The process of making yogurt involves culturing cream or milk with live and active bacterial cultures; this is accomplished by adding bacteria directly to the milk. Commercially made yogurt is usually made with a culture of Lactobacillus acidophilus and Streptococcus thermophilis. Some manufacturers use Lactobacillus bulgaricus rather than L. acidophilus. Yogurt made at home is usually started by adding a dab of commercially made yogurt to boiled milk, and then keeping the mixture at 112° Fahrenheit (45° Celsius).
(Written by Paulla Estes
Last Modified: 06 March 2010)
* 1 quart milk (any kind)
* 1/4 to 1/2 cup non-fat dry milk (optional)
* 2 tablespoons existing yogurt with live cultures (or you can use freeze-dried bacteria instead)
Heat milk to 185F (85C). Using two pots that fit inside one another, create a double boiler or water jacket effect. 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 the while. If you do not have a thermometer, 185F (85C) is the temperature at which milk starts to froth.
Cool the milk to 110F(43C). The best way to achieve 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 more frequently. Don't proceed until the milk is below 120F(49C), and don't allow it to go below 90F (32C). 110F (43C) is optimal.
Warm the starter. Let the starter yogurt sit at room temperature while you are waiting for the milk to cool. This will prevent it from being too cold when you add it in
Add nonfat dry milk, if desired. Adding about 1/4 cup to 1/2 cup nonfat 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 are using nonfat milk.
Add the starter. Add 2 tablespoons of the existing yogurt, or add the freeze-dried bacteria.
Put the mixture in containers. Pour your milk into a clean container or containers. Cover each one tightly with a lid or plastic wrap.
Allow the yogurt bacteria to incubate. Keep the yogurt warm and still to encourage bacteria growth, while keeping the temperature as close to 100F (38C) as possible. An oven with a pilot light is one option; see Tips for others. After seven hours you will have a custard-like texture, a cheesy odor, and possible some greenish liquid on top. This is exactly what you want. The longer you let it sit beyond seven hours, the thicker and more tangy it will become.
Refrigerate the yogurt. Place the yogurt in your fridge for several hours before serving. It will keep for 1-2 weeks. If you are going to use some of it as starter, use it within 5-7 days, so that the bacteria still have growing power. Whey, a thin yellow liquid, will form on the top. You can pour it off or stir it in before eating your yogurt
Add optional flavorings. Experiment until you develop a flavor that your taste buds fancy.
Use yogurt from this batch as starter for the next batch.
credited from http://www.wikihow.com/Make-Yogurt