Sour milk is a dairy product that has undergone fermentation, resulting in a tangy flavor and thicker texture. This process occurs when bacteria, such as Lactobacillus, convert lactose, the sugar in milk, into lactic acid. Sour milk can be made intentionally through methods like adding buttermilk or vinegar to fresh milk. While sour milk is often considered spoiled, it can still be safe to consume if it has been properly fermented. It is commonly used in recipes for baked goods, such as pancakes and muffins, where it adds moisture and a slight acidity that enhances flavor.