Yogurt starter culture usually refers to specific microorganisms used to make yogurt, primarily lactic acid bacteria. These lactic acid bacteria convert lactose in milk into lactic acid during the fermentation process, acidifying the milk and creating the distinctive texture and flavor of yogurt. Yogurt starter cultures can be obtained by purchasing commercial yogurt starter powder or by directly using yogurt with active cultures as a starter. When making yogurt at home, it is typically necessary to maintain a certain temperature (around 42-45°C) to facilitate the growth and fermentation of lactic acid bacteria. The fermentation time depends on the desired yogurt thickness and acidity, usually ranging between 4 to 12 hours.