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LWT 40 (2007) 225–231
Rheological characteristics of tamarind (Tamarindus indica L.)
juice concentrates
Jasim Ahmed, H.S. Ramaswamy, K.C. Sashidhar
Department of Food Science & Agricultural Chemistry, Macdonald Campus of McGill University, Ste. Anne de Bellevue, PQ, Canada H9X 3V9
Received 24 July 2005; received in revised form 4 November 2005; accepted 14 November 2005
Abstract
The steady-shear and small-amplitude oscillatory rheological properties of tamarind (Tamarindus indica L.) juice concentrate (TJC)
were studied in the temperature range of 10–90 1C using a controlled-stress rheometer. Under steady-shear deformation tests, shear
stress–shear rate data were adequately fitted to the Herschel-Bulkley and Casson model at lower (10–30 1C) and higher (5090 1C)
temperature range, respectively. The Carreau model was applied to describe the shear-thinning behaviour of the concentrate, and the
model parameters estimated empirically showed temperature dependence. Oscillatory shear data of TJC revealed predominating viscous
behaviour (G 004G 0) at lower frequency range while the elastic modulus predominating over the viscous one (G 04G 00) at higher frequency
range. The Cox–Merz rule that relates steady shear and dynamic material functions was tested and not followed by most of the
temperatures. The specific heat of TJC increased with temperature and the glass transition temperature of the product was found to be
70.74 1C.
r 2005 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.
Keywords: Tamarind juice concentrate; Herschel-Bulkley model; Casson model; Carreau model; Apparent viscosity; Complex viscosity; Cox–Merz rule
1. Introduction
Tamarind (Tamarindus indica L.) is a versatile fruit,
which can be used for many purposes. Tamarind pulp has
been used for many medicinal purposes and continues to be
used by many people in Africa, Asia and America
(Gunasena & Hughes, 2000). The pulp is believed to
improve loss of appetite and is used as a gargle for sore
throats, dressing of wounds (Benthal, 1933; Dalziel, 1937)
and is said to aid the restoration of sensation in cases of
paralysis. The unique sweet/sour flavour of the pulp ispopular in cooking and flavouring. Tamarind juice
concentrate (TJC) is a convenient product due to its ease
to dissolve and reconstitute in warm water. The product
can also be stored for long periods due to its moderate
water activity. TJC is manufactured by extracting pulp
with boiling water using the counter current principle,
where dilute extracts are used for extracting fresh batches
of the pulp. Following this process, an extract is obtained
containing 20% soluble solids. The extract is separated
from the pulp by sieving and is concentrated under vacuum
in a forced circulation evaporator. When the soluble solids
reach 68–70%, the pulp is placed in packed in cans or
bottles. The finished product sets like jam on cooling. The
yield of the concentrate is about 75% of the pulp used.
Several medicinal properties are claimed for preparations
containing tamarind pulp, leaves, flowers, bark and root
(Gunasena & Hughes, 2000).Rheological measurements have been considered as an
analytical tool to provide fundamental insights on the
structural organization of food and play an important role
in heat transfer to fluid foods. The rheological properties of
food products were strongly influenced by temperature,
concentration and physical state of dispersion. Therefore, it
is interesting to study the rheological properties of food
products as function of temperature. There are many
publications on flow properties of juice concentrates and
effects of temperature and concentration (Rao, 1977).
ARTICLE IN PRESS
www.elsevier.com/locate/lwt
0023-6438/$30.00 r 2005 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.lwt.2005.11.002
Corresponding author. Tel.: +15146314390; fax: +15143987977.
E-mail addresses: jahmed2k@yahoo.com, jasim.ahmed@mcgill.ca
(J. Ahmed).
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Most of the reported works were based on viscosimetry
data. Rheological characteristics of TJCs were also studied
by flow measurement at shear rate of 10–450 s1 and in the
temperature range of 25–70 1C (Manohar, Ramakrishna, &
Udayasankar, 1991). However, steady-shear viscosimetry
is not ideally suited if one wants to probe the rheological
characteristics of an unperturbed dispersion.Small-amplitude oscillatory shear (SAOS) test readily
affords the measurement of dynamic rheological functions,
without altering the internal network structures of materi-
als tested (Gunasekaran & Ak, 2000) and are far more
reliable than steady measurement (Bistani & Kokini, 1983).
Moreover, linear visco-elastic material functions measured
from oscillatory testing to be related to steady-shear
behaviour (Steffe, 1992) and more reliable relationships
can be established at low frequencies and shear rate. The
knowledge of the linear visco-elastic properties of TJC is of
great importance to obtain information in conditions close
to the unperturbed state, to characterize its microstructure
and, also, to predict its viscous flow behaviour through the
development of suitable nonlinear visco-elastic models.
Nevertheless, not many results have been published in
relation to the linear visco-elastic properties of TJC. Cox
and Merz (1958) found that the complex viscosity (Z*) and
steady-shear viscosity (Z) of polymeric materials is nearly
equal while the frequency (o) and shear rate (g) are equal
ðZ ¼ Zjo¼gÞ. This empirical rule is commonly valid for
simple flexible polymers to estimate the shear-thinning
behaviour (Bird, Armstrong, & Hassager, 1987) but fails
for structured fluids such as suspensions (Alhadithi,
Barnes, & Walters, 1992). Thus, the application of the
Cox–Merz rule to polymeric structured fluids like foodmaterials is questionable. In addition, the rule can apply
only if both steady-shear and dynamic measurements of
complex foods are exhibiting similar structural defects. Pre-
shearing of sample before dynamic measurement could
accomplish the validity of the Cox–Merz rule (Fujiwara,
Masuda, & Takahashi, 1991). The Cox–Merz rule and/or
its modified forms have been tested for many liquid and
semisolid foods with or without yield stress by various
researchers (Bistani & Kokini, 1983; Dus & Kokini, 1990;
Gunasekaran & Ak, 2000; Rao & Cooley, 1992).
Recently, food researchers have emphasized on phase
diagrams of food systems, which considered being systems
of water plasticized natural polymers (Hartel, 2001; Slade
& Levine, 1991). Materials with amorphous or partially
amorphous structure undergo a transition from a glassy
solid state to a rubbery viscous state at a material-specific
temperature range and the transition occurs over a range of
temperatures rather than single point (Roos, Karel, &
Kokini, 1996). Some of the physical properties of the
material change above glass transition temperature (T g). It
has been reported that T g has major impact on food
texture/rheology (Ahmed, Ramaswamy, & Pandey, 2006;
Hartel, 2001). Therefore, the knowledge of T g is essential in
assuring quality, stability and safety of various food
products.
In this paper, we have explored the steady state and
dynamic rheological behaviour of TJC. The objective of
the work was to study the rheological properties of TJC
with function of temperature and the applicability of
Cox–Merz rule to identify the variability of oscillation and
steady-shear measurement of TJC during thermal treat-
ment. The thermal properties of TJCs (specific heat andglass transition temperature) were also studied.
2. Materials and methods
TJC produce of India (Tamicon brand, Frespo Food
products, Jagatpur, India) was procured from a depart-
mental store in Montreal, Canada. Total soluble solids
(TSSs) content was measured using a hand refractrometer
and expressed in 1Brix. The water activity and pH of the
TJC were measured by a water activity meter and pH-
meter, respectively, at 20 1C.
2.1. Rheological measurements
A controlled-stress rheometer (AR 2000, TA Instru-
ments, New Castle, DE) attached with computer control
software (Rheology Advantage Data Analysis Program,
TA, New Castle, DE) was used to study both dynamic
oscillatory and steady-shear rheological behaviour of the
TJC. A 60 mm parallel plate attachment was used with a
gap of 1000 mm. The AR 2000 was supplemented with an
efficient Peltier temperature control system and the sample
temperatures were precisely controlled and monitored. For
each test, a measured volume (approximately 2 ml) of well-
mixed sample was placed on the bottom plate of therheometer. The sample temperature was ramped between
10 and 90 1C with incremental steps of 20 1C. The exposed
sample perimeter was covered with metal trap to minimize
evaporation at higher temperature. Dynamic oscillatory
tests were carried out at a frequency sweep from 0.1 to
10 Hz. The oscillation stress was selected based on linear
part of the visco-elastic range. Each time, a new sample was
used for rheological measurement. All the rheological
measurements were carried out in duplicate. Storage
modulus (G 0, a measure of elastic property), loss modulus
(G 00, a measure of viscous property), complex viscosity (Z*),
and phase angle (tan d, ratio of loss modulus to storage
modulus) were obtained directly from the software
(Rheology Advantage, TA version 2.3).
For steady flow measurements, the rheometer was
programmed for the set temperature and equilibrated for
10 min following which a two-cycle programmed shear
changing from 0.1 to 100 s1 in 5 min and back to 0 s1 in
next 5 min. Rheological parameters (shear stress, apparent
viscosity and shear rate) were obtained from the software.
Various rheological flow models based on shear stress–
shear rate (Newtonian, Bingham, Casson, power law,
Herschel-Bulkley) and apparent viscosity–shear rate
(Cross, Carreau, Sisko, Willamson, Ellis) were tested and
the best fit model was selected on the basis of standard
ARTICLE IN PRESS
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error, which is defined asX
½ðX m X cÞ2=n 20:5=Range 1000, (1)
where X m is the measured value, X c is the calculated value,
n is the number of data points and range is the maximum
value of X m —the minimum value.
2.2. Thermal analysis
For thermal analysis, TJC samples were scanned in a
differential scanning calorimeter (DSC) (TA Q100, TA
Instruments, Newcastle, DE, USA) calibrated with indium
for heat flow and temperature. The DSC was equipped
with a refrigerated cooling system that efficiently mon-
itored temperature down to 90 1C. Nitrogen was used as
purge gas at a flow rate of 50 ml/min. Hermetically sealed
aluminium pans were used to avoid any moisture loss
during the analysis. In the experiment, samples of TJC
were sealed, cooled to 901
C, held for 10 min forequilibration and then subjected to a programmed thermal
scan. The heating rate was set to 5 1C/min over a range of
90 to 150 1C. A four-axis robotic device on the system
was used to automatically load samples and the reference
pans to the DSC chambers. An empty aluminium pan was
used as a reference. All DSC measurements were done in
duplicate. DSC data were analysed with the Universal
Analysis Software (version 3.6C) for thermal analysis,
which was provided with the instrument (TA Instruments,
Newcastle, NJ). Heat capacity data were taken directly
from DSC during thermal scan at selected temperature
(90 to 120 1C).
2.3. Statistical analysis
Statistical analysis was carried out using Microsoft Excel
software. Trends were considered significant while means
of compared sets differed at P o0.05 (student’s t-test).
3. Results and discussion
A TSS of the paste was found to be 711 Brix while the
water activity and pH were 0.66 and 2.1 respectively. The
product contained 58 g carbohydrate, 2.5 g protein and
0.65 g fat per 100 g.
3.1. Flow models
3.1.1. Shear stress–shear rate model
Shear stress–shear rate data of TJC were tested for
various rheological models (Newtonian, Casson, Bingham,
power and Herschel-Bulkley), and it was found that the
Herschel-Bulkley model fitted adequately at lower tem-
perature range (10–30 1C) while Casson model described
well the data at higher temperature (50–90 1C) (Table 1).
The Herschel-Bulkley and Casson model are represented as
t ¼ t0 þ K _gn
, (2)
t0:5 ¼ t0:50c þ K c _g0:5, (3)
where t is the shear stress (Pa), t0 and t0c are the yield stress
(Pa) and Casson yield (Pa0.5), _g is the shear rate (s1), K is
the consistency coefficient (Pa sn), K c is the Casson constant
(Pa s)0.5 and n is the flow behaviour index (dimensionless).
The TJC samples exhibited definite yield stress due to
significant particle–particle interactions and crowding.
Yield stress is an important quality control parameter toprocess industries, particularly for comparing the overall
characteristics of products made on different production
lines (Ahmed, 2004). A true value of the yield stress could
be beneficial for the optimal design of food-processing
systems such as those required during thermal processing
(Steffe, 1992). The yield value obtained from both the
Herschel-Bulkley and the Casson model decreased
(P o0.05) with temperature since the stress level in the
fluid is increased, the structure responsible for the yield
stress is destroyed. The literature on concentrated suspen-
sions and materials which exhibit a yield stress is
voluminous. Manohar et al. (1991) reported TJC sample
(621Brix) did not exhibit any yield stress. This behaviour is
contrary to the present study. The differences in observa-
tion could be due to variation in soluble solids content,
type of instruments used and variation in shear rates.
Both the Herschel-Bulkley and the Casson model
parameters are presented in Table 2. The n values ranged
between 0.43 and 0.78 which indicate shear-thinning nature
of TJC. However, there is no trend for n values with
temperature. The consistency index decreased system-
atically with temperature (except 90 1C for Casson model).
The applicability of Casson model at elevated temperatures
indicated strong gel characteristics of TJC (possibly due to
protein and starch component); however, the stronginteraction among the molecules disrupted at 90 1C.
3.1.2. Apparent viscosity–shear rate model
At the start of a steady-shear flow, visco-elastic material
often exhibits a transient state, which is shear rate
dependent. At small shear rate, the viscosity reached the
steady-state value monotonically. The steady-state viscos-
ity as a function of shear rate at different temperatures is
shown in Fig. 1. At low shear rate, the viscosity was not
varied significantly in the temperature range of 50–90 1C,
which corresponds to the zero-shear rate viscosity (Z0). At
high shear rate, the viscosity decreased with shear rate and
ARTICLE IN PRESS
Table 1
Selection of flow models based on standard errors
Model Standard error at
10 1C
Standard error at
70 1C
Casson 11.58 6.62
Herschel-Bulkley 3.82 9.76
Bingham 29.42 38.14
Newtonian 48.12 151.9
Power 7.45 47.24
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the viscosity measurement became more difficult as the
material was spun out of the plates at high speed and it
represents the infinite-shear rate viscosity (ZN
). A non-
linear model (Carreau model, Eq. (3)) was used to describe
the apparent viscosity of the TJC at any given shear rate:
Z Z/=Z0 Z/ ¼ h1 þ ðK _gÞ2in=2, (4)
where Z is the measured apparent viscosity at a given shear
rate, Z0 is zero-shear viscosity (extrapolated low shear rate
value), ZN
is infinite-shear viscosity (high shear rate value),
K is the consistency coefficient, and n the flow behaviour
index.
The Carreau model parameters obtained from the
software are shown in Table 3. The rheological parameters
Z0 and ZN, K decreased with temperature, whereas flow
behaviour index increased; however, no significant differ-
ences were observed in rheological data (except ZN) attemperature range of 50–90 1C.
3.2. Effect of temperature on rheological characteristics of
TJC
Temperature has significant effect on rheological charac-
teristics of concentrated food products. Effect of temperature
on consistency coefficients obtained from the Herschel-
Bulkley model and apparent viscosity at constant shear rate
(10s1) of TJC are illustrated in Fig. 2. It indicates that an
increase in temperature reduced the shear stress/apparent
viscosity at a constant shear rate. However, an increase inmagnitude of K at 70 1C indicated gel characteristics (sol–gel)
of TJC.
Temperature dependency of consistency coefficient from
shear stress–shear rate and apparent viscosity at constant
shear rate (10 s1) are generally expressed by Arrhenius
relationship(Eqs. (5) and (6)):
K ¼ At expðE t=RT Þ, (5)
Z10 ¼ AZ expðE Z=RT Þ, (6)
where K t and Z10 are the consistency coefficient and
apparent viscosity at 10 s1, respectively, At, AZ are the pre-
exponential constants; E t and E Z are the corresponding
ARTICLE IN PRESS
0.01
0.1
1
10
100
0.1 1 10 100
Shear rate (s-1)
A p p a r e n t v i s c o s i t y ( P a . s
)
Fig. 1. Apparent viscosity–shear rate data of tamarind juice concentrate
at selected temperatures (n: 10 1C,&: 30 1C, B: 50 1C, J: 70 1C and :
90 1C).
Table 2
Rheological parameters of (a) Herschel-Bulkley and (b) Casson models of TJC at selected temperatures
Temperature (1C) Yield stress (Pa) K (Pa sn) Flow index (dimensionless) SE
(a) Herschel-Bulkley model
10 3.88 9.28 0.78 3.82
30 1.46 4.32 0.59 14.78
50 0.69 0.52 0.59 7.234
70 0.53 0.49 0.43 10.20
90 0.91 0.27 0.51 31.68
Temperature (1C) Yield stress (Pa) K (Pa sn) SE
(b) Casson model
10 6.07 2.63 11.58
30 2.32 0.29 33.72
50 0.73 0.06 4.43
70 0.70 0.03 6.62
90 1.79 0.21 14.75
Table 3
Carreau model parameters for tamarind paste
Temperature Z0(Pas)
ZN
(Pa s)
K (s) n
(dimensionless)
SE
10 447.5 3.82 625.2 0.60 5.93
30 275.5 0.26 578.0 0.81 1.78
50 21.96 0.12 34.38 0.84 0.72
70 13.15 0.05 20.43 0.86 0.73
90 18.28 6.07E-6 24.54 0.84 17.32
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activation energy values; R is the universal gas constant
and T is absolute temperature (Ahmed, 2004).
The coefficients of Eqs. (5) and (6) were computed using
the least-square technique. Magnitudes of the energy of
activation relating to consistency coefficient and apparent
viscosity ranged between 35.09 and 33.46 kJ/g mol, respec-
tively, while the corresponding magnitudes of constants
(At, AZ) were 1.76 106 and 1.58 106 Pa s, respectively.
The R2 for both cases were greater than 0.71 while
standard errors were less than 0.92. These values indicated
that both K obtained from Herschel-Bulkley model and
apparent viscosity at specific shear rate follow Arrhenius
model adequately. There was insignificant ðP 40:05Þdifference in activation energies. This difference is due to
a fitting in different models. The obtained activation
energies were higher compared to other pastes (Ahmed,
2004; Manohar et al., 1991). The high E values were
attributed by higher range of temperature studied and the
higher soluble solids content.
3.3. Dynamic shear rheology
A linearity test was performed to find the linear visco-
elastic region for the TJC at frequency range of 1 Hz
and an oscillation stress of 1 Pa was selected for the visco-
elastic measurement. Fig. 3 shows the typical mechanical
spectra describing the visco-elastic behaviour of TJC under
low-amplitude oscillatory deformation tests. The mechan-
ical response observed is characteristic of an entangled
network of disordered polymer coils (Ferry, 1980). At low
frequency, there is sufficient time for substantial chain
disentanglement and rearrangement within the time-
scale of the oscillation period. Hence, the predominant
response of the polymer to the imposed deformation is
dissipative viscous flow of energy (characterized by the
predominance of the loss modulus, G 00, over the storage
modulus, G 0). At higher o, as the oscillation period of the
applied deformation decreases, the time needed for chain
rearrangements exceeds the time-scale of the rate of
deformation; hence, elastic deformation of the entangled
network becomes progressively more significant and,
consequently, the two moduli cross. At such high o, the
system behaves as an elastic solid ðG 04G 00Þ. A crossover
point at approximately 1 Hz indicates gel characteristic of
the product.
3.4. Applicability of Cox–Merz rule
New generation rheometer interfaced with computer
produces dynamic and steady-shears data at selected
frequency and shear rate range precisely. The relationship
between dynamic complex viscosity (Z*) and the shear
viscosity (Z) data at frequency range of 0.1–10 Hz and shear
rate of 0.1–10 s1 were studied for TJC. The Cox–Merz
empirical rule (Eq. (7)) has been found to be applicable forTJC sample at temperature range of 10 1C. Fig. 4 illustrates
the Cox–Merz superposition (Cox and Merz, 1958) of
steady-shear viscosity and complex dynamic viscosity
plotted at equivalent values of shear rate and frequency.
However, the rule does not follow for other temperatures.
Compared with the steady-state viscosity shown in
Fig. 4, the dynamic viscosity was much higher. It is also
interesting to observe that frequency dependence complex
viscosity behaved differently with shear viscosity at similar
range of frequency/shear rate for TJC. Similar observa-
tions for some commercial food samples were earlier
reported by various researchers (Ahmed & Ramaswamy,2006; Bistani & Kokini, 1983). This difference, usually high
oscillation viscosity, is probably due to the interaction
among particles, and complex structural network, which
does not affect during small amplitude oscillation measure-
ment. In steady-state shear flow, the deformation is large
and the structural network ruptures at high shear rate
resulting in low shear viscosity.
Variations of both dynamic complex viscosity (Z*) and
the shear viscosity (Z) data with frequency/shear rate were
represented well (R2X0:964 and SE 0.248) by power-law-type relationship (Eq. (8)):
ðZ
¼ Zjo¼gÞ, (7)
ARTICLE IN PRESS
R
2
= 0.8195
R2 = 0.7061
0.1
1
10
100
0.0025 0.0027 0.0029 0.0031 0.0033 0.0035 0.0037
1/T (K-1)
K (
P a . s
n ) ; η
a t 1 0 s - 1 (
P a . s
)
Fig. 2. Temperature dependency of consistency coefficient obtained
from HB model and apparent viscosity of TJC at 10 s1 (n: consistency
coefficient and&: apparent viscosity).
1
10
100
1000
0.1 10
ω (Hz)
G ' , G ' ' ( P a )
10
100
δ
( ° )
1
Fig. 3. Typical mechanical spectra of tamarind juice concentrate at 50 1C.
J. Ahmed et al. / LWT 40 (2007) 225–231 229
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Fig. 5 also shows a typical DSC scanning curve for TJC.
Changes of TJC heat flow curve at temperature range of
60–90 1C indicated starch/protein gelatinization which also
supported rheological data discussed earlier. At lower
temperature ranges, thermogram exhibited glass transition
temperatures. The glass transition is a region that includes
a step change in heat capacity/heat flow in the thermogram.
A clear glass transition was observed during both warming
on heat flow signal and calculated the three points of glass
transition temperatures (onset, midpoint and end) using
the software attached with the DSC. The midpoint T g of
TJC was found to be 70.74 1C (ASTM, 1995) thatpredicates the shelf-life of the product. The T g for the
same material may differ slightly or significantly, depend-
ing on the definition, the experimental conditions (heating/
cooling rate) and the technique used (Mizuno, Mitsuiki, &
Motoki, 1999).
4. Conclusion
TJCs behaved as a true visco-elastic fluid. The steady-
shear flow pattern was well represented by the Herschel-
Bulkley, Casson and the Carreau models. The flow
activation energy calculated from the Herschel-Bulkley
model and the apparent viscosity at constant shear rate
ranged between 33.5 and 36 kJ/mol. Oscillatory rheology
data at lower frequency were characterized by the
predominance of the viscous modulus over the storage
modulus and a crossover point exhibited at 1 Hz indicated
gel characteristic of the product. The Cox–Merz rule was
not applicable for most of the temperature studied while at
10 1C TJC sample followed the rule adequately. The
product specific heat increased linearly with temperature.
Changes of rheological characteristics of TJC at selected
temperature ranges were duly supported by DSC thermo-
gram. The midpoint glass transition temperature of TJC
was found to be 70.74 1C.
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Steffe, J. F. (1992). Rheological methods in food process engineering.
Michigan, USA: Freeman Press (pp. 9–30).
ARTICLE IN PRESS
0
0.05
-0.05
-0.1
-0.15
-0.2-100 -50 0 50 100 150
Temperature (°C)
H e a t f l o w ( W / g )
-1
-0.5
0
0.5
1
1.5
2
2.5
H e a t c a p a c i t y
( J / ° C )
g
Fig. 5. Thermogram of tamarind juice concentrate (- - -: heat flow and -- -:
heat capacity).
J. Ahmed et al. / LWT 40 (2007) 225–231 231
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