When it comes to yogurt, it’s all about bacteria. In particular, two bacterial cultures: lactobacillus bulgaricus and streptococcus thermophilus. These two cultures convert some of the lactose (sugar) in the milk into lactic acid, which then causes the milk to curdle thermometer, and firm. There are plenty of ways to make plain yogurt your own. We’ve suggested jam, but you can also add honey or fresh fruit, too. For this recipe, any type of milk will do, even cream (higher milk fat equals creamier and smoother).