Yoghurt gets its natural tang from the lactic acid production of bacteria in the yoghurt during fermentation. When making yoghurt at home, you must supply these bacteria to the milk in the form of a starter. Commercial yoghurt and buttermilk both contain these bacteria and either can be used as a starter for your homemade yoghurt. Check the label on the buttermilk; verify that it contains bacteria, also called "live, active cultures." Use regular whole or skimmed milk as the base for your yoghurt for best results, reserving the buttermilk for the starter.
Warm the milk over medium low heat in the saucepan until it reaches 85 degrees Celsius.
Pour the milk into the resealable container.
Set the container into the ice water bath until the milk reaches 43.3 degrees Celsius.
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 yoghurt to the desired consistency. This could take two to four hours. Test the temperature of the yoghurt every 30 minutes with the candy thermometer to ensure it does not get above 44.4 degrees Celsius which could kill the bacteria. Alternatively, transfer the yoghurt to a Thermos and seal. Keep the Thermos at room temperature in a warm room for up to four hours or until the yoghurt reaches the desired thickness.
Remove the lid and refrigerate the yoghurt immediately after fermenting to slow the process.
Store your fresh yoghurt in the refrigerator for up to two weeks.