2. Homemade yogurt contains active probiotics, and it is easy to be infected with spoilage bacteria because no preservative measures are taken. Moreover, the probiotics in yogurt gradually decrease with the extension of storage time, so the shelf life of homemade yogurt is generally 3-5 days.

3. The basic principle of homemade yogurt is to inoculate lactic acid bacteria into milk, let it breed (ferment) at a suitable temperature (generally 40℃-42℃), and decompose lactose in milk into lactic acid. Because lactic acid is produced in the fermentation process, the acidity of the fermentation broth gradually decreases. When the pH value reaches about 4.6, casein in milk will slowly precipitate to form delicate jelly, and the viscosity of the whole solution will also increase, that is, yogurt will be formed.