Step 1

Heat the milk to 185ºF (85ºC). Using two large pots that fit inside one another, create a double boiler. This will prevent your milk from burning, and you should only have to stir it occasionally. If you cannot do this, and must heat the milk directly, be sure to monitor it constantly, stirring all of the while. If you do not have a thermometer, 185ºF (85ºC) is the temperature at which milk starts to froth. It is highly recommended that you obtain a thermometer in the range of 100 - 212ºF, especially if you plan to make yogurt on an ongoing basis.You can use any kind of milk, including whole milk, 2 percent, 1 percent, nonfat, pasteurized, homogenized, organic, raw, diluted evaporated, dry powdered, cow, goat, soybean, and more. UHP, or ultra-high pasteurized milk, is processed to a higher temperature, which breaks down some of the proteins that the bacteria need to make the milk into yogurt. Some have reported difficulties in making yogurt from UHP.

Step 2

Cool the milk to 110ºF (43ºC). The best way to do this is with a cold water bath. This will quickly and evenly lower the temperature, and requires only occasional stirring. If cooling at room temperature, or in the refrigerator, you must stir it more frequently. Don't proceed until the milk is below 120ºF (49ºC), and don't allow it to go below 90ºF (32ºC); 110ºF (43ºC) is optimal.

Step 3

Warm the starter. The starter is the bacteria that you add to the milk, which in turn cultivates more bacteria--bacteria that is necessary for the creation of yogurt. Let the starter yogurt sit at room temperature while you're waiting for the milk to cool. This will prevent it from being too cold when you add it in.All yogurt needs "good" bacteria. The easiest way to add this is to use existing yogurt. The first time you make your own yogurt, use store-bought plain (unflavored) yogurt. Be certain it has "active cultures" on the label. Yogurt cultures die over time so try and find the freshest yogurt that has a tart flavor, but no added flavors or other additives. Taste various plain yogurts before you get started. You will find that different kinds taste a bit different. Use one you like for your own starter. The different flavors are due to other bacteria than the two main bacteria needed to create yogurt.Alternatively, instead of using existing yogurt, use freeze-dried bacteria cultures (available in specialty stores or online), which are more reliable as a starter.In a pinch, you can use a flavored yogurt, but the taste of the resulting cultured yogurt will not be exactly the same as using plain yogurt.

Step 4

Add nonfat, reduced fat, or whole dehydrated milk, if desired. Adding about 1/4 cup to 1/2 cup dry milk at this time will increase the nutritional content of the yogurt. The yogurt will also thicken more easily. This is especially helpful if you're using nonfat milk.

Step 5

Add the starter to the milk. Add 2 tablespoons of the existing yogurt, or add the freeze-dried bacteria. Stir with a whisk or use a blender (such as a stick blender) to evenly distribute the bacteria throughout the milk. If you still have stringy ropes you may be heating your milk too fast or too long (scalding). Use a double boiler in that case, or at least stir regularly and check temperature with a thermometer. At high altitudes this can be more of a problem.