caracteristici fizice iaurt

8/17/2019 CARACTERISTICI FIZICE IAURT

1/12

International Dairy Journal 16 (2006) 40–51

Physical characteristics of yoghurts made using exopolysaccharide-

producing starter cultures and varying casein to whey protein ratios

T. Amatayakula, A.L. Halmosb, F. Sherkatb, N.P. Shaha,

aFaculty of Science Engineering and Technology, School of Molecular Sciences, Victoria University, Werribee Campus, PO Box 14428,

Melbourne City MC, Victoria 8001, AustraliabDepartment of Food Science, RMIT University, City Campus, GPO Box 2476V, Melbourne 3001, Victoria, Australia

Received 12 February 2004; accepted 11 January 2005

Abstract

This study investigated the physical characteristics of set and stirred yoghurts made at 9% (w/w) total solids with various casein

(CN)-to-whey protein (WP) ratios and with exopolysaccharide (EPS)-producing starter cultures (capsular or ropy) during storage.

The yoghurt was evaluated for composition, firmness and syneresis of set yoghurt, and for the flow curve and the area of hysteresis

loop between the upward and downward curve of stirred yoghurt. Viable counts of starter bacteria and concentrations of lactic acid

and EPS in the yoghurt were also determined. EPS concentration did not decrease during storage for 28 d. Firmness and syneresis of

set yoghurt decreased when the CN-to-WP ratio was reduced from ratio 4:1 to 1:1 and when EPS starter cultures (especially ropy

EPS) were used. Stirred yoghurt with a CN-to-WP ratio of 3:1 and made using ropy EPS-producing starter cultures had a higher

shear stress and hysteresis loop area than yoghurt made using capsular EPS- or non-EPS-producing starter cultures. The results

suggested that the physical characteristics of set and stirred yoghurts can be improved by varying CN-to-WP ratio and by the use of

EPS-producing starter cultures.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Yoghurts; Exopolysaccharide; Texture; Rheology; Storage

1. Introduction

Appearance and physical characteristics are impor-

tant quality parameters of yoghurt. Good quality

yoghurt should be thick and smooth with no signs of

syneresis. Set yoghurt with a high level of syneresis on

the surface may be regarded as a low quality product,

even though this is a natural phenomenon. Convention-ally, syneresis is reduced by increasing the total solids of

yoghurt mix to around 14% (w/w) with dry dairy

ingredients (Tamime & Deeth, 1980) or by using

stabilizers. Dry dairy ingredients such as skim milk

powder (SMP), whey protein isolate (WPI), whey

protein concentrate (WPC), sodium (Na)-caseinate or

calcium (Ca)-caseinate are commonly used to increase

the solids content of the yoghurt mix. Nevertheless,

fortification with these ingredients affects production

costs. The use of stabilizers including gelatin, modified

starches, or gums may affect the consumer perception of

yoghurt. The use of stabilizers is also prohibited in some

European countries (De Vuyst & Degeest, 1999).Yoghurts fortified with casein-based ingredients

(SMP, Na-caseinate or Ca-caseinate) showed an in-

crease in firmness (or viscosity) and a reduction in

syneresis compared with unfortified yoghurt (Modler,

Larmond, Lin, Froehlich, & Emmons, 1983; Guzmán-

Gonza ´ lez, Morais, & Amigo, 2000; Remuef, Mo-

hammed, Sodini, & Tissier, 2003). On the other hand,

there were no consistent trends between the physical

characteristics of yoghurts and the addition of whey

ARTICLE IN PRESS

www.elsevier.com/locate/idairyj

0958-6946/$- see front matter r 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.idairyj.2005.01.004

Corresponding author. Tel.: +61 3 92168289;

fax: +613 92168284.

E-mail address: [email protected] (N.P. Shah).

http://www.elsevier.com/locate/idairyjhttp://www.elsevier.com/locate/idairyj


2/12

protein (WP)-based ingredients (WPI or WPC). Modler

et al. (1983) and Guzma ´ n-Gonza ´ lez, Morais, Ramos,

and Amigo (1999) found that yoghurt supplemented

with WPC had lower apparent viscosity and firmness

than control yoghurt made without fortification. Baig

and Prasad (1996) and Bhullar, Uddin, and Shah (2002)

found that supplementation of milk with WPC im-proved the apparent viscosity and textural properties of

the resultant yoghurt. These differences may be due to

the variations in the composition of whey protein-based

ingredients. Remuef et al. (2003) showed that extending

the heating time of milk supplemented with WPC from 1

to 5min at 90 1C increased the apparent viscosity of

stirred yoghurt. On the other hand, the apparent

viscosity of stirred yoghurt fortified with Na- and Ca-

caseinate was not affected by the increase in the heating

time. Nonetheless, fortification with WPC reduced

syneresis dramatically.

There has been an increasing trend in the use of

starter cultures that are able to produce exopolysacchar-

ides (EPS). These EPS are heteropolysaccharides and

exist in two forms: capsular (associated with bacterial

cell surface) and ropy or slimy (secreted into the

environment) (Cerning, 1990). In some cases, bacteria

can produce both forms of EPS. EPS-producing starter

cultures are becoming increasingly popular due to their

high water binding and texture promoting abilities. Both

capsular and ropy EPS possess high water binding

ability resulting in increased water retention in yoghurt

(Wacher-Rodarte et al., 1993; Hassan, Frank, Schmid,

& Shalabi, 1996b; Jaros, Rohm, Haque, Bonaparte, &

Kneifel, 2002) and cheeses (Perry, McMahon, & Oberg,1997). Fermented dairy products produced using EPS-

producing starter cultures showed lower firmness than

yoghurts made using control culture (Hassan, Frank,

Schmid, & Shalabi, 1996a); in addition, these authors

found that each strain of EPS-producing starter culture

influenced the rheological properties of yoghurt differ-

ently. Stirred yoghurt made with ropy EPS-producing

starter cultures had higher apparent viscosity than

yoghurts made using capsular and non-EPS-producing

starter cultures, respectively (Marshall & Rawson,

1999).

The use of EPS-producing starter cultures in yoghurtmanufacture has the potential to replace or reduce the

use of stabilizers as well as added dairy ingredients.

Recent work by Puvanenthiran, Williams, and Augustin

(2002), who studied the effects of varying casein (CN)-

to-WP ratios on physical characteristics of set yoghurt,

showed improvements in physical characteristics (syner-

esis and gel strength) when the ratio of CN-to-WP were

reduced. By combining the use of EPS-producing starter

cultures with the alteration of CN-to-WP ratio, it may

be possible to maintain comparable physical character-

istics to normal yoghurts without the need for fortifica-

tion of milk or the use of stabilizers. The objective of

this study was to investigate the combined effects of

varying CN-to-WP ratio and the use of EPS starter

cultures on the physical characteristics of set and stirred

yoghurt, with a low total solids content, throughout a

28 d storage period at 4 1C.

2. Materials and methods

2.1. Experimental design and statistical analysis

Set and stirred yoghurts were produced using three

types of starter cultures (non-EPS-, capsular EPS- or

ropy EPS-producing starter cultures) and four CN-to-

WP ratios (4:1, 3:1, 2:1 or 1:1) in triplicate giving a total

of 72 batches. All measurements on yoghurts were

carried out in triplicate at 1, 7, 14, 21 and 28 d.

2.2. Microorganisms

Non-EPS-producing starter cultures (Streptococcus

thermophilus ASCC 1342 and Lactobacillus delbrueckii

ssp. bulgaricus ASCC 1466), capsular EPS-producing

starter culture (S. thermophilus ASCC 285), and ropy

EPS-producing starter culture (S. thermophilus ASCC

1275) were used in this study. These bacteria were

previously obtained from the Australian Starter Culture

Research Centre, Werribee, Australia, and were char-

acterized by Zisu and Shah (2003) for their EPS

production. Stock cultures were maintained at 80 1C

in 12% (w/w) sterile reconstituted skim milk (RSM) and

40% (v/v) sterile glycerol. The microorganisms wereactivated in 9% (w/w) sterile RSM and for 18 h. The

process was repeated three times prior to yoghurt

manufacture. S. thermophilus and L. delbrueckii ssp.

bulgaricus were incubated at 37 and 42 1C, respectively.

Viable counts in yoghurt were enumerated separately. S.

thermophilus was enumerated aerobically in M17 agar

(Amyl Media Pty. Ltd., Dandenong, Australia) at 37 1C

for 48 h, and L. delbrueckii ssp. bulgaricus was enumer-

ated anaerobically in MRS agar (Merck, Darmstadt,

Germany) at 42 1C for 48h (Dave & Shah, 1998).

2.3. Manufacture of yoghurts

Low heat SMP (34% (w/w) total protein, Murray

Goulbourne Co-operative Co. Ltd., Brunswick, Aus-

tralia), WPC 80 (76% (w/w) total protein, United Milk

Tasmania Ltd., Spreyton, Tasmania, Australia) and

lactose monohydrate (Merck, Darmstadt, Germany)

were blended at suitable quantities in order to vary CN-

to-WP ratios to 4:1, 3:1, 2:1 and 1:1. Lactose was added

to balance the level of total solids. Dry ingredients were

blended to 9% total solids (w/w) and hydrated with

distilled water overnight followed by heat treatment at

85 1C for 30 min (at natural pH of milk blends), cooled

ARTICLE IN PRESS

T. Amatayakul et al. / International Dairy Journal 16 (2006) 40–51 41


3/12

(42 1C) followed by inoculation with 1% (v/v) each of

non-EPS- or EPS-producing strain of S. thermophilus

(S. thermophilus ASCC 1342: non-EPS; S. thermophilus

ASCC 285: capsular; S. thermophilus ASCC 1275: ropy)

and L. delbrueckii ssp. bulgaricus ASCC 1466. Inocu-

lated mix (100 mL) was incubated in a closed-top plastic

container (top diameter 69 mm, bottom diameter 60 mmand 46mm height) at 421C until a pH of 4.70 was

reached; the yoghurts were then transferred to a walk-

in-cooler (4 1C). For making stirred yoghurt, set yoghurt

was pressed with a spoon through a sieve (pore size

1 mm2), and stored at 4 1C overnight.

2.4. Compositional analysis of liquid milk

The contents of total solids, fat, ash and protein

(Kjeldahl method) were determined according to meth-

ods described in AOAC (1995). Casein concentration

was determined by the difference between the level of total protein and non-casein nitrogen. The level of whey

proteins was determined by the difference between non-

casein nitrogen and non-protein nitrogen. Lactose was

quantified enzymatically using a sucrose/lactose/glucose

kit (Megazyme, Wicklow, Ireland). Proteins in samples

were precipitated by 12% (w/v) TCA (Sigma-Aldrich

Co., St. Louis, MO, USA) and filtered out using

Whatman no. 1 filter paper (Whatman International

Ltd. Maidstone, England). The pH of the clear filtrate

was adjusted to 4.50 using 2 M NaOH, and diluted 20

times with acetate buffer, pH 4.50. Quantification of

lactose was carried out according to the procedure

described in the test kit.

2.5. Determination of lactic acid concentration of yoghurt

The concentration of lactic acid was determined by

high performance liquid chromatography (HPLC). The

HPLC system (Varian, Varian Associates, Walnut

Creek, CA, USA) consisted of a solvent delivery system

(Varian, model 9012) connecting with an autosampler

(Varian, model 9100), a variable wavelength ultraviolet

(UV)–visible (VIS) light detector (Varian, model 9050)

and an organic acid analysis column (Aminex HPX-87H, 300 7.8 mm, Bio-Rad Lab, Richmond, CA,

USA). The method of Shin, Lee, Pestka, and Ustunol

(2000) was followed. The mobile phase used was 0.009 N

H2SO4, and the flow rate was set at 0.6 mL min1. The

temperature of the column was set at 65 1C. Lactic acid

was detected by UV detector at 220 nm. Five grams of

the sample were mixed with 100 mL of 15.8 N HNO3 and

5.9 mL of 0.009 N H2SO4 before centrifugation of a

1.5 mL aliquot at 11,600 g for 15 min at room

temperature. The supernatant was filtered through a

0.45mm membrane filter (Schleicher & Schvell, Dassel,

Germany) and 50 mL of the filtrate was injected. Lactic

acid standard was purchased from Sigma (Sigma

Chemical Co., St. Louis, MO, USA).

2.6. EPS purification and quantification in yoghurt

Proteins in 50 mL of diluted yoghurt sample (1:1

yoghurt:Milli-Q water; Millipore Corp, Bedford, MA,USA) were precipitated by adding 4 mL of 20% (w/v)

TCA (Sigma Chemical Co.). Precipitated proteins were

separated by centrifugation at 3313 g for 30min at

4 1C. The supernatant was adjusted to pH 6.8 with 40%

(w/v) NaOH followed by boiling the supernatant at

100 1C for 30 min to denature whey proteins. Denatured

whey proteins were separated by centrifugation at

3313 g for 30min at 4 1C. An equal volume (25 mL)

of cold absolute ethanol was mixed with the supernatant

to precipitate the carbohydrates from the supernatant.

The precipitation was carried out overnight at 4 1C, and

the precipitate was separated by centrifugation at

3313 g for 30 min at 4 1C. The resultant carbohydrate

pellet was completely dissolved by adding 10 mL of

Milli-Q water and the resultant suspension was sub-

jected to sonication for 1 h at room temperature. The

resultant solution was dialysed at 4 1C in a dialysis

membrane tube with molecular mass cut-off value of

13,000 Da (Carolina Biological Supply Company, NC,

USA) against tap water over a 2-week period. Water

was changed twice a day. The EPS concentration was

quantified using the phenol-sulphuric method of Du-

bois, Gilles, Hamilton, Rebers, and Smith (1956) and

was expressed as glucose equivalent.

2.7. Determination of spontaneous syneresis of

undisturbed set yoghurt

Spontaneous syneresis of undisturbed set yoghurt was

determined using a siphon method. The principle of the

siphon method was adapted from a method used by

Lucey (2001), who studied the amount of spontaneous

whey separated from acidified milk gels fermented in

volumetric flask. The method used by Lucey (2001) was

modified in order to prevent any effects of container

geometry, and the difference in heat transfer between

volumetric flask and the yoghurt cup, on syneresis. Inthis study, a cup of set yoghurt was taken out from the

walk-in-cooler (4 1C), weighed and kept at an angle of

approximately 451 to allow the whey on the surface to

collect on the side of the cup. A needle connected to a

syringe was used to siphon the liquid whey from the

surface of the sample, and the cup of yoghurt was then

re-weighed. The siphon was carried out within 10 s to

prevent further leakage of whey from the curd. The

syneresis was expressed as the percentage weight of the

whey over the initial weight of the yoghurt sample.

Measurement of syneresis by centrifugation and drai-

nage methods have been widely used, but were not

ARTICLE IN PRESS

T. Amatayakul et al. / International Dairy Journal 16 (2006) 40–5142


4/12

suitable for yoghurts with 9% (w/w) total solids made as

in this study. In addition, they did not represent the

actual values of syneresis of whey separation on the

surface of set yoghurt, and are likely to cause damage to

the structure of the yoghurt. Thus, the results would

represent the syneresis of broken curd.

2.8. Firmness of set yoghurt

Firmness of set yoghurt was defined as the maximum

force used in compression using TA-XT.2 Texture

Analyzer (Stable Micro System, Goaldming, UK) with

a P20 probe (diameter 20 mm) and 25 kg load cell. The

speed was set at 1 mms1. The ratio of the diameter of

yoghurt cup to diameter of probe ratio was 3.5:1.0.

According to Bourne (2002), it is generally accepted that

the boundary or wall effects will diminish when the

diameter of sample is at least three times higher than the

diameter of the probe. Yoghurt samples were com-pressed to 75% of their original height. The test was

carried out immediately after removing the samples

from the cold room.

2.9. Flow curve and the area of hysteresis loop of stirred

yoghurt

The flow curve of stirred yoghurt was constructed

using a RS 50 RheoStress (Haake Rheometer, Karls-

ruhe, Germany) using a coaxial measuring cell, Z20

DIN. A flow curve was constructed using the method of

Halmos and Tiu (1981), Ramaswamy and Basak (1991),

Hassan et al. (1996a) and Hassan, Ipsen, Janzen, and

Qvist (2003). The flow curve was constructed by

increasing shear rate from 10 to 50 s1 in 200 s (upward

curve) followed by decreasing shear rate from 50 to

10 s1 in 200 s (downward curve). The samples (10 mL)

were loaded into the measuring cell by using a spoon

and the temperature of the sample was allowed to drop

to 10 1C prior to commencing the measurement. Shear

rate in this range was chosen to prevent the slippage

effect as discussed by Haque, Richardson, and Morris

(2001). The use of high shear rate (1000 s1), as used by

other researchers (Teggatz & Morris, 1990; Bhattachar-

ya, 1999; Hassan et al., 1996a), would have severely

destroyed the structure of the stirred yoghurt resulting

in non-differentiation of flow behaviour between sam-

ples. Therefore, constructing the flow curve at low shearrate (0–50 s1) should be suitable for observing the flow

curve of yoghurt produced with low total solids. In

addition, the area of hysteresis loop between the upward

and downward curves was determined. This hysteresis

loop area represents the structural breakdown of stirred

yoghurt during shearing, as described by Halmos and

Tiu (1981), Ramaswamy and Basak (1991) and Hassan

et al. (1996a, 2003).

2.10. Statistical analysis

The data were analysed with one-way analysis of variance with SPSS version 10.0 for Windows (SPSS

Inc., NY, USA). The comparison between means of

data was carried out using the Tukey honestly sig-

nificant difference test.

3. Results and discussion

3.1. Composition of liquid milk blends

Table 1 shows the total solids, protein, fat, lactose and

ash contents of the milk blends with various CN-to-WP

ratios. All constituents in milk were kept constant, but

the CN-to-WP ratio was varied. The reduction in the pH

of yoghurts made using non-EPS-producing starter

cultures with low CN-to-WP ratios (ratio 1:1) was faster

than that for yoghurts with higher ratios (ratio 4:1)

(Fig. 1). This resulted in a shorter fermentation time of

yoghurt made with low CN-to-WP ratio. Puvanenthiran

et al. (2002) reported opposite results. The difference in

fermentation time between yoghurts made from milk

ARTICLE IN PRESS

Table 1

Compositional parameters of milk blends used in the manufacture of yoghurts

a

Compositional parameters Casein-to-whey protein ratio

4:1 3:1 2:1 1:1

Total solids (%, w/w) 8.8970.01a 8.8770.12a 8.8570.04a 9.0670.17a

Total protein (%, w/w) 3.0870.07a 3.0770.26a 3.0970.02a 3.0670.02a

Fat (%, w/w) o 0.1a o 0.1a o 0.1a o 0.1a

Lactose (%, w/w) 4.9770.07a 4.9970.11a 4.9670.05a 5.0170.09a

Ash (%, w/w) 0.4070.21a 0.4570.01a 0.4070.07a 0.3670.12a

Casein-to-whey protein ratio 4.3670.25a 2.9870.03b 2.1370.01c 1.2170.02d

aMilks were prepared by blending the desired levels of reconstituted low heat skim milk powder, whey protein concentrate (76% protein) and

lactose monohydrate to give varying casein-to-whey protein ratios. Presented values are the means of three replicate trials; 7 indicates standard

deviation from the mean. Mean values (7standard deviation) within the same row not sharing a common superscript differ significantly ( P o0.05).



5/12

blends with different ratios of CN-to-WP, as observed in

this study, may be due to the difference in the buffering

capacity of milk blends. According to Walstra and

Jenness (1984), whey proteins generally have lower

buffering capacity than casein. Therefore, lowering the

concentration of CN would result in a reduction in

buffering capacity. However, the reduction in bufferingcapacity may not be the only factor contributing to the

shorter fermentation time of yoghurt made with low

CN-to-WP ratios (Fig. 1). The increase in available

nutrients from WP may partly influence the growth of

yoghurt bacteria, and hence shortening the fermentation

time. Baig and Prasad (1996) and Dave and Shah (1998)

reported that WP stimulated the growth of S. thermo-

philus.

3.2. Lactic acid concentration

The concentration of lactic acid decreased when CN-

to-WP ratio was reduced (Table 2). Lactic acid

concentration varied from 0.60% (w/w) in yoghurt

made from a milk blend with a CN-to-WP ratio of 1:1 to

1.00% (w/w) in yoghurt made from milk blend with a

CN-to-WP ratio of 4:1. A similar trend was observed for

yoghurts made with any of the three types of starter

cultures. This could be due to the shorter fermentation

time of yoghurts produced from blends with low CN-to-

WP ratios, even though the pH at the end of the

fermentation was similar. This also suggests that milks

with low CN-to-WP ratios have low buffering capacity.In general, the types of starter cultures did not affect the

concentration of lactic acid. A slight increase in the

concentration of lactic acid was observed in all yoghurts

during the 28 d storage period.

3.3. Viable counts

Table 3 shows the viable counts of S. thermophilus

and L. delbrueckii ssp. bulgaricus in yoghurts made with

varying CN-to-WP ratio using the three types of starter

cultures, during the 28 d storage period. At 1 d, the

viable counts of S. thermophilus in yoghurts withdifferent ratios of CN-to-WP were similar, whereas the

ARTICLE IN PRESS

4.50 1 2 3 4 5

5.0

5.5

6.0

6.5

7.0

Fermentation time (h)

p H

Fig. 1. Reduction in pH of yoghurts made using non-EPS-producing

starter cultures with varying casein-to-whey protein ratios during

yoghurt manufacture. The milk blends were prepared from the desired

levels of reconstituted low heat skim milk powder, whey protein

concentrate (76% (w/w) protein) and lactose monohydrate to give

casein-to-whey protein ratios of 4:1 (J), 3:1 (K), 2:1 (B), and 1:1

(E). Each value is the mean of three replicate trials. See Section 2.2and Table 1 for details of starter culture and blend composition.

Table 2

Concentration of lactic acid (%, w/w) in yoghurts prepared from milk blends with varying casein-to-whey protein ratios, as described in Table 1, and

made using non-exopolysaccharide producing (control) cultures, capsular exopolysaccharide-producing starter culture or ropy exopolysaccharide-

producing starter culturesa

Starter culture Casein-to-whey

protein ratio

Storage period (d)

1 7 14 21 28

Non-exopolysaccharide-

producing starter cultures

4:1 0.9370.03g,A 0.9970.06e,A 0.9570.07d,e,A 0.9670.09d,e,A 0.9570.03d,A

3:1 0.8570.04d,e,f,g,A 0.8870.04d,e,A 0.8770.08b,c,d,e,A 0.9070.06c,d,e,A 0.8470.01b,c,d,A

2:1 0.7570.06b,c,d,A

0.8570.01c,d,A,B

0.7970.05a,b,c,d,A,B

0.8970.02b,c,d,e,B

0.8070.06a,b,c,A,B

1:1 0.6970.04a,,b,c,A 0.7570.04a,b,c,A,B 0.6970.03a,A 0.8070.03a,b,c,d,A,B 0.7870.05c,d,A,B

Capsular exopolysaccharide-


4:1 0.8570.03e,f,g,A 1.0170.04e,B,C 1.0270.02e,C 0.9770.09e,A,B,C 0.8970.04c,d,A,B

3:1 0.8070.05c,d,e,f,A 0.9370.06d,e,B 0.9470.03d,e,B 0.8670.03a,b,c,d,e,A,B 0.8070.06a,b,c,A

2:1 0.7270.05b,c,A 0.8270.05a,b,c,d,A,B 0.8670.05b,c,d,e,B 0.7770.02a,b,c,A,B 0.7770.08a,b,c,A,B

1:1 0.6070.01a,A 0.7270.04a,b,A 0.7170.02a,b,A 0.6970.07a,A 0.6970.08a,A

Ropy exopolysaccharide-


4:1 0.8970.02f,g,A,B 0.9070.01d,e,A,B 0.9970.05e,B 0.8670.08b,c,d,e,A 0.8970.02c,d,A,B

3:1 0.8470.02d,e,f,g,A 0.9170.05d,e,A 0.9270.06c,d,e,A 0.8970.03c,d,e,A 0.8870.04c,d,A

2:1 0.7570.04b,c,d,e,A 0.8470.03b,c,d,A 0.7870.09a,b,c,A 0.7970.08a,b,c,d,A 0.8570.04b,c,d,A

1:1 0.6970.01a,b,A 0.7070.04a,A 0.7970.05a,b,c,d,B 0.7270.02a,b,A,B 0.7270.03a,b,A,B

aPresented values are the means of three replicate trials; 7 indicates standard deviation from the mean. For details of starter cultures, see Section

2.2 and 2.3. Mean values (7standard deviation) within the same row not sharing a common superscript (A,B,C) differ significantly ( P o0.05). Mean

values (7standard deviation) within the same column not sharing a common superscript (a,b,c,d,e,f,g) differ significantly (P o0.05).



6/12

viable counts of L. delbrueckii ssp. bulgaricus decreased

as the ratio of CN-to-WP was reduced. This trend was

similar in yoghurts produced using non-EPS- or EPS-

producing starter cultures. This may be explained by the

differences in the fermentation time that affects the

population of this bacterium. During yoghurt manu-

facture, S. thermophilus is usually active during the first

few hours of fermentation until the pH reaches around

5.40; then, L. delbrueckii ssp. bulgaricus starts to grow

(Shah, 2003). Furthermore, during the storage, the

viable counts of S. thermophilus in the yoghurts madewith non-EPS- or ropy EPS-producing starter cultures

gradually decreased from day 1, whereas in the yoghurts

made with capsular starter cultures, the viable counts of

S. thermophilus increased slightly after 7 d of storage and

declined thereafter. The viable counts of L. delbrueckii

ssp. bulgaricus gradually decreased from day 1 in

yoghurts made with each of the starter cultures and

with CN-to-WP ratios in the range 3:1 to 1:1. The

yoghurts made with a CN-to-WP ratio of 4:1 showed a

slight increase in the population which reached the

highest counts at 14 d with the value of 7.86, 7.74 and

8.09 for those made with non-EPS-, capsular EPS- orropy EPS-producing starter cultures, respectively. At

28 d, the yoghurts produced using ropy EPS-producing

starter cultures showed higher viable counts than those

made using non-EPS- or capsular EPS-producing starter

cultures. This may suggest a protective effect of ropy

EPS on L. delbrueckii ssp. bulgaricus. No such protective

effects were observed for S. thermophilus.

3.4. EPS concentration

The concentration of EPS in yoghurts made using

EPS starter cultures ranged from 30 to 70 mg L1 (Table

4). Other researchers have reported EPS concentrations

in fermented milk products ranging from 40 to

400mgL1 (Marshall & Rawson, 1999; De Vuyst et

al., 2003; Toba et al., 1991). There are several possible

reasons for the difference between studies, including the

use of different strains and the level of inoculation of

starter cultures, the differences in fermenting conditions

and methods of isolation, purification and quantifica-

tion of EPS. The concentration of EPS in yoghurt

produced with 12% (w/w) dry matter in our preliminary

study was higher (100mgL

1, unpublished data) thanin yoghurt produced with 9% (w/w) dry matter in the

current study. Furthermore, the concentration of EPS in

yoghurt produced using ropy EPS-producing starter

cultures was slightly higher than that in yoghurts

produced using capsular EPS-producing starter cultures

(Table 4). This could be due to the differences in their

EPS production. Furthermore, it is interesting to note

that a low concentration of EPS (10mgL1) was

found in yoghurt produced with non-EPS-producing

starter cultures. The low value of EPS, detected in the

yoghurt made with non-EPS-producing starter cultures,

might be due to the residue of lactose remaining afterthe purification. Because the phenol-sulphuric method,

used in the determination of EPS, is not specific for the

quantification of EPS, the presence of lactose may

increase the value. However, there is a possibility that

the non-EPS-producing S. thermophilus produced small

amounts of EPS. Nonetheless, the physical character-

istics of yoghurts made using non-EPS-producing

starter cultures were significantly different from yo-

ghurts made using EPS-producing starter cultures as

discussed below (Figs. 2 and 3). For yoghurt made using

capsular EPS-producing starter cultures, there was no

difference in EPS concentration between yoghurts with

ARTICLE IN PRESS

Table 3

Viable counts (log10 cfu g1) of S. thermophilus (ST) and L. delbrueckii ssp. bulgaricus (LB) in yoghurts prepared from milk blends with different

casein-to-whey protein ratios, as described in Table 1, and made using different starter culturesa


protein ratio

Storage period (d)

1 7 14 21 28

ST LB ST LB ST LB ST LB ST LB

Non-exopolysaccharide-producing starter cultures 4:1 8.89 7.73 8.86 7.79 8.41 7.86 8.00 6.88 7.88 5.45

3:1 8.94 7.53 8.88 7.86 8.56 7.85 7.73 7.63 7.90 7.51

2:1 8.94 7.65 8.86 7.68 8.75 7.54 8.58 6.74 8.38 6.68

1:1 8.78 7.01 8.89 7.62 8.70 6.91 8.64 6.61 8.58 5.29

Capsular exopolysaccharide-producing starter cultures 4:1 8.85 7.64 8.84 7.69 8.61 7.74 7.71 6.67 7.44 6.37

3:1 8.79 7.71 8.56 7.69 8.53 7.66 8.35 7.11 8.18 6.68

2:1 8.84 7.51 8.70 7.46 8.56 7.59 8.61 6.95 8.52 6.47

1:1 8.63 7.59 8.44 7.32 8.41 7.51 8.36 6.51 7.72 6.46

Ropy exopolysaccharide-producing starter cultures 4:1 8.82 7.73 9.05 7.82 8.70 8.09 8.37 7.69 8.53 7.50

3:1 8.80 7.85 8.84 7.67 9.33 8.03 8.53 7.46 8.49 7.67

2:1 8.85 7.76 9.03 7.64 8.69 7.59 8.58 7.74 7.73 7.22

1:1 8.81 7.11 8.89 7.45 8.90 7.09 8.67 7.28 7.72 7.47

a

Presented values are the means of three replicate trials. For details of starter cultures, see Sections 2.2 and 2.3.



7/12

different CN-to-WP ratios. This may be due to the

similar viable counts of S. thermophilus (capsular EPS-

producing strain) in yoghurts with different CN-to-WP

ratios. For yoghurt made using ropy EPS-producing

starter cultures, the highest concentration of EPS was

found in the yoghurt made with a CN-to-WP ratio of

3:1, whereas the lowest EPS concentration was found in

the yoghurt with a CN-to-WP ratio of 1:1. Zisu andShah (2003) showed that the addition of 0.5% (w/w)

WPC to 10% (w/v) RSM increased the level of EPS

produced by ropy starter cultures. The increase in EPS

concentration with the increase in concentration of WP

was also observed in the current study. It can be seen

that the reduction in fermentation time of milks with

CN-to-WP ratios ranging from 4:1 to 1:1 by EPS-

producing starter cultures did not reduce the concentra-

tion of EPS in the resultant yoghurts. Furthermore, the

concentration of EPS in yoghurt made using ropy

starter cultures increased during storage. This was not

found in yoghurt made using capsular EPS-producingstarter cultures, in which the EPS concentration

remained constant during storage at 4 1C. In other

studies (Degeest & De Vuyst, 1999; Pham, Dupont,

Roy, Lapointe, & Cerning, 2000), the degradation of

EPS during prolonged fermentation (72 h) in a complex

media has been reported. These studies reported a large

spectrum of glycohydrolytic enzymes (a-D-glucosidase,

b-D-glucosidase, a-D-galactosidase, b-D-galactosidase, b-

D-glucuronidase and a-L-rhamnosidase) during pro-

longed fermentation. This might suggest that the storage

at 4 1C helps to suppress the activity of those glycohy-

drolytic enzymes.

3.5. Firmness and syneresis of set yoghurt

Firmness and syneresis of set yoghurts with varying

CN-to-WP ratios and three types of starter cultures are

shown in Fig. 2. The firmness of yoghurts made using

capsular EPS- (Fig. 2b) or ropy EPS- (Fig. 2c)

producing starter cultures was generally lower than that

in yoghurts made with non-EPS-producing startercultures (Fig. 2a). Yoghurt made using ropy EPS-

producing starter cultures had the lowest firmness.

Similar results were reported by other researchers

(Marshall & Rawson, 1999; Hassan et al., 1996b; Hess,

Roberts, & Ziegler, 1997). The incompatibility between

EPS and proteins may be the explanation. The EPS and

proteins have like charges at pH values above the

isoelectric point of the protein (de Kruif & Tuinier,

2001), as occurs in yoghurt. The incompatibility between

EPS and proteins may result in depletion-induced

attraction of casein micelles by EPS, leading to the

formation of an acid milk gel filled with EPS mass (deKruif & Tuinier, 2001; Tolstoguzov, 1997). This in turn

may cause a difference in protein aggregation mechan-

isms between yoghurt made using non-EPS- and EPS-

producing starter cultures, and lead to differences in the

structure of the protein networks. Hassan, Frank,

Farmer, Schmidt, and Shalabi (1995) and Hassan and

Frank (1997) observed that the microstructure of

yoghurt and rennet curd made using capsular EPS-

producing starter cultures had larger spaces between the

protein matrix and bacteria compared to the corre-

sponding curds acidified by glucono-d-lactone or non-

EP-producing starter cultures. Nevertheless, the protein

ARTICLE IN PRESS

Table 4

Concentration of exopolysaccharide (mg L1) in yoghurts prepared from milk blends with different casein-to-whey protein ratios, as described in

Table 1, and made using different starter culturesa


protein ratio

Storage period (d)

1 7 14 21 28

Non-exopolysaccharide-producing starter cultures

4:1 9.1072.11a,A 10.3071.19a,A 9.4870.09a,A 10.4471.34a,A 8.5473.71a,A

3:1 8.0370.71a,A 9.7470.14a,A 9.6870.97a,A 8.2770.86a,A 9.6871.40a,A

2:1 9.2071.04a,A,B 9.0470.51a,A,B 8.8670.81a,A,B 9.7070.83a,B 7.9670.71a,A

1:1 7.9571.20a,A 10.0170.26a,A 8.8571.90a,A 8.2872.47a,A 8.7671.69a,A

Capsular exopolysaccharide-


4:1 32.4170.73b,c,B 28.8970.94b,A 30.8672.96b,c,A,B 36.9071.00b,c,C 33.5170.71b,B

3:1 32.5470.31b,c,A,B 29.3770.93b,A 33.4373.92b,c,B 30.3471.55b,A,B 32.4071.06b,A,B

2:1 29.1672.84b,A 28.8371.27b,A 28.5373.43b,A 31.4870.87b,A 31.8675.66b,A

1:1 38.3578.56b,c,d,A 31.8573.80b,A 32.8574.22b,c,A 36.1572.17b,c,A 36.0671.80b,A

Ropy exopolysaccharide-


4:1 41.5778.19c,d,e,A 47.7873.42d,A 49.69713.75d,e,A 53.9775.63d,e,A,B 66.4575.49d,B

3:1 43.2073.04d,e,A 47.7272.02d,A,B 60.06711.79e,B,C 63.21712.47e,B,C 75.4176.70e,C

2:1 48.6279.99e,A,B 41.4874.78c,A 58.4178.79e,B 50.13711.59d,A,B 62.1372.67c,d,B

1:1 36.6474.15b,c,d,A 42.4773.81c,A,B 42.3974.02c,d,A,B 44.7072.72c,d,B 57.3372.38c,C

aPresented values are the means of three replicate trials; 7 indicates standard deviation from the mean. For details of starter cultures, see Section2.2 and 2.3. Mean values (7standard deviation) within the same row not sharing a common superscript (A,B,C) differ significantly ( P o0.05). Mean

values (7standard deviation) within the same column not sharing a common superscript (a,b,c,d,e) differ significantly (P o0.05).



8/12

matrix of yoghurt containing EPS was more compact

(Hassan et al., 1995; Hassan & Frank, 1997). These

observations suggested that EPS affected the micro-

structure of yoghurt and, thereby, affect the physical

characteristics yoghurt.

Increasing the CN-to-WP ratios from 1:1 to 4:1

resulted in higher firmness values. Tamime, Kalab, and

Davies (1984) reported that yoghurts made using milkwith a CN-to-WP ratio of 4.62 were firmer than those

made using milk with CN-to-WP ratios of 3.20 to 3.40.

A reduction in the firmness with a decrease in CN-to-

WP ratio was observed in yoghurts made with the three

types of starter cultures evaluated (Fig. 2a–c). The

reduction in firmness as the CN-to-WP ratio was

reduced contradicts to the results reported by Puva-

nenthiran et al. (2002), who found that the gel strength

(representing yield point, or the first fracture point

during the first bite in the compression test) increased as

the CN-to-WP ratio was reduced. The difference in

firmness or gel strength between the current study and

that of Puvanenthiran et al. (2002) could be due to the

difference in sizes of protein aggregates as a result of

heating of milks at different pH values. Puvanenthiran

et al. (2002) reported the increase in size of particles in

milk heated at pH 7.0 when the CN-to-WP ratio

decreased. They found a relationship between the

increase in particle size and gel strength, and explained

that heating of milk at pH 7.0 promoted the formationof dissociated k-casein–whey protein aggregates, whey

protein–whey protein aggregates and conglomerates of

whey protein aggregates as the CN-to-WP ratios were

decreased. Vasbinder and de Kruif (2003) reported that

heating milk at pH above 6.55 promoted the formation

of soluble denatured whey protein aggregates as

compared with heating at pH 6.35, at which all

denatured whey protein coated on the surface of casein

micelles. This may imply that heating milks with various

CN-to-WP ratios at natural pH had different effects on

the size of protein aggregates compared with heating

milk at pH 7.00. Furthermore, Cho, Singh, and Creamer

ARTICLE IN PRESS

0 7 14 21 28

0.2

0.4

0.6

0.8

1.0

0 7 14 21 28

0.2

0.4

0.6

0.8

1.0

F i r m n e s s ( N )



0 7 14 21 280.2

0.4

0.6

0.8

1.0

Storage period (d)

(c)

(b)

(a)

0 7 14 21 28

0.0

2.0

4.0

6.0

8.0

10.0

0 7 14 21 28

0.0

2.0

4.0

6.0

8.0

10.0

S y n e r e s i s ( % , w / w )

S y n e r e s i s ( % , w / w )

S y n e r e s i s ( % , w / w )

0 7 14 21 280.0

2.0

4.0

6.0

8.0

10.0

Storage period (d)

(d)

(e)

(f)

Fig. 2. Firmness (a, b, c) and syneresis (d, e, f) of yoghurts made using non-exopolysaccharide-producing (control) cultures (a, d), capsular

exopolysaccharide-producing starter cultures (b, e), or ropy exopolysaccharide-producing starter cultures (c, f). The yoghurts were prepared from

milk blends with varying casein-to-whey protein ratios of 4:1 (J), 3:1 (K), 2:1 (E), and 1:1 (E). Each value is the mean of three replicate trials; errorbars represent standard deviations from the mean. See details of starter culture and milk blends composition in Section 2.2 and Table 1, and details of

tests in Sections 2.7 and 2.8.



9/12

(2003) studied on the effect of heat treatment on the

mixtures of k-casein and b-lactoglobulin at pH 6.70

using polyacrylamide gel electrophoresis and reported

that the size of protein aggregates decreased as the ratio

of k-casein to b-lactoglobulin was decreased. This may

also suggest that the reduction of the CN-to-WP ratio of milk blends in our study promotes the formation of

small protein aggregates after heat treatment. According

to the correlation between the increase in particle size

and gel strength of set yoghurt, as observed by

Puvanenthiran et al. (2002), the small protein aggregates

formed as a result of heat treatment of a milk blend with

a low CN-to-WP ratio (ratio 1:1) at natural pH would

result in a lower firmness in set yoghurt than in yoghurt

made from milk with a high CN-to-WP ratio (ratio 4:1).

Guzma ´ n-Gonza ´ lez et al. (1999) reported that a decrease

in the ratio of denatured whey protein-to-total protein

(kept constant at 4.3%, w/w) was positively correlated

with the decrease in the apparent viscosity of yoghurts

made from milks supplemented with WPC. In our study,

the decrease in the level of whey protein denaturation in

milk blended to low CN-to-WP ratio (ratio 1:1) or

increased proportion of WP might have contributed to

the decrease in the firmness of set yoghurt. Duringstorage, the firmness of yoghurts made with EPS-

producing starter cultures did not change significantly.

On the contrary, the firmness of yoghurts produced with

non-EPS-producing starter cultures (especially from

milks with a CN-to-WP ratio of 4:1 or 3:1) increased

during the first few weeks of storage and thereafter

remained constant.

In general, syneresis in yoghurts decreased when the

ratio of CN-to-WP was reduced, regardless of the type

of starter cultures (Fig. 2d–f ). No syneresis was found in

yoghurt with a CN-to-WP ratio of 1:1. This result was

similar to that reported by Puvanenthiran et al. (2002).

ARTICLE IN PRESS

1 7 14 21 280

50

100

150

200

1 7 14 21 280

50

100

150

200

L o o p a r e a ( P a s - 1 )

L o o p a r e a ( P a s - 1 )

L o o p a r e a ( P a s - 1 )

1 7 14 21 280

50

100

150

200

Storage period (d)

(f)

10 20 30 40 502

6

10

14

18

10 20 30 40 502

6

10

14

18

S h e a r s t r e s s ( P a )

S h e a r s t r e s s ( P a )

S h

e a r s t r e s s ( P a )

10 20 30 40 502

6

10

14

18

Shear rate (s-1

)

(a) (d)

(e)

(b)

(c)

Fig. 3. Shear stress as a function of increasing and decreasing shear rate (a, b, c) for yoghurts that were prepared from milk blends with varying

casein-to-whey protein ratios of 4:1 (J), 3:1 (K), 2:1 (B), and 1:1 (E). Loop area as a function of the corresponding areas of shear stress/shear rate

hysteresis loops (d, e, f) for yoghurts that were prepared from milk blends with varying casein-to-whey protein ratios of 4:1 (T), 3:1 (’), 2:1 ( ), and

1:1 ( ). Yoghurts were made using non-exopolysaccharide-producing (control) starter cultures (a, d), capsular exopolysaccharide-producing starter

cultures (b, e), or ropy exopolysaccharide-producing starter cultures (c, f). Each value is the mean of 3 replicate trials; error bars represent standard

deviations from the mean. See details of starter culture and milk blend composition in Section 2.2 and Table 1, and details of tests in

Section 2.9.



10/12

Guzma ´ n-Gonza ´ lez et al. (1999) reported that the

addition of WPC to milk reduced the level of syneresis

in yoghurt. Bhullar et al. (2002) and Remuef et al. (2003)

reported similar results. Harwalkar and Kalab (1986)

found that the level of syneresis in yoghurt decreased as

the density of protein matrix increased. Puvanenthiran

et al. (2002) suggested that an increase in the compact-ness of yoghurt microstructure, as the CN-to-WP ratio

was reduced, led to immobilization of a high level of free

water. Further, whey proteins and EPS are known to

have high water binding capacity (Walzem, Dillard, &

German, 1996; Cerning, 1990; De Vuyst & Degeest

1999). Whey proteins and EPS may act synergistically in

retaining water in the gel structure. Our results showed

that yoghurts made using EPS-producing starter cul-

tures (both capsular and ropy) had a lower level of

syneresis than yoghurt produced with non-EPS-produ-

cing starter cultures (Fig. 2d–f ). Similar results have also

been reported by others (Marshall & Rawson, 1999;

Wacher-Rodarte et al., 1993). Apart from the water

binding property of EPS, the modification of yoghurt

microstructure by EPS may also partly affect syneresis

levels. Yoghurt made using capsular EPS-producing

starter cultures had the lowest level of syneresis,

especially when the CN-to-WP ratio was 4:1. Interest-

ingly, there appeared to be no difference in the syneresis

between yoghurts made with capsular EPS- and ropy

EPS-producing starter cultures at CN-to-WP ratios of

2:1 and 1:1. It seems that the water binding ability of

WP at CN-to-WP ratios of 2:1 and 1:1 surpassed water-

binding ability of capsular EPS and ropy EPS. During

storage, the level of syneresis between yoghurts did notappear to change, regardless of the type of starter

cultures or the CN-to-WP ratios used.

3.6. Flow curve and hysteresis loop area of stirred

yoghurt

The flow curves and hysteresis loop area of the different

stirred yoghurts are shown in Fig. 3. Similar flow curves

were observed for stirred yoghurts made using the different

three types of starter cultures. Stirred yoghurt made with

ropy EPS-producing starter cultures (Fig. 3c) had a higher

shear stress (on increasing shear rate, corresponding to theupward curve) than yoghurts made using the other types

of starter cultures. Several researchers have found that

yoghurt made with ropy EPS-producing starter cultures

had higher apparent viscosity than that made with non-

EPS- or capsular EPS-producing starter cultures (Wacher-

Rodarte et al., 1993; Hassan et al., 1996a, 2003). Stirred

yoghurts made using non-EPS-producing starter cultures

(Fig. 3a) had a similar shear stress to those made using

capsular EPS-producing starter cultures (Fig. 3b). For

yoghurts made with EPS- or non-EPS-producing starter

cultures, a CN-to-WP ratio of 1:1 gave a shear stress that

was significantly lower than that found at the higher CN-

to-WP ratios. For stirred yoghurts made using EPS-

producing starter cultures (both capsular and ropy), the

highest shear stress was found at a CN-to-WP ratio of 3:1.

The area of hysteresis loop between the up and down

shear rate versus shear stress curves was determined. The

hysteresis loop area of stirred yoghurts made with ropy

EPS-producing starter cultures (Fig. 3f ) was higher thanthose made with non-EPS- (Fig. 3d) and capsular EPS-

producing starter cultures (Fig. 3e). As all stirred yoghurts

had the same level of total solids and were made using the

same procedure and given the incompatibility between

EPS and milk proteins (de Kruif & Tuinier, 2001), the

presence of ropy EPS could be solely responsible for the

results observed. According to Morris (1995), most

polysaccharides exist in solution as random coils and can

form entangled networks depending on their numbers

(proportion to concentration) and molecular volume (size).

The entangled network causes an increase in the viscosity

of solution (Sworn, 2004).

Hassan, Frank, and Qvist (2002) observed the

microstructure before and after stirring of milk fermen-

ted with a single strain of L. delbrueckii ssp. bulgaricuss

RR (ropy starter culture). They reported that the EPS

segregated from the proteins and the EPS formed a

more extensive structure in the stirred gel compared with

the set gel. They suggested that stirring promoted the

interactions between molecules of ropy EPS. However,

they did not specify whether the interactions are

chemical or physical (entanglement) type. However, it

is likely that the interactions, as stated by Hassan et al.

(2002), are physical types or the entanglement of

polysaccharides. The entangled networks of ropy EPSare also expected to occur in our study.

Theoretically, the area of hysteresis loop between

upward and downward flow curves represents the

structural breakdown of stirred yoghurt during shearing

(Halmos & Tiu, 1981; Ramaswamy & Basak, 1991;

Hassan et al., 1996a, 2003). The formation of entangled

networks of ropy EPS, as an additional structure in

stirred yoghurt, may explain the increase in the area of

hysteresis loop of products made using ropy EPS-

producing starter cultures. It also implies that there was

no entangled network of EPS formed in the products

made using capsular EPS-producing starter cultures asthe value of hysteresis loop was comparable to that in

yoghurt made with non-EPS-starter cultures.

The area of hysteresis loop of stirred yoghurts made

using non-EPS- or capsular EPS-producing starter

cultures decreased as the CN-to-WP ratio was de-

creased. As the presence of capsular EPS did not

contribute to the hysteresis loop area, the results can

be interpreted as a decrease in structural breakdown of

proteins during shearing or an increase in the damage of

protein structure of initial stirred yoghurts made with

low CN-to-WP ratios compared with yoghurts with high

ratios. In this study, stirred yoghurt was produced by

ARTICLE IN PRESS



11/12

pressing the gel of set yoghurt with a spoon through a

sieve. Applying the same procedure to the soft gel (set

yoghurt made at a CN-to-WP ratio of 1:1) could result

in higher structural damage of the protein structure than

that with a firm gel (set yoghurt made at a CN-to-WP

ratio of 4:1). Stirred yoghurt made with a CN-to-WP

ratio of 3:1 and using ropy EPS-producing startercultures had the highest loop area. During storage, a

slight increase in the area of hysteresis loop was

observed in stirred yoghurts. The change in loop area

of the products made with ropy EPS-producing starter

cultures correlated with the increase in its EPS

concentration (Table 4). This relationship was not

observed in those made with non-EPS- or capsular

EPS-producing starter cultures. In addition, after 14 and

21 d, the loop area decreased slightly in stirred yoghurts

made using either non-EPS- or EPS-producing starter

cultures (both capsular and ropy). However, as the

phenomenon was also observed in stirred yoghurt made

using non-EPS-producing starter cultures, the decrease

in loop area may be due to changes in milk proteins.

4. Conclusions

A decrease in the ratio of CN-to-WP of set yoghurt

reduced the level of syneresis and firmness. The use of

EPS-producing starter cultures also reduced the level of

syneresis and gel firmness. In stirred yoghurt made using

ropy EPS-producing starter cultures, there was an

increase in shear stress and the area of hysteresis loop

between the up and down shear rate versus shear stresscurves; the increase in loop area represents an additional

structure of EPS—EPS in stirred yoghurt. The results of

this study suggest that there is a possibility of reducing

the level of added dairy ingredients and stabilizers in the

manufacture of stirred yoghurt by using ropy EPS-

producing starter cultures. For set yoghurt, a CN-to-

WP ratio of 3:1 and the use of non-EPS-producing

starter cultures gave optimal firmness and syneresis

levels.

Acknowledgments

The authors would like to thank Mr. Micheal

Kakoullis, Laboratory Manager, Department of Food

Science, RMIT University, City Campus, Melbourne,

Australia for the assistances in using texture analyser

and rheometer.

References

AOAC. (1995). Official methods of analysis of AOAC international

(16th ed.). Arling, VA, USA: AOAC International.

Baig, M. I., & Prasad, V. (1996). Effect of incorporation of cottage

cheese whey solids and Bifidobacterium bifidum in freshly made

yogurt. Journal of Dairy Research, 63, 467–473.

Bhattacharya, S. (1999). Yield stress and time-dependent rheological

properties of mango pulp. Journal of Food Science, 64, 1029–1033.

Bhullar, Y. S., Uddin, M. A., & Shah, N. P. (2002). Effects of

ingredients supplementation on textural characteristics and micro-

structure of yoghurt. Milchwissenschaft, 57 , 328–332.Bourne, M. (2002). Food texture and viscosity: Concept and measure-

ment (2nd ed.). New York: Academic Press.

Cerning, J. (1990). Exocellular polysaccharides produced by lactic acid

bacteria. FEMS Microbiology Reviews, 87 , 113–130.

Cho, Y., Singh, H., & Creamer, L. K. (2003). Heat-induced

interactions of b-lactoglobulin A and k-casein B in a model

system. Journal of Dairy Research, 70, 61–71.

Dave, R. I., & Shah, N. P. (1998). Ingredient supplementation effects

on viability of probiotic bacteria in yogurt. Journal of Dairy

Science, 81, 2804–2816.

Degeest, B., & De Vuyst, L. (1999). Indication that the nitrogen source

influences both amount and size of exopolysaccharides produced

by Streptococcus thermophilus LY03 and modelling of the bacterial

growth and exopolysaccharide production in a complex medium.

Applied and Environmental Microbiology, 65, 2863–2870.de Kruif, C. G., & Tuinier, R. (2001). Polysaccharide protein

interactions. Food Hydrocolloids, 15, 555–563.

De Vuyst, L., & Degeest, B. (1999). Heteropolysaccharides from lactic

acid bacteria. FEMS Microbiology Reviews, 23, 153–177.

De Vuyst, L., Zamfir, M., Mozzi, F., Adriany, T., Marshall, V. M.,

Degeest, B., & Vaningelgem, F. (2003). Exopolysaccharide-produ-

cing Streptococcus thermophilus strains as functional starter

cultures in the production of fermented milks. International Dairy

Journal , 13, 707–717.

Dubois, M., Gilles, K. A., Hamilton, J. K., Rebers, P. A., & Smith, F.

(1956). Colorimetric method for determination of sugars and

related substances. Analytical Chemistry, 38, 350–356.

Guzma ´ n-Gonza ´ lez, M., Morais, F., & Amigo, L. (2000). Influence of

skimmed milk concentrate replacement by dry dairy products in a

low-fat set-type yoghurt model system. II: Use of caseinates, co-

precipitate and blended dairy powders. Journal of the Science of

Food and Agriculture, 80, 433–438.

Guzmán-González, M., Morais, F., Ramos, M., & Amigo, L. (1999).

Influence of skimmed milk concentrate replacement by dry dairy

products in a low fat set-type yoghurt model system. I: Use of whey

protein concentrates, milk protein concentrates and skimmed milk

powder. Journal of the Science of Food and Agriculture, 79,

1117–1122.

Halmos, A. L., & Tiu, C. (1981). Liquid foodstuffs exhibiting yield

stress and shear-degradability. Journal of Texture Studies, 12,

39–46.

Haque, A., Richardson, R. K., & Morris, E. R. (2001). Effect of

fermentation temperature on the rheology of set and stirred yogurt.

Food Hydrocolloids, 15, 593–602.Harwalkar, V. R., & Kalab, M. (1986). Relationship between

microstructure and susceptibility to syneresis in yoghurt made

from reconstituted nonfat dry milk. Food Microstructure, 5,

287–294.

Hassan, A. N., & Frank, J. F. (1997). Modification of microstructure

and texture of rennet curd by using a capsule-forming non-ropy

lactic cultures. Journal of Dairy Research, 64, 115–121.

Hassan, A. N., Frank, J. F., Farmer, M. A., Schmidt, K. A., &

Shalabi, S. I. (1995). Formation of yogurt microstructure and

three-dimensional visualization as determined by confocal scanning

laser microscopy. Journal of Dairy Science, 78, 2629–2636.

Hassan, A. N., Frank, J. F., Schmid, K. A., & Shalabi, S. I. (1996a).

Rheological properties of yogurt made with encapsulated nonropy

lactic cultures. Journal of Dairy Science, 79, 2091–2097.

ARTICLE IN PRESS



12/12

Hassan, A. N., Frank, J. F., Schmid, K. A., & Shalabi, S. I. (1996b).

Textural properties of yogurt made with encapsulated nonropy

lactic cultures. Journal of Dairy Science, 79, 2098–2103.

Hassan, A. N., Frank, J. F., & Qvist, K. B. (2002). Direct observation

of bacterial exopolysaccharide in dairy products using confocal

scanning laser microscopy. Journal of Dairy Science, 85,

1705–1708.

Hassan, A. N., Ipsen, R., Janzen, T., & Qvist, K. B. (2003).Microstructure and rheology of yogurt made with cultures differing

only in their ability to produce exopolysaccharides. Journal of

Dairy Science, 86 , 1632–1638.

Hess, S. J., Roberts, R. F., & Ziegler, G. R. (1997). Rheological

properties of nonfat yogurt stabilized using Lactobacillus del-

brueckii ssp. bulgaricus producing exopolysaccharide or using

commercial stabilizer systems. Journal of Dairy Science, 80,

252–263.

Jaros, D., Rohm, H., Haque, A., Bonaparte, & Kneifel, W. (2002).

Influence of the starter cultures on the relationship between dry

matter content and physical properties of set-style yogurt.

Milchwissenschaft, 57 , 325–326.

Lucey, J. A. (2001). The relationship between rheological parameters

and whey separation in milk gels. Food Hydrocolloids, 15, 603–608.

Marshall, V. M., & Rawson, H. L. (1999). Effects of exopolysacchar-ide-producing strains of thermophilic lactic acid bacteria on the

texture of stirred yoghurt. International Journal of Food Science and

Technology, 34, 137–143.

Modler, H. W., Larmond, M. E., Lin, C. S., Froehlich, D., &

Emmons, D. B. (1983). Physical and sensory properties of yogurt

stabilized with milk proteins. Journal of Dairy Science, 66 , 422–429.

Morris, E. R. (1995). Polysaccharide rheology and in-mouth percep-

tion. In A. M. Stephen (Ed.), Food polysaccharides and their

applications (pp. 517–546). New York: Marcel Dekker, Inc.

Perry, D. B., McMahon, D. J., & Oberg, C. J. (1997). Effect of

exopolysaccharide-producing cultures on moisture retention in low

fat mozzarella cheese. Journal of Dairy Science, 80, 799–805.

Pham, P. I., Dupont, I., Roy, D., Lapointe, G., & Cerning, J. (2000).

Production of exopolysaccharide by Lactobacillus rhamnosus R and

analysis of its enzymatic degradation during prolonged fermenta-tion. Applied and Environmental Microbiology, 66 , 2302–2310.

Puvanenthiran, A., Williams, R. P. W., & Augustin, M. A. (2002).

Structure and visco-elastic properties of set yoghurt with altered

casein to whey protein ratios. International Dairy Journal , 12,

383–391.

Ramaswamy, H. S., & Basak, S. (1991). Rheology of stirred yogurts.

Journal of Texture Studies, 22, 231–241.

Remuef, F., Mohammed, S., Sodini, I., & Tissier, J. P. (2003).

Preliminary observations on the effects of milk fortification and

heating on microstructure and physical properties of stirred yogurt.

International Dairy Journal , 13, 773–782.

Shah, N. P. (2003). Yogurt: The product and its manufacture. In B.

Caballero, L. C. Trugo, & P. M. Finglas (Eds.), Encyclopedia of

food science and nutrition (Vol. 10. 2nd ed., pp. 6252–6259). UK:

Academic Press.

Shin, H. S., Lee, J. H., Pestka, J. J., & Ustunol, Z. (2000). Growth and

viability of commercial Bifidobacterium spp. in skim milk contain-ing oligosaccharides and insulin. Journal of Food Science, 65,

884–887.

Sworn, G. (2004). Hydrocolloid thickeners and their application. In P.

A. Williams, & G. O. Phillips (Eds.), Gums and stabilisers for the

food industry 12 (pp. 12–22). UK: The Royal Society of Chemistry.

Tamime, A. Y., & Deeth, H. C. (1980). Yogurt: Technology and

biochemistry. Journal of Food Protection, 43, 939–977.

Tamime, A. Y., Kalab, M., & Davies, G. (1984). Microstructure of set-

style yoghurt manufactured from cow’s milk fortified by various

methods. Food Microstructure, 3, 83–92.

Teggatz, J. A., & Morris, H. A. (1990). Changes in the rheology and

microstructure of ropy yoghurt during shearing. Food Structure, 9,

133–138.

Toba, T., Uemura, H., Mukai, T., Fuji, T., Itoh, T., & Adachi, S.

(1991). A new fermented milk using capsular polysaccharide-producing Lactobacillus kefiranofaciens isolated from kefir grains.

Journal of Dairy Research, 58, 497–502.

Tolstoguzov, V. (1997). Protein–polysaccharide interactions. In S.

Damodaran, & A. Paraf (Eds.), Food proteins and their application

(pp. 176–177). New York: Marcel Dekker, Inc.

Vasbinder, A. J., & de Kruif, C. G. (2003). Casein–whey protein

interactions in heated milk: The influence of pH. International

Dairy Journal , 13, 669–677.

Wacher-Rodarte, C., Galvan, M. V., Farres, A., Gallardo, F.,

Marshall, V. M., & Garcia-Garibay, M. (1993). Yogurt production

from reconstituted skim milk powders using different polymer and

non-polymer forming starter cultures. Journal of Dairy Research,

60, 247–254.

Walstra, P., & Jenness, R. (1984). Dairy chemistry and physics

(pp. 194–196). New York: Wiley.Walzem, R. L., Dillard, C. J., & German, J. B. (2002). Whey

components: Millennia of evolution create functionalities for

mammalian nutrition: What we know and what we may be

overlooking. Critical Reviews in Food Science and Nutrition, 42,

354–375.

Zisu, B., & Shah, N. P. (2003). Effects of pH, temperature,

supplementation with whey protein concentrate, and adjunct

cultures on the production of exopolysaccharides by Streptococcus

thermophilus 1275. Journal of Dairy Science, 86 , 3405–3415.

ARTICLE IN PRESS


caracteristici fizice iaurt

Documents