Make Your Own Yogurt
8 cups milk — whole or 2% are best, but skim can also be used
1/2 cup plain commercial yogurt containing active cultures
3 quart or larger Dutch oven or heavy saucepan with a lid
Instant-read or candy thermometer (one that can clip to the side of the pan)
Small measuring cup or small bowl
Heat the milk. Pour the milk into the Dutch oven and set over medium to medium-high heat. Warm the milk to right below boiling, about 200°F. Stir the milk gently as it heats to make sure the bottom doesn't scorch and the milk doesn't boil over. According to the National Center for Home Food Preservation, this heating step is necessary to change the protein structure in the milk so it sets as a solid instead of separating.
Cool the milk. Let the milk cool until it is just warm to the touch, 112°F to 115°F. Stir occasionally to prevent a skin from forming. (Though if one does form, you can either stir it back in or pull it out for a snack!) You can help this step go faster by placing the Dutch oven in an ice water bath and gently stirring the milk.
Thin the yogurt with milk. Scoop out about a cup of warm milk with a measuring cup and add the yogurt. Whisk until smooth and the yogurt is dissolved in the milk.
Whisk the thinned yogurt into the milk. Pour the thinned yogurt into the warm milk while whisking gently. This inoculates the milk with the yogurt culture.
Transfer the pot to the (turned-off) oven. Cover the Dutch oven with the lid and place the whole pot in a turned-off oven — turn on the oven light or wrap the pot in towels to keep the milk warm as it sets (ideally around 110°F, though some variance is fine). You can also make the yogurt in a dehydrator left at 110°F or using a yogurt maker.
Wait for the yogurt to set. Let the yogurt set for at least 4 hours or as long as overnight — the exact time will depend on the cultures used, the temperature of the yogurt, and your yogurt preferences. The longer yogurt sits, the thicker and more tart it becomes. If this is your first time making yogurt, start checking it after 4 hours and stop when it reaches a flavor and consistency you like. Avoid jostling or stirring the yogurt until it has fully set.
Cool the yogurt. Once the yogurt has set to your liking, remove it from the oven. If you see any watery whey on the surface of the yogurt, you can either drain this off or whisk it back into the yogurt before transferring to containers. Whisking also gives the yogurt a more consistent creamy texture. Transfer the to storage containers, cover, and refrigerate. Homemade yogurt will keep for about 2 weeks in the refrigerator.