The basic ingredients
Milk and culture. Part of the beauty of homemade yogurt is that you can control what goes into it. You’ll get a pure and delicious yogurt with these simple ingredients:
Milk: I use cow’s milk and make a half gallon at a time. Skim milk makes the firmest yogurt, while milk with higher fat content makes creamier yogurt. Goat’s milk can also be used but turns out a thinner yogurt. Ditto for soy milk, plus some brands will result in a smoother consistency than others. (After taking this photo, I tried Silk and now prefer it over West Soy.)
Culture: I’ve had the most reliable results with freeze-dried cultures. Store bought yogurt also works as a starter culture, as long as it has live active cultures in it. Once you’ve made your yogurt, you can use it to culture your next batch. After several rounds of re-culturing, however, you’ll need a new starter culture.
If you want to experiment with the flavor and texture, here are some possibilities:
Thickeners: For a thicker yogurt or a yogurt that doesn’t weep whey, stir dried milk powder, starch thickeners (I like arrowroot), or softened gelatin or agar powder into the milk as you are heating it. Adding a thickener may be particularly desirable with goat’s milk and soy milk yogurts.
Flavorings: You can sweeten your milk and/or flavor it as you heat it: with a shot of espresso (pictured), a little almond extract, or the scrapings of a vanilla bean. If your soy milk doesn’t already contain some sugar, you’ll need to add a couple of teaspoons to give the cultures something to feed on.
Fruit yogurt: Stir in cooked and sweetened fruit or fruit preserves after the yogurt has been made. Or, put the preserves at the bottom of the container in which the yogurt will incubate.
Heat milk to 185 to 195°F
Over a moderate heat, warm the milk, along with any sweeteners, flavors, or thickeners you are using, stirring occasionally, until it is very hot and steamy, not quite boiling, 185 to 195°F. Hold the milk in this range for 10 minutes.
Soy milk does not have to be heated this high. You just need to warm the milk enough to get any thickeners going— try 3 teaspoons of arrowroot starch per quart of soy milk, heated between 150 and 160°F. If you aren’t using a thickener, then just warm the milk to 110°F, and move to the next step.
Cool the milk to 110°F
Transfer the milk into a bowl, place the bowl in a cool water bath, and stir gently. This will cool the milk fairly quickly. Monitor the temperature closely and remove the bowl when it gets to around 115 to 120°F. As you stir the cultures in and transfer the mixture to its incubation resting spot, it will cool a little more, ideally landing you in the 108 to 115°F range. This is the temperature at which the bacteria thrive most. If the temperature is lower, they’ll just grow more slowly. A higher temperature range kills the culture.
Mix in the culture
Stir the culture into a small portion of milk. Use either the amount indicated on the pack of powdered culture or a heaping tablespoon of yogurt per quart of milk.
Then add it to the rest of the milk, making sure the culture is well blended throughout.
Soy milk users: A store-bought soy milk yogurt such as Whole Soy is a non-dairy culture option.
Let the yogurt set for about 8 hours
The trick is to maintain the temperature of the yogurt for several hours. I let the cultures do their work for a relatively long time, at least 8 hours. The longer it incubates, the more acidic the yogurt becomes (which is fine for tangy yogurt lovers). Depending on your culture, however, you may just need 5 hours of incubation.
These days, I use an insulated container designed for yogurt-making (it's called a "Yogotherm"). But you don’t need special equipment. As you'll see in the next slide...
Making yogurt without an actual yogurt maker
A slow cooker makes a great yogurt maker and allows you to incubate several individual servings of yogurt at once. Fill the slow cooker with enough 110°F water to come up the sides of the yogurt containers and set it on warm. When the yogurt-to-be is ready to go in, turn off the pre-heated cooker, put the lid on, cover it with a heavy beach towel, and leave it alone until the yogurt was ready.
You can use a cooler in much the same way. Also, I haven’t tried it, but an oven with a "proof mode" or a pilot light should work, too, but may take a little longer, given the slightly lower temperature.
Ready to Eat
When the incubation period is done, the yogurt should be smooth and firm. If you like warm yogurt, it’s ready to eat. Otherwise, chill it and eat it with your favorite mix-ins, make a smoothie with it, use it in dressings and marinades. If you like a thick, Greek-style yogurt, drain the yogurt in colander lined with a super fine mesh cheesecloth (butter muslin) or a coffee filter and let the whey drip out for a few hours or until you get the consistency you want.