Yes all the store bought kefir and yogurt is pasteurized. This is why I use the Raw Goats milk, Fermented Cow kefir and Fermented Fish Stock, all from Answers Pet Foods. We also rotate with them every week.
So this is just my experience.. I’m only an undergrad, and this is an ongoing project in my school’s micro lab.
I use the same powdered starter, but two variables with the milk.
I used pasteurized and raw cow.
When I start and grow the cultures with the pastuerized milk, that gives me a pretty good baseline of bacteria.
When I start out and grow in raw milk, the bacteria numbers are through have about 35,000 more ppm. But then they die off after about 24 hours. My conclusion is that the raw milk brings its own established bacteria and yeasts into the experiment, that they compete with the kefir cultures and destroy them.
The best results I’ve had, is to start off the powder in pasteurized milk, get the cultures established, then slowly add small amounts of raw milk over a course of a couple batches.
My hypothesis is that we need to start off with baby cultures, and have to feed them clean baby food (pasteurized milk) until they become a hungry angry little army. Only then are they able to process the lactose in raw milk with becoming destroyed. Once the colony is established, the evolved cultures can handle straight milk.
I won’t have this project finished until the end of next semester, but I think i have a pretty good handle on it, it’s repeatable, and I’ve been getting the same results so far.