This what you'll need:
- a medium-large cooler
- 2-3 glass or plastic quart-sized containers (like pitchers, empty soda bottles, etc.)
- a kitchen thermometer
- about a dozen glass or ceramic cups for the yogurt
- 1/2 gallon - 1 gallon milk (lowfat to whole; I personally haven't had much luck with nonfat)
- small container of unsweetened, unflavored yogurt with live cultures (I've had the best luck with Stonyfield Farms) - for a more consistent result, you might want to try a commercially-available starter
- (optional) powdered milk, if you want your yogurt thicker - with a good starter, this usually isn't necessary. I do not use it.
1. Heat the milk
The milk needs to be heated to about 170-180F (76-82C). This is for two reasons:
- It kills any other bacteria that might be in the milk that would compete against the bacteria that convert milk to yogurt.
- It changes the milk protein in a way that allows it to culture and firm up.
Keep stirring the milk and do not let it go past 180F. If it scorches, your yogurt will taste bad.
2. Sterilize the containers
I use boiling water from an electric kettle to sterilize my (cleaned) containers. Add the boiling water, let it sit for 5-10 minutes, and then pour out the hot water.
3. Cool the milk and inoculate
Once the milk reaches 170-180F (76-82C), turn off the burner and continue to stir it as it cools. You only need to stir it for another 2-3 minutes, to prevent any of the milk from scorching at the bottom of your pot.
Once it reaches 105-110F (40-43C), mix up your plain yogurt in its container until it's liquid, and add it to the pot of warm milk. Stir it for a couple of minutes for the yogurt to dissolve well into the milk. This will spread the bacteria (Lactobacillus, Bifidus, and other bacteria, depending on your source) throughout the milk and allow it to start to grow.
4. Pour into containers
Pour the inoculated milk into the containers.
5. Load into cooler and put in heat sources
Put all the containers into the cooler, along with the pitchers full of hot water from the tap. The heat from those containers will keep the containers warm. The heat should be maintained and stable throughout the process, so try to avoid opening the container at all until the end of the process. The fermentation takes anywhere from 4-8 hours (about 6 is ideal).
If the cooler/kitchen was a bit cool before you put everything in, you might want to quickly take those containers out at about the 1-2 hour point and refresh with hot water. But keep the cooler closed as much as possible, to avoid letting the temperature of the yogurt containers from dropping.
Also, avoid jostling the cooler, even if you have to open it up to refresh the hot water. It needs stillness to firm up.
6. Check yogurt to see if done
After about 6-8 hours (or a bit longer, if the temperature in your cooler is below 100F/39C), the yogurt should be firm. Test by gently turning it to see if it keeps its shape.
There will be some slightly yellowish, viscous liquid on the top. This is whey. You can either pour it off, or just mix it into the yogurt when you eat it.
7. Cover and refrigerate!
Cap or cover your yogurt and put them into your refrigerator. They'll last about 2 weeks.
If you want to use one of these yogurts as a starter for your next batch, use the yogurt within 5-7 days, or else freeze some of the fresh yogurt, and let it thaw before using it to inoculate the sterilized milk.
If you like thick, Greek yogurt, which is strained, you can use a coffee filter to strain the resulting yogurt to drain away the excess whey and leave you with a much thicker product. The longer you strain it, the more it becomes almost like sour cream in consistency.
If you don't want to use a cooler, there are a few incubators you can buy to maintain the right temperature during the fermentation process. A couple of the best are available from Amazon.