Raw milk protects against allergies and asthma. However, its consumption is discouraged for food safety reasons. Research is looking for ways to preserve the benefits of raw milk and ensure food safety. Raw milk was fermented into yoghurt, on the one hand mild yoghurt with pH 4.4-4.5, on the other hand acidic yoghurt with pH 3.9-4.0 (n=10). After fermentation and after 14 days, the yoghurt was tested for Escherichia coli, Staphylococcus aureus, Salmonella spp., STEC and Listeria monocytogenes. In a second experiment, challenge tests with an STEC model and with a mixture of four Listeria innocua strains were performed 3-fold with 104-105 cfu/ml each. E. coli and S. aureus could be detected in the mild fresh yoghurt, but not in the acidic fresh and neither in all 14-day yoghurt. Salmonella spp. and L. monocytogenes were not detectable in fresh or 14-day yoghurt. STEC genes were detected in one out of ten fresh mild and one out of ten acidic 14-day yoghurts. However, no STEC could be isolated from the positive samples. In the challenge tests, the bacterial count of both the STEC model germ and L. innocua was reduced over 14 days, in mild yoghurt by 101-102, in acidic by 106 to below the detection limit. The acidic raw milk yoghurt produced in this way are safe. However, the results show that periodic monitoring of raw milk for STEC is useful in addition to the other parameters.
