Viability of probiotic bacteria and concentrations of organic acids in commercial yoghurts during refrigerated storage

Yoghurts (four varieties), prepared using Streptococcus thermophilus and Lactobacillus delbruckii ssp. bulgarcus and containing three probiotic strains, Bifidobacterium animalis Bb-12, L. acidophilus La-5 and L. rhamnosus GG, were examined for viability over a month of refrigerated storage. At 1, 7, 14, 21 and 28 days of refrigerated storage, the yoghurts contained stable viable populations of B. animalis Bb-12, L. acidophilus La-5 and L. rhamnosus GG, with no significant changes between intervals (P > 0.05), populations remaining at 8.0 to 8.7 log(10) viable cells/mL for the projected shelf life. Decreases in pH between storage intervals appeared to reduce the viable populations of probiotic bacteria in each yoghurt (P > 0.05). Low pH levels (<= 5 pH 4.0) were due to lactic and acetic acids and acids derived from fruit juice flavourings. The concentration of lactic acid was significantly greater than acetic acid in each yoghurt (P > 0.05).