Yogurt is both delicious and nutritious. Not only is it creamy and tangy, yogurt also contains beneficial bacteria that can help your digestion and immune system function better. Because most people buy yogurt from the grocery store, it’s easy to forget that plain yogurt is just fermented milk. It is created by introducing live cultures, specifically Lactobacillus bulgaricus and Streptococcus thermophiles, and allowing them to change the chemistry of the milk. This process is easy and inexpensive to do right at home.
To make your own homemade yogurt, the only ingredients you’ll need are a half-gallon of milk and three tablespoons of plain “live culture” yogurt or freeze-dried yogurt starter culture. Live culture yogurts still contain the living organisms that caused the milk to ferment. Some commercial yogurt brands heat treat yogurt after production to kill these beneficial organisms. To find live culture yogurt, check the label of the brands at your grocery store — organic brands like Stonyfield Farm include live cultures — or buy some directly from a local dairy.
Other things you’ll need are a large stock pot (at least 8 quarts), a smaller pot (4-5 quarts) with a lid, a large metal spoon, a cooking thermometer with a clip, and a heating pad.
Before beginning, set aside the three tablespoons of yogurt or live culture, and allow it to come to room temperature. Set a heating pad to medium and place it on a clear spot on your kitchen counter.
Set the larger pot in the sink and place the smaller pot inside it. Fill the larger pot until water reaches about halfway up the side of the smaller pot. You’ll probably need to hold the smaller pot down to keep it from floating. Drop the cooking thermometer and spoon into the large pot of water, and turn the small pot upside down over the top of the larger pot. Bring the water in the large pot to a boil. This will sterilize your tools. Remove the spoon and thermometer, and set the smaller pot back inside the larger one.
Pour the half-gallon of milk into the small pot, and clip on the thermometer, making sure that the sensor reaches into the milk. Make certain that the milk in the small pot and the water in the large pot reach the same level for even heating. Add or remove water as needed. Heat the milk to 185° F, stirring frequently. The milk should begin to froth once it is near the correct temperature.This means the milk is ready to accept the live cultures.
Fill your kitchen sink with 3-4 inches of cold water. Once the milk reaches 185° F, plunge the pot into the cold water and cool the milk back down to 110° F. This is the temperature at which yogurt cultures thrive and reproduce. Immediately add the live culture yogurt or starter culture to the milk and stir it well to ensure that the cultures are incorporated evenly.
Cover the pot with a lid and a towel, place it on top on the waiting heating pad, and leave it to incubate for seven hours. If you don’t have a heating pad handy, there are other methods you can use to keep the yogurt warm. Some people use a crockpot or rice cooker, set on medium heat. Some put the pot inside the oven on low heat. It’s also possible to fill an airtight cooler with a few inches of warm water (the same depth as the milk in the pot) and set the pot into it. No matter which method you use, it is very important that the yogurt stay at about 110° F for a full seven hours. You’ll need to monitor the temperature throughout the incubation period.
After seven hours, remove the yogurt from the heat and stir it until the curds disappear. Don’t worry if the yogurt appears runny or greenish. That’s perfectly normal. It will thicken up and begin to look more like yogurt in the next step.
Pour the yogurt into sterile sealing containers, such as mason jars or plastic tubs, and chill it in the refrigerator overnight. This will thicken the yogurt and give it its trademark tart flavor. After that, you can flavor it with honey, fruit, or any other add-in you like (just be sure to set aside three tablespoons of plain yogurt to start your next batch), and enjoy!