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Simultaneous Effects of Total Solids Content, Milk Base, Heat Treatment Temperature and Sample Temperature on the Rheological Properties of Plain Stirred Yogurt

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Response surface methodology was used to establish a relationship between total sol- ids content, milk base, heat treatment temperature, and sample temperature, and consis- tency index, flow behaviour index, and apparent viscosity of plain stirred yogurts. Statisti- cal treatments resulted in developments of mathematical models. All samples presented shear thinning fluid behaviour. The increase of the content of total solids (9.3–22.7 %) and milk base heat treatment temperature (81.6–98.4 °C) resulted in a significant increase in consistency index and a decrease in flow behaviour index. Increase in the sample tempera- ture (1.6–18.4 °C) caused a decrease in consistency index and increase in flow behaviour index. Apparent viscosity was directly related to the content of total solids. Rheological properties of yogurt were highly dependent on the content of total solids in milk.
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by donper america frozen yogurt store on November 29th, 2012 at 11:51 am
It`s an interesting study. I didn`t know about all the properties of yogurt. I like yogurt and it`s very healthy.
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A.L. BARRETTO PENNA et al.: Rheological Properties of Yogurt, Food Technol. Biotechnol. 44 (4) 515–518 (2006)
515
ISSN 1330-9862
original scientific paper
(FTB-1529)
Simultaneous Effects of Total Solids Content, Milk Base,
Heat Treatment Temperature and Sample Temperature on
the Rheological Properties of Plain Stirred Yogurt
Ana Lúcia Barretto Penna1*, Attilio Converti2 and Maricê Nogueira de Oliveira3
1UNESP-Universidade Estadual Paulista, Dept. Food Technology and Engineering,
Rua Cristóvão Colombo 2265, 15054 000 São José do Rio Preto, Brazil
2Università degli Studi di Genova, Dipartimento di Ingegneria Chimica e di Processo,
Via Balbi 5, I-16126 Genova, Italy
3 USP–Universidade de São Paulo, Dept. Pharm. Biochem. Technology,
Av. Lineu Prestes 580, 05508 900 São Paulo, Brazil
Received: July 6, 2005
Accepted: January 30, 2006
Summary
Response surface methodology was used to establish a relationship between total sol-
ids content, milk base, heat treatment temperature, and sample temperature, and consis-
tency index, flow behaviour index, and apparent viscosity of plain stirred yogurts. Statisti-
cal treatments resulted in developments of mathematical models. All samples presented
shear thinning fluid behaviour. The increase of the content of total solids (9.3–22.7 %) and
milk base heat treatment temperature (81.6–98.4 °C) resulted in a significant increase in
consistency index and a decrease in flow behaviour index. Increase in the sample tempera-
ture (1.6–18.4 °C) caused a decrease in consistency index and increase in flow behaviour
index. Apparent viscosity was directly related to the content of total solids. Rheological
properties of yogurt were highly dependent on the content of total solids in milk.
Key words: yogurt, rheology, apparent viscosity, consistency, flow behaviour
Introduction
stirred yogurt depends on acidity and aroma percep-
tions and on the textural properties of the product (5).
Yogurt is a fermented product made from milk forti-
Milk components and their concentrations are also im-
fied with milk solids using Lactobacillus delbrueckii ssp.
portant, especially fat and non-fat solids (6). Shelf life of
bulgaricus and Streptococcus thermophilus (1) as starter
yogurt should be around twenty days, under refrigera-
cultures. It is considered the most popular fermented
tion and the product should maintain its own character-
milk and it is characterized by a soft, viscous gel consis-
istics during storage (3). Beyond that time, the number
tency and a delicate flavour (2). In many countries, yo-
of starter culture bacteria may drop, acidity increase,
gurt is classified according to the fat content (whole,
syneresis often occurs, off-flavours are likely to appear,
semi-skimmed or skimmed), but the most used classifi-
and texture can change.
cation refers to the physical structure of the gel, being
set, stirred or fluid. Set yogurt should have a firm enough
Different technological factors influence the rheolog-
body to be spooned, texture should be fine and smooth,
ical properties of yogurt, such as: (i) factors involved in
without clots or granules and no fissures, and it should
the preparation of the milk base and its heat treatment,
have a typical acid taste (3,4). Consumer acceptance of
(ii) temperature of incubation and the type of culture
*Corresponding author; Phone: ++55 17 32 212 266; Fax: ++55 17 32 212 299; E-mail: analucia@ibilce.unesp.br

516
A.L. BARRETTO PENNA et al.: Rheological Properties of Yogurt, Food Technol. Biotechnol. 44 (4) 515–518 (2006)
employed, and (iii) the cooling process (7,8). In the in-
scribed by the Ostwald-de Waele and Herschel-Bulkley
dustrial production of yogurt, the use of certain equip-
models as used by Penna et al. (10). Apparent viscosity
ment can affect yogurt consistency. Structural losses in
was calculated at shear rate 100 s–1.
stirred yogurt can occur at several places from the incu-
bation tank to the packaging machine. The fermentation
Experimental design
tank must have an agitator to mix the starter culture
The trials were made according to an orthogonal se-
into the milk and, optionally, to break the curd after fer-
cond-order design described by Barros Neto et al. (11).
mentation. The agitation speed is critical. Low agitation
This was composed of 18 trials, 14 axial points and 4
speeds are used to optimize the mixture efficiency and
central points. The independent variables were the con-
to decrease losses of yogurt consistency (6). During the
tent of total solids (TS, 9.3, 12, 16, 20 and 22.7 %), milk
cooling stage, the yogurt is subjected to both shear and
base heat treatment temperature (TT, 81.6, 85, 90, 95 and
time effects. The breakdown of the structure is directly
98.4 °C), and sample temperature (ST, 1.6, 5, 10, 15, and
related to the geometry of the equipment used and the
18.4 °C) (Table 1). The effect of these variables on yogurt
process conditions, especially temperature and flow rate
rheological properties (consistency index, flow behav-
(8).
iour index and apparent viscosity) was studied. The sig-
Based on these considerations, the purpose of this
nificance of the model was tested by analysis of vari-
research was to study how much the content of total sol-
ance (ANOVA) and the influence of the variables was
ids, milk base heat treatment temperature and sample
shown by a three-dimensional representation of the re-
temperature influence the composition and the rheologi-
sponse. In all analyses a significance level of 5 % was
cal properties of plain stirred yogurts. No data are avail-
considered.
able in the literature on the mathematical modeling of
the simultaneous effects of these variables on the rheo-
logical properties of plain stirred yogurt.
Table 1. Coded and actual levels of three variables
Variables
Symbola
Coded level of variablesa
Material and Methods
–1.68
–1
0
+1
+1.68
Total solids
TS/%
X1
9.3
12
16
20
22.7
Yogurt production and control
Treatment
TT/°C
X
temperature
2
81.6
85
90
95
98.4
A volume of 1500 mL of pasteurized whole milk (Pau-
lista, São Paulo, Brazil) was used in each trial. The con-
Sample
ST/°C
X3
1.6
5
10
15
18.4
tent of total solids of the milk was determined with an
temperature
Ackermann calculator (9). Skim milk powder (Molico,
aThe passage from coded variable level to the origin level is
Nestlé, Brazil) was used to standardize the pasteurized
given by the following equations: X1= (TS-16)/4, X2=(TT-90)/5,
whole milk, to reach the desirable total solids (9.3–22.7
and X3=(ST-10)/5
%). The milk base was heat treated in a continuous, in-
direct, helically-coiled tube up to 81.6, 85, 90, 95 or 98.4
°C, and held at that temperature for 3 min in an oil
Results and Discussion
bath. Then it was cooled at 43 °C and inoculated with
0.1 % normal viscous freeze-dried mixed-starter culture
Rheological parameters for yogurt samples mea-
of Lactobacillus delbrueckii ssp. bulgaricus and Streptococ-
sured by the Ostwald-de Waele and the Herschel-Bulk-
cus thermophilus (Chr. Hansen’s, Horsholm, Denmark)
ley models showed adequate fit of the flow curves by
direct vat set type. The inoculated milk was mixed and
both models (data not shown).
incubated at 42 °C. Fermentation was stopped until
All yogurts could be characterized as non-Newto-
pH=(4.3±0.3). Afterwards, the curd was manually stirred
nian fluid (shear thinning), regardless of the variables
by up and down movements for almost 3 min with a
used in the study, and indicated the presence of yield
stainless spoon, according to a standardized protocol,
stresses ranging from 0.21–3.01 Pa. The increase of shear
cooled in an ice water bath, and packed into 150-mL
rate due to handling the curd lowered the yogurt viscos-
plastic cups. The yogurt was stored for 16 h at 5–8 °C
ity and, according to Tamime and Robinson (6), low vis-
before evaluation.
cosity is one of the most common defects of yogurt. An
increase of both the content of total solids (9.3–22.7 %)
Rheological measurements
and milk base heat treatment (81.6–98.4 °C) promoted
Measurements were carried out at temperatures rang-
an increase in the consistency index K and decrease in
ing from 1.6–18.4 °C (according to the trial), using a ro-
the flow behaviour index n. Sample temperature (1.6–18.4
tational rheometer (Rheotest 2.1 model, Freital, Lebke-
°C) also influenced the rheological properties; when the
strabe, Germany) with coaxial cylinder geometry (gap
sample temperature was increased, there was a decrease
0.275 mm). Shear rates ranging from 0.21–1851.81 Pa/s,
in the consistency index and an increase in the flow be-
under upward curves, and the corresponding shear stress
haviour index. Similar effect was reported by Ramaswa-
data were obtained. The data were acquired via a per-
my and Basak (12).
sonal computer using Microcal Origin software, 5.0 ver-
In our study, through second-order design it was
sion (Northampton, MA, USA). A controlled-temperature
possible to obtain quadratic polynomial models describ-
bath circulated water through the jacket surrounding the
ing the three rheological responses: yogurt consistency
rotor and cup assembly to maintain the specified tem-
index (YE1), flow behaviour index (YE2) and apparent vis-
perature used in each trial. The flow curves were de-
cosity (YE3).

A.L. BARRETTO PENNA et al.: Rheological Properties of Yogurt, Food Technol. Biotechnol. 44 (4) 515–518 (2006)
517
Values of consistency index ranged from 0.06–10.31
Pa·sn, using the Ostwald-de Waele model. The second
degree equations for the effect of the content of total sol-
ids (X1), milk base heat treatment temperature (X2), and
sample temperature (X3) on the consistency index (YE1),
adjusted by multiple regression of the 18 trials are pre-
sented in Table 2.
The result of the fitted model for yogurt consistency
showed the dependency of the variable X1 (total solids)
and it was independent of the variables X2 (milk base
heat temperature) and X3 (sample temperature) (Eq. 1,
Table 2). The quadratic coefficient of X1, X2, X3 and their
interactions showed no statistical significance. Thus, with
the increase in the content of total solids of the yogurt,
there is an increase in the consistency index.
Hess et al. (13), while studying the rheological prop-
erties of nonfat yogurt stabilized using Lactobacillus del-
brueckii
ssp. bulgaricus that produces exopolysaccharide
or using a commercial stabilizer, noted that the consis-
tency index (K) increased significantly with each incre-
mental increase in nonfat solids (SNF). K for yogurts fer-
Fig. 1. Effect of the content of total solids, milk base, heat treat-
mented with strains that produce exopolysaccharide
ment and sample temperature on the flow behaviour index of
(Eps) was significantly lower than for non-ropy yogurts.
yogurt
Consistency of yogurts formulated to contain 10 and 12
% SNF was not significantly different; however, consis-
tency was significantly higher for yogurts containing 14
Commercial yogurts evaluated by Ramaswamy and
% SNF. According to Tamime and Robinson (6), consis-
Basak (12) showed apparent viscosity between 68.7–109
tency improves when the content of milk total solids in-
mPa, smaller than that obtained in this study, which
creases from 12–20 %, and a small difference in consis-
varied from 22–425 mPa. This could be explained by the
tency is achieved when the content of total solids varies
structural breakdown during storage and distribution.
from 16–20 %. Thus, there is little interest in the use of
Besides this, Labropoulos et al. (17) showed that gel
concentrations above 16 %.
strength and apparent viscosity were the highest for yo-
gurt made from milk preheated at 82 °C/30 min, fol-
The Eq. 2 (Table 2) was obtained to describe the si-
lowed by milk preheated at 63 °C/30 min, while milk
multaneous effects of the content of total solids and
preheated by a UHT process (149 °C/3.3 s) produced
milk base heat treatment temperature on flow behaviour
yogurt with the lowest gel strength and apparent viscos-
index (YE2). A smaller flow behaviour index was found
ity. Parnell-Clunies et al. (18) studied the effect of pre-
when the content of total solids ranged from 16.8–22.2 %,
heating milk by different methods on the physical prop-
and sample temperature varied between 4.5–16 °C (Fig. 1).
erties of yogurt. They found the following order of the
The apparent viscosity model (Eq. 3) is shown in Ta-
effect of heat treatment on yogurt firmness and apparent
ble 2. It was shown that the content of total solids di-
viscosity: vat-heated (85 °C/10–40 min)>HTST (98 °C/
rectly affected apparent viscosity (YE3). With an increase
0.5–1.87 min)>UHT (140 °C/2–8 s)>unheated milk,
in the content of total solids (9.3–22.7 %), there was an
while water-holding capacity, and protein hydration in-
increase in yogurt apparent viscosity. There was a signi-
dices of yogurts were the highest when manufactured
ficant dependency of total solids on acid milk gel for-
from HTST or UHT-treated milk, and the lowest from
mation that could be attributed to differences in casein
unheated milk; intermediate values were obtained for
micelle composition (14). Apparent viscosity is also a
yogurts manufactured using vat-heated (85 °C/10–40
function of aggregate size (15). Remeuf et al. (16) ob-
min) milk. They found that apparent viscosity and curd
served a relationship between micelle solvation and yo-
firmness were highly correlated with whey protein de-
gurt microstructure, as well as micelle size in milk base
naturation, with apparent viscosity showing higher cor-
and yogurt graininess.
relation coefficient.
Table 2. Adjusted models describing the simultaneous effect of the content of total solids, milk base heat treatment temperature and
sample temperature on the consistency index, flow behaviour index, and apparent viscosity (at 100 s–1) of yogurt
Significance
Standard
Response
Adjusted model
R2
level/%
error
Consistency index
YE1 = 3.85 +3.25 X1 /1/
96
2.27
0.802
Flow behaviour index
YE
2
2
2 = 0.32 – 0.13 X1 +0.06 X1 +0.08 X3 /2/
99
0.08
0.859
Apparent viscosity (at 100 s–1)
YE3 = 160.14 + 126.84 X1 /3/
97
82.20
0.612
X1: total solids (TS/%), X2: milk base heat treatment temperature (TT/°C), X3: sample temperature (ST/°C), R2: determination coeffi-
cient

518
A.L. BARRETTO PENNA et al.: Rheological Properties of Yogurt, Food Technol. Biotechnol. 44 (4) 515–518 (2006)
Conclusions
6. A.Y. Tamime, R.K. Robinson: Yoghurt, Science and Technol-
ogy, CRC Press, Boca Raton, USA (1999).
All yogurt samples investigated in this work showed
7. S.M. Schellhaass, H.A. Morris, Rheological and scanning
non-Newtonian fluid behaviour (shear thinning). With
electron microscopic examination of skim milk gels ob-
increase in the content of total solids (9.3–22.7 %) the
tained by fermenting with ropy and non-ropy strains of
consistency index and apparent viscosity increased and
lactic acid bacteria, Food Microstruct. 4 (1985) 279–287.
flow behaviour index decreased. Increasing the tempera-
8. T. Benezech, J.F. Maingonnat, Flow properties of stirred
yogurt: Structural parameter approach in describing time-
ture of heat treatment (81.6–98.4 °C), an increase in con-
-dependency, J. Food Eng. 21 (1994) 447–473.
sistency index (K) and decrease in flow behaviour index
9. P.H.F. Silva, D.B.C. Pereira, L.L. Oliveira, L.C.G. Costa Ju-
(n) were observed. Higher sample temperatures (1.6–18.4
nior: Físico-química do leite e derivados. Métodos Analíticos
°C) promoted a decrease in consistency index, and in-
(Physical Chemistry of Milk and Other Products. Analytical
creased the flow behaviour index. Apparent viscosity
Methods), Oficina de Impressão Gráfica e Editora Ltda.,
was strongly affected by the content of total solids of
Juiz de Fora, Brazil (1997).
yogurt; with an increase in the total solids there was an
10. A.L.B. Penna, M.N. Oliveira, A.Y. Tamime, The influence
increase in apparent viscosity. Rheological properties of
of carrageenan and total solids content on the quality of
yogurt were highly dependent on the content of total
lactic beverage made with yogurt and whey, J. Texture
solids of milk. Thus, the choice of type and quantity of
Stud. 34 (2003) 95–113.
dry matter fortification of milk should be considered in
11. B.B. Barros Neto, I.S. Scarmínio, R.E. Bruns: Planejamento e
improving rheological properties of yogurt.
Otimização de Experimentos (Planning and Experimental Opti-
mization)
, Unicamp, Campinas, Brazil (1995).
12. H.S. Ramaswamy, S. Basak, Rheology of stirred yogurts, J.
Acknowledgements
Texture Stud. 22 (1991) 231–241.
We thank »FUNDUNESP«, »FAPESP«, and »CNPq«
13. S.J. Hess, R.F. Roberts, G.R. Ziegler, Rheological properties
for their support.
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ssp. bulgaricus producing exopolysaccharides or using com-
mercial stabilizer systems, J. Dairy Sci. 80 (1997) 252–263.
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