studies on shrikhand rheology

www.elsevier.com/locate/jfoodeng

Journal of Food Engineering 74 (2006) 169–177

Studies on shrikhand rheology

Chandrashekhar Kulkarni, Nilesh Belsare, Ashish Lele *

Polymer Science and Engineering Division, National Chemical Laboratory, Pune 411 008, India

Received 2 July 2004; accepted 12 February 2005

Available online 22 April 2005

Abstract

Shrikhand, a popular Indian dessert made from yogurt, is manufactured on an industrial scale using several chemical engineering

unit operations such as mixing, filtration and heat transfer. Understanding the rheology of shrikhand is not only relevant for design-

ing large scale mixers but can also provide quantitative means for linking the microstructure of shrikhand to its perception of

texture, consistency and taste. We show here that shrikhand exhibits a combination of several rheological properties such as weak

gel-like viscoelasticity, an apparent yield stress, thixotropy and long structural recovery time scales. For instance, the elastic modulus

is always higher than the loss modulus over the measurable frequency range and that both moduli show only weak frequency depen-

dence that is a characteristic of gel-like consistency. Forward and reverse rate sweep tests show a distinct hysteresis loop, which is a

signature of thixotropic character. In an attempt to trace the origins of these rheological properties in shrikhand we characterized its

microstructure and showed that there exist two different microstructures: one composed of crystallites of milk fats having a length

scale of �50–100 lm, and the other composed of aggregates of colloidal casein micelles of �0.5–10 lm size. Our studies show that

while the temperature sensitivity of the viscoelastic parameters is dominated by the semicrystalline milk fat microstructure, the shear

sensitivity is largely dictated by the protein network.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Shrikhand; Rheology; Microstructure

1. Introduction

Shrikhand is a popular Indian dessert prepared by

the fermentation of buffalo milk. It has semi-soft consis-

tency and has a sweet and sour taste. Typically shrik-

hand constitutes 39.0% moisture and 61.0% of total

solids of which 10.0% is fat, 11.5% proteins 78.0% car-

bohydrates and 0.5% ash, on a dry matter basis. It has

a pH of about 4.2–4.4 (Boghra & Mathur, 2000).

The method of manufacture of shrikhand depends onits scale of production. The traditional method is still

predominantly used for small-scale production. In this

method boiled milk after cooling is fermented using a

culture to make yogurt, which is then filtered using a

0260-8774/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2005.02.029

* Corresponding author. Tel.: +91 20 5893300; fax: +91 20 5893041.

E-mail address: [email protected] (A. Lele).

muslin cloth to remove whey. The thickened mass

known as chakka is pressed over a strainer to get asmooth product which when mixed with sugar gives

shrikhand.

On an industrial scale shrikhand is prepared by using

different mechanical devices (Aneja, Vyas, Nanda,

& Thareja, 1977). In this process, pasteurized milk or

skim milk is inoculated with the culture. The yogurt thus

prepared is centrifuged in a continuous quarg separator

to produce chakka, which is then mixed with cream,ground sugar and flavours in a scraped surface heat ex-

changer for manufacture and pasteurization of the prod-

uct. Post-processing heat treatment (70 �C for 5 min) for

the shrikhand is found to yield a product with superior

overall quality and increased shelf life (Prajapati, Upad-

hyay, & Desai, 1992).

The composition of shrikhand and the processing

conditions determine its microstructure, which in turn

mailto:[email protected]

170 C. Kulkarni et al. / Journal of Food Engineering 74 (2006) 169–177

decides the texture and consistency of the product.

Rheology has been widely used to qualify the texture

of food products since the flow properties of complex

fluids can be correlated to their microstructure. It is

hoped that industrial process parameters for manufac-

turing better and consistent products can be optimizedand controlled using measurable rheological properties.

Although the rheology of shrikhand has not been re-

ported so far, the flow behaviour of its precursor yogurt

has been widely studied and reviewed. Several resear-

chers have reported the pseudoplastic behaviour, thixot-

ropy and viscoelastic characteristics of yogurt. The weak

gel-like viscoelasticity of yogurt was reported by Afonso

and Maia (1999) and Skriver (1995). Various phenome-nological models have been used to fit rheological data.

For example the Herschel–Bulkley model was used by

de Kee, Code, and Turcotte (1983), Benezech and Main-

gonnat (1993), Rohm (1992, 1993), Ramaswamy and

Basak (1991a, 1991b) and Rohm and Schmid (1993).

The Casson Model was used by Skriver, Roemer, and

Qvist (1993), the power law model was utilized by

O�Donnell and Butler (2002, Parts I & II), and the Welt-mann model was used by Weltmann (1943) and O�Don-

nell and Butler (2002a, 2002b). van Marle, van den

Ende, de Kruif, and Mellema (1999) used a microrhe-

ological model to explain effect of shear on the viscosity

of stirred yogurt. The kinetics of structural breakdown

due to shear was explained using a structural kinetic

model by Abu-Jadyil (2003).

The effects of processing parameters such as shearrate, time of mixing (O�Donnell & Butler, 2002, Parts I

& II), and temperature (Afonso, Hes, Maia, & Melo,

2003) have also been studied. Abu-Jadyil and Moham-

med (2002) studied the influence of storage time on the

flow properties of concentrated yogurt. The effect of

change of process variables at various stages during pro-

cessing such as heating, stirring and cooling on the phys-

ical and rheological properties and hence on the productperformance has been reported by several researchers

(Arshad, Paulsson, & Dejmek, 1993; Benezech & Main-

gonnat, 1994; Bouzar, Cerning, & Desmazeaud, 1997;

Haque, Richardson, & Morris, 2001; Lucey, 2001;

Lucey, Teo, Murno, & Singh, 1997; Parnell-Clunies,

Kakuda, de Man, & Caazzola, 1988; Remeuf, Moham-

med, Sodini, & Tissier, 2003).

The microstructure of yogurt has been characterizedusing techniques like scanning electron microscopy

(SEM) (Modler & Kalab, 1983), transmission electron

microscopy (Barrantes, Tamime, Sword, Muir,

& Kalab, 1996), confocal scanning laser microscopy

(CSLM) (Lucey, Munro, & Singh, 1999) and optical

microscopy (OM) (Skriver, 1995). The formation of

the microstructure and its firmness as affected by the

type of culture, extent of syneresis, and thermal treat-ment has been studied by Lucey et al. (1999), Lucey

(2001), Haque et al. (2001). Puvanenthiran, Williams,

and Augustin (2002) used an Instron Universal Testing

machine for determining the strength and texture of

yogurt.

The rheological behavior of shrikhand is expected to

be different from that of yogurt for several reasons: first,

the water content (whey) of shrikhand (�30 wt%) isconsiderably less than that in yogurt, second, the fat

content in shrikhand (�20 wt%) is significantly higher

than that in yogurt, and third, the quantity of sugar

in shrikhand (�50 wt% of chakka) is substantially

higher than that in yogurt. The relationships between

the microstructure, rheology, processing parameters

and properties of shrikhand have not been studied so

far. In this paper, we report on the effects of processingparameters such as temperature and shear rate on the

rheological properties of commercial shrikhand samples

and attempt to understand them from microstructural

viewpoint.

2. Experimental

2.1. Materials

Samples of commercially manufactured shrikhand

were purchased as 250 g packs in plastic containers.

The samples were stored between 0 and 5 �C throughout

the period required for experimental work and were

used as such for all rheological experiments. Based on

the observations of Upadhyay, Dave, and Sannabhadti(1985), who reported that shrikhand deteriorates within

40–50 days at 7 ± 2 �C and that its pH changes by 7%

within 50 days, we carried out all tests within 20 days

after the date of packing.

2.2. Chemical characterization

Shrikhand samples were characterized for their fat,protein and moisture contents. Defatting was carried

out by subjecting the samples to Soxhlet extraction

using hexane at 69 �C for 7 h. The defatted sample

and the extracted fat were weighed to determine the

fat content. To determine the moisture content, the sam-

ples were vacuum dried at 60 �C for 6 h and the moisture

content was calculated from the difference in weight of

the samples before and after drying. Nitrogen contentof the samples was determined using elemental analysis

and the protein content was estimated using the formula

(Reddy, Henderson, & Erdman, 1976).

Total protein content ¼ % total nitrogen� 6.25 ð1Þ

The above chemical characterization tests were carried

out for a minimum of three samples and the average re-sults showed fat, moisture and protein contents to be

22%, 36% and 7%, respectively, which agreed with the

numbers reported by Boghra and Mathur (2000).

1

10

100

1000

10000

0 1 10 100

Vis

cosi

ty (

Pa.s

)

Batch No.1

Batch No.2

Batch No.3

T=10oC

Shear rate (s-1)

Fig. 1. Batch-to-batch variations in the steady shear viscosity of

shrikhand. Experiments were performed at 10 �C.

C. Kulkarni et al. / Journal of Food Engineering 74 (2006) 169–177 171

2.3. Rheological experiments

Two different rheometers viz., a strain-controlled

rheometer (ARES, Rheometric Scientific) and a stress-

controlled rheometer (CVO-50, Bohlin Instruments

Ltd.) were used to carry out rheological experimentson the shrikhand samples. Temperature control on

ARES was achieved using a forced convection oven,

while a recirculating water bath was used with the Boh-

lin rheometer. For samples tested using the ARES

rheometer a thin coat of low viscosity silicon oil was ap-

plied to the free surface of the sample to prevent mois-

ture evaporation. Gap setting was performed slowly so

as to avoid high shear rates during sample loading andthe same protocol was followed every time.

Dynamic strain sweep and frequency sweep experi-

ments were performed over the temperature range of

5–60 �C using the ARES rheometer. The tests were car-

ried out on 25 mm disposable aluminum parallel plates

with a gap of 1.0 mm between plates. Steady shear, shear

start-up and thixotropic loop test were also performed on

the ARES rheometer using cone & plate geometry with25 mm plate, 0.1 rad cone. A 40 mm plate and 0.4 rad

cone was used to generate the data of creep and yield

stress ramp on the stress controlled (Bohlin) rheometer.

2.4. Microstructural analysis

Thermal characterization of shrikhand was performed

using differential scanning calorimetry (DSC-7, Perkin-Elmer). Samples were heated in crimped aluminum dis-

posable pans from 5 �C to 60 �C at 10 �C/min followed

by cooling down to 5 �C at 10 �C/min and reheating to

60 �C in the second heat. Baseline scans were carefully

recorded for each sample. Cross-polarized optical

microscopy (Olympus Model BX-50) was used to visual-

ize the effect of temperature on the microstructure. A hot

stage (Mettler, Model Toledo FP90) was used for theisothermal measurements. The microstructure at a smal-

ler length scale was observed using scanning electron

microscopy (Leica, Model S-440). Samples were pre-

pared as per the protocol suggested by Kalab (1981).

Shrikhand was first defatted by Soxhlet extraction using

hexane as the solvent. The defatted shrikhand was fixed

with glutaraldehyde for 4 h followed by gradual removal

of moisture from the sample by using a graded acetoneseries (50:50, 75:25, 95:5 and 100:0 volume ratios of dried

acetone and water). The dry sample thus obtained was

fractured, mounted on an SEM stub and sputtered with

a thin film of gold before observing under the SEM.

3. Results and discussion

Steady shear experiments were performed to measure

typical batch-to-batch variations of the rheological

properties of shrikhand. Fig. 1 shows that as much as

70–80% variation in the viscosity of shrikhand was ob-

served between three different batches, the manufactur-

ing dates of which spanned twelve weeks. While this

suggests that rheology can be used as a sensitive toolto quantify some of the attributes of shrikhand such

as its texture and consistency, it also highlights the diffi-

culty in reporting quantitative rheological data. The

large batch-to-batch variations in rheological properties

could arise due to several factors (Afonso & Maia,

1999). These include among others, variations in the

milk constituents, protein functionality, processing

parameters and storage history. In particular, the roleof processing parameters such as shear and temperature,

and the effect of long structural relaxation times (e.g.,

during storage) on the rheology of shrikhand are

highlighted in the following discussions. The various

rheological data presented below also show the highly

non-Newtonian characteristics and viscoelastic charac-

teristics of shrikhand.

3.1. Effects of temperature on rheology and

microstructure

The temperature dependence of viscoelastic parame-

ters was measured by performing oscillatory small

amplitude frequency sweep experiments. The linear vis-

coelastic regime was first delineated from the non-linear

regime using dynamic strain sweep tests, which were per-formed at different temperatures in the range 5–60 �Cover a strain amplitude range of 0.1–50% at 1 rad/s fre-

quency. Linear viscoelastic response was observed below

0.1% strain and the strain at incipient non-linear

behaviour was found to increase with temperature.

Subsequently, the dynamic frequency sweep tests were

carried out at strain amplitudes of less than 0.1% at

different temperatures in the range 5–60 �C. The datafor storage modulus (G 0) and loss modulus (G00) at var-

ious temperatures is shown in Fig. 2A and B. For all

temperatures used in this study G 0 was found to be

greater than the G00 over the measured frequency range

10

100

1000

10000

0 1 10 100

G' (

Pa)

5C 10C 20C30C 40C 50C60C

(A)

10

100

1000

0 1 10 100

G"

(Pa)

5C 10C 20C 30C40C 50C 60C

(B) Frequency (rad/sec)

Frequency (rad/sec)

Fig. 2. Dynamic oscillatory frequency sweep data at various temper-

atures: (A) elastic (storage) modulus, (B) loss modulus. Experiments

were done at 0.1% strain amplitude.

0

1000

2000

3000

4000

5000

0 10 20 30 40 50 60

G' (

Pa)

I

II

III

70

Temperature (oC)

Fig. 3. Dynamic oscillatory temperature sweep experiment at 0.1%

strain amplitude and 1 rad/s frequency.

5 10 15 20 25 30 35 40 45

Hea

t Flo

w E

ndo

Up

(mW

)

Temperature (oC)

Fig. 4. The first heat scan of shrikhand in a differential scanning

calorimetry experiment. The Y-axis scale is arbitrarily shifted.


of 0.1–100 rad/s, and both were weakly dependent on

the frequency. Both G 0 and G00 showed a distinct de-

crease in magnitudes between 30 and 40 �C. Below

30 �C, G00 showed a minimum at intermediate frequen-

cies while G 0 was nearly independent of frequency.

Above 30 �C, the frequency dependence of G 0 increased

while G00 did not show a distinct minimum.The principle of time–temperature superposition in

general, failed to yield a mastercurve for the viscoelastic

data shown in Fig. 2. However, it was found that the

data for 40�, 50� and 60 �C could be shifted reasonably

well to yield a master-curve, while the data for temper-

atures below 30 �C could not be shifted to any master-

curve. A temperature sweep experiment was performed

to quantify the temperature dependence of G 0 and G00.As seen in Fig. 3, G 0 decreased significantly with increase

in temperature from 10 to 40 �C, but showed only a

moderate decrease after 40 �C, thus indicating a possible

microstructural transition below 40 �C.Differential scanning calorimetry (DSC) experiments

were carried out to determine the nature of the transi-

tion. The first heat scan shown in Fig. 4 suggested a

broad melting transition in the range of 10–20 �C fol-lowed by a smaller one at 37 �C. The enthalpy changes

associated with both transitions were very small. Base-

line corrections were meticulously employed to ensure

the existence of the transitions. We believe that the ther-

mal transition may be associated with the melting of

globules of homogenized milk fats, which constituted

approximately 22% of our shrikhand samples. It is well

known that milk fat has a broad melting range between

�40 �C and 40 �C (Boudreau & Arul, 1993) but is often

rigid enough to exhibit self-standing property at room

temperature. Milk lipids contain more than 400 fatty

acids, ranging from 4 to 28 carbon numbers (Jensen,

Ferris, & Lammi-Keefe, 1991) that are bound to a gly-cerol backbone to form triglycerides. Lipids contain

low molecular weight fractions, which melt at intermedi-

ate and lower temperatures, and high molecular weight

fractions that have higher melting temperatures. van

Aken and Visser (2000) showed that low melting triglyc-

erides (LMT)-a polymorph remain liquid above approx-

imately 10 �C, the medium melting triglycerides (MMT)

melt around 14–21 �C, and the high melting triglycerides(HMT)-b 0 polymorph starts melting above 21 �C. Themelting transitions shown in Fig. 4 are likely to corre-

spond to the melting of MMT and HMT fractions.

Direct microstructural evidence for the melting of fat

globules in shrikhand was obtained by heating the sam-

ple in a hot stage and observing its structure under a

cross-polarized optical microscope. At room tempera-

ture fine birefringent structures were seen uniformly dis-tributed in the entire sample as seen in Fig. 5A. More

prominent birefringent structures can be seen in Fig.

5A around an entrapped air bubble. We believe that

birefringence is caused by crystallized fat globules that

are uniformly dispersed in the shrikhand during manu-

Fig. 5. Cross-polarized optical micrographs of shrikhand samples taken at (A) ambient temperature (�23 �C), (B) 40 �C, and (C) 2 min (D) 30 min,

(E) 6 h, and (F) 12 h after cooling down to ambient temperature.


facturing. This is supported by the fact that shrikhand

sample after Soxhlet extraction showed no such birefrin-

gent structures, while the extracted fat at room temper-

ature showed a high degree of birefringence. Intensivemixing often causes small amount of air entrapment.

The air bubbles provide a hydrophobic surface for the

fat to accumulate, which can be seen as the strongly

birefringent structures in Fig. 5A. It may be noted that

microscopic observations at various locations show that

there are only a few air bubbles in the shrikhand sam-

ples. On heating the sample, the birefringent structure

was seen to completely disappear at 40 �C. Fig. 5Bshows that apart from the slightly birefringent interface

of the entrapped bubble, the majority of the birefringent

structure has disappeared at 40 �C.We propose that homogenization of shrikhand dur-

ing its manufacture can result in the formation of par-tially coalesced tiny fat droplets, which crystallize at

the lower storage temperature. Given the high fat con-

tent in shrikhand (�22% in our samples), it is possible

that the crystallized globules could form a volume ex-

cluded sample-spanning network. On the other hand it

might also be possible that protein molecules that are

adsorbed on the fat globules could link up to form a net-

work with the crystallized fat globules acting as effectivecross-links. The presence of a network is clearly


suggested by the weak frequency independence of G 0

and the occurrence of a distinct minimum in G00 below

30 �C as seen in Fig. 2. Melting of fat above 40 �C causes

disruption of this network and consequently the fre-

quency dependence of G 0 increases while the G00 does

not show a distinct minimum. Similarly, the rheologicaltransition seen in Fig. 3 can be linked to the melting of

the milk fat in shrikhand.

On cooling down to 25 �C from above the melting

point of fat, the fine birefringent structures were ob-

served to crystallize out from the sample. Recovery of

the structure after 2 min, 30 min, 6 h and 12 h is shown

in Fig. 5C–F. Recovery of the birefringent structure was

perhaps not complete even after 12 h. This may be be-cause of the fact that, while the HMT fractions can crys-

tallize faster, the recrystallization of the MMT and

LMT fat fractions is expected to be much slower. It

has been reported that MMT and HMT-b 0 polymorph

co-crystallizes in about 20 min at temperature less than

20 �C and once such crystals have formed, the redistri-

bution of triglycerides in the partially solidified fat is

strongly impeded so that recrystallization into more sta-ble crystal forms and compositions proceeds extremely

slowly (van Aken & Visser, 2000). Dynamic oscillatory

time sweep experiments were used to track the kinetics

of recovery of the structure formed by the recrystalliza-

tion of fat. The changes in G 0 were monitored as a func-

tion of time to indicate the structural recovery. The

experiments were carried out at 10 �C using a strain

amplitude of 0.1% and a frequency of 1.0 rad/s. Afterrunning an initial time sweep experiment at 10 �C, thesample was heated to 40 �C, held for 2 min and cooled

down to 10 �C, whereupon a second time sweep experi-

ment was performed. Fig. 6 shows the elastic modulus at

10 �C before and after heating the sample to 40 �C. Themodulus was constant before heating the sample, but

dropped by an order of magnitude after the heating

cycle. The recovery of G 0 after annealing was observedto occur in two-steps: a fast initial recovery within

100

1000

10000

1 10 100 1000 10000

Recovery after annealing

Initial scan

G' (

Pa)

Time (s)

Fig. 6. Recovery of the elastic modulus of shrikhand measured at

10 �C after annealing the sample at 40 �C for 2 min followed by

cooling down to 10 �C.

approximately 8 min followed by a slow and partial

recovery process.

3.2. Effect of shear on the rheology of shrikhand

Steady shear experiments were performed on a stress-controlled rheometer and the data is shown in Fig. 7.

The data at 5 �C showed a distinct region at intermedi-

ate shear rates in which the stress was nearly constant

(�140 Pa) and consequently the viscosity of the sample

dropped significantly indicating an apparent yield-like

transition. Interestingly, steady shear experiments on

the strain-controlled rheometer over the same range of

shear rates showed a non-monotonic region of flowcurve implying multiplicity of shear rates as can be seen

in Fig. 7. For example at a value of shear stress between

105 and 120 Pa (at 5 �C), there could be more than one

value of steady shear rates achieved in a strain-con-

trolled steady shear experiment. The multiplicity of

shear rates and the apparent yield-like transition

strongly suggests a flow instability that might be caused

by the breaking down of a microstructure. Such a struc-ture must be different than that formed by the fat

network because similar shear sensitivity was also ob-

served at 40 �C (>m.p. of fat) although the value of

the plateau stresses (�13 Pa) was much lower. It is pos-

sible that a high shear might melt some fraction of the

fat. An adiabatic temperature rise due to steady shear

flow can be estimated using

DT ¼ shz _chzDtqCp

ð2Þ

where, shz is the shear stress, _chz is the shear rate, q is the

density, Cp is the specific heat of shrikhand and Dt is

the time duration of the imposed shear. Assuming that

the sample was sheared at 25 s�1 shear rate for 1000 s

at 5 �C and using the corresponding value of the shear

stress from Fig. 7 at this shear rate, Eq. (2) givesDT � 3 �C. Given the broad melting range of fat and

1

10

100

1000

1.E-06 1.E-04

5 deg, stress-control5 deg, rate-control40 deg, stress-control40 deg, rate-control

~ 140 Pa

~ 13 Pa

Shear rate(s-1)

1.E-02 1.E+00 1.E+02 1.E+04

She

ar s

tres

s (P

a)

Fig. 7. Comparison of steady shear data generated using stress-

controlled and strain-controlled rheometers for the same sample at

5 �C and 40 �C.


small heat of melting, such a small temperature rise

might partly melt the LMT and MMT fractions of the

fat. The actual temperature rise in the thickness of

the sample may be expected to be much lower

than the above estimate since boundaries of the sample,

i.e., the parallel plate geometry, were maintained at aconstant temperature throughout the test. We may

therefore expect that the large shear sensitivity exhibited

by the sample cannot be due to the melting of the fat

microstructure.

It is possible that there exists another level of micro-

structure that is shear sensitive and is different than the

temperature sensitive crystalline fat structure. The shear

sensitivity of the structure was measured in several otherexperiments. Fig. 8a shows that the viscosity measured

in steady shear mode was found to be lower than the

complex viscosity measured in the dynamic oscillatory

mode when compared at similar values of shear rates

and frequencies, suggesting invalidity of the empirical

Cox–Merz rule. It is evident that the structure probed

in the steady shear experiment was different than that

probed in the dynamic experiment. Specifically, thelower steady shear viscosity suggests that the structure

might have broken down by the imposed steady shear,

while the small strain conditions of the dynamic experi-

ment preserved the structure. Fig. 8b shows results of a

shear start-up experiment at 10 �C when a shear rate of

25 s�1 was suddenly imposed. A large overshoot in stress

0.1

10

1000

100000

0.1 1 10 100 1000 100

Com

plex

vis

cosi

ty (

ηη*, P

as),

Vis

cosi

ty (

η, P

as)

(a)

0.1

1

10

100

0.01 0.1 1 10 100 1000 10000

Vis

cosi

ty (

Pa.

s)

1st run

2nd run

10oC

(b)

Frequency (ω, (ω, rad/sec), Shearrate (γ, s-1)

Time (s)

Dynamic

Steady shear

Fig. 8. Shear sensitivity of shrikhand measured using different tests: (a) comp

after inception of steady shear of 25 s�1; (c) thixotropy loops in which the sam

shear rate ramps; (d) recovery of the elastic modulus after the application o

growth followed by a slow approach to steady state can

be seen. Stress overshoot is an indication of non-linear

viscoelastic behaviour and could have implications dur-

ing the design of motors for commercial scale mechani-

cal mixers such as those used in the manufacture of

shrikhand. A second shear start up test after a rest per-iod of 5 min showed a significantly smaller stress

overshoot and rapid evolution to steady viscosity indi-

cating that the structure had not recovered 5 min after

the first run. Fig. 8c shows the results of thixotropy test

in which the sample was subjected to two consecutive

loops of increasing and decreasing shear rates at

40 �C. The results showed a large hysteresis in the first

thixotropic loop, while in the second loop the shearstress values in the increasing rate ramp matched with

the values in the decreasing rate ramp. This suggests that

the structure must have broken down in the initial

increasing rate ramp and does not recover rapidly giving

rise to a large hysteresis in the first loop. The structure

remains broken down so that the second consecutive

thixotropic loop does not show hysteresis. The fact that

hysteresis was observed at 40 �C (>m.p. of fat) furthersupports the existence of another hierarchy of micro-

structure. Fig. 8d shows time evolution of the elastic

modulus at 10 �C measured in a time sweep test before

and after shearing the shrikhand sample at a steady

shear rate of 25 s�1 for 5 min. The modulus of the sam-

ple immediately after shearing was lower than that of

00 1

10

100

1000

0 1 10 100

Stre

ss (

Pa)

Forward 01 Reverse 01

Forward 02 Reverse 02

T=40oC

(c)

100

1000

10000

1 10 100 1000 10000

Before shearing

Recovery : after shearing

T=10oC

(d)

Shear rate(s-1)

Time (s)

G' (

Pa)

arison of steady shear viscosity and complex viscosity; (b) stress growth

ple was subjected to two consecutive loops of increasing and decreasing

f a pre-shear of 25 s�1 for 5 min.


the unsheared sample by an order or magnitude and

recovery proceeded extremely slowly with the modulus

recovering to only 45% of its unsheared value after 2 h.

The rheological features of thixotropy, apparent yield

stress, failure of the Cox–Merz rule, stress overshoots

and slow stress relaxation have also been observed inthe case of yogurt as discussed in the Introduction sec-

tion. We believe that the structure that is largely respon-

sible for the shear sensitivity as well as to some extent,

for the soft-gel like viscoelastic characteristics (see Fig.

2) is formed by a network of aggregated casein micelles.

A scanning electron micrograph of shrikhand sample is

shown in Fig. 9. A dense network of aggregated casein

micelles is clearly seen in the micrograph, which issimilar to what has been widely reported in the case of

yogurt (for e.g., Modler & Kalab, 1983; Puvanenthiran

et al., 2002; Remeuf et al., 2003). The distinctly low

porosity of the aggregated casein structure in shrikhand

is due to the fact that much of the whey from yogurt is

removed during the preparation of shrikhand. van

Marle et al. (1999) proposed a microstructural model

for explaining the link between the casein network struc-ture and the steady shear viscosity of stirred yogurt. The

model proposes a fractal structure in which primary

protein particles form aggregates, which further coalesce

to form superaggregates. At low shear the superaggre-

gates break down into aggregates, which break down

into the primary particles at still higher shear rates. This

model was able to explain the �kink� observed in experi-

mental data of steady shear viscosity for stirred yogurt.Our steady shear experimental data on shrikhand shows

a similar kink in the viscosity over a shear rate range

of �1–20 s�1 as seen in Fig. 1. It remains to be seen

whether models such as these can explain the various

non-Newtonian and viscoelastic features of shrikhand

reported above.

Fig. 9. Scanning electron micrograph of a shrikhand sample that was

prepared as discussed in Section 2.

4. Conclusions

In this paper we have reported the effects of tempera-

ture, shear and time on the rheology of shrikhand. It was

shown that the viscoelastic parameters of shrikhand were

particularly sensitive to temperature in the range of10–40 �C. Heating to 40 �C led to a large drop in modu-

lus, which showed rapid initial recovery followed by a

slow recovery over a period of about 2 h. Shrikhand sam-

ples also showed high shear sensitivity at all tempera-

tures. The samples showed an apparent yield

transition, multiplicity of shear rates, failure of the

Cox–Merz rule, thixotropy, large stress overshoots in

shear start-up experiments, and slow and incompleterecovery of the storage modulus after a large pre-shear.

It was shown that there exists at least a dual hierarchy

of microstructure, one caused by a lipid network and

the other formed by aggregated casein micelles. The lipid

structure was found to melt in the range of 10–40 �C and

thereby impart temperature dependence to the viscoelas-

tic parameters. The protein microstructure was believed

to be the cause of high shear sensitivity of shrikhand.

Acknowledgments

The authors would like to acknowledge the help of

Dr. Gaikwad of the Centre for Materials Characteriza-

tion at NCL for his help with SEM and would like to

thank Mr. Sanjay Nene of NCL for useful discussions.

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