Survival of Escherichia coli O157:H7 in yogurt during preparation and storage at 4 degrees C

Cow's milk was inoculated with cn 10(3) and 10(7) cfu ml(-1) Escherichia coli O157:H7. After fermentation at 42 degrees C for 0-5 h, the yoghurt was stored at 4 degrees C. Two kinds of yoghurt were used: traditional yoghurt (TY), made with Streptococcus thermophilus and Lactobacillus bulgaricus starter cultures, and 'bifido' yoghurt (BY), made with the two starter cultures plus Bifidobacterium bifidum. After 7 d E. coli O157:H7 decreased from 3.52 to 2.72 log(10), cfu ml(-1) and from 7.08 to 5.32 log(10) cfu ml(-1) in TY, and from 3.49 to 2.73 log(10) cfu ml(-1) and from 7.38 to 5.41 log(10) cfu ml(-1) in BY. The pH values of yoghurt dropped from 6.6 to 4.5 and 4.4 in TY (for low and high pathogen inocula, respectively), and from 6.6 to 4.6 and 4.5 in BY (for low and high pathogen inocula, respectively).