Homemade Yogurt

Whisk the optional milk powder into the cold milk to dissolve any lumps, then pour into a heavy-bottomed pot. Heat over medium, stirring often and scraping the bottom so it doesn't catch, until the milk reaches 180 to 185°F (82 to 85°C) — just below a boil, with steam rising and a fine ring of foam at the edge. Hold it there for 5 minutes. This step is not about killing bacteria; it denatures the whey proteins so they cross-link into a thick, set curd instead of a thin, weepy one.

Cool the milk to 110 to 115°F (43 to 46°C) — body temperature, warm but not hot to a clean fingertip. Stir gently now and then to keep a thick skin from forming, or set the pot in an ice-water bath to speed it up. This window matters: hotter than about 120°F kills the culture, cooler than 105°F and it works sluggishly or not at all. If a skin did form, lift it off so it doesn't make lumps.

Make sure the starter yogurt is fresh and labeled with live or active cultures. Whisk it together with a cup of the warm milk in a small bowl until completely smooth and lump-free, then stir that slurry back into the pot and whisk thoroughly so the culture is evenly dispersed. Don't overdo the starter — a little goes a long way, and too much actually crowds the bacteria and gives a sourer, thinner result.

Hold the seeded milk at a steady 105 to 115°F (40 to 46°C) for 6 to 10 hours, undisturbed — in a turned-off oven with the light on, a cooler packed with warm water jars, a wrapped warm pot, or a yogurt maker. Do not move, stir, or jostle it while it sets; agitation breaks the forming curd. Start checking at 6 hours: tip the jar gently and watch for a clean set. Longer means thicker and tangier. When it's set to your taste, refrigerate at least 4 hours to firm it fully and stop the souring. For Greek-style, line a sieve with cloth and drain the chilled yogurt 1 to 2 hours.