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 85?C/20 min and 90?C/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 43?C. Samples were incubated until pH 4.6 was reached. Samples were immediately cooled to 4?C 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 85?C/20 min, obtained by stirring of gel 1 day after production had a higher viscosity than sample produced from milk heat treated at 90?C/10 min. On the other hand, samples produced from milk heat treated at 90?C/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 razlicite kolicine demineralizovane surutke u prahu (DSUP), pri cemu su dobijeni uzorci mleko A (sa 9.71% SM) i mleko B (sa 10.75% SM). Svi uzorci mleka su termicki tretirani na 85?C/20 min i 90?C/10min, respektivno, a kao kontrolni uzorak je korisceno termicki netretirano mleko. Nakon termickog tretmana, mleko je ohladjeno do 43?C i inokulisano sa 2.5% komercijalne jogurt kulture (Lb. delbrueckii subsp. bulgaricus i Str. thermophilus u odnosu 1:1). Uzorci su inkubirani na 43?C do postizanja pH 4.6. Uzorci su zatim brzo ohladjeni na 4?C i drzani na toj temperaturi do analize. Uzorci kiselog kazeinskog gela su razbijani pomocu elektricne mesalice u toku 30 sekundi nakon 1, 7 i 14 dana skladistenja. Merenje viskoziteta je vrseno pomocu viskozimetra Brookfield DV-E. Upotrebljen je spindl No 3, pri 30 obrt/min. Fermentacija traje krace kada se za standardizaciju suve materije mleka koristi DSUP. Uzorci tecnog jogurta proizvedeni od mleka termicki tretiranog na 85?C/10 min, dobijeni razbijanjem gela 1 dan nakon proizvodnje imaju vece vrednosti viskoziteta od uzoraka proizvedenih od mleka termicki tretiranog na 90?C/10 min. Nakon 7 i 14 dana skladistenja, vece vrednosti viskoziteta su imali uzorci tecnog jogurta proizvedeni od mleka termicki tretiranog na 90?C/10?, sto ukazuje na jacu hidrofilnost i vise izrazeno bubrenje kazeinskih cestica tokom skladistenja.