Learning how to make yogurt is a trial and error process until you find what works best for you. The first time that I made this, I preheated the oven as instructed then turned it off and placed the mixture inside overnight. I did not keep the oven light on, and I was asleep during the thickening process so I did not do additional 1 minute preheats and woke up to find a liquid mess.
This time around, on a Sunday when I knew I was not going to use the oven that day, I kept the light on and did additional 1 minute preheats every few hours. 8 hours later, I was happy to report that I made yogurt: thick, yummy yogurt.
For my next batch I will see what happens if I keep the oven temperature on warm (approximately 100 degrees F) with the mixture inside the oven (without the towel) for 3-4 hours continuously. I will update this post following my experiment.
4 cups milk
2 heaping tsp plain yogurt *
Add milk to a heavy bottom pot and heat over medium-high heat, stirring occasionally until the temperature reaches 180 degrees F. (This denatures the milk proteins so that they do not interfere with the incubation process.)
Note: If you are using good store-bought milk, you do not have to heat it that high, you can just warm to 115 degrees F and then add the starter yogurt. However, if your milk has gone sour (which is the perfect time to make yogurt), you will need to heat it to 180 before starting.
Remove from heat. Let cool until the temperature drops to 115 degrees F. Stir in 2 heaping teaspoons of plain store bought yogurt. (Yogurt is a starter for making more yogurt)
Preheat the oven to any temperature and turn off after 1 minute. Turn the oven light on. Cover the pot with a lid and wrap in a towel. Close the oven, keeping the light on, and let the mixture incubate. (If you feel that the oven might be getting too cold, you can do additional 1 minute preheats every couple of hours.
Now just wait and let the yogurt incubate for 8 hours. The mixture will become thick and then you will have your very own homemade yogurt.
Note: If your yogurt comes out too runny, do not throw it away. Just put it back in the oven on warm heat for a few more hours.
Pour the yogurt into a sieve to strain some, not all, of the liquid. Transfer the yogurt to a container with a lid and whisk to smooth it out. You can add 1/2 tbsp of vanilla extract at this point or you can keep it plain and add fruit and honey right before eating. Store in refrigerator for up to one week.
*Make sure that the store bought yogurt you use as the starter has LIVE cultures. It will say so in the ingredients list.
Source: instructions from Annies-Eats and notes from Adel Elsohly