Milk is heated to destroy bacteria that are present in milk and could compete with the yogurt cultures, Lactobacillus bulgaricus and Streptococcus thermophilus. Heating changes the structure of the proteins in milk that gel, causing the finished yogurt to have a thick consistency. You don't have to heat the milk, but if you want to eat the finished yogurt with a bowl and spoon, you might want to. There is a way to make raw milk yogurt with the help of gelatin and some different cultures.