This step does two things. First, it kills native bacteria in the milk that might compete with the introduced cultures. Second, it alters the structure of the milk protein, casein, which helps to make a thicker and more firm yogurt.
It's important to raise the temperature slowly. The faster the temperature is raised, the more congealed lumpy proteins.
Holding the milk at between 180-200, while constantly stirring, will concentrate the milk and make a thicker yogurt. Do this for a bout 20 minutes if possible.
If time doesn't permit holding the 200 degree temperature, take 1/4 cup dry milk, add about a cup of the warmed milk from the pot, whisk very vigorously until all the lumps are gone, and add that back to the pot.
When finished warming the milk, remove from heat and bring back down to around 110-115 degrees. This makes sure the temperature isn't too high to kill the active culture.
Whisk in 1/2 cup of yogurt with active cultures.
Pour into containers and culture at 115 degrees for 8-10 hours. Then refrigerate.
