Yogurt starts as fresh milk or cream. It is often first pasteurized, then fermented with various live bacteria cultures, and incubated at a specific temperature to encourage bacteria growth. The culture ferments the lactose. This produces lactic acid, which gives yogurt its distinctive flavor. Yogurt starts as fresh milk or cream. It is often first pasteurized, then fermented with various live bacteria cultures, and incubated at a specific temperature to encourage bacteria growth. The culture ferments the lactose. This produces lactic acid, which gives yogurt its distinctive flavor, Yogurt starts as fresh milk or cream. It is often first pasteurized, then fermented with various live bacteria cultures, and incubated at a specific temperature to encourage bacteria growth