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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: hazim@just.edu.jo (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