Yogurt is a kind of fermented milk. It can be made from cow's, sheep's or goat's milk. It forms a fairly liquid curd that is slightly acidic. This fermented milk is formed by the action of two lactic bacteria: Lactobacillus bulgaricus which gives the yogurt its acidity and Streptococcus thermophilus which develops its flavors. The bacteria must be cultured simultaneously and be alive in the yogurt.