1. Boil four 8-ounce jars on a rack inside a large pot for at least ten minutes to sterilize.
2. Pour 1 quart nonfat, low-fat, or whole milk (cow, soy, goat, or your choice) in a small saucepan. Affix a candy thermometer to the side of the pot. Heat milk to 180 degrees to kill all bacteria. Turn off heat and let milk cool to 105 degrees.
3. Stir in 1/4 cup premade plain yogurt or yogurt starter (available at natural foods stores). Pour mixture into jars and cap.
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4. Keep the jars undisturbed at 100–110 degrees for four to six hours. There are different ways to accomplish this:
Pour 115-degree water into a Styrofoam cooler and place jars inside; cover.
Place yogurt jars in a large covered pot and put into a gas oven. The pilot light will keep them warm.
Put the jars in a clean sink with hot water; continue to add hot water as necessary.
5. Tilt each jar gently to determine whether yogurt has thickened to a creamy consistency. Refrigerate for up to one week. Save 1/4 cup of your new yogurt to use as a starter for the next batch.