1 gallon pasteurized organic whole milk
1 cup plain organic full fat yogurt (regular or Greek, depending on your preference) at room temperature, either from the previous batch or store bought. The purchased yogurt must have live l. bulgaricus and s. thermophilus bacteria.
Natural unbleached cheesecloth
Pour the milk into a slow cooker set to High and close the lid. Heat until 180° F, the milk will be bubbly on top without actually boiling. This should take two to four hours depending on your slow cooker. An instant read thermometer will work best for checking temperatures.. Turn off the slow cooker, close the lid and cool to between 110 and 115 ° F. Add some of the warm milk to the room temperature yogurt and stir. Gently stir the mixture back into the milk in the crock pot . Close the lid.
Wrap the slow cooker crock with a large bath towel and let it sit undisturbed in a non-drafty spot at room temperature for 10-12 hours or overnight (an unheated oven works well).
The milk will thicken and the whey will have started separating from the milk. For Greek yogurt line a colander with cheesecloth and pour the yogurt into it. Set the colander over a large glass bowl for several hours until yogurt has reached desired thickness. The length of the straining time will also depend on the gauge of the cheesecloth. For thick Greek yogurt plan on at least 4 hours. You will have approximately 8 cups of whey and 7-8 cups of yogurt. The whey is an excellent substitution for water or milk in your baking, soups or smoothies.
Store the yogurt in an airtight container for up to 2 weeks.
Note: Occasionally you may have a “fail” and not have the separation into whey. This is still quite usable and will make excellent yogurt drinks and smoothies and still have all the health benefits.