Try making a batch of cheese with it - if anything, I would guess that the ph is going to be low to start with, so you may not need to add as much culture to it.
If you aren't happy with the results, you can low-temp pasturize the milk before making cheese. Just warm it to 145 degrees Fahrenheit, hold there for 20 minutes, then quick cool to the starting temp for whatever cheese you are making.
That way, you will kill off the organisms that may compete with your culture. Though pasturized, it will still be better than storebought p/h milk, IMHO.
I used to freeze milk for cheesemaking, too. It worked just fine. If you leave the cream on it, you will want to warm the milk to about 95 degrees initially to melt the butterfat back into the milk.