Kefir: commercial, home-brew, and continuously cultured
After I discovered that kefir was something good to eat/drink, I learned that it could be prepared at home for about half the cost of the commercial product. That included the price of the milk, and a packet of culture organisms. There are two culture-options in my store. One of them described heating the milk to near boiling, letting it cool to near room temperature, then adding a packet of culture (sort of like a yeast packet). That worked fine. But then, the other product just said "save a 1/4 cup of your last batch, and add a quart of milk to it to culture the next". I decided to combine the two: heating the milk, and adding a part of last week's kefir after it cools. If I recall correctly, I'm now culturing my 7th "generation" from one initial packet, with no change in the product. A double-boiler keeps the milk from scorching, and a floating dairy thermometer helps me track the temperature. I believe that heating the milk will prevent unwanted organisms from growing in it. They might grow slowly, but persist to contaminate future batches.
I use kefir in two ways, consuming a quart a week. Five days a week, I put some on my overnight oats (with fresh fruit, pumpkin seeds, and sunflower seeds). On Sunday mornings, I use up to one cup (with a cup of flour, a tablespoon of sugar, a tablespoon of oil, an egg, and 1-1/2 teaspoons of baking powder) to make pancakes (for two). Kefir and flour alone can make a batter that's too thick, so I add milk to get the right viscosity. (Of course, I reserve 1/4 cup for the next batch.) For flour, I usually blend 50/50 whole-wheat and buckwheat. If there's any left when the pancakes are done and the next batch of kefir under way, I just scrape out the jar onto a stack of pancakes.
Give it a try. The flavor is more like that of cottage cheese than yogurt; very mild. Much of the lactose has been digested, though, so there's less of it to worry about. It is supposed to be pro-biotic, but I have no whey of verifying that. ;-)