Making cultured milk. Milk for making yogurt should be very fresh. Any type is suitable except condensed; the more cream it contains, the sweeter the taste. To obtain a thicker curd, add up to 3 tablespoons of powdered milk per quart of liquid. In an enamel, stainless steel, or glass pan, bring 1 quart milk slowly to a boil.
Let it cool to 105°F. (If you have no thermometer, sprinkle a few drops on your wrist; it should feel neither hot nor cold. ) Add 2 tablespoons starter from a health food store or 4 tablespoons unflavored commercial yogurt. Stir thoroughly, cover, and set the milk to incubate for at least 3 hours.
To keep the incubation temperature around 105°F. use an electric yogurt-maker or set the milk in any slightly warm place such as the top of a radiator, refrigerator, or stove pilot light. Some people wrap the container in a thick towel or a blanket. Although yogurt can be cultured as Iow as 70°F, it takes longer to thicken and may acquire a sour taste. Above 115°F the culturing bacteria are killed.
The yogurt is ready to be refrigerated when it retains the impression of a spoon pressed lightly into its surface. It will keep for about a week.