560
International Journal of Food Science and Technology 2006, 41, 560–568
Original article
Compositional and physical properties of yogurts
manufactured from milk and whey cheese concentrated
by ultrafiltration
Renata B. Magenis,1* Elane S. Prudêncio,1 Renata D. M. C. Amboni,1 Noel G. Cerqueira Júnior,1
Ricardo V. B. Oliveira,2 Valdir Soldi2 & Honório D. Benedet1
1 Department of Food Science and Technology, Center of Agricultural Science, Federal University of Santa Catarina, Rod.
Admar Gonzaga, 1346, Itacorubi, 88034-001, Florianópolis, SC, Brazil
2 Department of Chemistry, Federal University of Santa Catarina, Campus Universitário, Trindade, Florianópolis, SC, Brazil
(Received 17 December 2004; Accepted in revised form 15 July 2005)
Summary
Keywords
Yogurts made with 80% milk retentate (MR) [Volume Reduction Factor (VRF) ¼ 1.5]
and 20% cheese whey retentate (WR; VRF ¼ 8.0) (yogurt 1) and yogurts made with 100%
MR through ultrafiltration have been evaluated as to flow, texture profile analysis (TPA)
and syneresis index. As with MR and WR, their physico-chemical composition was also
determined. The yogurt to which WR had been added showed; less apparent viscosity and
greater tixotrophya; less firmness and adhesiveness and greater cohesiveness; higher
syneresis index, less protein and mineral content, and greater lipid content in comparison
with the yogurt made only with MR.
Flow properties, milk, syneresis, texture, ultrafiltration, whey cheese, yogurts.
Introduction
Yogurt is defined as a fermented milk product
produced with thermophilic lactic bacteria, usually
Streptococcus thermophilus and Lactobacillus
delbrueckii ssp. bulgaricus (Mottar et al., 1989;
Ginovart et al., 2002) and is one of the most
popular fermented milk products (Lucey et al.,
1999). Three types of yogurts are commercially
available: set-type, stirred and drinking yogurt
(Benezech & Maingonnat, 1994). The formulation
of yogurt products with optimum consistency and
stability to syneresis is of primary concern to the
dairy industry (Biliaderis et al., 1992). Factors
influencing yogurt texture and syneresis include
total solids (TS) content, milk composition (proteins, salts), homogenization, type of culture,
acidity resulting from growth of bacterial cultures
and heat pretreatment of milk (Harwalkar &
Kalab, 1986).
*Correspondent: Fax: 55 48 331 9943;
e-mail: [email protected]
Whey is the soluble fraction of milk that
separates from the curds during the manufacture
of cheese or casein (Morr & Ha, 1993; Quaglia
et al., 1993). The whey produced from rennetcoagulated casein or cheese is referred to as sweet
whey or cheese whey and is an opaque liquid
possessing a greenish-yellow colour, with TS
content generally ranging from 6.0% to 6.5%
(w/v) and a biological oxygen demand (BOD) of at
least 30 000 mg of O2 L)1 (Morr & Ha, 1993;
Brandão, 1994; Pintado et al., 2001). Whey composition varies according to the type of cheese
produced and the technological process used in its
production (Quaglia et al., 1993). It is a source of
lactose, calcium, proteins and soluble vitamins,
and is thus considered as a source of valuable
nutrients (González-Martı́nez et al., 2002). Whey
proteins are well known for their high nutritional
value and versatile functional properties in food
products (de Wit, 1998) and represent 12–15%
total whey dry extract, capable of concentration
and purification by several procedures (Fauquant
et al., 1985). Brazil produced 480 000 tons of
doi:10.1111/j.1365-2621.2005.01100.x
2006 Institute of Food Science and Technology Trust Fund
Yogurts from milk and whey cheese concentrated by ultrafiltration R. B. Magenis et al.
cheese in 2003, yielding 4.32 million L of cheese
whey (Brazil, 2004), 50% of which was used to
feed animals, to treat effluents and as soil fertilizer
(Wasen, 1998). Broome et al. (1982) state that the
incorporation of whey solids into milk derivates
helps dairy industries reduce problems with their
disposal.
Ultrafiltration (UF) is a membrane screening
technology used by dairy industries to concentrate
or separate milk and whey constituents resulting in
a retentate or concentrate (particles bigger than
the membrane pores) which contain protein, fat
and colloidal minerals in higher ratios than those
found in milk not submitted to the process; it also
contains permeate or filtrate (particles smaller
than the membrane pores) which consist of water,
soluble minerals, lactose, non-protein nitrogen
and water soluble vitamins (Rosenberg, 1995;
Rattray & Jelen, 1996). The use of membrane
technologies for the fortification of milk for the
production of fermented dairy products has been
reported (Chapman et al., 1974; Kosikowski,
1979; Abrahamsen & Holmen, 1980; Marshall &
El-Bagoury, 1986; Becker & Puhan, 1989; Biliaderis et al., 1992; Ozer & Robinson, 1999;
Schkoda et al., 2001). Milk concentrated by UF
has been shown to produce a good quality yogurt
(smooth, creamy and with typical acid flavour)
without the need for homogenization (Chapman
et al., 1974). Abrahamsen & Holmen (1980)
observed an increased viscosity and curd firmness
using UF milk for yogurt production. UF also
contributes to an increase of the nutritional value
of fermented milk because of higher protein,
calcium and phosphorus content in final product
(Becker & Puhan, 1989).
UF is a technology applied also to cheese whey,
mainly for the retrieval of the protein fraction
(Rattray & Jelen, 1996; Siso, 1996; Zydney, 1998;
de la Fuente et al., 2002). However, the process
effectiveness is limited by the presence of whey
phospholipids, which slows down the permeate
flow (Fauquant et al., 1985).
Rheological properties are important for foods,
such as fermented dairy products, in the design of
flow processes, quality control, storage and processing and in predicting the texture of foods
(Benezech & Maingonnat, 1994; Aichinger et al.,
2003). On the other hand, and even more importantly, rheological properties determine product
2006 Institute of Food Science and Technology Trust Fund
texture, thereby affecting sensory perception and
ultimately the acceptance of a product by the
consumer (Aichinger et al., 2003). Viscous properties are of primary importance with respect to
the quality of the products. Foodstuffs rarely obey
Newton’s law of viscosity; they exhibit a variety of
non-Newtonian effects, such as shear thinning,
yield stress, viscoelasticity and time-dependency
(Benezech & Maingonnat, 1994).
The flow curves have been described by the
power law model, as used by Benezech &
Maingonnat (1994); Shaker et al. (2000); Penna
et al. (2001) and Koksoy & Kilic (2004). This
model has been used to determine the consistency
and the flow behaviour indices of the samples
using the shear stress data obtained from increasing shear rate measurements as follows:
r ¼ jcg
where r is shear stress, j is the consistency index, c
is shear rate, and g is the flow behaviour index,
which is <1 for pseudoplastic behaviour.
The flow behaviour of stirred yogurt is of
importance in the definition and qualification of
the quality of the product. It is generally admitted
that yogurt exhibits an irreversible time-dependent
effect or irreversible thixotropy (Ramaswamy &
Basak, 1991; Benezech & Maingonnat, 1994;
Afonso & Maia, 1999). de Lorenzi et al. (1995)
defined the yogurt as a material with non-Newtonian flow properties and with strong time dependence of both the thixotropic and viscoelastic types.
di Cagno et al. (2004) noticed pseudoplastic
behaviour in fermented milks made from mare’s
milk.
Another important aspect of a milk gel is whey
separation, which refers to the appearance of a
liquid (whey) on the surface of milk gel. It is a
common defect in fermented milk products such
as yogurt (Lucey, 2001). Syneresis is defined as the
shrinkage of gel and this occurs concomitantly
with expulsion of liquid or whey separation and is
related to instability of the gel network resulting in
the loss of the ability to entrap all the serum phase
(Walstra, 1993). According to Lucey (2001) some
possible causes of wheying-off in acid gels are very
high incubation temperatures, excessive treatment
of the mix, low TS content (protein and/or fat) of
the mix, movement or agitation during or just
after gel formation, very low acid production
International Journal of Food Science and Technology 2006, 41, 560–568
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Yogurts from milk and whey cheese concentrated by ultrafiltration R. B. Magenis et al.
(pH ‡ 4.8), and the extent of wheying-off will
depend on the combinations of these conditions.
Considering that there is little information
about yogurts manufactured from milk and liquid
cheese whey concentrated by UF, the purpose of
this research work was to evaluate the flow
properties, texture profile and syneresis of yogurts
manufactured with 80% of milk retentate (MR)
and 20% of cheese whey retentate (WR) and
manufactured with 100% of MR.
Materials and methods
modifications). The MR (VRF 1.5) was added 10%
(w/w) sucrose and pasteurized at 95 C for 5 min
while the WR (VRF 8.0) was heated at 65 C for
30 min. The retentates were cooled at 42 C and
employed in the manufacturing of the following
yogurts: yogurt (1) ¼ 80% MR and 20% WR, and
yogurt (2) ¼ 100% MR, to which a lactic culture
was added before incubation at 42 C. Fermentation was stopped at pH 4.5, and the yogurts were
cooled at 4 C, gently stirred and stored at
4 ± 1 C, till the analyses were done. Retentate
percentages, as well as VRF had been determined in
previous studies (results unpublished).
Materials
Milk, cheese whey from the making of fresh Minas
cheese, milk thermophilic culture (YC-X11 Yo
Flex, Chr. Hansen, Hønsholm, Denmark) and
sucrose have been used. All the reagents were of
analytical grade.
Ultrafiltration
Milk, previously skimmed and pasteurized at
72 C for 15 s, and cheese whey with the lipoproteic fraction removed (Fauquant et al., 1985)
were ultrafiltered in a pilot unit, with a mineral
membrane (SCT - P1940 GL of 50 nm pores and
0.24 m2 of useful filtering area, Pall Exekia, Bazet,
France). The following operational parameters
were used during the process: (a) 2 bar inlet
pressure and 1 bar outlet pressure for milk and
cheese whey; (b) 32 ± 8 C temperature,
45 ± 4 L h)1m)2 permeate flux, 600–700 L h)1
flow and 0.75 m s)1 flow velocity for milk, and
27 ± 8 C temperature, 117 ± 16 L h)1 m)2 permeate flux, 700–800 L h)1 flow and 0.85 m s)1
flow velocity for cheese whey. UF was carried out
to the point when volumetric reduction factor
(VRF) was 1.5 for milk and 8.0 for cheese whey.
After each UF stage, the equipment was cleaned
following the manufacturer’s instructions. The
experiment was carried out in triplicate.
Yogurt manufacture from milk and cheese whey
retentates obtained from ultrafiltration
Yogurt made from milk (MR) and cheese
whey (WR) retentates followed the methodology described by Lucey & Singh (1998) (with
Physico-chemical characteristics
Milk retentate, WR, yogurt (1) and yogurt (2)
were submitted to the following physico-chemical
analyses: moisture [% (w/w)]; ash [% (w/w)]; lipids
[% (w/w)]; proteins [% (w/w)]; TS [% (w/w))
[Association of Official Analytical Chemists
(AOAC), 1998] and pH. Carbohydrate values [%
(w/w)] were obtained by difference. The measurements of pH were taken with a pH meter (MP 220
Metler Toledo, Greinfensee, Switzerland). All the
analyses were carried out in duplicate.
Physical testing of yogurts
Flow properties measurements, texture profile
analysis (TPA) and syneresis of the yogurts were
evaluated after 5 days of storage at 4 ± 1 C.
Flow properties measurements
The flow properties measurements of the yogurts
were made using a Brookfield rotational rheometer (Brookfield Engineering Laboratories, model
LVDV III, Stoughton, MA, USA), with cone
geometry. The instrument was equipped with a
device that allows continuous speed variation of
the internal cone (CP 51). A controlled ramped
shear rate was carried out to determine the
rheological characteristics of the samples. The
shear rates were increased linearly from 8 to
196 s)1 in 8 min (upward curve) and subsequently
reduced back to 8 s)1 in the next 8 min (downward curve) (rpm ranging from 2 to 50, increasing
1.0 rpm each 10 s). The data were acquired via a
personal computer using Rheocalc software
International Journal of Food Science and Technology 2006, 41, 560–568
2006 Institute of Food Science and Technology Trust Fund
Yogurts from milk and whey cheese concentrated by ultrafiltration R. B. Magenis et al.
(Brookfield Engineering Laboratories). The temperature of the sample cup was adjusted to
5 ± 0.1 C, selected as the usual consumption
temperature, and kept constant with a cooled
water jacket. All experiments were carried out in
duplicate.
The flow curves were described by the power
law model. Viscosity values in the upward viscosity/shear rate curves at a shear rate of 50 s)1 were
taken as the apparent viscosity of the yogurt
samples. This value would represent the approximate viscosity felt in the mouth as the shear rate in
mouth is approximately 50 s)1 (Bourne, 2002).
Thixotropic behaviour of the samples was evaluated by calculating the area of the hysteresis loop
between the upward and downward shear stress/
shear rate curves.
Texture profile analysis
A universal testing machine (Stable Micro System,
Model TA-XT2, Texture Expert, Surrey, UK),
operating software Texture Expert, was used for
the instrumental TPA of yogurts (1) and (2). A
25 mm (P25/L) acrylic probe was used, having the
analysis been carried out in a 50 mL aluminium
capsule, the sample at 5 ± 1 C. Test velocity,
time and distance were ¼ 2.0 mm s)1, 5.0 s and
5.0 mm, respectively. All measurements were
made six times.
From the TPA curves, the following texture
parameters were obtained: firmness, springiness,
cohesiveness and adhesiveness (Fig. 1). Firmness
was defined by peak force during the first compression cycle. Cohesiveness was calculated as the
ratio of the area under the second curve to the area
under the first curve. Springiness was defined as a
ratio of the time recorded between the start of the
second area and the second probe reversal to the
time recorded between the start of the first area
and the first probe reversal. Adhesiveness was the
negative area under the curve obtained between
cycles.
Syneresis
The index of syneresis of yogurts (1) and (2) was
evaluated according to the method proposed by
Modler & Kalab (1983). A 100 mL sample of each
yogurt was drained through a 100-mesh stainless
screen placed on the top of a long stemmed funnel,
which was introduced in a graduated cylinder to
collect the liquid. The liquid quantity (mL) per
100 mL of sample was taken as an index of
syneresis after 2 h of draining at 5 ± 1 C. All
experiments were carried out in duplicate.
Statistical analysis
The mean values, standard deviation, variance
analysis (5% significant) were calculated with
Statsoft software, Statistica version 6.0 (Statsoft
Inc., 2001).
Results and discussion
Physico-chemical characteristics
Force (g) 1
23
4
56
100.0
80.0
Area 1
60.0
Area 2
40.0
20.0
0.0
0.0
–20.0
10.0
20.0
30.0
40.0
50.0
60.0
Time (s)
–40.0
–60.0
Area 3
Figure 1 Typical force by time plot through two cycles of
penetration to determine texture profile analysis parameters.
Firmness ¼ peak 2; cohesiveness ¼ area 2/area 1; springiness ¼ relation between time pass away points 4:5 and 1:2;
adhesiveness ¼ area 3.
2006 Institute of Food Science and Technology Trust Fund
The average results of milk and cheese WRs, and
yogurts (1) and (2) physico-chemical compositions
are shown in Table 1. Of note was the significant
difference (P < 0.05) between yogurts as to
protein, lipid and ash contents, yogurt (2)
displaying higher protein and ash contents and
lower lipid content than yogurt (1) (P < 0.05).
The addition of WR to MR in the manufacturing
of yogurt has contributed to the decrease of the
protein content and to the increase of the lipid
content of the yogurt, because of the chemical
characteristics of retentates (Table 1). There was
no significant difference (P > 0.05) in TS, carbohydrate and moisture contents and pH values
between yogurts.
International Journal of Food Science and Technology 2006, 41, 560–568
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Yogurts from milk and whey cheese concentrated by ultrafiltration R. B. Magenis et al.
Table 1 Results of the average physico-chemical composition of milk and cheese whey retentates, and yogurts (1) and (2)
Analyses
Whey retentate (VRF 8.0)
TS [% (w/w)]
Proteins [% (w/w)]
Lipids [% (w/w)]
Moisture [% (w/w)]
Ash [% (w/w)]
Carbohydrates [% (w/w)]
pH
9.48
2.91
1.25
90.52
0.58
4.74
6.12
±
±
±
±
±
±
±
0.63
0.33
0.12
0.56
0.04
0.21
0.02
Milk retentate (VRF 1.5)
9.68
4.18
0.14
90.27
0.81
4.59
6.49
±
±
±
±
±
±
±
0.31
0.19
0.02
0.34
0.04
0.12
0.04
Yogurt (1)
16.91
3.31
0.56
83.10
0.68
12.36
4.46
±
±
±
±
±
±
±
Yogurt (2)
a
0.21
0.12a
0.02a
0.21a
0.01a
0.29a
0.05a
16.93
3.54
0.46
83.07
0.72
12.36
4.46
±
±
±
±
±
±
±
0.13a
0.10b
0.02b
0.12a
0.01b
0.26a
0.03a
Mean values with the same superscript letter in same line are not significantly different (P < 0.05).
Yogurt (1): yogurt with 80% of milk retentate (MR) and 20% of whey retentate (WR).
Yogurt (2): yogurt with 100% of MR.
VRF, Volumetric Reduction Factor; TS, total solids.
Physical testing of yogurts
Flow properties measurements
The apparent viscosity of the yogurt samples
decreased with increasing shear rate, indicating
non-Newtonian behaviour (Fig. 2a and b). This
result is in accordance with results of previous
studies on labneh (Abu-Jdayil & Mohameed,
(a)
1000
900
Viscosity (mPa.s)
800
700
600
500
400
300
200
2002) and on fermented mares milk (di Cagno
et al., 2004). The shear thinning behaviour was
expected in yogurts as the texture of fermented
milk products is affected by weak physical bonds,
electrostatic and hydrophobic interactions (Kinsella, 1984). Therefore, the fall in the apparent
viscosity of yogurts with shear rate was found to
be a result of the destruction of the interactions.
The power law model was found to be suitable
in this study to fit the shear stress data of yogurts
samples at increasing shear rate (Table 2). The
correlation coefficient for the model fit was above
0.98 in all cases. The apparent viscosity of yogurts
was decreased with the addition of whey in yogurt
formulation. According to Tamime & Robinson
(1991), fermented beverages added to cheese
whey present the characteristics of lower
viscosity. Protein content also determines viscosity
100
0
0
50
100
150
200
Shear rate (s–1)
(b)
Table 2 Rheological parameters of yogurts (1) and (2)
obtained by power law model (r ¼ jcg) at 5 ± 0.1 C
Sample
of
yogurt
1000
900
Apparent
Consistency Flow
behaviour viscosity Thixotropy
index
(K, mPaÆsg) index (g) (mPaÆs)1)a (PaÆs)1)b
800
Viscosity (mPa.s)
564
700
600
500
400
300
200
Upward curve
Yogurt (1) 2.66
Yogurt (2) 2.80
0.36
0.35
181
207
186
69
Downward curve
Yogurt (1) 1.82
Yogurt (2) 2.16
0.43
0.41
–
–
–
–
100
0
0
50
100
150
200
Shear rate (s–1)
Figure 2 Apparent viscosity · shear rate relationship of
yogurt (1) (a) and yogurt (2) (b) at 5 ± 0.1 C.
Yogurt (1): yogurt with 80% of milk retentate (MR) and 20% of
whey retentate (WR).
Yogurt (2): yogurt with 100% of MR.
a
Apparent viscosity at shear rate of 50 s)1.
b
Hysteresis loop area between the upward and downward
shear stress/shear rate curves.
International Journal of Food Science and Technology 2006, 41, 560–568
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Yogurts from milk and whey cheese concentrated by ultrafiltration R. B. Magenis et al.
2006 Institute of Food Science and Technology Trust Fund
(a) 20
18
Shear stress (Pa)
16
14
12
10
8
6
4
2
0
0
50
100
150
200
150
200
Shear rate (s–1)
(b)
18
16
Shear stress (Pa)
(Abu-Jdayil, 2003). According to Table 1, yogurt
(2) has displayed a higher protein content than
yogurt (1) and also greater viscosity. Similar
results were found by Abu-Jdayil (2003) who
found greater viscosity in yogurts of the type
labneh, with higher protein content.
According to Benezech & Maingonnat (1994)
and Penna et al. (2001), the main characteristic of
the relationship shear stress/shear rate is the
development of a hysteresis curve; the higher the
area below the curve, the higher the thixotropic
effect. When a sample is sheared at increasing and
then at decreasing shear rates, the observation of
the hysteresis area between the curves representing
shear stress values indicates that the sample’s flow
is time dependent (Tárrega et al., 2004). The area
enclosed between the up-and-down curves (hysteresis loop) is a measure of the extent of the
structural breakdown during the shearing cycle
(Ramaswamy & Basak, 1991). Mottar et al. (1989)
calculated the areas of the hysteresis-loop curves
observed as a degree of thixotropy. Figure 3
demonstrates the occurrence of the hysteresis of
the rheological behaviour of yogurts, in which
they were subjected to a cycle of increasing and
decreasing shear rate. It also shows that yogurt is a
shear thinning material, which exhibits a thixotropic behaviour. It is generally admitted that
yogurt exhibits an irreversible time-dependent
effect or irreversible thixotropy (Benezech &
Maingonnat, 1994).
Referring to Table 2, the thixotropy was higher
with the addition of WR. Thixotropy is caused by
the structural break down in a dispersion under
shear. Weak particles in a suspension or the weak
interparticle bonds can be broken under shear
(Shoemaker & Figoni, 1984). Teo et al. (2000)
related that the thixotropy in heated whey proteins
suspension was attributed to particle breakage, or
to breakage of disulphide bonds, van der Waals,
ionic and hydrophobic interactions between the
protein particles. Particle breakage and breakage
of weak bonds between particles could also cause
thixotropy in yogurt.
At 5 ± 0.1 C consistency indices calculated by
the power law model ranged from 2.66 to
2.80 mPaÆsg (upward curves), and from 1.82 to
2.16 mPaÆsg (downward curves). Both yogurt types
behaved as pseudoplastic fluid (g < 1), thus
confirming a non-Newtonian behaviour.
14
12
10
8
6
4
2
0
0
50
100
Shear rate (s–1)
Figure 3 Shear stress · shear rate relationship (flow curves)
for yogurt (1) (a) yogurt (2) (b) at 5 ± 0.1 C, during a
programmed cycle up and down shearing between shear
rates of 0 and 196 s)1.
Texture profile analysis
Texture profile analysis results for yogurt samples
are shown in Table 3. Four parameters were
obtained; firmness, adhesiveness, springiness and
Table 3 Results of the average Texture Profile Analyse
(TPA) and syneresis index of yogurts manufactured from
milk (MR) and cheese whey (WR) retentates at 5 ± 1 C
Parameters
Firmness (g)
Adhesiveness (gÆs)
Springiness
Cohesiveness
Syneresis
index [mL (100 mL))1]
Yogurt (1)
9.60
)5.41
0.91
0.78
40.00
±
±
±
±
±
Yogurt (2)
b
0.34
1.12a
0.03a
0.02a
0.00a
14.14
)12.94
0.91
0.71
36.00
±
±
±
±
±
0.97a
3.28b
0.03a
0.05b
0.60b
Mean values with the same superscript letter in same line are
not significantly different (P < 0.05).
Yogurt (1): yogurt with 80% of milk retentate (MR) and 20% of
whey retentate (WR). Yogurt (2): yogurt with 100% of MR.
International Journal of Food Science and Technology 2006, 41, 560–568
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Yogurts from milk and whey cheese concentrated by ultrafiltration R. B. Magenis et al.
cohesiveness. There was an observable significant
difference among all the parameters, except for
springiness. It has also been observed that the
addition of WR (yogurt 1) contributed to the
increase of cohesiveness and to a decrease in
firmness and adhesiveness when compared with
the yogurt made only with MR (yogurt 2).
The firmness of yogurt is dependent on TS
content (Tamime & Deeth, 1980; Gastaldi et al.,
1997; Penna et al., 1997; Kristo et al., 2003), on
the protein content of the product (Trachoo &
Mistry, 1998; Abu-Jdayil, 2003), and on the type
of protein (Cho et al., 1999). Although TS content
did not significantly vary between yogurts, protein
content was significantly lower in yogurt 1
(P < 0.05) which may have resulted in a lower
firmness of the yogurt substituted by WR. These
results are consistent with those from Oliveira
et al. (2001), who found lower firmness of the
fermented milk enriched with whey, and from
Antunes et al. (2003), who found the best results
for firmness with higher levels of protein concentration in acid gels. Puvanenthiran et al. (2002)
reported that decreasing the casein: whey protein
ratio in milk destined for yogurt manufacture by
substituting whey protein concentrated caused a
lower firmness of the final yogurt.
The addition of WR to MR in the manufacturing of yogurt has contributed to lower adhesiveness in yogurt (1). This result could indicate a
tendency of the yogurt with higher protein content
to become associated with the surface of the
texturometer solid rod. Cheese whey proteins
which show better gelatinizing properties are alactoalbumin and ß-lactoglobulin, the latter being
considered the main gelatinizing agent because of
the presence of free sulphhydryls (Rattray & Jelen,
1997). Therefore, WR addition in the manufacturing of yogurt (1) may have influenced in the
increase of cohesiveness once this parameter is
related to the forces involved in the internal bonds
of the product.
Syneresis
The yogurt made with WR has presented a higher
index of syneresis (P < 0.05) than that yogurt
made only with MR, as shown in Table 3. This
behaviour may be attributed to the higher protein
content of the yogurt made with MR (Table 1).
These results are similar to those found by Modler
et al. (1983) who, by adding different lactic protein
concentrations to yogurts, verified that the decrease of the syneresis index might be related to
the greater protein concentration in yogurt,
because of intensified water retention by the
protein matrix (Mangino, 1984).
The addition of whey proteins to yogurt through
the incorporation of WR in its formula may also
have contributed to an increase in syneresis. These
observations were similar to those reported by
Modler & Kalab (1983) who, by adding whey
protein concentrated through UF to the yogurt,
obtained an increase in the syneresis index.
Conclusion
The power-law model was applied successfully to
describe the flow properties of yogurt. Both types
of yogurt behaved as pseudoplastic fluid (g < 1),
confirming a non-Newtonian behaviour. The
addition of WR contributed to the increase of
the thixotropy. Yogurt with WR showed higher
cohesiveness and lower firmness and adhesiveness
than yogurt manufactured only with MR. The
lower protein content of the product and the type
of protein of the WR may be responsible for the
increase in the syneresis and a decrease in the
firmness of the yogurt (1).
Acknowledgments
The authors wish to thank Coordenação de
Aperfeiçoamento de Ensino Superior (CAPES)
for financial support; Federal University of Santa
Catarina (UFSC); Techniques Industrielles Apliquèes (TIA); Victoria Alimentos Ltda and Borsato Industrial.
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