Authentic Homemade Yogurt

Method

1. Whisk the nonfat dry milk powder into the whole milk in a heavy pot, then heat over medium-low, stirring often and scraping the bottom, until it reaches 185 degrees F (85 degrees C). Hold it there for 10 minutes, adjusting the heat as needed; this denatures the whey proteins and is the whole reason homemade yogurt sets thick.
2. Take the pot off the heat and let the milk cool to 110 to 115 degrees F (46 degrees C), about 30 minutes, or faster in a sink of cold water. Any hotter and the culture dies on contact.
3. Whisk 1 cup of the warm milk into the plain yogurt until smooth, then stir that mixture back into the pot with a few gentle strokes rather than vigorous beating.
4. Pour into clean jars, cover, and hold at 105 to 115 degrees F (45 degrees C) for 6 to 10 hours, undisturbed: an oven with only the light on, a cooler filled with warm water, or a yogurt maker all work. Moving or stirring the jars during this time breaks the set.
5. Check at 6 hours by tilting a jar. It is done when the yogurt pulls away from the side in one mass and the surface looks smooth. Longer incubation makes it tangier and firmer.
6. Refrigerate at least 4 hours before eating; it firms up considerably as it chills. For Greek-style yogurt, strain it through cheesecloth for 2 hours.
7. Save 1/2 cup of each batch as the starter for the next, and keep the finished yogurt refrigerated and used within 2 weeks.