When I was a kid—maybe in middle school?—we got a yogurt maker. It was a contraption that plugged into the wall, and it came with six or eight plexiglass containers that were about the size of single-serve yogurts (6 oz or so). I think the drill was that you stuck some yogurt in each container, filled them with milk, covered them, and plugged the thing in. Several hours later, you had yogurt. (I don't remember how we came to own this contraption, but it may have been something we acquired through the Green Stamp program at Schmidt's.)
The yogurt we made with this thing was kind of runny, but tasty. Every once in a while, over the last few years, I've run across blog posts explaining how to make your own yogurt. While there are other steps, the key one is incubating milk with a bit of yogurt starter for several hours at about 110°F. One trick some people advocate is incubating in a gas oven that has a pilot light, because apparently these ovens often are right around the right temperature when they are "off," due to the heat from the pilot light. But our oven doesn't use a pilot light—this saves us energy, but kept me from trying to make homemade yogurt.
Then, a couple of weeks ago, I read a blog post by an old high school classmate, who has been making his own yogurt (using the oven-with-pilot-light method). And right around the same time, I happened to read another blog post, somewhere else (I've lost the URL, alas), with a much simpler (for me) incubation method.
So I decided to give it a try, and it's worked quite well. Batch #3 is incubating right now. Here's my method:
- Let about 2 tablespoons of plain yogurt (from your last batch, or from the store—I used Brown Cow yogurt the first time) come to room temperature.
- Fill two quart mason jars with boiling water, and place in a small cooler bag, and zip it closed. This sterilizes the jars (enough) and preheats the cooler bag.
- Measure 1/3 c. of dry milk into a 1 quart liquid measuring cup. Top it off to the quart mark with milk (I'm using 2%, but any fat level is fine). Whisk until well mixed and pour into a saucepan. (The dry milk is optional, but will yield a firmer yogurt.)
- Heat over a very low flame, using a diffusion plate, stirring regularly. Alternatively, you could use a double-boiler. The goal is to raise the temperature slowly, and not to scald the milk. Let the milk come up to 185°F. If you don't have a good thermometer, you can heat the milk until it begins to foam up. (I can't remember whether we did this step when we made yogurt with our yogurt maker years ago, but heating the milk to high temperature denatures the whey proteins, which yields a firmer yogurt. Raising the temperature slowly and using a diffusion plate (or a double boiler) makes scalding the milk less likely, but also allows the milk more time at high heat to denature the whey.)
- Take the milk off of the heat, and leave the milk covered for 15 minutes (I don't know if this really is necessary, but keeping the milk at above 165°F for 15-30 minutes total—remember, it was at this temperature for some time during the heating process, too—should denature at least half of the whey, and perhaps even more than that). Then, uncover the saucepan and let the milk cool down to around 110°F (anything above 130°F or so risks killing the yogurt cultures you'll be adding, and anything too cool will keep the cultures from multiplying at a decent rate). If you're impatient, you can put the saucepan into an ice water bath and stir. Meanwhile, bring another quart of water to boil.
- Add a couple tablespoons of the lukewarm milk to the yogurt, stir, and then add the yogurt-milk mixture to the milk. (Adding milk to the yogurt first tempers the yogurt, avoiding thermal shock. I doubt this is necessary, given that the yogurt is at room temperature and the milk is luke warm, but it can't hurt.) Mix thoroughly with a whisk. Empty one of the two quart jars and fill with the yogurt-milk mixture. Empty the other quart jar and fill with the fresh boiling water (this merely ensures that this jar is freshly hot—this second jar is there to keep the yogurt jar warm enough during the incubation step). Wrap both jars with tea towels, and return to the cooler and zip it up.
- Let sit for 4-8 hours. (If I do this after dinner, I just leave it out overnight.) The live cultures from your yogurt starter (a couple types of "good" bacteria) will consume most of the lactose in the milk and multiply. That multiplication process is also what "makes" the yogurt out of the milk. After 4-8 hours, the yogurt should be fully set, and then it can be cooled in the refrigerator. Once cold, it's ready to eat. (Tip: reserve 2 tablespoons out of the jar now, so that you don't accidentally find yourself without the yogurt you need to start the next batch.)