1 qt. skim milk
1/3 c. powdered noninstant nonfat milk
1 to 6 Tbs. plain yogurt with active culture (the starter)
Put the powdered milk into a clean container in which you want the yogurt
to "make." In another container heat the skim milk to 120 degrees F.
(microwave works great). Add some of the heated milk to the powdered milk
and blend until smooth. Stir in the yogurt (the more you add, the quicker
the mixture sets). Stir in the remainder of the heated milk. Put the
mixture where the temperature can stay between 100-120 degrees F. for about
5 hours. The longer it "sets," the firmer and more tart it gets.
Notes:
Temperatures: Under 100 degrees F., the yogurt will not set; over 120
degrees, the culture may die. The ideal temperature range is 110-114.
I make a half-gallon at a time and most often put the jar in my gas oven,
which has a pilot light, with an occasional teakettle full of boiling water
placed in the oven to warm it. I've also successfully put the jar of mix
in a large stockpot and wrapped loosely around it a heating pad set at the
lowest temperature.
If you are making smaller batches, a warmed wide-mouthed thermal jar is the
perfect container.
For your first batch, use a plain yogurt of a brand that you like. For
subsequent batches, use a small amount of the yogurt you've made.
In many years of yogurt making, I've had failures only twice by this
method--once because I got impatient and overheated the yogurt and once
because I used a starter that was too old. Both times, I was able to use
the resulting product for baking (instead of buttermilk).