Traditional yoghurt (TY) (Streptococcus thermophiles+ Lactobacillus bulgaricus) and Bifidus yoghurt (BY) samples
(Streptococcus thermophiles+ Lactobacillus bulgaricus+ Bifidobacterium bifidum). Traditional & Bifidus yoghurt
samples were inoculated with 10⁶ cfu/g L. monocytogenes and stored for 21 days at 4±1oC. Samples were taken at zero
time (fresh samples) then after 3, 5, 7, 10, 14 and 21 days of storage for titratable acidity measurement and bacterial count
of the culture starters and L. monocytogenes counts. The results revealed that the titratable acidity of TY & BY was
increased during the 21 days of storage, while the population of L. monocytogenes became non-detectable level in 10^th
days of storage in BY and still detected in TY till the day 14th of storage and became 2.20± 0.31 log10 cfu/g. The count
of all starter cultures (S. thermophiles+ Lb. bulgaricus+ B. bifidum) in both yoghurts increased during the fermentation
and cold storage and remained stable with values >6 log10 cfu/g throughout the storage period at 4±1°C.