The primary ingredient used in making yogurt is milk. Yogurt is produced by fermenting milk with specific bacterial cultures, typically Lactobacillus bulgaricus and Streptococcus thermophilus. These bacteria convert lactose, the sugar in milk, into lactic acid, which thickens the milk and gives yogurt its distinct tangy flavor. While cow's milk is the most common source, yogurt can also be made from goat, sheep, or plant-based milks. The fermentation process not only enhances the nutritional value of milk by making it easier to digest but also introduces probiotics, beneficial bacteria that contribute to gut health.