The aim of the study was to define the growth of yogurt bacteria in UF milk retentates as well as examination their survival in acidified retentates during their storage at 6°C and -18°C for the period of 22 months. Retentates (concentration factor between 4.0–4.5 and 5.0–5.3) were obtained from pasteurized milk (3.2% fat) by its concentration using ultrafiltration. Retentates after pasteurization (72°C/15s) were fermented (temp. 44 ± 1°C) to the pH 5.1–5.2 by yogurt cultures: YC-X11 and YC-180. During the fermentation process the acidification rate of retentate and pasteurized milk by measuring the pH and titratable acidity were investigated. After 24 hours from processing in retentates acidified to pH 5.1–5.2 the plate count of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus were determined. The survival of yogurt bacteria in acidified retentates were determined by measuring their counts in retentates after 1, 2, 3, 4, 5, 6, 16 and 22 months of storage at 6° and -18°C. It was found out that retentates showed different rates of acidification compared with milk because of their higher buffer capacity. In general yogurt cultures showed good growth in milk highly concentrated by UF. In retentates acidified to pH 5.1–5.2 the mean plate count of S. thermophilus was really high (2.8×109 cfu/g). L. delbrueckii ssp. bulgaricus counts were depended from yogurt’s culture type used for fermentation and was lower for retentates acidified with YC-X11 culture (2.6×105 cfu/g) than for retentates in which YC-180 culture was used (2.7×108 cfu/g). Finally both yogurt bacteria showed much better survival in acidified retentates stored at -18°C than stored at 6°C.
ultrafiltration, retentate, yogurt culture