Yogurt starter culture refers to microorganisms, mainly lactic acid bacteria, used in yogurt production. These bacteria ferment lactose in milk into lactic acid, acidifying the milk and developing yogurt's unique taste. To create yogurt, one can purchase commercial starter culture or use yogurt with live active cultures. Maintaining an incubation temperature of 42 to 45 degrees Celsius is essential for optimal fermentation. The fermentation time varies from 4 to 12 hours based on desired consistency and acidity, reflecting personal taste preferences.