1gallonwhole milk, 2% or skim milk (whole milk is recommended for thickest, creamiest results)(see note about using raw milk) *this method does NOT work with non-dairy alternatives
1cupplain yogurt at room temperature, either from previous batch or store-bought (**The yogurt must have live cultures in it to work. Check the ingredients to make sure they include l. bulgaricus and s. thermophilus.)
IF USING A STOCK POT: If you're in a hurry or don't have a slow cooker you can use a stock pot. Simply heat the milk in a large stock pot over low to medium heat. Just be sure to stir it regularly, especially as it gets hotter, to prevent the milk from scorching. Then follow the rest of the instructions as written.IF USING A SLOW COOKER: Pour the milk into a slow cooker set to HIGH and close the lid. It will take 2-3 hours before the temperature of the milk is up to 180 degrees F. Check it with an instant-read thermometer. Once it's at 180 degrees, turn off the slow cooker, close the lid, and let the milk cool to 110 degrees F. This will take another 2-3 hours. Once the temperature is between 110 and 115 degrees F, add some of the warm milk to the plain yogurt, stir to combine, and then gently but thoroughly stir the mixture back into the milk in the slow cooker using up and down, left and right motions (not circular).
Close the lid, wrap the slow cooker or stock pot with a large bath towel, and let it sit undisturbed in a non-drafty place at room temperature (or warmer) for 10-12 hours or overnight. If the environment is too cool the yogurt will not set. During this process the milk will have thickened and the whey will have started separating from the milk.
Line a colander with a cheesecloth and pour the yogurt into it. Set the colander over a large glass bowl and let it strain at room temperature for several hours, until you've achieved the desired thickness (length of straining time will also depend on the gauge of the cheesecloth. I use this cheesecloth - excellent quality and you get a lot of it for a great price) For thick Greek yogurt plan on at least 4 hours. You'll end up with about 8 cups of whey and 7-8 cups of Greek yogurt.
Store the yogurt in the fridge in an airtight container, preferably a glass bowl for up to 2 weeks. When ready to make another batch of yogurt, use a cup of the previous batch as the starter culture.
**UPDATE: Yes, you CAN use raw milk. It's more temperamental though. If after the wait time the yogurt still isn't firm, turn the slow cooker back on to "warm" and let it sit another 4 hours or until thickened. Raw milk yogurt often tends to be clumpier. **You don't have to use the slow cooker method. For a quicker method simply heat the milk in a large stock pot over low to medium heat. Just be sure to stir it regularly, especially as it gets hotter, to prevent the milk from scorching. Then follow the rest of the instructions as written.