Tamarind rheology

8/17/2019 Tamarind rheology

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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).

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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: [email protected], [email protected]

(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

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J. Ahmed et al. / LWT 40 (2007) 225–231226


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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

J. Ahmed et al. / LWT 40 (2007) 225–231 227


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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

J. Ahmed et al. / LWT 40 (2007) 225–231228


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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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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

Documents

Tamarind rheology