At the beginning of this month BBC 2's "Trust Me I'm a Doctor" did an experiment to discover whether or not various probiotics were having positive effects on people's micro biomes, and as expected the homemade culture out performed anything you could buy in a shop.
The specific culture they used was Milk Kefir, and ever since I've been inundated with questions about it.
It's actually really easy and typically one of the first fermented foods I recommend people try. You can culture it at room temperature, with no special equipment, and once you get a stable colony going it's fairy resistant to neglect.
You can use any mammalian milk with no problems, you can also use some plant milks, although that gets a bit trickier, and if you are using raw milk you will need to give your grains a little pasteurised milk every now and then to recover (the enzymes and beneficial bacteria in the raw milk start to over power the Kefir - they aren't dangerous, but they also aren't Kefir). The higher the fat content of the milk, the more the Kefir seems to thrive. If you don't have access to raw milk I recommend using full fat, and/or adding a little cream.
Heres my basic guide to culturing your own Kefir:
What are Kefir grains?
They look like little florets of cauliflower, but they are actually colonies of yeast and bacteria living together in a little bundle.
You will need a clean jar, a sieve, some milk and about a tbsp of Kefir grains.
Place the grains in the jar with about 7:1 ratio of milk:kefir. If you have a tbsp of grains this will be approximately 100ml of milk.
You then need to cover the jar, but still allow the Kefir to breathe. I recommend a piece of paper towel with an elastic band to secure it.
Place it somewhere dark, inside a cupboard for example, and leave it to grow. After around 24 hours you will hopefully see a thickened Kefir with liquid whey beginning to separate. You can now taste the Kefir and decide if you like it or want to leave it a little longer. When the ferment has reached your desired flavour/consistency you simply push it through a sieve to rescue your grains and you're ready to start your next batch.
If you need a break between batches, simply place your grains in a clean jar of fresh milk and store in the fridge for 1-2 weeks.
Milk Kefir uses
Add it to a smoothie, eat it like yoghurt with some honey and granola, let it really thicken and strain it to make a cream cheese, give it a second ferment with some fruit purée to make a lassi style drink, use it as a base for salad dressings, make ice cream.... a quick google will find you all kinds of fun recipes.