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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 membranetechnologies 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; Bilia-

deris 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 pro-

cessing and in predicting the texture of foods

(Benezech & Maingonnat, 1994; Aichinger et al.,

2003). On the other hand, and even more import-

antly, rheological properties determine product

texture, thereby affecting sensory perception and

ultimately the acceptance of a product by the

consumer (Aichinger et al., 2003). Viscous prop-

erties are of primary importance with respect to

the quality of the products. Foodstuffs rarely obey

Newtons 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 increas-

ing shear rate measurements as follows:

r jcg

wherer is shear stress, j is the consistency index, c

is shear rate, and g is the flow behaviour index,

which is

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(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

Materials

Milk, cheese whey from the making of fresh Minas

cheese, milk thermophilic culture (YC-X11 Yo

Flex, Chr. Hansen, Hnsholm, Denmark) and

sucrose have been used. All the reagents were ofanalytical grade.

Ultrafiltration

Milk, previously skimmed and pasteurized at

72 C for 15 s, and cheese whey with the lipop-

roteic 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, 600700 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 per-

meate flux, 700800 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 manufacturers 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 methodo-

logy described by Lucey & Singh (1998) (with

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) 80% MR and 20% WR, and

yogurt (2) 100% MR, to which a lactic culture

was added before incubation at 42 C. Fermenta-

tion 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 beendetermined in

previous studies (results unpublished).

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 measure-

ments 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 rheom-

eter (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. Theshear 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 (down-

ward 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

Yogurts from milk and whey cheese concentrated by ultrafiltration R. B. Mageniset al.562

International Journal of Food Science and Technology 2006, 41, 560568 2006 Institute of Food Science and Technology Trust Fund

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(Brookfield Engineering Laboratories). The tem-

perature 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 visco-

sity/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 approxi-

mate viscosity felt in the mouth as the shear rate in

mouth is approximately 50 s)1 (Bourne, 2002).

Thixotropic behaviour of the samples was evalu-

ated 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 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 com-

pression 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 tocollect 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

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 theprotein 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, carbo-

hydrate and moisture contents and pH values

between yogurts.

Area 1

Force (g)

Time (s)

100.0

80.0

60.0

40.0

20.0

0.00.0 10.0 20.0 30.0 40.0 50.0 60.0

20.0

40.0

60.0

21 3 4 56

Area 2

Area 3

Figure 1 Typical force by time plot through two cycles of

penetration to determine texture profile analysis parameters.

Firmness peak 2; cohesiveness area 2/area 1; springi-

ness relation between time pass away points 4:5 and 1:2;

adhesiveness area 3.

Yogurts from milk and whey cheese concentrated by ultrafiltration R. B. Mageniset al. 563

2006 Institute of Food Science and Technology Trust Fund International Journal of Food Science and Technology 2006,41, 560568

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Physical testing of yogurts

Flow properties measurements

The apparent viscosity of the yogurt samples

decreased with increasing shear rate, indicatingnon-Newtonian behaviour (Fig. 2a and b). This

result is in accordance with results of previous

studies on labneh (Abu-Jdayil & Mohameed,

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 (Kin-sella, 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

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) Milk retentate (VRF 1.5) Yogurt (1) Yogurt (2)

TS [% (w/w)] 9.48 0.63 9.68 0.31 16.91 0.21a 16.93 0.13a

Proteins [% (w/w)] 2.91 0.33 4.18 0.19 3.31 0.12a 3.54 0.10b

Lipids [% (w/w)] 1.25 0.12 0.14 0.02 0.56 0.02a 0.46 0.02b

Moisture [% (w/w)] 90.52 0.56 90.27 0.34 83.10 0.21a

83.07 0.12a

Ash [% (w/w)] 0.58 0.04 0.81 0.04 0.68 0.01a 0.72 0.01b

Carbohydrates [% (w/w)] 4.74 0.21 4.59 0.12 12.36 0.29a 12.36 0.26a

pH 6.12 0.02 6.49 0.04 4.46 0.05a 4.46 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.

(a)

(b)

0 50 100 150 2000

100

200300

400

500

600

700

800

900

1000

Viscosity(mPa.s

)

0

100

200

300

400

500

600

700

800

900

1000

V

iscosity(mPa.s

)

Shear rate (s1)

0 50 100 150 200

Shear rate (s1)

Figure 2 Apparent viscosity shear rate relationship of

yogurt (1) (a) and yogurt (2) (b) at 5 0.1 C.

Table 2 Rheological parameters of yogurts (1) and (2)

obtained by power law model (r jcg) at 5 0.1 C

Sample

of

yogurt

Consistency

index

(K, mPasg)

Flow

behaviour

index (g)

Apparent

viscosity

(mPas1)aThixotropy

(Pas1)b

Upward curve

Yogurt (1) 2.66 0.36 181 186

Yogurt (2) 2.80 0.35 207 69

Downward curve

Yogurt (1) 1.82 0.43

Yogurt (2) 2.16 0.41

Yogurt (1): yogurt with 80% of milk retentate (MR) and 20% of

whey retentate (WR).

Yogurt (2): yogurt with 100% of MR.aApparent viscosity at shear rate of 50 s)1.bHysteresis loop area between the upward and downward

shear stress/shear rate curves.



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(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 samples flow

is time dependent (Ta rrega et al., 2004). The area

enclosed between the up-and-down curves (hys-

teresis loop) is a measure of the extent of the

structural breakdown during the shearing cycle(Ramaswamy & Basak, 1991). Mottaret 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 thixo-

tropic 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 mPasg (upward curves), and from 1.82 to

2.16 mPasg (downward curves). Both yogurt types

behaved as pseudoplastic fluid (g < 1), thus

confirming a non-Newtonian behaviour.

Texture profile analysis

Texture profile analysis results for yogurt samples

are shown in Table 3. Four parameters were

obtained; firmness, adhesiveness, springiness and

Shearstress(Pa)

Shearstres

s(Pa)

Shear rate (s1)

(a)20

18

16

14

12

10

8

6

4

2

00 50 100 150 200

Shear rate (s1)

(b)

0 50 100 150 200

18

16

14

12

10

8

6

4

2

0

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.

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 Yogurt (1) Yogurt (2)

Firmness (g) 9.60 0.34b 14.14 0.97a

Adhesiveness (gs) )5.41 1.12a )12.94 3.28b

Springiness 0.91 0.03a 0.91 0.03a

Cohesiveness 0.78 0.02a 0.71 0.05b

Syneresis

index [mL (100 mL))1]

40.00 0.00a 36.00 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.



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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 thefermented milk enriched with whey, and from

Antunes et al. (2003), who found the best results

for firmness with higher levels of protein concen-

tration 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 manufactur-

ing of yogurt has contributed to lower adhesive-

ness 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 a-

lactoalbumin and -lactoglobulin, the latter being

considered the main gelatinizing agent because of

the presence of free sulphhydryls (Rattray & Jelen,

1997). Therefore, WR addition in the manufactur-

ing 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 de-

crease 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 typesof 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 Coordenacao de

Aperfeicoamento de Ensino Superior (CAPES)

for financial support; Federal University of Santa

Catarina (UFSC); Techniques Industrielles Ap-

lique` es (TIA); Victoria Alimentos Ltda and Bor-

sato Industrial.

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