Bring milk to almost scalding, then LET COOL to body temperature [Milk is scalded by heating it to 180-185°F (82°C).].
Mix well. Transfer to a sterile container, like glass or a thermos.
Cover and keep warm overnight. The inside of a foam cooler is ideal.
The starter has inoculated the milk, and this will incubate the culture, thickening into real buttermilk.
Recipe may be doubled for quarts or gallons with a 1 to 4 ratio of buttermilk to milk.
If the buttermilk is very old, however, the lactic acid bacteria may have lost their ability to produce acid rapidly. About once a week, a fresh batch of buttermilk may be made from an older batch. Take great care to prevent contamination during the reinoculation process. Cultures may be carried successfully through many inoculations if care is taken. However, it is best to start with new buttermilk from the store after five or six inoculations.