The strain you want has a huge headstart, and there shouldn't be much bacteria in boiled milk to begin with. Containers can be also sterilized with boiling wate

The strain you want has a huge headstart, and there shouldn't be much bacteria in boiled milk to begin with. Containers can be also sterilized with boiling water. Then keep it cool after it's done (14 hours). It doesn't seem likely that some other strain would dominate, but who knows.