Yogurt can be made from reconstituted dry milk, pasteurized milk, or raw milk. It does not necessarily have to be pasteurized before the yogurt starter is added, but the end result (both texture and flavor) is usually better when the desired bacteria does not have to compete with other organisms when it is growing. To kill off competing microorganisms heat milk in a saucepan or double boiler to 160–185°F slowly, so as not to burn it in the process. Pour into clean/sterile glass jars. Allow to cool to approximately 110°F. To do this the jars can be left at room temperature, dipped in cool water bath, or refrigerated for faster cooling.
Next inoculate with bacteria starter. This can be done with commercial starter, acidophillis tablets (available at health food stores), or plain yogurt with active live cultures. The most common are Streptococcus thermophilus and Lactobacillus bulgaricus. Probiotic yogurts can be cultured at home also, although in my experience they usually yield a thinner yogurt than their store-bought counterparts. Use approximately ¼ cup yogurt per quart of milk. Fruit flavored yogurt may have other undesirable yeasts or bacteria that make it a less potent starter than plain yogurt. Stir starter thoroughly through the milk and then tightly seal jars.
Now the task is to simply keep the mixture as close to 110°F as possible for 6–8 hours and allow the bacteria to culture, creating thick yogurt. This can be done in a variety of ways. A good thermos, a water bath in a small cooler or large pot, a gas oven with a pilot light, an electric oven with 100 watt or higher light bulb, or a heating pad are all possible heat regulating methods and other methods are certainly possible. The important factor here is not the method but rather little temperature fluctuation. If the temperature rises above 120°F the beneficial bacteria will begin to die off and if dips below 100° F they will begin to go dormant and stop reproducing.
After the yogurt has grown sufficiently thick it may be chilled for several hours before eating. Sweeteners, flavoring, fruit, and other additions may be added before or after chilling as well. Store refrigerated in sealed container.
Some of my favorite yogurt additions are:
- Puréed berries or bananas
- Granola sprinkled on top right before eating