leenia, it's all been properly cold with its lid on most of the time, and it has been a quantity thing.   I'm having problems getting the right milk in the right size at the moment. Some has to be decanted and frozen. I'm very careful, and go by smell and taste, as well as heating the milk. My mother, who was brought up on a farm, taught me about making cheese from turned milk, and I have her dairy thermometer to check when I scald the stuff. I'm still here! I am careful, and dump anything at all iffy. I've never felt ill after any of it.