Yogurt gets its natural tang from the lactic acid production of bacteria in the yogurt during fermentation. When making yogurt at home, you must supply these bacteria to the milk in the form of a starter. Commercial yogurt and buttermilk both contain these bacteria and either can be used as a starter for your homemade yogurt. Check the label on the buttermilk; verify that it contains bacteria, also called "live, active cultures." Use regular whole or skim milk as the base for your yogurt for best results, reserving the buttermilk for the starter.
Warm the milk over medium low heat in the saucepan until it reaches 185 degrees Fahrenheit.
Pour the milk into the resealable container.
Set the container into the ice water bath until the milk reaches 110 degrees Fahrenheit.
Stir the buttermilk into the warmed milk and seal the container.
Wrap the container with a towel and place it on top of a heating pad to ferment the yogurt to the desired consistency. This could take two to four hours. Test the temperature of the yogurt every 30 minutes with the candy thermometer to ensure it does not get above 112 degrees Fahrenheit which could kill the bacteria. Alternatively, transfer the yogurt to a Thermos and seal. Keep the Thermos at room temperature in a warm room for up to four hours or until the yogurt reaches the desired thickness.
Remove the lid and refrigerate the yogurt immediately after fermenting to slow the process.
Store your fresh yogurt in the refrigerator for up to two weeks.