I'm sure we've all opened a bottle of milk to find that it's turned to cheese while we weren't looking. Well, this wasn't quite that unexpected, but, well...
We bought some milk that was already at its use-by date, so needed using up quickly. At the same time, I was wondering whether I could come up with any lacto-fermented product that Ian would like, and I put the two together. Whey (from milk) is sometimes used as a starter for sauerkraut, so why not the other way round? I warmed a pint or so of milk and stirred in about a tablespoonful of juice from the sauerkraut. I expected the milk to curdle almost immediately with the acid, but it didn't, so I put in a bowl in a warm-ish place, and left it.
I say left it... I peered at it, stirred it, sniffed it, and tasted it fairly frequently. The smell and taste weren't too off-putting, so I kept it until, after two and a half days, it separated out into curds and whey.
I'd been hoping it might thicken into yoghurt or something similar, but that isn't what I got. I strained out the curds and tasted them. Certainly not yoghurt... not exactly cheese either, but something along those lines. It had a strong tang, so I rinsed the curds under the tap to make them a bit milder, but they're still quite strong. Not necessarily unpleasant, but not what you expect from something that looks like cottage cheese.
I added a pinch of salt and a tiny bit of finely chopped rosemary for flavour, then variously wrapped and rolled it to squeeze out more of the strong-tasting whey. I left it sitting over the sieve for about half an hour, but it didn't look like it was going to lose any more liquid, so I transferred it to a little plastic pot for the fridge.
It holds together reasonably well, but it's still very soft and easily spreadable. It's a ricotta-type cheese, though stronger in flavour. It's not the yoghurt I was hoping for (Ian doesn't like cheese, much) and I'm not sure I'd choose this flavour for cheese, but it was certainly an interesting experiment and the result is far from inedible. I also have a bottle of whey in the fridge, now. I'm not sure what I'm going to do with that.