Cultured Buttermilk: You can make your own buttermilk at home, from reconstituted powdered milk. You will need commercial buttermilk as a starter. Mix up a quart of reconstituted milk by combining 3 3/4-cup slightly warm water with 1 1/3 cups dry milk powder. Add 1/2 cup of fresh buttermilk, purchased from the store. Cover and allow to stand at room temperture until clabbered (6-12 hrs or just leave it overnight). Refrigerate after clabbering, makes a little more than a quart. Buttermilk will keep in the refrigerator for 2-3 weeks. It may be frozen. It will be necessary to use a fresh start of buttermilk occasionally.