I put in Meiji full fat 1 litter 3 table spoon Yolida yougurt, cover it and leave outside for 24 hours, after let separate the way through cheese clot over night in fridge. The best yougurt, can mix with few blueberies and pomegranates.
I used weekly 10 litter raw milk from nearby milk processing fabric. I used it for making kefir, butter from cream and occasinally quark. Never had any problems. The cultures of Kefir grains kill bad bacteria in competition with milk cultures.