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 85ºC/20 min and 90ºC/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 43ºC. Samples were incubated until pH 4.6 was reached. Samples were immediately cooled to 4ºC 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 20ºC. 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 90ºC/10 min had a higher viscosity than samples produced from milk heat treated at 85ºC/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 istraživanja korišćeno je rekonstituisano obrano mleko u prahu sa 8.44% SM, mleko sa 9.65% SM i 10.84% SM. Za fermentaciju je korišćeno termički netretirano mleko i mleko termički tretirano na 85ºC/20 min i 90ºC/10min. Fermentacija je izvršena na temperaturi od 43ºC, sa dodatkom 2.5% tehničke jogurtne kulture (Lb. delbrueckii subsp. bulgaricus i Str. thermophilus u odnosu 1:1). Fermentacija je prekinuta pri pH 4.6. Uzorci su zatim brzo ohlađeni na 4ºC i skladišteni na toj temperaturi u toku 14 dana. Kiseli kazeinski gel je pre merenja viskoziteta razbijan nakon 1, 7 i 14 dana skladištenja na 4ºC. Sva merenja su vršena pri brzini rotacije spindla od 30 obrt/min tokom 2 min, na temperaturi od 20ºC. Na osnovu istraživanja može se zaključiti da na vrednost viskoziteta tečnog jogurta dobijenog nakon razbijanja kiselog kazeinskog gela utiču primenjeni termički tretman i sadržaj suve materije. Sa povećanjem suve materije povećava se i viskozitet tečnog jogurta. Najmanji viskozitet je imao jogurt proizveden od termički netretiranog mleka sa 8,44% SM, a najveći jogurt proizveden od mleka sa 10,84% SM. Primenjeni termički tretmani mleka imali su značajan uticaj na viskozitet tečnog jogurta dobijenog razbijanjem kiselog kazeinskog gela nakon 7 i 14 dana skladištenja. Tečni jogurt dobijen od mleka termički tretiranog na 90ºC/10 min imao je veći viskozitet od uzoraka proizvedenih od mleka termički tretiranog na 85ºC/20 min. Period skladištenja je uticao i na srednju vrednost viskoziteta tečnog jogurta. Kod tečnog jogurta sa 8,44% SM srednja vrednost viskoziteta se smanjuje tokom perioda skladištenja bez obzira na primenjeni termički tretman mleka. Najveće srednje vrednosti viskoziteta imali su uzorci proizvedeni od mleka sa 10,84% SM.