Who would have thought homemade yogurt could be so easy and yet so yummy! Making homemade yogurt is not only frugal, but you get to control all that is added to your final product. Unfortunately, most store bought yogurts are loaded with sugar and preservatives.
Yogurt is a fermented milk product which was apparently broght to Turkey by the mongols millenia ago. It is produced by adding a "starter" of active yogurt containing a mixed culture of Lactobacillus bulgaricus (or occasionally L. acidophilus ) and Streptococcus thermophilus. These produce lactic acid during fermentation of lactose. The lactic acid lowers the pH, makes it tart, causes the milk protein to thicken and acts as a preservative since pathogenic bacteria cannot grow in acid conditions. The partial digestion of the milk when these bacteria ferment milk makes yogurt easily digestible. In addition, these bacteria will help settle GI upset including that which follows oral antibiotic therapy by replenishing non-pathogenic flora of the gastrointestinal tract.
Cool!! Looks like it's got it's benefits for us, even thought I don't like the idea of eating Streptococcus.
Yogurt is preserved by its acidity which inhibits the growth of putrefactive or pathogenic bacteria, though we we'll have to be extra carefull to implement a proper cleansing and heat treatment of glassware that will contain the final product if we don't want to spoil everything we worked for!
1 quart (946 mL) milk (any kind but if you use "ultra-high pasteurized" or "UHP" or "UHT" then you can skip step one, as the milk has already been heated to this temperature before the pack was sealed) 1/4 to 1/2 cup non-fat dry milk (optional) 1 tablespoons white sugar to feed the bacteria pinch salt (optional) 2 tablespoons existing yogurt with live cultures (or you can use freeze-dried bacteria instead)
1 Heat the milk to 185ºF (85ºC). Using two 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.
Heat to 185ºF (85ºC) in a double boiler.
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.
Add sugar and (optionally) a pinch of salt and cool to about 110ºF (43ºC).
3 Warm the starter. 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.
4 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're using nonfat milk.
5 Add the starter. Add 2 tablespoons of the existing yogurt, or add the freeze-dried bacteria. Stir it in or better yet, use a blender to evenly distribute the billions of bacteria throughout the milk.
Inoculate the yogurt with a little yogurt from the last batch.
6 Put the mixture in containers. Pour the milk into a clean container or containers. Cover each one tightly with a lid or plastic wrap.
7 Allow the yogurt bacteria to incubate. Keep the yogurt warm and still to encourage bacterial growth, while keeping the temperature as close to 100ºF (38ºC) as possible. An oven with a pilot light left on is one option; see the "Tips" section for other ideas.
Incubate the yogurt for several hours.
After seven hours, you'll have a custard-like texture, a cheesy odor, and possibly some greenish liquid on top. This is exactly what you want. The longer you let it sit beyond seven hours, the thicker and tangier it will become.
8 Refrigerate the yogurt. Place the yogurt in your fridge for several hours before serving. It will keep for 1 to 2 weeks. If you're going to use some of it as starter, use it within 5 to 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.
Many commercial yogurts include a thickening agent, such as pectin, starch, gum, or gelatin. Don't be surprised or concerned if your homemade yogurt has a somewhat thinner consistency without these thickeners.
9 Add optional flavorings. Experiment until you develop a flavor that your taste buds fancy. Canned pie filling, jams, maple syrup, and ice-cream fudges are good flavorings. For a healthier option, use fresh fruit, with or without a small amount of sugar or honey.
10 Use yogurt from this batch as the starter for the next batch.
Here's a quick microwave powered recipe tutorial for Greek Yogurt.