Pour 1/2 gallon of whole milk into the slow cooker and set it to low heat. Heat for 2.5 to 3 hours until the milk reaches 180°F (82°C).
Turn off the slow cooker and let the milk cool to 110°F (43°C), which may take 2 to 3 hours. For faster cooling, remove the ceramic insert.
In a bowl, mix 1 cup of the warm milk from the slow cooker with 1/4 cup of plain yogurt until well combined.
Stir the yogurt mixture back into the slow cooker.
Cover the slow cooker and wrap it with a thick towel to maintain warmth. Allow it to culture for 8 to 12 hours. I use a big bath towel for this. (Your room temperature should remain above 68℉ ( 20℃), or the yogurt may not thicken.
After incubation, check that the yogurt has thickened. For Greek-style yogurt, place a cheesecloth in a strainer, set it over a bowl, and pour the yogurt in. Let it strain in the fridge for a few hours or up to 24 hours until it reaches your desired thickness.
Transfer the yogurt to containers and refrigerate to enhance firmness and flavor.
Notes
Reserve about 1/4 cup of your finished yogurt to use as a starter for your next batch. This not only saves money but also maintains the live cultures needed for fermentation.Ensure that all tools and containers are clean to prevent contamination.The incubation period can be adjusted depending on how tangy you like your yogurt; longer periods result in tangier yogurt.