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1
Place four 10-ounce jars in a slow cooker. Fill slow cooker with water up to 1/3 inch of the jar rims. Remove jars and set aside.
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2
Cover slow cooker and preheat on High while you prepare the yogurt for about 20 minutes.
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3
Pour milk and milk powder into a saucepan and whisk to dissolve. Heat over medium heat until just steaming and a thermometer reads 179 degrees F (82 degrees C), about 10 minutes. Remove from heat and place the bottom of the pan into a bowl of ice water. Cool milk mixture until the temperature has dropped to 114 degrees F (46 degrees C). Lift pan out of the ice water.
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4
Measure about 1 cup of milk and pour into a bowl. Stir in yogurt until thoroughly blended. Return yogurt mixture to the pan of milk. Pour milk mixture into the 4 glass jars, filling up to 1/2 inch of the rims.
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5
Check the temperature of the water in the slow cooker is no higher than 114 degrees F (46 degrees C). If it is cooler, cover slow cooker again, and allow to come to temperature. If it is too hot, uncover and turn off slow cooker.
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6
Set jars in the warm water; the water level should be up to the level of the yogurt in the jars. Cover with a folded dish towel and then with the slow cooker lid. Turn slow cooker off. Allow to set for 6 hours, keeping the water at 114 degrees F (46 degrees C) as best you can. Check the temperature with your thermometer after about 2 hours, taking care not to disturb the yogurt. If the water is lower than 114 degrees F (46 degrees C), turn slow cooker to Warm for 10 minutes, then shut off again. Check temperature again at the 4-hour mark and repeat. Do not stir or poke the yogurt at all during this time.
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7
Check the yogurt after 6 hours by pressing gently on the top or tipping the jars to see if it is set; it is done when the yogurt is firm and there is a thin layer of yellowish liquid on the top. Remove the jars from the water and dry off. Top with clean lids and place in the refrigerator.
