1/2 gallon whole milk
1 cup plain yogurt with live active cultures
1. Pour milk into a thick pot or dutch oven & heat to just before boiling, approximately 200°F, over med-high heat, stirring to prevent scorching.
2. Remove from burner & allow to cool to 115°. Stir occasionally to prevent skin from forming. Just stir skin back into milk if it does form. You can place pot in fridge or in ice water to speed process. It only takes about 20-30 minutes to cool by itself though.
3. When the temp reaches 115°, remove about 1 cup of water & whisk it into the 1 cup of yogurt. Then pour the mixture back into the pot of milk.
4. Place a lid on the pot, place the pot into the oven, wrap the pot with a towel, & leave it for 4-6 hours. Don't touch it!! You want the temp to stay around 115°. Just leaving the oven light on should keep it at the correct temp. If it's cold out though, preheat your oven to the lowest setting (mine's 150°) and turn off, leaving the light on, before putting the wrapped pot into the oven. You can turn the oven back on for a minute to keep the temp up if necessary. Usually though, just leaving the light on is enough.
5. After 4-6 hours, Tilt the pot slightly. If it's watery, leave it for 4 more hours. You can leave it overnight if necessary. It won't spoil.
6. Set 1 cup to the side to use as your next yogurt starter & flavor the rest as desired or leave it plain. You can add vanilla extract, vanilla beans, lemon extract, lemon juice/zest, chopped or blended fruit, granola... the possibilities are endless.
To make thicker or Greek yogurt, after the initial 4-6 hours of thickening, simply pour into a strainer or colander lined with cheesecloth, coffee filters, or paper towels, cover with plastic wrap & allow to sit in the fridge (over a bowl) for 1 hour to overnight. 1 hour will drain about 20% of the liquid, 3-4 hours will drain about 50%, & overnight will get you a nice thick Greek yogurt.