Step 1

This recipe will work only with ultra-pasteurized milk. Most organic milk is ultra-pasteurized; this will be noted on the label. Be sure to use freshly opened containers of milk. Any type of plain yogurt (whole-milk, low-fat, Greek, etc.) that contains live cultures can be used. You can halve this recipe, if preferred. Be sure to save ½ cup of yogurt to use as the starter for a subsequent batch.

Step 2

Bring milk to 115 degrees in saucepan over medium heat, 2 to 5 minutes. (If your milk gets hotter than 115 degrees, let it cool to 115 degrees before proceeding. Higher temperatures can kill some cultures.) Off heat, whisk yogurt into milk until very well combined. Divide mixture evenly between two 1-quart Mason jars (they will be very full; a funnel can be helpful for this step). Add lids, then screw on rings until just finger-tight.

Step 3

Adjust oven rack to lowest position and turn on oven light. Bring 5 quarts water to boil in large covered pot. Place jars in second large pot. Add water until jars are submerged to their necks. (If jars are taller than pot, add water to 1¼ inches from rim of pot.) Remove jars and bring water to 120 degrees over medium heat. Return jars to pot of 120-degree water. Transfer both pots to oven. Incubate yogurt, without opening oven door, for 5 hours.

Step 4

Remove 1 jar from water bath. Using clean spoon, taste yogurt for tartness and consistency. If tarter, thicker yogurt is desired, return jar to water bath and shut oven door. (Bear in mind that yogurt will thicken significantly as it cools but can be loosened up by stirring prior to serving.) Set oven to 350 degrees and heat for 3 minutes (start your timer as soon as you set the dial; oven will not reach 350 degrees). Turn off oven and let incubate for up to 10 more hours, repeating heating step each time oven door is opened. Remove jars from water bath and refrigerate until fully cooled and set, about 8 hours. (Yogurt can be refrigerated for up to 2 weeks.)