I've been pondering MDee's post (in quotes and purple):
"Can you ferment "pasteurized and homogenized" milk? Technically yes...very controlled fermentation think of making yogurt. In this instance you add or inoculate the milk with what you want then incubate it until it is at the point of microbe "ecosystem" that you desire. Then you stop everything at that spot (refrigeration) and consume it. Kefir has more forgiveness than yogurt but the same thing applies."
My comment: please see Tim Hall's post at "Yogurt Fermentation History". Raw milk has been "innoculated" for centuries.