Physico-chemical and rheological properties of yoghurt manufactured with ewe's milk and skim milk

African Journal of Biotechnology Vol. 8 (9), pp. 1938-1942, 4 May, 2009
Available online at http://www.academicjournals.org/AJB
ISSN 1684–5315 © 2009 Academic Journals




Ful Length Research Paper
Physico-chemical and rheological properties of yoghurt
manufactured with ewe’s milk and skim milk

Chougrani Fadela1*, Cheriguene Abderrahim1 and Bensoltane Ahmed2

1Laboratoire de Microbiologie et Biologie Végétale, Département de Biologie, Faculté des Sciences, Universit de
Mostaganem, BP 227 Mostaganem 27000 Algeria.
2Laboratoire de Microbiologie Alimentaire et Industriel e, Département de Biologie, Faculté des Sciences, Université
d Oran Es Senia. Algeria.

Accepted 9 April, 2009

Yoghurts were prepared with commercial strains "CHR HANSEN” Denmark using two types of milk,
ewe's milk and skim milk, ‘’CELIA’’. A dose of 2.5% of commercial starters was used for the preparation
of the yoghurt. Physico-chemical, organoleptic and rheological properties of yoghurts prepared with
both types of milk were studied in order to determine the best preparation depending on the type of
milk. Sensory analysis revealed that the product made with ewe's milk was better compared to that
made from skim milk. Furthermore, the effect of type of milk was clearly observed on the acidity,
cohesiveness and number of lactic acid bacteria in this type of milk.

Key words: Yoghurt, fermentation, ewe’s milk, lactic acid, viscosity, cohesiveness, adhesiveness.


INTRODUCTION

In recent years, there has been increasing demand for a
period, the product undergoes physico-chemical and
new range of dairy products, including yoghurts, which
rheological changes that may affect its organoleptic
are similar to traditional products but have a low fat
quality. The aim of this work is to study these variations
content (Begona et al., 2000). Yoghurt is one of the most
during the period of fermentation as wel as storage
popular fermented dairy product widely consumed al
period using two types of milk; ewe's milk and skim milk,
over the world. It is obtained by lactic acid fermentation of
‘’CELIA’ .
milk by the action of a starter culture containing

Streptococcus thermophilus and Lactobacil us delbruecki

ssp. bulgaricus. The role of these two genera in yoghurt
MATERIALS AND METHODS
manufacture can be summarized as milk acidification and

synthesis of aromatic compounds (Sera et al., 2009),
Analytical methods
(Sahan et al., 2008).

Al analytical determinations were performed at least in triplicate.
Yoghurt is more nutritious than many other fermented
Values of different parameters were expressed as the mean ±
milk products because it contains a high level of milk
standard deviation (X ± SD).

solids in addition to nutriments developed during the

fermentation process. Different forms of yoghurt are now

available in the market like stirred, set, frozen and liquid
Yoghurt manufacture


yoghurt.
The yoghurt was manufactured according to international standards
To preserve its inherent quality during storage and, in
of yoghurt manufacture (IDF, 1987 standards). The milk is homo-
particular, its physicochemical and sensory characteris-
genized and heated to 90°C for 3 min for pasteurization, then
tics, packaging is essential (Saint Eve et al., 2008).
cooled to 45°C. It is then inoculated with 2.5% of a mixed lactic
Yoghurt can be stored for up to four weeks. During this
starter (2:1 Streptococcus salivarius ssp. thermophilus and L.

delbruecki ssp. bulgaricus). Yoghurt samples were elaborated of
quantities of 100 ml for each sample and the experiment was

realized in triplicate.

The inoculated milk is incubated to 45°C until a pH of 4.4 was
*Corresponding author. E-mail: chougranifadela@yahoo.fr.
attained in approximately 4 h (the pH end point). When the pH end

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Fadela et al. 1939



Table 1. Physico-chemical analysis of yoghurt.

Analysis
Ewe’s milk
Skim milk
Analysis of variance
Titratable acidity
85.667±27.21
101.533±15.268
NS
pH
5.569±0.318 (a)
4.34±0.338 (b)
**
Viscosity
0.304±0.135 (b)
24.39±6.68 (a)
**

NS: Not significant ; *Significant ( 0.05); **Highly significant ( 0.01).
a,b,c : The homogeneous groups after comparing means.



point was achieved, the yoghurts were cooled at 6°C and stored at
This technique was performed using a spoon. Judged samples
the same temperature during al period of post-acidification (for 21
were then evaluated for adhesiveness which reflects the power
days).
required to defeat the forces of links between the surface of the

coagulum and the surface of materials (this technique is also

performed with a spoon).
Experimental analysis
The texture can be defined as a property on the sensory touch.

The analysis of the texture al ows an objective measure by
Physico-chemical analyses were carried out according to AOAC
mechanical action. The importance of texture in the assessment of
(2005). During the fermentation period, the tests were conducted
food varies depending on the expectations of consumers.
for 2 h and between 3 and 4 h for skim milk, while the period of

post-acidification were made weekly for a period of 21 days.


Microbiological analysis of yoghurt


Measurement of pH and acidity
Yoghurt is produced by fermentation of milk with two bacteria, L.

The pH of yoghurt samples was measured at 1, 7, 14 and 21 days
bulgaricus and S. thermophilus, which act together. The enume-
of storage at 6°C using a pH meter with a glass electrode over the
ration of S. thermophilus and L. delbruecki ssp. bulgaricus was
performed as described by the International Dairy Federation (IDF
range 6.8 to 4.0. Acidity was determined by titration with 0.1 N
NaOH using phenolphthalein as an indicator color Results were
Standard 306, 1995). The fol owing media were selected as suita-
expressed as degree Dornic (Afnor, 1980).
ble for enumeration: M17 agar and aerobic incubation at 42°C for

24 h for the selective enumeration of S. Thermophilus while for the

enumeration of L. delbruecki ssp. Bulgaricus, MRS agar incubated
Measurement of viscosity
at 42°C for 24 h was applied. Microbiological count data are

expressed as colony forming units (cfu) per gram of yoghurt. Four
Viscosity was measured using a viscometer model model HAAKE
dilutions were carried out to determine the number of bacteria
Viscosimeter (Mess Technik GmbH) (Brookfield Engineering Labo-
during storage.
ratories Inc., Stoughton, MA) using a glass tube and a normalized

bal equipped with a chronometer at 25°C and was expressed as

mPas. Every experiment was repeated 3 to 5 times to have some
RESULTS AND DISCUSSION
meaningful results after statistical analyses. Viscosity was

expressed as mil ipoises (m.p.s).
Physico-chemical properties of yoghurt




Organoleptic tests
The results obtained for chemical characterization of the

yoghurt samples are presented in Table 1. During the
Throughout the period of post-acidification (7th, 14th and 21st day
period of fermentation, we noticed a remarkable de-
of storage), the organoleptic quality of yoghurt products was
crease in pH for yoghurt samples manufactured with skim
evaluated by a jury of 10 panelists with a seven point scale
milk. This decrease is less important for yoghurt
(Meilgaard et al., 1999) and involved three parameters. Flavour,
manufactured with ewe's milk with an average of 5.56
cohesiveness and adhesiveness of the samples were evaluated
fol owing the recommendations of International Dairy Federation
compared to 4.34 for the other samples. These results
standard 99A (1987).
can be explained by the composition of each kind of milk.
Flavour was assessed by the estimation of acidity developed by
However, in a previous study, our results showed that the
specific lactic acid bacteria in the samples.

titratable acidity of ewe’s milk is slightly low and does not
A strict protocol was imposed to panelists to minimize variability.
exceed 21°D (Table 2). Our results are in keeping with
At each session, subjects tasted samples of yoghurt (5 g) at 10°C.
They were asked to keep the yoghurt in the mouth for 12 s and then
those reported by Sokolinska et al. (2004), who indicated
swal ow. This time was chosen after preliminary tests conducted
that the pH values of milk decreased during the
with 10 subjects. The subjects tasted samples of yoghurt in the
manufacturing process, from the time it was inoculated
most natural possible way by keeping their mouth closed and
with bacterial cultures to the time when it was manufac-
swal owing the product.
tured ranging from 6.7 to 4.34. The same observations
The yoghurt samples were presented in random order. Water
were reported by O’Neil et al. (1979).
was used for rinsing between samples. A smal period of several
minutes was required between tasting samples.

Moreover, according to Luquet (1990), lactic strains
Panel members were then asked for cohesiveness which reflects
have the ability to ferment lactose into lactic acid, with an
the maximum capacity of deformation of the sample before break.
increase in acidity and a decrease in pH of fermented

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1940 Afr. J. Biotechnol.



Table 2. Physico-chemical analysis of ewe’s milk.

Analysis
pH
°D
Dry matter (%)
Fats (%)
Ewe’s milk
6.7
21
11.5
6.5
Standards
6.5-6.85
22-25
18.3
7.1



Table 3. Sensorial analysis ( ranks n= 40).

Analysis
Ewe’s milk
Skim milk
Analysis of variance
Flavour
70.50 (b)
49.50 (a)
**
Cohesiveness
75 (b)
45 (a)
**
Adhesiveness
77 (b)
44 (a)
**

*Significant ( 0.05); **Highly significant ( 0.01).
a,b,c : The homogeneous groups after comparing means.



milk, which reveals the influence on the composition of
thermophilus is not completely stopped, but it is less
the inoculum on the rate of bacteria growth such as
important compared to that of L. bulcaricus which not
Streptococcus. Indeed, the development of these germs
only produces lactic acid, but probably a smal amount of
seem to be proportional to the rate of protein (and
texturising agents (Luquet, 1994).
certainly the rate of amino acids) in the medium. In

addition, during the total experimental period, the values

of acidity are even higher.

Rheological properties of yoghurt
The analysis of variance showed that the fermentation


period and storage have a significant effect on reducing
The panelists found the flavour of yoghurt inoculated at a
the pH. Similar changes were observed in the level of
rate of 2.5% prepared with commercial strains using
titratable acidity in the yoghurt during storage.
ewe's milk as the best in relation to skim milk with an
We noted that there is a proportional relationship
average sum of ranks of 70.50 against 49.50, respec-
between the rate of inoculation strains and acidity; the
tively (Table 3). This difference in flavour was probably
degree of acidity increases proportional y to inoculation
the result of important fermentation of lactose by lactic
rate (Chougrani et al., 2008).
acid bacteria (Bourgeois and Larpent, 1989). However,
In addition, we recorded low values of acidity for
the panelists described the flavour of yoghurt prepared
yoghurt prepared using skim milk with an mean of
with ewe's milk as more acceptable. In terms of statistical
101.53°D (Table 1). Gueimonde et al. (2003) and
analysis, the factor type of milk has a highly significant
Salvador and Fiszman (2004) found similar results in pH
effect on the evolution of this parameter.
and titratable acidity when they studied the quality of
General y, cohesiveness is more satisfactory in the
plain yoghurt.
fermented milk made from ewe's milk ; it is an average of

75 against 45 of the sample produced from skim milk.

According Rawsan and Marshal (1997), the assess-
Evolution of the viscosity during fermentation

ment of rheological properties of yoghurt (adhesiveness

and cohesiveness) is probably linked to exopolysac-
The viscosity of yoghurt prepared with ewe’s milk varies
charides produced by specific strains of yoghurt S.
with time. The results indicate that the values differ from
thermophilus and L. bulgaricus. The analysis of variance
one type of milk to another. We noted that viscosity is
showed significant effect of the factor, kind of milk on the
much greater in the yoghurt made from skim milk
development of cohesion in tested yoghurt.
compared to those made from ewe's milk, despite the
Adhesiveness increased in the prepared yoghurt milk
richness of the fat. This may be due to some factors that
sheep between the 1st and 14th day using ewe’s milk
affect the quality of ewe’s milk such as the physiological
(Figure 1; Table 4). At the end of the experiment, during
status of the animal, its diet, race and the climate.
the 21st day, a slight decrease was observed (Figure 2).
According to Rawson and Marshal (1997), S.
Furthermore, in the yoghurt made from milk powder, this
thermophilus are the most germs incriminated in the
is growing at the end of fermentation, it is about 77
production of exocel ular texturising agents cal ed
against 44 on average compared to skim milk. Similar
exopolysaccharides that might interact with the protein
remarks were reported by Rawson and Marchal (1997)
content of milk and increase the viscosity and rheologial
and Katsiari et al. (2002) who reported an increase in
quality of products.
adhesiveness of ewe’s yoghurt during storage.
During the post-acidification period, the activity of S.
The analysis of variance showed a significant effect

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Fadela et al. 1941





Figure 1. Evolution of rheological properties in ewe’s milk yoghurt.



Table 4. Sensory evolution of flavour, cohesiveness and adhesiveness of experimental yoghurts prepared with ewe’s
milk and skim milk during fermentation and post-acidification period ( ranks n= 40).

Period of
Ewe’s milk
Skim milk
days
Flavour Cohesiveness Adhesiveness Flavour Cohesiveness Adhesiveness
1
19.50
18.5
19
10.50
11.50
11
7
18
19
19
12
11
11
14
17.50
19.50
20
12.50
10.50
10
21
15.50
18
18
14.50
12
12





Figure 2. Evolution of rheological properties in skim milk yoghurt.



of the factor, kind of milk on the development of
and Death, 1980), it is general y accepted that yoghurt
adhesiveness in tested yoghurt.
should contain 107 CFU of viable bacteria (S.

thermophilus and L. bulgaricus) per ml of yoghurt.

During the fermentation process, the number of S.
Microbiological analysis

thermophilus is relatively higher compared to L.

bulgaricus. In paral el, the presence of these germs is
Yoghurt consumption is beneficial to human health
much greater in skim milk compared to ewe’s milk;
because of the bacteria the yoghurt contains. Although
21.6.106 CFU/ml in skim milk against 1.8.106 CFU/ml in
quantitative standards for yoghurt bacteria differ (Tamine
ewe’s milk.

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1942 Afr. J. Biotechnol.







In terms of analysis of variance, we reported a highly
Meilgaard M, Civil e GV, Carr BT (1999). Sensory evaluation techniques
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Rawson HL, Marschal VM (1997). Effect of ‘ropy’ strains of

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