Here is how I make my homemade yogurt almost every week. It has turned out perfect every time. I am including pictures of my favorite yogurt container, because it can go from microwave to refrigerator, and there is very little clean up involved.
Pour 8 cups of milk of choice into container (I use 1%.) Heat in microwave for 8 minutes. Stir with a candy thermometer and check temperature (important to stir, because microwave heats unevenly and there are pockets of warmer and colder–we all know this,right?) You will generally have to heat for another 2-6 minutes, depending on the power of your microwave. The important point is that you want to heat to 190-200 degrees (not quite boiling.) Then, you have to let it cool down to 110-120 degrees. This is also important, because if you add the yogurt ‘culture’* when it is hotter, you will kill the bacteria. For 8 cups of milk, you need 1/2 cup of yogurt. When the milk has cooled to 110-120 degrees, stir 1/2 cup of this milk into the yogurt until smooth. Then add this thinned yogurt to the heated milk, stirring all the time. Wrap the yogurt container in a big dishtowel, and ‘incubate’ for 4 hours. If my microwave is going to be free for 4 hours, I like to put the dishtowel-wrapped yogurt container in the microwave and close the door, because I think the temperature might be more stable in there, especially if I am running my air conditioner. After 4 hours, remove towel, stir yogurt, put lid on, and refrigerate. If you prefer a tarter yogurt, you can incubate for a longer period of time. It doesn’t seem to change the texture. If you want thicker yogurt, you can strain it through coffee filters.
To start, use any plain yogurt–Dannon is usually recommended. But I have used the cheapest store brand with no problem. I like to look and see if the only ingredients are milk and bacteria. I get the most consistent, best results with Safeway’s store brand (Lucerne.) After the first time, I just save 1/2 cup from the current yogurt batch to add to the next batch.
THAT IS ALL THERE IS TO IT! Perfect homemade yogurt every time.
**If I want to make ‘greek’ yogurt, I strain it, using a coffee filter that fits inside a mesh strainer. I usually strain it for 20-30 minutes. Makes a thick and creamy yogurt. I save the whey liquid for smoothies.