by Christopher Weeks on Thu Feb 19, 2015 8:46 am
You can do it until you can't; that's all. Feel free to do it until the taste or consistency isn't the way you like, then start again with a new culture.
Most yogurt cultures are a bunch of different organisms, each doing their own thing. If you leave them to their own devices, some will outperform others and the distribution of the bugs will change over time, with some massively proliferating and others dying out completely. And yet others colonizing the heritage culture from the air, the milk and your hands. And the direction that goes will be different on your counter, in your milk, than it will be at my house.
Maybe that process will produce a stable culture over time that makes a yogurt unique to your house, that's just as goodas , but a little different from, the yogurt that your starter culture produced. Or maybe it won't ever firm and will throw off-flavors. Try it and see!
And remember that any live yogurt from the grocer can act as a culture for your milk -- you don't have to buy specialty cultures.