Reserve ¾-1 quart of yogurt from your prior batch.
Pour the gallon of milk into your soup or stock pot.
Scald the milk on low-medium heat until it reaches 165℉, then turn off heat and allow to cool to between 110℉-115℉.
Evenly distribute the yogurt from your prior batch into 4-5 quart-sized mason jars.
Pour cooled, scalded milk into each of mason jars and stir the contents of each jar thoroughly.
Place in dehydrator at 110℉ for 24 hours to allow bacteria to colonize. After 24 hours, you can remove and place in the refrigerator.
Notes
If you don't have a dehydrator, you can use your oven light. Simply place your jars in a cool oven, then keep the door propped open so the light remains on but the temperature inside doesn't get too hot. If you have the time, straining the yogurt through a cheesecloth produces delicious, creamy, and thick Greek-style yogurt. I skip this step when I'm too busy, and the kids don't seem to notice much!