymerisation
= (mol. wt. of dextran)/l62
c = concentration of dextran (gidecilitre)
k,, k,, k3, k;, k;,
k; = parameters to be fitted by least squares regression.
Estimates of the parameters and a measure of their reliability are shown in Table
I, while Figs. 1 and 2 show the fit for each of the predicted equations. It should be noted
that the equations were fitted to both sucrose-and molasses data simultaneously. The
data shown in these two figures represent the average values of viscosity for both
molasses and sucrose solutions measured at a variety of conditions. It is apparent that
the generalised fit of the data by the above equations is very satisfactory considering
the wide range of conditions that were used in the various experiments.
To apply the above equations to cane dextrans an estimate must be made of the
typical molecular weight of cane dextran. For the present, a value of 2 X 10' is
assumed (this is discussed further in the next section). A reasonable estimate of the
viscosity increase due to dextran is then given by:
/ l D E X = pTS[l
2.2c1.']
(4)
or ,UDEX= /lTsexp[l. lc"Y]
(5)
/LDEX
= viscosity
,UTS =
+
TABLE I-Prediction
of equation parameters (eqns. 2 and 3) by least squares regression (note:
concentration is expressed i n gldecilitre solution)
----M95% Confidence
Limits
Equation
Parameter
Parameter
Value
Lower
Bound
Upper
Bound
Multiple
Correlation
Coefficient
FORTY-FIFTH CONFERENCE
line
ALL points represent
average of a l l runs at:
that particular dextran
concentrotion
1C
1%
Concentration of Dextran
(wt/voL solution)
Fig. l-Effect
of Dextran Concentration on Viscosity of Sugar Solutions and Molasses (Equation 2
As an example, a concentration of dextran in solution of 1% wi'v would increast
the viscosity to about 300% of the dextran free solution at the same total solid:
concentration. This is of the same order as that predicted by Ruiz et al. (1975) anc
referred to earlier. The main source of error is likely to be in the estimate of the weigh]
average molecular weight of dexiran produced in the cane. A further source oj
variation in the values of the parameters may be due to the different molecular weigh1
distributions found in cane dextrans. This would mean that while the forms of tht
equations would be the same, individual mills would have to estimate their owr
parameters. Evidence suggests that this problem is unlikely to be very seriour
(Covacevich and Richards, 1974).
Effect of Dextran Concentration and Molecular Weight
Both the absolute dextran concentration and its average molecular weight affeci
the increase in viscosity of a degraded cane sugar solution. While concentration i!
readily measured in first expressed juice, an estimate of dextran molecular weight i:
difficult to obtain. The molecular weight of dextran produced during cane degradatior
is considered to be of relatively high order. Bovey (1 959) found values ranging from 51
X 106 to 120 X 106for the weight average molecular weight of dextrans, producec
from cultures of L. mesenteroides. In studies of dextrans produced in sugar solution:
with B-512 L. mesenteroides Jeanes et a1 (1954) measured values of M. around 50 X
106molecular association during the analytical procedure. Along with Tsuchiya (1955
FORTY-FIFTH CONFERENCE
.
Mw=2x107
rn=2x106
A Mw=5x105
Mw=6.9x104
,
Least squares
line
0
L1 points represent
that particular dextran
concentration
1%
Concentration of Dextran
( w t / v o l solution)
Fig. 2-Effect
of Dextran Concentration on Viscosity of Sugar Solutions and Moiasses (Equation 3)
they also showed that the molecular weight distributions of dextrans in degraded cane
lvere either bimodal or trimodal. In conclusion, it can be said that there still remains
zonsiderable uncertainty regarding an average molecular weight for cane dextran: a
value of 20 X 106 appears a reasonable approximation.
Effect of Sucrose Concentration
Table I1 indicates that the concentration of low molecular weight components (in
particular sucrosc) does not markedly affect the relative increase in v~scositydue to
TABLE Il-Effect
Dextran
Molecular
Weight
MW
of varying sucrose concentration on relative viscosity increase due to dextran
Sucrose
Concentration
Brix
Dextran
Concentration
gldecilitre
Relative Increase in
Solution Viscosity
Temperature
Measured
Predicted
(Eqn 2)
FORTY-FIFTH CONFERENCE
Leost Squares
Line
P
%
Fig. %-Effect
.l%
Concentration o f Dextran
1%
fi 2 ~ 1 0 ~ ( w t / v osto h )
of Temperature on Relative Increase in Solution Viscosity Due to Dextran (Sucrosc
Solution 66.7 Brix)
dextran, although some of the charged components in molasses may exert associalivc
effects. The range of sugar concentrations tested was quite wide and although tht
number of molasses samples tested at this stage is relatively small, the agreement is verj
good between the different samples.
It is noticeable that the above equations suggest that lou7 molecular weigh
compounds should have little effect on solution viscosity apart from their contributior
to total solids. This agrees with the conclusions of Atherton (1959) and Awang anc
White (1976).
Effect of Other Polymers
The possibility of dextran interaction with other polymeric species which are
present in cane juice and molasses is stressed. Further work is anticipated on measuring
the effect of dextran on sucrose viscosity in the presence of the various components thal
make up molasses-in particular starch, gums, occulant and some salts. The effect of
flocculant carryover from the clarifier is unknown at this stage but should it promote
association of the dextran at a later processing stage, much larger viscosity increases
than those reported in this paper are likely to be encountered.
Effect of Temperature
Over the experimental temperature range, the data suggest that the relatikr
viscosity increase due to dextran is temperature independent, i.e. the effect ol
temperature on dextran contaminated solutions is no different from that on dextrar
free solutions. (Figs. 3 and 4). It has been shown previously that the absolute viscositj
of sugar solutions is related to the absolute temperature by an equation of the form
y = A.lOBjT
(6,
where A and B are constants.
FORTY-FIFTH CONFERENCE
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Fig. 4-Effect
Least Squares
Line
of Temperature on Relatwe Increase In Molasses V ~ s c o s ~ Due
t y t o Dextran
referto
= 2 x 1 0 6 , 0 , A , Mreferto ~ w = 2 x 1 0 7 )
(O,n,
Dixon (1976) has shown that a similar type of relationship exists for dextran
zontalning sucrose solutions. Because the relative viscosity increase is unaffected by
Lemperature, only the change in the reference viscosity with temperature need be
;onsidered in predicting molasses or sucrose solution viscosities.
Effect of Dextran Degradation
Fulcher and Inkerman (1976) have shown that dextran can be degraded
economically by the addition of the enzyme dextranase. The significance of the above
results is that they suggest that there is an optimum processing strategy for such
removal, which is dependent on the degradation kinetics. Since degradation does not
improve sucrose concentration in any absolute fashion but improves processing
performance and final yield, there is little reason to degrade the polymer beyond the
point where such improvements become marginal.
This can be illustrated by an example. A reduction in M,from 50 X 106to 1 X 106
at the same weight concentration (20 000 ppm) produces a much greater change in
absolute viscosity than does a further 50 fold reduction to a M, of 20 000. This is
slightly simplistic since it does not take into account the degradation kinetics and the
possibility of an absolute reduction in weight concentration of dextran, but it does
suggest that the initial reaction is likely to be of most significance in its effect on
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FORTY-FIFTH CONFERENCE
'i978
solution viscosity. Hence, care should be taken to minimise the dextranase input for a
given incubation time. Inkerman and Riddell (1977) s h o w d this same result with
actual dextranase studies.
Conclusions
The effect of the polymer dextran on the viscosi:y of sugar and molasses solutions
has been measured and an expression developed to predict the fractional increase in
viscosity over a solution of equivalent total solids but zero dextran levels. This viscosity
increase can be represented approximately for a typical case of degraded cane molasses
by either of the following equations
p ~ ~ ~ = / l T s [ 2l . 2 ~ ~ ~ 1
or D E x = /(TS exp[l. lcO9]
In this expression, a value of 20 X 10Vor the weight average molecular weight of
the dextran is assumed. It is recommended that work be carried out to speclfy more
exactly this property of cane dextran.
+
Acknowledgements
The financial assistance of the Bureau of Sugar Experiment Stations and the
Sugar Research Institute is appreciated.
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