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.