2 quarts milk
2 tsp. plain yogurt
¼ cup instant dry milk powder (optional)
1 tbsp. vanilla extract (optional)
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Add 2 quarts of milk to a saucepan and heat over medium-high heat, stirring occasionally. (You can use whatever type of milk you prefer. I like 2%, but skim, 1% or whole are all fine too.) Continue heating until the temperature reaches 180˚ F. (This denatures the milk proteins so that they do not interfere with the incubation process.) Remove from the heat.
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Set aside and let cool, stirring occasionally, until the temperature has dropped to between 110-120˚ F. It is important that the temperature is within this range so that the bacterial cultures can do their thing. If the temperature is too hot, the cultures will be killed. If it is too low, they won’t incubate properly.
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Add the milk to a ceramic or glass bowl and stir in 2 teaspoons of plain yogurt. This will provide the live active cultures needed to make your yogurt work – essentially, yogurt is a starter for making more yogurt. (Some people say that you shouldn’t use your homemade yogurt as the starter for more yogurt because it may cause a sour taste. Some people say it is fine to use your own yogurt as a starter. You’ll have to experiment and see what works for you.) If you are using instant dry milk, whisk it in at this time. (I have tried it both with and without the dry milk powder, and I like it both ways but prefer it without.)
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Preheat the oven (to any temperature), shutting the oven off after 1 minute. This serves to slightly warm the oven, taking any chill out of the air. Turn the oven light on. Cover the dish and wrap the covered bowl in a couple of thick kitchen towels. (I use a Pyrex dish that has a lid, but I’m sure you could use a mixing bowl and cover it with a plate just fine.) Close the oven and let the mixture incubate in the warm oven. (It is important that the mixture stay within the aforementioned temperature range during the entire incubation period. If you feel that the oven may be getting too cold, you can do additional 1 minute preheat periods every couple of hours. I find this unnecessary and anyway, I’m asleep while this is going on in my kitchen.)
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Now you just wait and let the yogurt incubate. The incubation period can vary significantly. It can take as little as 8 hours but mine takes closer to 12. As such, I prep my milk mixture before going to bed at night and let the yogurt incubate overnight.
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When the yogurt magic has happened, you will know because the mixture has become thick, gelatinous and, well, yogurt! At this point the yogurt will have a lot of excess liquid and be fairly runny. Place a fine mesh sieve over a large bowl, and line with a thick paper towel, coffee filter, or cheesecloth. Pour the yogurt into the sieve, place the straining set up in the refrigerator, and strain until most of the liquid has been drained from the yogurt. (This liquid is known as whey. Some people save it and use it for other things. I discard it.)
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Place the yogurt in a storage container, whisk to smooth it out (I like to add a tablespoon of vanilla extract), and store in the refrigerator. This keeps for at least a week.