Making Yogurt by the Numbers

     fine cooking magazine offers these 6 simple steps for making your own yogurt:

Ingredients

1/2 gallon milk,preferable not ultra-pasteurized
3 Tbsp homemade or store bought plain yogurt with active cultures, at room temperature

Directions

1.  Affix a probe or instant read thermometer to the side of a heavy-duty 6 to 7 quart pot.  Pour in the milk, and heat over medium-high heat, without stirring, to 180F.  Adjust the heat to maintain that temperature for 5 minutes. 

2. Turn off the heat, and use a spoon to lift off and discard any skin that has formed on its surface.  This is a normal occurrence when you heat milk, but you don't want that skin in your yogurt.

3.  Allow the milk to cool to 115F before adding the yogurt starter so that the active cultures don't die, frequently stirring gently and checking the temperature, 25 to 30 minutes.  Occasional stirring can help speed this process and will remind you to keep an eye on the temperature.  If the milk drops below 115F, rewarm it.

4.  Put the yogurt in a medium bowl and whisk in 1 cup of the milk to temper it. Gently stir the mixture back into the pot, remove the thermometer, and cover it with a lid.  Transfer to the oven and drape a kitchen towel over the pot.  Turn on the oven light and let sit, undisturbed, until thick with a slight wobble when you jiggle the pot, 8 to 16 hours.  A layer of cloudy whey on top is perfectly normal.  

5.  After 8 hours, taste a spoonful of the yogurt for consistency and tanginess, being very careful not to agitate the rest.  Then quickly cover it and close the oven door if it's not done.  Bear in mind that the yogurt will get a little thicker and more tart when it cools.

6.  Set aside 1/4 cup of the yogurt to culture future batches, if you like; cover and refrigerate for up to 1 week, or freeze for up to 3 months (thaw in the refrigerator before using).  Transfer the rest of the yogurt to a large container or divide among small containers, cover, and refrigerate until cool and set.  Whisk to break up any curds before serving.  If small lumps remain, add an ice cube and gently whisk for 20 to 30 seconds more, then discard the ice cube.  The yogurt can be refrigerated for 7 to 10 days.