I use my yogurt maker: http://www.amazon.com/Yogourmet-Multi-Electric-Yogurt-Starter/dp/B0016HM77A/ref=sr_1_1?ie=UTF8&qid=1328754307&sr=8-1
That said, for several years I made yogurt and creme fraiche in a Mason jar wrapped in a heating pad set on Low, secured with rubber bands. Works like a charm. The ratio of 16 ounces heavy cream to 4 ounces of buttermilk is perfect. With a yogurt maker or a heating pad, 12 hours is usually sufficient.
I forgot about yogurt/sour cream Sam1148, but you will have to make sure they have active cultures, the good thing about buttermilk is that it almost always has active cultures in it.
Another hint is after you warm the creme and mix the 'starter'. Put it in the microwave (no power). It a good place in the kitchen to have a draft free space. A cup of water boiled before hand on the side and left in the microwave, also helps to hold thermal mass overnight. (or better yet a old fashion water bottle with hot tap water on the side).
Sanitize a vessel. Add 2 cups heavy creme. Add 1/4 cup buttermilk. Cover lightly. Set in a warm place (70-75 degrees Fahrenheit) for 24 hours or until thickened significantly.
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That said, for several years I made yogurt and creme fraiche in a Mason jar wrapped in a heating pad set on Low, secured with rubber bands. Works like a charm. The ratio of 16 ounces heavy cream to 4 ounces of buttermilk is perfect. With a yogurt maker or a heating pad, 12 hours is usually sufficient.