You can heat the milk on the stove or in a microwave. To boil milk on the stove, use a large, deep pot so the milk has room to crawl up the sides a bit. Stir the milk occasionally to prevent a skin from forming, and keep a close eye on the pot, lowering the flame if necessary, as milk has a tendency to boil up quickly and can scorch.
To boil milk in the microwave, use a microwave-safe bowl that's large enough to allow for bubbling—a large glass measuring cup with a spout (for easy pouring) is perfect. Heat the milk in short increments, and check the temperature frequently—after a few batches, you'll know how long your microwave takes.
3. Cool It Back Down
Once you've heated the milk, you must let the temperature drop to 110°F to 115°F: Use a thermometer to monitor the milk's temperature and to speed the process, use an ice bath, submerging the pot of milk in a large bowl or sink filled with ice. Stir the milk occasionally to help cool it down.
4. Add the Starter Culture
Once the milk's temperature is in the optimal incubation range of 110°F to 115°F, it's time to add your starter. If using powdered starter, simply whisk in the amount specified on the packaging. If using yogurt as a starter, in a separate bowl, stir together the starter yogurt and a small amount of the warm milk, then add this to the rest of the warm milk and stir to incorporate it. While some experts, including English, have success without this step, adding cold starter yogurt directly to the milk can potentially drop the temperature too much, impeding the incubation process. Plus, says Mendelson, "It's just easier to mix the starter yogurt evenly with the main amount of milk if you first combine the starter with a small amount."
5. Incubate
Once your milk and starter are combined, all that's left is to keep the yogurt at a steady temperature (110°F to 115°F). Have you ever Googled "how long to make yogurt?" Here's your answer: 5 to 10 hours, which allows the good bacteria to flourish. Leave it alone, undisturbed, for the entire time. This incubation stage can be accomplished with a yogurt machine or any DIY incubators. And note that transferring the mixture from a pot to small jars can be messy business. A ladle (particularly one with a spout) can come in handy here.
How long yogurt needs to incubate depends on a number of factors, including the incubation temperature (higher temperatures make for faster incubation, but take it too high and your cultures will die); how many active cultures are in your starter (this can vary from batch to batch); and your personal preferences for flavor and texture. "Shorter and cooler incubation periods create sweeter, thinner yogurts, while longer and hotter periods create a thicker, tarter yogurt," explains English.
Do not disturb! As mentioned above, during incubation, do not move or jostle the mixture. "Stick it in a warm place and don't touch it," says Jenny Ammirat. Mendelson says, "Tell yourself that the yogurt deities will leap out and curse you if you do anything more drastic than sneaking a cautious peek during the incubation period." She explains that the matrix that the casein in milk forms around the whey when you make yogurt is very fragile: "This means fragile, fragile, fragile. Jostling, shaking, transferring from one vessel to another are all going to stop the yogurt from setting up properly."
6. Check for Doneness
Ammirati recommends that you begin to taste the mixture hourly after the first four or five hours; the yogurt will get a bit more acidic as it matures, so stop incubating shortly before it reaches the acidity you like. If you don't want to taste the mixture (and how many people really want to taste warm yogurt?), you can also judge fairly well by appearance. The solids and whey will start to separate and the yogurt will begin to look firm, explains Jenny, adding, "If it's still milk, leave it longer."
