Vol.4, No.9B, 29-32 (2013) Agricultural Sciences
http://dx.doi.org/10.4236/as.2013.49B005
Rheological properties of low fat yogurt containing
cress seed gum
Armi t a Behni a1* , Hojjat Karazhiyan2, Razieh Niazmand3, Abdol Reza Mohammadi Nafchi4
1Department of Chemical Engineering, Islamic Azad University, Shahrood Branch, Shahrood, Iran;
*Corresponding Author: armita.behn i a@ gmail.com
2Department of Food Science and Technology, Islamic Azad U niversity, Torbat-Heydarieh Branch, Mashhad, Iran
3Department of Food C hemistry, Research Institut e fo r Food Science and Tech nology (RIFST), Mashhad, Iran
4Department of Food Science and Technology, Islamic Azad U niversity, Damghan Branch, Damghan, Iran
Received Ju l y 2013
AB STRACT
Yogurt—a milk based mix fermented by lactic
acid bacteria is a valuable health food for both
young and old. Milk is the main ingredient of
yogurt. However, most yogurts contain addi-
tional solids such as milk solids nonfat to boost
the nonfat milk solids. Stabilizers such as natu-
ral gums are added to im prove and maintain gel
firmness and consistency, while also for many
people to improve appearance and mouth feel.
Hydrocolloids sp ecifical ly stabili ze gel structure,
increase viscosity and either from networks with
milk constituents and establish a separate gel
structure. In current research, a natural local
plant Iranian hydrocolloid, cress seed gum, is
added to yogurt formulation and its rheological
properties are evaluated using a rotational vis-
cometer. Different famous rheological models
have been used to fit shear stress-shear rate
data’s. The results demonstrated that cress seed
gum has a good potential to be used as a stabi-
lizer in yogurt formula.
Keywords: Y ogurt; Hydrocolloids; Cress Seed Gum;
Rheological Properties; Viscosity
1. INTRODUCTION
The growing awareness of the relationship between
diet and health has led to an increased demand for food
products that support health above and beyond providing
basic nutrition. One of these products is yogurt that is
made from milk. Yogurt essentially has all the nutritive
components of milk [1]. Yogurts are prepared by fermen-
tation of milk with bacterial cultures consisting of a mix-
ture of Streptococcus subsp. Thermophiles and Lactoba-
cillus delbrueckii subsp. Bulgaricus [2]. Recently, con-
sumption of whole dairy products (e.g. full fat yogurt)
has declined due to the awareness of the probable harm-
ful effect of fat on consumer ’s health, thus dietary habits
of consumers have been changed and market interest has
tended to change in favor of low or nonfat dairy products
[3]. According to the Code of Federal Regulations of
FDA, low fat yogurt and nonfat yogurt are similar in
description to yogurt but contain 0.5% to 2% and less
than 0.5% milk fat, respectively [4,5]. Milk fat has an
important role in the texture, flavor and color develop-
ment of dairy products [6]. Because of reduction of fat
and subsequently reduction of total solids content in low
fat and nonfat yogurts, they exhibit weak body, poor tex-
ture, and whey separatio n unle ss variou s stabilizer blends
or ropy strains of yogurt bacteria are used [7]. It is a big
challenge for many food scientists to produce a suitable
fat substitute to provide the functionality of the missing
fat [8]. Therefore, manufacturers have followed different
strategies including the milk solid nonfat in yogurt milk,
in addition of nondairy based stabilizers, and usage of
milk proteins as fat substitutes [9]. Although, enhancing
the total solid content of skim yogurt similar to full fat
products is a traditional and common method which
leads to improvement in viscosity and water binding in
yogurt [10]. Hydrocolloids are used in food products as
thickeners, stabilizer, gellin g agents and emul sifiers. They
improve the texture of the products, increase water reten-
tion while enhancing lower energy value; they are often
employed in low-calorie foods [11]. Stabilizers are used
to produce a thick, cohesive body, smooth texture and to
prevent wheying off [12] and to improve consistency
(increase viscosity) and reduce syneresis [13]. Stabilizers
such as pectin or gelatin are often added to the milk base
to enhance or maintain the appropriate yogurt properties
including texture, mouth feel, and appearance viscosity/
consistency and to prevent whey separation [2]. The ef-
fects of some stabilizers such as waxy maize starch, ge-
latin, xanthan gum, low methoxy pectin, guar gum, lo-
cust bean gum, and λ-carrageenan on the microstructure

A. Behnia et al. / Agricultural Scienc es 4 (2013) 29-32
30
and rheology of yogurt have been studied [14,15]. Ref-
erence [16] was the first research on cress seed gum. The
researches on this area were continued by the same au-
thor [17-20]; however, literature review still shows the
lack of complete and sufficient information about this
new Iranian hydrocolloid and its possible use in food
formulations. Thus, the objectives of this stud y were first
to evaluate the physicochemical properties of yogurt con-
taining different concentrations of the gum and second to
evaluate the rheological properties of this food formula
using differe nt rheological models.
2. MATERIALS A ND METHODS
2.1. Yogurt Treatments
Five yogurts treatments were made as follows: control
low fat yogurt; low fat yogurt fortified with 0.05, 0.07,
0.1 and 0.15 g/100mL o f cress seed gum.
2.2. Materials
Skim milk with 0.5% fat content and Starter culture
were obtained from Pegah Dairy Company (Mashhad, Iran).
Cress seeds were provided by a medical plant supplier in
Mashhad, Iran. All impure matter such as dust, dirt, stone,
chaff and broken seed were manually removed from the
seeds.
Extracts of dry cress seeds were prepared according to
the method presented in [17]. Briefly, cress seed was
soaked in preheated de-ionized water at a water/seed ratio
of 30:1. 0.1 mol/L NaOH solutio n was used to adjust the
pH to 10. The slurry was stirred continuously for about
15 minutes in constant temperature (35˚C). An extractor
with a rotating rough plate was e mployed to cut the gum
layer off the seed. This degummed seeds were discarded;
final ly, the slurry wa s dried with the 60˚C air forced oven
and milled to powders. They were kept in cool and dry
condition.
2.3. Preparation of Yogurt Starter Culture
The yogurt culture combination of Lactobacillus del-
brueckii ssp. Bulgaricus and Streptococcus thermophiles
was weighted (10 g) and added to 100 mL of sterile skim
milk. One milliliter of this mixture was inoculated per
100 mL of yogurt mi x.
2.4. Preparation of Yog urt Mixes
In this experiment, lo w fat yogurt samples were made
by adding different concentration of cress seed gum into
skim milk (0.05, 0.07, 0.1 and 0.15) as stabilizer. Control
sample was made from skim milk without hydrocolloid.
After blending, each mix was pasteurized at 85˚C for 30
minutes, then cooled to 42˚C and inoculated with 0.1%
yogurt starter, dispensed into plastic containers and in-
cubated at 42˚C for approximately 4 - 4.5 hours; u ntil the
pH reached 4.6. Then cooled to 4˚C and kept at refrige-
rator.
2.5. Rheological Analysis
The rheological parameters were determined using ro-
tatio nal visco mete rs (mode l RV, DV-III ULTRA, BRO O K-
FIELD). The temperature of the system was set and main-
tained at ambient temperature (25˚C) for the flow curve.
The flow curves of the low fat yogurts were determined
usin g shear ra te rangin g from 0 to 85
1
s
−
. T he rhe ologi-
cal properties were fitted to three models including: the
Power law, Herschel-Bulkley and Casson models.
The equations for these models are:
Power law model:
()
n
K σγ =
Herschel-Bulkley model :
()
n
K
σσ γ
°
= +
Casso n model :
1/21/21/2
()K σσ γ =+ 
Where
σ
is the shear stress (Pa),
σ
°
is the yield
stress,
γ

is the shear rate (
1
s
−
),
K
is the consistency
index (Pa. n s) and
n
is the flow behavior index (di-
mensio nless).
2.6. Statistical Analysis
The experiments were performed in two duplicates.
Analysis of variance (ANOVA) was performed using the
Duncan ’s multip le-range test to compare treatment mea n s.
Significance was defined at p < 0.05.
3. RESULTS AND DISCUSSIONS
The flow curves for samples containing cress seed gum
with different concentratio ns at 2 5˚C are depicted in F ig-
ure 1 . A shear-thinning behavior was observed for all con-
centrations (Figure 2) and becomes more prominent as
the concentration is increased shown by a decrease in
values of the flow behavior index (data for power law
model, Table 1). For all sa mples, an increase in concentra-
tion was accompanied by an increase in pseudoplasticity,
Figure 1. Flow curves of yogurt samples containing cress
seed gum.

A. Behnia et al. / Agricultural Scienc es 4 (2013) 29-32
31
Figure 2. Viscosity curves for different yogurt samples.
Table 1. Calculated flow model parameters for yogurt samples
with different concentrations (25˚C).
Mod e l
Sample
Power law Hersch el -Bulkley Cas son
R2 K n R2 σ0 K n R2 σ0 K
Control
0.05%
0.07%
0.1%
0.15%
0.99
0.97
0.98
0.73
0.91
41.39
27.20
46.63
24.49
16.18
0.32
0.28
0.23
0.20
0.24
0.99
0.99
0.97
0.98
0.93
20.54
27.19
14.63
34.66
16.56
29.01
7.33
34.62
0.10
3.43
0.37
0.51
0.27
1.31
0.51
0.95
0.98
0.92
0.84
0.93
50.30
32.04
53.66
26.23
18.60
0.70
0.46
0.47
0.31
0.29
shown by a decrease in values of the flow behavior index
(Table 1). This suggested that the deviation from the
Ne wtonian behavi or (n = 1) increased with incre a sin g t he
solids concentration of hydrocolloid.
In addition the consistency coefficient generally showed
an increment with the concentration of cress seed solu-
tions (0.07% concentration) (Table 1). This behavior is
generally explained to arise from disentanglement of the
polymer network and the partial chain orientation or alig n-
ment of micro-struct ure in the dire ction of the shear flo w,
thus, reducing the local drag [21]. The viscosity of solu-
tions decreased with increasing shear rate (Figure 2).
With further shear rate increasing the intermolecular in-
teractions (particularly the entanglements) may be de-
clined due to a micro struct ural anisotrop y resulting fro m
the shear deformation and consequently the shear stress
is further decreased [22].
A pronounced shear-thinning behavior of samples may
be interpreted by its rather rigid chain conformation that
gives rise to a highly entangle d macromolecular solutio n
[16] and the presence o f gel-like structure whi ch is chiefly
related to the tendency of molecular association, demon-
strated by the high-hydro gel content of the extract (76%)
and its high M/G ratio (8/2) [18].
All models used in this study were directly applied to
the experimentally measured shear stress-shear rate data
define flow behavior. Although all these mentioned mo d-
els with yield stress indicate high R2, from the point of
being in better agreement with the experimental data and
consistency with theories, Herschel-Bulkley model do
the best (Table 1). Such behavior was obtained other
concentrations. The parameters obtained for the different
models are summarized in Ta ble 1. The results showed
that n values were less than unity conforming that these
products are pseudo-plastic materials at all concentra-
tions studied. The coefficients of determination (R2) ob-
tained were high, indicating that all model were ade-
quately suitable for describing the flow behavior of sam-
ples.
The value of consistency coefficient (K), flow beha-
vior index (n), and yield stress (
σ
°
) ranged from 0.1 to
29.01 Pa. n
s, 0.3 to 1 and 14.63 to 34.66 Pa, respec-
tively. The existence of
σ
°
indicates that there is a
cross-li nked or other interactive in a material which must
be broken down before flow can occur at an appreciable
rate [23,24].
Results show that samples containing hydrocolloids
have a higher viscosity, because hydrocolloid can bond
water in samples and consequently increase the viscosity,
but this increment in up to 0.1% gum concentration
which shows some possible variations in protein-protein
interactions in three dimensional protein networks in
samples. The data’s of water loss confirm these results
(data are not presented here).
4. CONCLUSI O N
Cress seed gum exhibited a positive relationship with
yogurt quality parameters. All samples showed a shear
thinni ng pr ofile. Hersc hel-Bul kley mod el fo und to b e the
best model to describe the rheological behavior. Yogurts
showed to have a yield stress which indicates a cross
linked structure in samples. Addition of cress seed gum
due to its high pharmaceutical properties and simple ex-
traction and availability in the yogurt mixes could be
plausible measure to improve yogurt gel qua lity.
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