I have a small culture of water kefir grains, which I use to make… water kefir. Recently, I wondered if I could make something similar to milk kefir using soy milk and my grains. I tried:
* Putting kefir grains in soy milk
* Putting already-made water kefir (which also contains ferments) in soy milk
Both methods produced kind of the same result, which was a separation of curd and "water". The grains' result was much faster and impressive, but the grains were really hard to remove (unlike in water, you can't really sieve them, since the curd itself will not pass the sieve). I tried skimming the result for various amounts of time, which made :
* Something really similar to milk kefir, which didn't really have kind of soy-like taste. If you told someone this was fermented cow milk and they were not an expert I'm pretty sure it could fool them.
* When skimmed for longer, something more like spreadable cheese. This did have a soy taste, and with a little lemon and salt it was very good.
So yes, these experimentations were really successful. I wonder if it could make actual cheese, but of course this is way longer to make.