1 packet of direct-set buttermilk starter culture (like this)
OR 1/8th teaspoon mesophilic starter culture (like this)
OR 1 cup cultured buttermilk from the store*
*If using 1 cup of cultured buttermilk as your starter, reduce the amount of whole milk to 3 cups.
Instructions
Gently stir the starter culture into the milk (I use a mason jar) and cover it with a towel and rubber band. Avoid capping it tightly with a lid, as the culture needs room to breathe.
Allow the milk to culture at room temperature for 12-24 hours. When it is complete, the buttermilk will be thick and have a delicious tangy smell.
Store your finished buttermilk in the fridge (it usually lasts at least several weeks for me)