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1
In a medium saucepan or saucier, heat milk gently over medium heat until it reaches 180u00b0F (82u00b0C) on an instant-read thermometer. Keep the milk between 180 and 190u00b0F (82 to 88u00b0C) for at least 10 minutes and up to 30 minutes (how long you hold the milk at this temperature will change how much water steams off and how concentrated the milk proteins and fats end up, changing the final texture of the yogurt; there's no one right way to do it).
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2
Allow the milk to cool to about 110u00b0F (43u00b0C). In a small bowl, stir together the yogurt with a few spoonfuls of the warm milk, then scrape the yogurt mixture into the pot of lukewarm milk. Stir well to distribute the yogurt culture.
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3
Transfer the cultured milk to glass jars, the vessels of a yogurt maker, an Indian clay yogurt pot, or whatever incubating device you may have, such as a slow cooker or Instant Pot.
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4
Cover the jars and keep warm; how you do this depends on what you have at home. You can submerge the jars up to their necks in the temperature-controlled water of an immersion circulator (set to 110u00b0F or the incubation temperature of your choosing); place them in a turned-off oven with the light switched on; wrap them in kitchen towels and hold them in a warm place; submerge them in 110u00b0F water held in a cooler; use a yogurt maker, etc.
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5
How long it takes the yogurt to set will depend on the temperature at which it is held. This can be as short as 3 or 4 hours and as long as 18 hours. Once the yogurt has set, allow it to sit out at room temperature for up to an additional 12 hours to ensure a strong culture; if you're working with a culture you know well, you may not need to let it sit out for so long, especially if you don't want it to grow too sour. There's no one good rule here except to give the yogurt the time it needs to sour and thicken properly.
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6
Transfer the yogurt to the refrigerator and, if you can, let it set for another 2 or 3 days before eating it (all this time is simply to allow the culture to grow strong).
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7
, if desired. If you plan to use this yogurt to inoculate future batches, make sure to set a few tablespoons aside.
