The influence of investigated factors on viscosity of stirred yogurt

Abstract

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.

Obrano mleko u prahu je rekonstituisano tako da se dobije mleko sa 8.44% SM koje je zatim standardizovano dodatkom različite količine demineralizovane surutke u prahu (DSUP), pri čemu su dobijeni uzorci mleko A (sa 9.71% SM) i mleko B (sa 10.75% SM). Svi uzorci mleka su termički tretirani na 85ºC/20 min i 90ºC/10min, respektivno, a kao kontrolni uzorak je korišćeno termički netretirano mleko. Nakon termičkog tretmana, mleko je ohlađeno do 43ºC i inokulisano sa 2.5% komercijalne jogurt kulture (Lb. delbrueckii subsp. bulgaricus i Str. thermophilus u odnosu 1:1). Uzorci su inkubirani na 43ºC do postizanja pH 4.6. Uzorci su zatim brzo ohlađeni na 4ºC i držani na toj temperaturi do analize. Uzorci kiselog kazeinskog gela su razbijani pomoću električne mešalice u toku 30 sekundi nakon 1, 7 i 14 dana skladištenja. Merenje viskoziteta je vršeno pomoću viskozimetra Brookfield DV-E. Upotrebljen je spindl No 3, pri 30 obrt/min. Fermentacija traje kraće kada se za standardizaciju suve materije mleka koristi DSUP. Uzorci tečnog jogurta proizvedeni od mleka termički tretiranog na 85ºC/10 min, dobijeni razbijanjem gela 1 dan nakon proizvodnje imaju veće vrednosti viskoziteta od uzoraka proizvedenih od mleka termički tretiranog na 90ºC/10 min. Nakon 7 i 14 dana skladištenja, veće vrednosti viskoziteta su imali uzorci tečnog jogurta proizvedeni od mleka termički tretiranog na 90ºC/10’, što ukazuje na jaču hidrofilnost i više izraženo bubrenje kazeinskih čestica tokom skladištenja.