The influence of dry matter, applied heat treatment and storage period on the viscosity of stirred yogurt

Abstract

Skim milk powder reconstituted to 8.44% TS, 9.65% TS and 10.84% TS respectively was used for investigation. Untreated milk and milk heat treated at 85?C/20 min and 90?C/10 min, respectively, were used for the investigation. Milk was inoculated with 2.5% of 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 during 14 days. Acid casein gel was stirred after 1, 7 and 14 days of storage. Measurements were done at 30 rpm during 2 min, at 20?C. According to the investigation, it could be concluded that both applied heat treatment and dry matter content influence viscosity of stirred yogurt. Viscosity increases when dry matter content increases. The smallest viscosity had yogurt produced from untreated milk with 8.44% TS, while samples produced from milk with 10.84% TS had the highest viscosity. Applied heat treatments had significant influence on viscosity of yogurt gained by stirring of acid casein gels after 7 and 14 days of storage. Stirred yogurt produced from milk heat treated at 90?C/10 min had a higher viscosity than samples produced from milk heat treated at 85?C/20 min. Storage period influenced average viscosity of stirred yogurt. Samples of stirred yogurt produced from milk with 8.44% TS showed a decrease of average viscosity during storage regardless of the applied heat treatment of milk. The highest average viscosity had samples produced from milk with 10.84% TS.U okviru ovih istrazivanja korisceno je rekonstituisano obrano mleko u prahu sa 8.44% SM, mleko sa 9.65% SM i 10.84% SM. Za fermentaciju je korisceno termicki netretirano mleko i mleko termicki tretirano na 85?C/20 min i 90?C/10min. Fermentacija je izvrsena na temperaturi od 43?C, sa dodatkom 2.5% tehnicke jogurtne kulture (Lb. delbrueckii subsp. bulgaricus i Str. thermophilus u odnosu 1:1). Fermentacija je prekinuta pri pH 4.6. Uzorci su zatim brzo ohladjeni na 4?C i skladisteni na toj temperaturi u toku 14 dana. Kiseli kazeinski gel je pre merenja viskoziteta razbijan nakon 1, 7 i 14 dana skladistenja na 4?C. Sva merenja su vrsena pri brzini rotacije spindla od 30 obrt/min tokom 2 min, na temperaturi od 20?C. Na osnovu istrazivanja moze se zakljuciti da na vrednost viskoziteta tecnog jogurta dobijenog nakon razbijanja kiselog kazeinskog gela uticu primenjeni termicki tretman i sadrzaj suve materije. Sa povecanjem suve materije povecava se i viskozitet tecnog jogurta. Najmanji viskozitet je imao jogurt proizveden od termicki netretiranog mleka sa 8,44% SM, a najveci jogurt proizveden od mleka sa 10,84% SM. Primenjeni termicki tretmani mleka imali su znacajan uticaj na viskozitet tecnog jogurta dobijenog razbijanjem kiselog kazeinskog gela nakon 7 i 14 dana skladistenja. Tecni jogurt dobijen od mleka termicki tretiranog na 90?C/10 min imao je veci viskozitet od uzoraka proizvedenih od mleka termicki tretiranog na 85?C/20 min. Period skladistenja je uticao i na srednju vrednost viskoziteta tecnog jogurta. Kod tecnog jogurta sa 8,44% SM srednja vrednost viskoziteta se smanjuje tokom perioda skladistenja bez obzira na primenjeni termicki tretman mleka. Najvece srednje vrednosti viskoziteta imali su uzorci proizvedeni od mleka sa 10,84% SM.