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Effect of solids concentration on the rheology of labneh(concentrated yogurt) produced from sheep milk
Hazim A. Mohameed a,*, Basim Abu-Jdayil a, Ali Al-Shawabkeh b
a Department of Chemical Engineering, Jordan University of Science and Technology, P.O. Box 3030, 22110 Irbid, Jordanb Department of Chemical Engineering, University of Jordan, 11942 Amman, Jordan
Received 7 November 2002; accepted 14 April 2003
Abstract
The effect of solids concentration on the apparent viscosity of labneh made from sheep milk has been investigated using a rotaryviscometer. Sheep labneh was manufactured following the traditional method by using cloth bags. Apparent viscosity of labneh with
four different solids concentration was studied as a function of the shear rate. It is found that sheep labneh with different solids
concentration exhibits shear thinning and thixotropic behavior. The power law model was found to fit the apparent viscosityshear
rate experimental data, satisfactorily. Both the consistency coefficient and the flow behavior index were correlated as a function of
solids concentration. The predicted apparent viscosity showed an absolute average error of less than 5%. Time-dependent viscosity
measurements were performed at four different shear rates. Using the structural kinetic approach, a 1.5-order kinetic model was
found best for correlating the experimental data. Completely destructed labneh, after 4-h preshearing, was also studied as function
of shear rate. Comparison between labneh made from cow milk and sheep milk was performed, whenever data were available.
2003 Elsevier Ltd. All rights reserved.
1. Introduction
The traditional fermented milk foods made from the
milk of cows, sheep, and goats are still competitive with
newer products. In the Middle East region, concentrated
yogurt (labneh) is highly appreciated and consumed
with bread all year a round. Labneh is an important
supplement to the local diet and provides vital elements
for growth and good health. According to Lebanese
standards, labneh is defined as a semisolid food derived
from yogurt by draining away part of its water and
water-soluble compounds (Lebanese Standards, 1965).
Usually, labneh is prepared with two solids concentra-
tion ranges either around 22 wt.% or around 40 wt.%
(labneh anbaris). The former is prepared to be con-sumed within two weeks and usually stored in refriger-
ators; the other one is stored in vegetable oil at room
temperature and can be consumed within two years
(Keceli, Robinson, & Gordon, 1999).
Usually, sheep are moved in bands or flocks in order
to graze land from one area to another. Flocks are
maintained for their valuable wool and meat. The
availability of sheep milk is limited during late winter
and early spring, when the breeding ewes are mated torams and produce lambs. Therefore, the peak production
of sheep labneh is during the spring season and to a less
extent in other seasons. Sheep milk is rich in nutrients,
having about 19% solids concentration compared to cow
milk with 1213% (Path, 1995). This high solids con-
centration, coming from fat and protein content, is ex-
cellent ingredients for manufactured dairy products like
cheese and labneh. Nutrient composition of both sheep
and cow milk are shown in Table 1 (Posati & Orr, 1976).
Labneh made from sheep milk is still less popular and
produced in much less amounts than cow labneh. The
cow milk availability is not the only issue but also the
sensorial acceptance of cow labneh is higher becausesheep labneh has a sharp flavor.
Recently, and specially for Jordan many dairy fac-
tories started to offer sheep labneh in the grocery stores
for daily consumption. Due to the high fat content of
sheep milk, the traditional methods used to make cheese
and other yogurt products (like labneh) from cow milk
may not be suitable for sheep milk (Ag-Ventures, 2000).
Therefore, more studies related to labneh produced
from sheep milk are needed. Investigation of the rhe-
ological properties are one of the those important
studies. Structure, firmness, and viscosity are important
Journal of Food Engineering 61 (2004) 347352
www.elsevier.com/locate/jfoodeng
* Corresponding author. Tel.: +962-2-7201000; fax: +962-2-
7095018.
E-mail address: [email protected] (H.A. Mohameed).
0260-8774/$ - see front matter 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0260-8774(03)00139-0
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quality properties of labneh. These properties are pri-mary criteria for their quality evaluation regarding
labnehs sensorial acceptance.
The present study aims to investigate the effect of the
solids concentration of labneh made from sheep milk on
its rheological properties. The labneh investigated in this
study is made from fresh sheep milk and manufactured
using the traditional home made method, where the
whey is removed using cloth bags. Although, many re-
searchers have investigated the rheological behavior of
labneh produced from cow milk, the rheological prop-
erties of sheep labneh have not received much attention.
2. Materials and methods
2.1. Labneh manufacture
Labneh was made by the procedure normally used in
homes. Fresh sheep milk, brought from a local dealer, is
boiled for a few minutes to destroy vegetative bacterial
cells and fungi (sterilization). The milk is cooled to
about 45 C and inoculated with %2% starter cultureand stirred so that the starter is distributed through the
milk mass. The starter culture is a commercial fermented
product of the yogurt type. The inoculated milk is
poured into 1-l plastic containers and its temperature is
allowed to drop slowly to the ambient temperature for
about 3 h. The plastic containers were incubated in a
refrigerator at 5 C overnight. The coagulum in each
plastic container is placed in a hanging cloth bag to
drain the whey. The drainage was achieved at room
temperature. To produce labneh with different solids
content, the cloth bags were allowed to drain for dif-
ferent time periods. The volume of the drained whey was
an indication of the solid content. The accurate solid
content of labneh was determined by the methods in
AOAC (1995). The four solid concentrations investi-gated in this study are 17.9, 18.8, 20.9, and 24.2 wt.%.
They are assigned the following symbols S1, S2, S3 and
S4, respectively.
2.2. Viscometer
Rheological properties of sheeps labneh were moni-
tored using a rotational viscometer (Haake VT500/
MV3, Searle type) with a fixed outer cylinder and ro-
tating measuring bob. The radius of the rotating cylin-
der was 20.04 mm, the length of the cylinder and the gap
width are 60 and 0.96 mm, respectively. Values of shearstress and viscosity were recorded after the shear stress
signal attained a constant value for 30 s. The shear rate
range was 2.2219.80 s1. The labneh samples were kept
at room temperature to equilibrate before being loaded
into the viscometer. Temperature of the samples was
controlled by circulating water in the jacket of the bob
cylinder arrangement. All the measurements in this
study were performed at 25 C.
To study the effect of solids concentration on the
rheological properties of labneh made from sheep milk,
three set of experiments were performed using the ro-
tational viscometer: (i) The apparent viscosity of freshsamples as a function of shear rate. The measurements
were carried out by increasing (forward) and by de-
creasing (backward) the shear rate. (ii) Time-dependent
viscosity measurements for each solids concentration at
four different shear rates. The four shear rates applied in
this study were 3.65, 10.21, 28.38, and 219.80 s1. (iii)
The fresh samples were subjected to a shear rate of 79.02
s1 for 4 h till the structure of labneh completely de-
structed. Then the flow properties of the destructed
labneh were measured.
The time-dependent flow properties could be modeled
by applying the structural kinetic approach, which is
adopted by using the analogy with chemical reactions.
The final form of the model is
g geg0 ge
1m m 1kt 1 1
where g0 is the initial apparent viscosity at t 0(structured state), and ge is the final apparent viscosity
as t! 1 (equilibrium structured state), k k _cc is thebreakdown rate constant and it is function of shear rate.
The rate constant k is considered as an indication of the
rate of the thixotropic breakdown. The exponent m is
the order of the breakdown reaction. Details and
assumptions of this equation are to be found in Abu-Jdayil (2002). In this work, the value of the initial
viscosity, g0, is the first measured value after 30 s from
starting the experiment.
3. Results and discussion
3.1. Apparent viscosity versus shear rate
The effect of the solids concentration on the apparent
viscosity of labneh is depicted in Fig. 1. The measure-
Table 1
Nutrient composition of whole milk per 100 g (Posati & Orr, 1976)
Milk Energy (kcal) Fat (g) Cholesterol (mg) Protein (g) Calcium (mg) Phosphorus (mg) Carbohydrate (g)
Cow 61 3.34 14 3.29 119 93 4.66
Sheep 108 7.00 5.98 193 158 5.36
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ment was carried out with both increasing the shear rate
(forward measurement) and decreasing the shear rate
(backward measurement, not shown). The apparent
viscosity decreased as the shear rate increased, so labneh
exhibited a shear thinning behavior. It can be seen that
the change in the apparent viscosity was not linear withsolids concentration, where a 5% increase in the solids
concentration led to duplicate in the apparent viscosity
(from 26 to 60 Pa s at 2.2 s1). This is an indication that
the control of the solids concentration is an important
quality factor and it may affect the final acceptance of
labneh by the consumer.
The fall in viscosity with shear rate might be due to
the destruction of the interactions within the labneh
network structures. These interactions are electrostatic
and hydrophobic ones, which are considered as weak
physical bonds. The shear rate range applied in this
study was enough to destroy these physical bonds.As shown experimentally by Ozer, Stenning, Grand-
ison, and Robinson (1998), the preparation of labneh
using a cloth bag had the highest protein and fat con-
tent, since the cloth bags allow, mainly, the separation
of lactose and minerals into the whey. Therefore the
sample S4 with the highest solids concentration also had
the highest protein content and consequently S4 had the
highest apparent viscosity, as shown in Fig. 1.
Fig. 2 demonstrates an example of the hysteresis of
the rheological behavior of sheep labneh, in which the
labneh was subjected to a cycle of increasing and de-
creasing shear rate. This figure shows that sheep labneh
is a shear thinning material, which exhibits a thixotropicbehavior. This non-Newtonian behavior for sheep lab-
neh was also observed for labneh made from cow milk
(Abu-Jdayil & Mohameed, 2002; Abu-Jdayil, Shaker, &
Jumah, 2000). The thixotropic behavior was a charac-
teristic of sheep labneh regardless of the solids concen-
tration.
The non-Newtonian viscosity of labneh was modeledusing the two-parameter power law model given by
g m _cc n1 2
where m is the consistency coefficient and n is the flow
behavior index. Continuous lines on Fig. 1 represent the
power law model. The regressed parameters, m and n,
for both forward and backward measurements are listed
in Table 2. The correlation factor was one for all the
fittings.
For the forward measurement, the flow behavior in-
dex n decreases with the increase of solids concentration.This shows that the deviation from the non-Newtonian
behavior (n 1) increases with increasing the solidsconcentration of labneh. It is interesting to note that the
n values have the opposite trend in the backward mea-
surement, where the n values were found to increase
with increasing the solids concentration. Generally, n is
not a strong function of the variation in solids concen-
tration. The decrease of n for the forward curves is
probably due to the thixotropic behavior; during the
experiment, the time increases and then the viscosity
deceases, so m decreases.
It was also found that the value of m, a measure of
viscosity, was highly dependent on the solids concen-tration of labneh for the forward measurement, and less
Fig. 1. Effect of solids concentration on the apparent viscosity of fresh
samples, forward measurement. Continuous line is power law fit.Fig. 2. Examples of hysteresis loops for sheep labneh at different solids
concentration.
Table 2
Power law model parameters for fresh samples
Sample Solids concentration
(wt.%)
m (Pasn) n
Forward Backward Forward Backward
S1 17.9 49.80 41.19 0.142 0.115
S2 18.8 63.98 46.84 0.131 0.136
S3 20.9 72.42 46.89 0.117 0.140
S4 24.2 123.81 59.08 0.087 0.182
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dependent for the backward measurement. For both
measurements, m values increase as the solids concen-
tration of labneh increases. A similar trend was observed
for other foodstuff like tomato juice (Steffe, 1986),
sweetened condensed milk (Hough, Moro, Segura, &
Alvo, 1988) and soybean milk (Son & Singh, 1998).
The consistency coefficient, m and the flow behavior
index, n in Eq. (2) were correlated as a function of solids
concentration as the following:
m 1 103 a1S a2S2 3
and
n 1 b1S b2S2 4
where S is the solids concentration expressed as mass
fraction. The correlation parameters a1, a2, b1, and b2were found by least squares fit and their values are
summarized in Table 3. The correlation factors were
0.97 and 0.89 for Eqs. (3) and (4), respectively. The form
of the equation for m and n is selected so that it satisfiesthe following physical conditions:
i(i) As S! 0, n ! 1 (Newtonian fluid) and g is the vis-cosity of water (g m 0:001 Pa s).
(ii) As S! 1, g ! 1 for the solid material.
In order to check the fit goodness, the average ab-
solute deviation (AAD) was used:
AAD Xpi1
gexp;i gcal;i
gexp;i
" #
100
p5
where p is the number of experimental data. gexp is theexperimental apparent viscosity and gcal is the apparent
viscosity calculated from Eq. (2) after inserting Eqs. (3)
and (4). The AAD was found to be 4.7% for all the
experiments. This shows that the least squares fit for
Eqs. (3) and (4) was adequate to describe the apparent
viscosity as function of shear rate and solids concen-
tration.
3.2. Transient flow properties
Although, the results extracted from Fig. 2 were
useful in the sense they demonstrated that sheep labnehhas a non-Newtonian, shear thinning, thixotropic be-
havior, these results cannot be used for engineering de-
sign and scale up of equipment and processes (Nguyen,
Jensen, & Kristensen, 1998). Therefore, time-dependent
rheological data should be attained and modeled.
Typical experimental results for the time-dependent
apparent viscosity were depicted in Figs. 3 and 4. The
effect of the solids concentration on the transient ap-
parent viscosity, at constant shear rate equal to 10.21 s1,
is shown in Fig. 3. The apparent viscosity decreases
rapidly within the first 10 min of shearing and then
reaches a steady state value (equilibrium state) after 40
min. Labneh with a solids concentration of 24.2 wt.%
(S4) is considerably more viscous than S1, meanwhile,
the two samples with close solids concentration of 18.8
wt.% (S2) and 20.9 wt.% (S3) have a closer viscositytime
curves. It is clear that, increasing the solids concentration
leads to an increase in the extent of thixotropy. This re-
sult can be quantified by modeling the experimental data.
Small changes in labnehs solids concentration will affect
highly its rheological properties.The effect of shear rate at constant solids concentra-
tion (S1 sample as an example) is shown in Fig. 4. The
apparent viscosity of labneh is decreasing as the applied
shear rate increases. The rate and extent of viscosity
reduction is a function of both the solids concentration
and the shear rate.
The transient viscosity data of labneh at constant
shear rate were correlated using m 1:5 i.e. with 3/2-order irreversible kinetic model. A comparison between
the experimental data and the model in Eq. (1) with
m 1:5 is presented by the continuous lines in Figs. 3
Table 3
Correlation parameters for Eqs. (3) and (4) found by least squares fit
for fresh samples
Parameter Value
a1 (Pasn) )350.57
a2 (Pasn) 3517.40
b1 )7.62
b2 15.97
Fig. 3. Time-dependent viscosity data at shear rate equal to 10.21 s1.
Effect of solids concentration. Continuous line is the fit of the kinetic
model (m 1:5).
Fig. 4. Time-dependent viscosity data for sample S1 17.9 wt.%. Effectof shear rate. Continuous line is the fit of the kinetic model (m 1:5).
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and 4. As a measure for the extent of thixotropy the
ratio of the initial to final viscosity (g0=ge) was used.This approach was applied successfully by Nguyen and
Boger (1985) to describe the time-dependent properties
of concentrated mineral suspensions with m 2, byNguyen et al. (1998) to aqueous starch pastes with
m 3, and by Abu-Jdayil and Mohameed (2002) tolabneh made from cow milk with m 2.
Table 4 illustrates the values of k and g0=ge as a
function of shear rate and solids concentration. Thevalue ofk was increasing as the shear rate increases for
all the solids concentration investigated in this study, as
it is expected for thixotropic structured materials. On
the other hand, at constant shear rate, the value of k
decreases as the solids concentration increases, i.e. the
degree of thixotropy decreases with increasing the solids
concentration. Increasing the solids concentration, due
to the presence of protein and fats, may enhance for-
mation of the gel structure as a result of different in-
teractive forces. This gel structure for yogurt has been
studied by many investigators (Ozer et al., 1998; Roefs
& van Vliet, 1990; Ross-Murphy, 1990). It is believed
that the gel structure was involved in sheep labneh at allsolids concentration but to different degrees. Therefore,
the results suggest that the rate shear induced break-
down of the internal structure in labneh decreased with
increasing the solids concentration. The same qualitative
results were found for labneh produced from cow milk.
It was found that the k values were decreasing with the
storage time (Abu-Jdayil & Mohameed, 2002). Increas-
ing the storage time enhanced the gel structure forma-
tion as the solids concentration does, and the same trend
for k value was found.
The extent of thixotropy, g0=ge, generally increases
with increasing shear rate, except for sample S3 and_cc 28:38 s1. It may be also seen in Table 4 that g0=ge isgenerally larger for S4 than S1, but there is no trend
between S2 and S3. This may be explained by analyzing
the experimental results (see Fig. 3), where S3 and S2 are
very close and they have close values of both the initial
and the equilibrium viscosity.
3.3. Steady state flow properties
For the steady state flow properties, a shear rate of
79.02 s1 was applied for 4 h in order to break down,
completely, the thixotropic structure observed in Fig. 2.
Then, the apparent viscosity was measured as a function
of shear rate in order to study the sheep labneh flow
properties after reaching steady state (completely de-
stroyed state). The effect of solid content of the com-
pletely destroyed labneh on the apparent viscosity is
shown in Fig. 5. The viscosity of sheep labneh at all
solids concentration came very close to each other and
approached the viscosity of the lowest solid concentra-
tion, sample S1, This is an indication that, applying a
shear rate for a long time had broken the interactive
forces in the labneh structure and led to a collapse of the
gel structure. The slight differences in viscosity shown in
Fig. 5 may be due to the solid amounts that behave as
solid aggregates i.e. labneh became more like a thick
suspension. However, the viscosity of destroyed labneh
is lower than fresh labneh, as is expected. An example to
show this is depicted in Fig. 6.
Fig. 5. Effect of solids concentration on the apparent viscosity of de-
structed labneh. Continuous line is power law fit.
Table 4
Structural breakdown parameters for the 1.5-order irreversible kinetic model
Sample Solids con-
centration
(wt.%)
_cc 3:65 s1 _cc 10:21 s1 _cc 28:38 s1 _cc 219:80 s1
k (s1) g0=ge k (s1) g0=ge k (s
1) g0=ge k (s1) g0=ge
S1 17.9 0.0033 1.22 0.0038 1.33 0.0040 1.40 0.0068 1.85
S2 18.8 0.0033 1.37 0.0035 1.57 0.0036 1.58 0.0048 1.74
S3 20.9 0.0023 1.31 0.0034 1.54 0.0034 1.45 0.0037 1.80S4 24.2 0.0023 1.69 0.0030 1.85 0.0031 1.86 0.0035 1.98
Fig. 6. Comparison between fresh and destructed labneh at S3 20.9wt.%.
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The data were also described using the power law
model introduced before (see Eq. (2)). Table 5 summa-
rizes the parameters of the power law model for the
completely destroyed labneh at different solids concen-
tration. The correlation factor was one for all experi-
ments. The values of m and n were almost the same for
all solids concentration, confirming the results in Fig. 5.
The values of m and n for the backward and forward
were also the same showing that the thixotropic hys-
teresis was diminished. Compared to cow labneh,
shearing for 2 h at the same shear rate was enough to
overcome the thixotropic behavior (Abu-Jdayil & Mo-
hameed, 2002).
4. Conclusions
The variation in solids concentration of sheep labneh
has an obvious effect on the apparent viscosity. Changes
in solids concentration of 5% had led to doubling the
viscosity of labneh. It was also found that labneh pro-
duced from sheep milk has a higher viscosity than the
one produced from cow milk. The power law model wasfound to correlate the experimental data accurately.
Moreover, The consistency coefficient and the flow be-
havior index were correlated as a function of solids
concentration. The calculated apparent viscosity showed
an average absolute error of less than 5% for all the
experiments. Sheep labneh exhibits a shear thinning and
thixotropic behavior. To investigate the thixotropic be-
havior of sheep labneh, time-dependent viscosity ex-
periments at different shear rates were performed. The
structural kinetic approach was followed and a 1.5-
order kinetic model could correlate the experimental
data well. The breakdown constant and the extent ofthixotropy were found to increase with increasing the
shear rate. After a 4-h application of 79.02 s1 shear rate
to labneh samples of different solids concentrations, the
hysteresis effects have disappeared and the labneh sam-
ples reached a completely destroyed state (destructed
labneh). The destructed labneh was also modeled satis-
factorily using the power law.
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Table 5
Power law model parameters for completely destructed labneh
Sample Solids concentration (wt.%) m (Pasn) n
Forward Backward Forward Backward
S1 17.9 39.79 39.56 0.063 0.063
S3 20.9 41.44 40.99 0.081 0.082
S4 24.2 41.60 41.66 0.108 0.107
352 H.A. Mohameed et al. / Journal of Food Engineering 61 (2004) 347352