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

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 deg C/20 min and 90 deg C/10 min, respectively, were used for the investigation. Milk was inoculated with 2.5% of yoghurt culture at 43 deg C. Samples were incubated until pH 4.6 was reached. Samples were immediately cooled to 4 deg 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 deg C. According to the investigation, it could be concluded that both applied heat treatment and dry matter content influence viscosity of stirred yoghurt. Viscosity increases when dry matter content increases. Applied heat treatments had significant influence on viscosity of yoghurt gained by stirring of acid casein gels after 7 and 14 days of storage. Stirred yoghurt produced from milk heat treated at 90 deg C/10 min had a higher viscosity than samples produced from milk heat treated at 85 deg C/20 min. Storage period influenced average viscosity of stirred yoghurt. Samples of stirred yoghurt 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.