Yogurt is a coagulated, sour dairy food. Human beings have probably consumed fermented milks, such as yogurt, since they first milked cows and other animals. Before modern times, however, people did not know of the need for sanitation, adequate refridgeration, and heat treatment of milk to prevent spoilage. Milk fermented naturally (or spoiled) if it was left unconsumed. The quality of the product depended on the microorganisms present in the original milk.