Culturing your own Kefir
Kefir, also called the "champaign" of dairy drinks, is cultured using both a yeast and a lactic bacteria group like that used for making yogurt. Because of the symbiotic yeasts, Kefir can be a little fizzy but it mostly just tastes a lot like yogurt to me. I like Kefir because it cultures at room temperatures and it has a wide range of tolerance for culturing times (mine cultures in about 8-12 hours, but is still sweet and drinkable after as long as 24-36 hours.) Kefir can be used as-is for super quick morning "smoothies" and it also makes a good substitute for buttermilk in salad dressings and recipes. Like yogurt, you can also drain the whey from the Kefir solids and make a sort of spreadable "cream" cheese from it. I make a half gallon of Kefir pretty much every week, which gives me five 8 ounce servings for quick school day breakfasts on the go, one larger amount to use as buttermilk substitute for salad dressings or recipes during the week, and about half a cup or so to set back to use for culturing the next batch.
Yogourmet Kefir Culture is the culture I'm using right now. I got a good deal on several packages a few months ago and plan to continue using it until it's used up - but I plan to buy Kefir "grains," probably from these folks, after that for a more sustainable Kefir solution. I've heard the biotics of grain Kefir are better for you and the Kefir itself is a lot tastier, but I don't know that for sure as I've not tried them yet.
When I make Kefir, I follow this very basic procedure: Pour one quart of refrigerator temperature milk into a very clean two-quart pitcher, and heat the other quart up in a microwave safe cup for about 2 minutes on high. This should make the milk very warm, but not warm enough to form a "skin" or to boil over. I then combine the warm and cold milk in the pitcher and stir it well. The reason I do this is to bring the temperature of the cold milk up enough to culture the Kefir without having to let it sit all day on the counter, growing things other than Kefir as it very slowly warms up...
Once the milk is about 70 - 75 degrees, I either open a packet of culture and stir it in, or dump in the half cup or so of last week's culture that I saved from my last batch and stir that in. I've found I can re-culture my Kefir at least 5 or 6 times before it starts to change texture and become more sour than I like it. (This is why it will probably take me a year to go through all the packets I bought a few months ago!) After innoculation, I cover the milk and culture mix with a lid or a clean kitchen towel, and set it back on the counter out of the way.
After about 8 hours or so I begin to check the Kefir to see if it has thickened up. Once it has thickened up to my satisfaction, I give it a good stir to break it up a bit, and pour it into my single serving size bottles, reserving another half cup for next week's culture. I put my new batch of Kefir into the fridge, clean up the pitcher, and I'm all set for a week's worth of smoothies. Actual hands on time once you have made a few batches and know what you're doing will probably be less than 10 minutes a week.