Yogurt Starter Cultures - Pack of 5 Freeze-dried Culture Sachets for Pure Acidophilus Yogurt (5) Boil the whole milk first. This will kill all kind of non-yogurt bacteria in it and evaporate the water in the milk. Cool the milk down to a lukewarm state in order to start the fermentation process. Somewhere between 40 and 45 degrees Celsius is the ideal temperature, which will make the bacteria thrive and start producing your yogurt. Just touch the container with the milk and if you feel it warm this means that the temperature is ideal. Introduce the bacteria, with the help of the starter, into the milk and stir gently. Next, allow the milk to sit in a warm and protected place, without bothering it for a while. During this time, the bacteria will work and will process nutrients, and will separate the milks sugar, called lactose, into simpler components for easy absorption by your body. Lactic acid is also produced meanwhile, which gives the yogurt its well-known acidity. Make sure to check on the yogurt and see when it has the consistency you desire. Still, do not allow the yogurt to ferment for more than 2 days because it can get way too sour if the bacteria multiply too much. Once the fermentation process is complete, make sure to store it in the fridge, so you will stop the bacteria from multiplying and ruining the taste of your yogurt. If not stopped by a cooler temperature, the bacteria will continue consuming the milk, until its food source is depleted, which explains the spoiled taste yogurt can get or the visible separation of the milk.