In fact, the principle of homemade yogurt is to inoculate lactic acid bacteria into milk, let the lactic acid bacteria breed (ferment) at a suitable temperature (usually 40℃-42℃), decompose lactose in milk into lactic acid, and cool the milk for a period of time to form solidified yogurt.

Historical evidence shows that yogurt has been used as food for at least 4500 years. The earliest yogurt may be milk packed in sheepskin bags by nomads, which was naturally fermented by bacteria attached to the bags and turned into cheese. More than 2000 years ago BC, the ancient Thracians living in northeast Greece and Bulgaria also mastered the technology of making yogurt. They started with goat's milk. Later, yogurt technology was spread to other parts of Europe by the ancient Greeks.