Hi smallyiyi,
I enclose a few literature extracts on some bacterial pathogenic possibilities in conventional yoghurt (ie not further heat treated). I have later, briefly, added my own deductions for the heat-treated case since literature data seems limited.
Regarding conventional yoghurt –
The heat treatment of the yoghurt milk at 85-95 degC is sufficient to kill the majority, if not all, of the vegetative cells of microorganisms associated with raw milk , but spore-formers and some heat-stable enzymes will remain. This reduced competition ensures that the heated milk will provide a good growth medium for the yoghurt starter culture, but nevertheless, the bacteriological quality of the raw milk and any dry ingredients used in the milk base is of great importance.
Regarding B.cereus –
Spore germination and growth of B. cereus in fermented milks are prevented by low pH. However, growth of B. cereus has been shown in yoghurt milk at 31 °C, although, as the pH dropped, the growth rate declined, and it ceased at pH 5.7. Although it is possible that high levels could be reached when initial acid production is slow, B. cereus is not normally considered a hazard in fermented milks.
Fermented milks have a good safety record in terms of foodborne disease, and there are very few recorded incidents of food poisoning associated with these products. Milk used in fermentations is generally subjected to a severe heat treatment sufficient to destroy vegetative pathogens.
Owing to its acidic nature, fermented milks limit the growth of pathogenic microorganisms. Especially, some pathogens including Listeria monocytogenes, Salmonella spp., coliforms, and Staphylococcus aureus are rarely present in yogurt and other fermented milks. These pathogens are rapidly inhibited within 24 h after the manufacture . The level of survivability of the pathogenic organisms in fermented milk depends on the severity of contamination and the pH of the product . Mild yogurts with pH values of >4.5 can allow the survival of salmonellae for up to 10 days.
Regarding pathogenic E.coli -
Postfermentation contamination of verotoxigenic Escherichia coli O157:H7 carries a high risk for consumers’ health . Although E. coli O157:H7 is negatively affected by high acidity, it can survive in yogurt during cold storage.
Experiments to determine the survival of foodborne pathogens in yoghurt and other fermented milks tend to produce quite variable results. Survival times can be influenced by pH, acidity and the characteristics of the starter culture used.
Regarding further stage of heat treatment to eliminate LAB, Y&M, other vegetative bacterial contaminants, enzymes and extend shelf-life –
I’m not familiar with the temp.-times used but I assume the major risk is again post-pasteurization contamination, ie dependent on hygiene for environment/personnel/equipment/packaging. I presume the pH is similar to initial level so most of previous pathogen-related comments unchanged.
