Protecting each piece of equipment from contaminating your yogurt is a crucial first step, because you don't want any unwanted microorganisms in there to contaminate the process. Remember, it's all about the right bacteria.
Step 1: Sterilize
Thoroughly wash all of your equipment -- the boiler, the spoon, measuring cups, the thermometer and the container(s) that will hold your finished product. Run everything through the dishwasher, do it by hand with soap and hot water or kill a couple of birds with one stone and just boil everything in the boiler.
Also now, preheat your oven to 200 degrees F (93 degrees C). Once it reaches 200 degrees F, turn it off. While you're preparing the yogurt, make sure the oven stays above 100 degrees F (37.8 degrees C).
Step 2: Set Up
Fill the larger pot of the boiler with water so that it reaches about halfway up the outside of the smaller pot when you insert it.
Also now, fill your sink with several inches of ice water.
Step 3: Heat
Pour the quart of milk into the smaller pot and place the double boiler on the stove with the thermometer inserted into the milk. Heat the milk, stirring lightly, to 200 degrees F (93.3 degrees C). Keep it there for about 10 minutes, careful to maintain the 200 degrees F temperature.
Step 4: Cool
Remove the boiler from the heat and place the smaller pot (with the milk) into the cold water bath in your sink. (The water should not be high enough to get in the pot.) Keep watching the thermometer. When the milk's temperature is between 112 degrees F and 115 degrees F (44.4 and 46.1 degrees C), remove the pot from the water.
Step 5: Add
Remove 1 cup of milk from the pot and blend it with 1/4 cup of the plain yogurt or with the appropriate amount (see package) of packaged yogurt starter culture. Pour the combined liquid back into the milk pot. The milk should be at or around 111 degrees F (43.9 degrees C).
Step 6: Incubate
Quickly pour the warm milk into the container(s) you've pasteurized to hold the yogurt. Place the containers in the oven, which needs to stay between 108 degrees F and 112 degrees F (42.2 and 44.4 degrees C), so keep reading your oven thermometer, and turn the oven off and on to stay in that temperature range. You'll incubate the yogurt for 4 hours.
Step 6: Refrigerate
Remove the container(s) from the oven and immediately put them in the refrigerator. It'll keep for between one and two weeks.
That's it -- your most basic yogurt recipe. On the next page, we'll look at what else you can do to customize your homemade yogurt.