Making yogurt at home is both easy and economical; you can turn just a couple tablespoons of your favorite store-bought yogurt into a quart of your own. The fundamental steps are pretty consistent: Heat the milk to 180ºF to denature the proteins, cool to 110ºF, add the starter, and incubate in a warm environment for several hours. During this process, the live cultures incubate, thickening the milk into a curd.
During this period, the goal is to keep the yogurt at a fairly consistent temperature of about 110ºF, the optimal temperature for the bacteria to do its thing. are designed to do exactly that, but if you don’t want to invest in an appliance that does only one thing, there are alternatives.
In her book Yogurt Culture, author Cheryl Sternman Rule outlines a few options (plus offers 115 recipes for what to do with your finished yogurt).
Use your oven. If you have a gas oven with a pilot that stays on, it may be the right ambient temperature. Alternatively, switch on the light. The heat from the light should elevate the temperature sufficiently. Just be careful not to turn the oven on with your yogurt inside.
Find a warm spot in your house. Does your kitchen (or another room) have a warm, sunny window? Wrap your yogurt jars in a towel and place them there. You can also place them near (but not directly on) heating vents or other heating units.
Use a heating pad -- but not just any heating pad. Look for a heating pad that allows you to disable the auto-shutoff option, so you can maintain a consistent temperature, and that has a low setting of around 110ºF. The heating pad can be wrapped around your vessel for the duration of incubation. You may need to place a towel in between to temper the heat.
A water bath kept at a stable temperature is an excellent way to incubate yogurt as well, and there are a few ways to achieve this.
Fill a cooler with 130ºF water just high enough to cover the jars to the neck. The yogurt will initially bring down the temperature of the water closer to 110ºF. Closing the cooler will stabilize the temperature during incubation.
You can use a slow cooker to keep a water bath at a stable 110ºF as well. If your slow cooker doesn’t run low enough, you can put it on a dimmer switch.
If you have an immersion circulator, you can also use that to maintain the temperature of the water bath. ChefSteps recommends setting it to 109ºF.
Depending on the temperature you are incubating at, getting a good curd can take anywhere from five to 12 hours; the cooler, the longer. Do not disturb the yogurt during incubation, as that may cause the curd to break, and you won’t get a good set. The longer you incubate, the more assertive and tart the flavor of your yogurt will be; in time you’ll find a method and time that results in the texture and flavor you like best.