In reply to Murderous_Crow:
Due to the apparent impossibility of getting Scandinavian Filmjölk in the UK, we went through a phase of growing our own - and tried a few other cultures.
I'm highly sceptical about the supposed health benefits, but it is very tasty!
I bought a starter variety pack (Buttermilk, Filmjölk, Keffir, Viili and Piimä), and we tried them all, though Filmjölk remains our strong favourite. You can buy these easily online; I think I found it on ebay.
In terms of how to grow it, while there's a lot of new-age nonsense around these products it isn't hard to find growing instructions. I worked from this:
http://www.savvyteasandherbs.com/viili-fil-mjolk-caspian-piima-and-buttermi...
Make sure you keep the temperature about right, and keep different types of culture apart so they don't cross-contaminate. I'm pretty sure we had a Filmjölk culture hijacked by Piimä at one point, possibly from spores in the fridge.
You can vary the end product quite a bit by using milks with different fat content, or adding in some cream; this is worth playing with a bit.
Note that you can freeze small amounts of any of these cultures for later use if you like - it lets you try other ones in the meantime, plus it's a good idea in case your main culture goes off, but they don't last forever in the freezer so you have to renew those 'backup' stocks too.
It's easy to do and good fun, but you do need to either eat or discard fairly large quantities, and there can also be the problem of what to do with your pet culture while you're away for a few days - you don't want it to get too old before starting the next batch, but you need to be around to deal with the new batch a bit until it's ready to put in the fridge.
We eventually got a bit bored and stopped, but it was worth doing for a while and I'll do it again some time.
