Twelve batches of fermented milk were prepared at 42ºC where the studied variables were total solids content (12 and 15%), sucrose concentration (0 and 8%), and co-culture type (Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus ; Streptococcus thermophilus and Lactobacillus acidophilus ; Streptococcus thermophilus and Lactobacillus rhamnosus. Kinetic parameters to decrease pH until 4.5 were calculated. Physico-chemical and microbiological determinations were carried out on products after 1 and 7 days of storage at 4ºC. The increase in total solids and the addition of sucrose resulted in water activity decrease, which affected the time to reach pH 4.5 according to the milk and co-culture employed. The milk fermented by Streptococcus thermophilus and Lactobacillus acidophilus (STLA) presented higher post acidification. Besides, the milk fermented by Streptococcus thermophilus and Lactobacillus rhamnosus (STLR) was the more stable. The milks containing higher levels of total solids reached higher acidity independently of the co-culture. Increasing sucrose amounts and total solids of milk resulted in higher firmness using the co-cultures STLR e STLA. After seven days of storage, there were no major differences in the initial counts of yogurt and probiotic bacteria in all fermented milks. In all batches, the number of probiotic bacteria was higher than suggested by literature.
yogurt; probiotics; acidification; viability
