THE INFLUENCE OF INVESTIGATED FACTORS ON VISCOSITY OF STIRRED YOGURT

Skim milk was reconstituted to obtain milk with 8.44% DM, which was standardized with demineralized whey powder (DWP) to obtain milk sample A (9.71% DM) and milk sample B (10.75% DM). Milk samples were heat treated at 85oC/20 min and 90oC/10 min, respectively. Untreated milk was used as control. Milk samples were inoculated with 2.5% of commercial yogurt culture (containing Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus in the ratio 1:1) at 43oC. Samples were incubated until pH 4.6 was reached. Samples were immediately cooled to 4oC and held at that temperature until analyses. Samples of acid casein gels were stirred after 1, 7 and 14 days of storage. Measurements of viscosity were done with Brookfield DV-E Viscometer. Spindle No 3 at 30 rpm was used for all samples. Duration of fermentation decreased when DWP was used for standardization of milk dry matter content. Yogurt samples produced from milk heat treated at 85oC/20 min, obtained by stirring of gel 1 day after production had a higher viscosity than sample produced from milk heat treated at 90oC/10 min. On the other hand, samples produced from milk heat treated at 90oC/10 min had a greater viscosity after 7 and 14 days of storage, which indicates a greater hydrophilic properties and a more pronounced swelling of casein micelles.

Jelen et al., 1987al., , Maćej et al. 1997, S a v e l l o and D a r g a n , 1997, T a m i m e and D e e t h , 1980, T a m i m e and R o b i n s o n , 1988).Also, an addition of stabilizers is used to obtain as good as possible rheological properties (caseinates, gelatin, carragenan etc.) (M a ć e j et al. 1994, 1995).
Over the last twenty years, an increase of healthy food consumptions has been observed, which practically initiates the application of whey proteins due to their high nutritive value (d e W i t , 1998, D e n i n -D j u r d j e v i ć , 2001, D j o r d j e v i ć , 1987, H a m b r a e u s , 1986 ).
As elsewhere discussed, addition of whey proteins in milk changes technological properties of milk and has a great effect on the rheological properties of products.There are numerous articles that discuss the influence of whey protein addition on viscosity (D e n i n -D j u r d j e v i ć et al.  al., 1986, 1988a,b).The complex between casein and whey proteins is formed during heating of milk.According to C o r r e d i g and D a l g l e i s h , 1999, the amount of ß-lactoglobulin is a significant factor that influences complex formation.However, the amount of α-lactalbumin associated with casein micelle influences hydrophilic properties of casein micelles at pH 4.60 (M o t t a r et al., 1989).
The aim of this work was to determine the influence of different amounts of added demineralized whey powder, applied heat treatments and storage period before stirring on the viscosity of stirred yogurt.

Materials and Methods
Skim milk powder was reconstituted to obtain milk with 8.44% DM, which was standardized with different amounts of demineralized whey powder (DWP) to obtain milk A (with 9.71% DM) and milk B (with 10.75% DM), respectively.Skim milk powder and DWP were obtained from the dairy "IMPAZ" Zaječar.
For the experiments, untreated milk and milk heat treated at 85ºC/20 min and 90ºC/10 min, respectively, were used.Milk was inoculated with 2.5% of commercial yogurt culture (containing Lactobacillus delbrueckii subsp.bulgaricus and Streptococcus thermophilus in the ratio 1:1) at 43ºC.Samples were incubated until pH 4.6 was reached.Samples were immediately cooled to 4ºC and held at that temperature during 14 days.
Samples of acid casein gels were stirred during 30 sec after 1, 7 and 14 days of storage.Samples gained by stirring of acid casein gels after 1 day of storage are named as samples of stirred yogurt after 1 day of storage.Samples gained by stirring of acid casein gels after 7 days of storage are named as samples of stirred yogurt after 7 days of storage.Samples gained by stirring of acid casein gels after 14 days of storage are named as samples of stirred yogurt after 14 days of storage.

Analyses and measurements
Dry matter content: AOAC method 16

Measurements of viscosity
Measurements of viscosity were done with Brookfield DV-E Viscometer.Spindle No 3 at 30 rpm was used for all samples, as formerly described by D e n i n -D j u r d j e v i ć et al., 2002a.
Viscosity was monitored during storage at 4ºC after 1, 7 and 14 days.

Quality parameters of milk and yogurt
Chemical composition of milk A and B as well as yogurt samples are shown in Table 1 As can be seen from the results shown in Table 1., nitrogen content of yogurt samples is lower than in milk, which indicates that applied LAB carries out proteolysis (D e n i n -D j u r d j e v i ć et al., 2002a., S h a h and S h i h a t a , 1998).

The influence of added demineralized whey powder on the fermentation process
The fermentation process of milk A and B as well as of milk with 8.44% DM is shown in Fig. 1., 2. and 3.

Changes of viscosity of stirred yogurt at steady spindle rotation
The influence of applied heat treatment and storage period before stirring on viscosity change of stirred yogurt produced from milk A is shown in Table 2. and Fig. 4.   Stirred yogurt produced from milk A heat treated at 85ºC/20 min had after 30 sec smaller viscosity than yogurt produced from milk heat treated at 90ºC/10 min.However, yogurt produced from milk A heat treated at 85ºC/20 min had higher viscosity during the rest of shearing.Decrease of viscosity during time of shearing was 7.90 mPas and 13.53 mPas, respectively, for samples produced from milk heat treated at 85ºC/20 min and 90ºC/10 min.
As can be seen from Table 2. and Fig. 4. stirred yogurt produced from milk heat treated at 85ºC/20 min had higher viscosity after 7 than after 14 days of storage.Decrease of viscosity during time of shearing was 10.03 mPas and was by 2.13 mPas greater than after 1 day.Viscosity value of stirred yogurt after 14 days of storage was 44.03 mPas after 30 sec and 36.90 mPas after 120 sec.Decrease of viscosity during time of shearing was 7.13 mPas.
Table 2. and Fig. 4. show that viscosity values of stirred yogurt produced from milk heat treated at 90ºC/10 min were between 56.47 mPas (after 30 sec) and 43.00 mPas (after 120 sec).Decrease of viscosity during time of shearing was 13.47 mPas and was approximately such as decrease of viscosity after 1 day of storage.
After 14 days of storage, yogurt samples produced from milk A heat treated at 90ºC/10 min had a highest viscosity values, which ranged from 61.50 mPas, after 30 sec, to 45.83 mPas, after 120 sec.Decrease of viscosity during time of shearing was 15.67 mPas, which was the greatest decrease in this group.
The influence of applied heat treatment and storage period before stirring on viscosity change of stirred yogurt produced from milk B is shown in Table 3. and Fig. 5.
T a b .As results show, stirred yogurt produced from untreated milk B had the smallest viscosity, ranging from 30.53 mPas after 30 sec, to 22.40 mPas after 120 sec.Viscosity decreased by 8.13 mPas during shearing.
Yogurt samples produced from heat treated milk had a higher viscosity after 1 day of storage.Viscosity ranged from 50.53 mPas and 49.17 mPas after 30 sec to 43.87 mPas and 38.07 mPas, after 120 sec, for yogurt samples produced from milk heat treated at 85ºC/20 min and 90ºC/10 min, respectively.
Table 3. and Fig. 5. show that after 7 days stirred yogurt produced from milk B heat treated at 85ºC/20 min, had the highest viscosity (51.47 mPas after 30 sec), which was by 0.94 mPas greater than of that after 1day.However, viscosity decreased by 9.27 mPas during shearing, which resulted in smaller viscosity after 120 sec by 1.67 mPas than after 1 day.Stirred yogurt produced from milk B heat treated at 85ºC/10 min had the smallest viscosity after 14 days of storage.Decrease of viscosity during shearing was 7.50 mPas.
As Table 3. and Fig. 5. show, viscosity of yogurt samples produced from milk B heat treated at 90ºC/10 min was influenced by storage period of gel before stirring.Also, these samples had higher viscosity after 7 and 14 days than samples produced from milk heat treated at 85ºC/20 min.
Samples of stirred yogurt produced from milk B heat treated at 90ºC/10 min had the highest viscosity after 14 days of storage and the smallest after 1 day of storage.After 7 days, viscosity values of stirred yogurt ranged from 55. 30  According to the gained results, it could be concluded that hydrophilic properties of acid casein gel produced from milk heat treated at 90ºC increase during storage, which has positive influence on the viscosity of stirred yogurt.However, in view of the fact that manufacturers stir yogurt immediately after fermentation and cooling, a better solution is heat treatment at 85ºC/20 min.

The influence of added demineralized whey powder on the viscosity of stirred yogurt
The influences of added demineralized whey powder and applied heat treatment on viscosity changes of stirred yogurt, after 1 day of storage, compared with viscosity of stirred yogurt produced from milk with 8.44% DM, are shown in Figs.6., 7. and 8. Fig. 6. shows that yogurt samples produced from untreated milk standardized with DWP had less pronounced decrease of viscosity during shearing.Also, these  -The influence of added demineralized whey powder on viscosity of stirred yogurt produced from milk heat treated at 90ºC/10' Fig. 7. shows that samples of stirred yogurt produced from milk A and B heat treated at 85ºC/10 min had remarkably higher viscosity than samples with 8.44% DM.On the other hand, when heat treatment at 90ºC/10 min was used, viscosity values of stirred yogurt samples A and B were similar to those of yogurt with 8.44% DM as Fig. 8. shows.
Gained results indicate that aport from the amount of added DWP, applied heat treatment has a great influence on rheological properties of stirred yogurt after 1 day of storage.Contrary to set-style yogurt samples produced from milk standardized with DWP (D e n i n -D j u r d j e v i ć et al., 2002), which had a greater viscosity if heat treatment at 90ºC/10 min had been used, stirred yogurt samples had a higher viscosity if heat treatment at 85ºC/10 min had been used.Our results indicate that DWP could be used for milk standardization, but selection of heat treatment depends on the type of yogurt, namely, is the milk designed for production of stirred or set-style yogurt.

C o n c l u s i o n
According to all aforementioned, it could be concluded: Standardization of milk dry matter content with DWP leads to shorter duration of fermentation with lactic acid bacteria, which effects costs of production.Milk samples standardized with 2% DWP had shorter fermentation than milk samples standardized with 1% DWP did.
Samples of stirred yogurt produced from milk heat treated at 85ºC/20 min obtained by stirring of acid casein gel stored 1 day had a greater viscosity than samples produced from milk heat-treated at 90ºC/10 min, regardless to the amount of added DWP.
Samples produced from milk heat treated at 90ºC/10 min had a greater viscosity after 7 and 14 days of storage, which indicates greater hydrophilic properties as well as a more pronounced swelling of casein micelles during storage.
Samples of stirred yogurt produced from milk heat treated at 85ºC/20 min had a greater viscosity than samples produced from milk without added DWP.It can be concluded that DWP could be used for milk dry matter standardization.
, 2002, L u c e y et al., 1998, Lucey et al., 1999), firmness (J e l e n et al., 1987, M o d l e r et al., 1983) and microstructure (M o d l e r and K a l a b , 1983, M o d l e r et al., 1983, T a m i m e et al., 1984) of set-style yogurt.Also, the influence of whey proteins (B o ž a n i ć et al., 2000) and whey powder addition (A b d E l -S a l a m et al., 1991), D e n i n -D j u r d j e v i ć , 2001, T o d o r i ć and S a v a d i n o v i ć , 1973) on rheological properties of stirred yogurt is often investigated.However, according to general opinion, the addition of more than 2% of whey proteins induces undesirable sensory properties of yogurt (D e n i n -D j u r d j e v i ć , 2001, J e l e n et al., 1987, M o d l e r et al., 1983, T a m i m e and D e e t h , 1980, T a m i m e and R o b i n s o n , 1988).The next crucial factor that influences viscosity of stirred yogurt is the application of heat treatment (D a v i e s et al., 1978, K a l a b et al., 1976, L a b r o p o u l o s et al., 1984, P a r n e l l -C l u n i e s , et

Fig. 1 .Fig. 3 .Fig. 1 .
Fig. 1. -Exchange of pH value during fermentation Fig. 2. -Exchange of pH value during of milk with 8.44% TS as influenced by fermentation of milk A as influenced applied heat treatment by applied heat treatment after 1 day of storage.Decrease of viscosity during time of shearing was 8.73 mPas.

Fig 4 .-
Fig 4. -The influence of applied heat treatment and storage period before stirring (gels were stored 1 day (full symbol, full line); 7 days (empty symbol, full line) and 14 days (empty symbol, dashed line)) on viscosity of stirred yogurt produced from milk A

Fig 5 .-
Fig 5. -The influence of applied heat treatment and storage period before stirring (gels were stored 1 day (full symbol, full line); 7 days (empty symbol, full line) and 14 days (empty symbol, dashed line) on viscosity of stirred yogurt produced from milk B mPas after 30 sec. to 45.40 mPas after 120 sec.Viscosity decreased by 9.90 mPas during shearing.After 14 days of storage, viscosity values of stirred yogurt ranged from 57.83 mPas after 30 sec. to 46.60 mPas after 120 sec.Viscosity decreased by 11.23 mPas during shearing.Gained results agree with the results of B o ž a n i ć et al., 2000, D e n i n -D j u r d j e v i ć et al., 2001, D e n i n -D j u r d j e v i ć et al., 2002, L a b r o p o u l o s et al., 1984, and indicate that stirred yogurt shows thixotropic behaviour.
samples had higher viscosity values.Contrary to set-style yogurt, where addition of DWP leads to decrease of viscosity (D e n i n -D j u r d j e v i ć et al., 2002), addition of DWP in milk for the production of stirred yogurt leads to increase of viscosity, which agrees with the results of D e n i n -D j u r d j e v i ć , 2001, and T o d o r i ć and S a v a d i n o v i ć , 1973.

Fig. 6 .
Fig.6.-The influence of added demineralized Fig.7.-The influence of added demineralized whey powder on viscosity of stirred yogurt whey powder on viscosity of stirred produced from untreated milk yogurt produced from milk heat treated at 85ºC/20' R E F E R E N C E S UTICAJ ODABRANIH FAKTORA NA PROMENU VISKOZITETA TEČNOG JOGURTA Jelena Denin Djurdjević 1 , O. Maćej 1 i Snežana Jovanović 1

Table 2 .
19a b .2.-The influence of applied heat treatment and storage period of acid casein gel before stirring on viscosity change of stirred yogurt produced from milk A and Fig.4.show that stirred yogurt produced from untreated milk A had the smallest viscosity values (28.33 mPas at the beginning and19.60 mPas at 3.-The influence of applied heat treatment and storage period of acid casein gel before stirring on viscosity change of stirred yogurt produced from milk B