I have a friend that is giving me the "extra" cow's milk (left over from feeding his calves). Up until about a week ago, everything was fine, cheeses were coming out great. I pasteurize ALL the milk at 145 degrees for 30 minutes (standardized process here), and place it into "skimmers" to save the cream for butter and to re-add to cheese recipes where needed. Over the last week, I have been finding that the milk has started to coagulate after pasteurization in the skimmer. The milk smells normal (sweet), and tastes good, but it will not form a proper curd for making cheese, instead, it looks like vinegar has been added when it gets to temperature (without any additives)... Have any of you experienced this before? Any ideas?