Sour milk is milk 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 produced intentionally for use in recipes or can occur unintentionally when milk is left out too long. Sour milk is often used in cooking and baking, as it can add moisture and flavor to dishes. It is a key ingredient in products like buttermilk and yogurt. While sour milk is safe to consume if properly fermented, spoiled milk should be discarded to avoid foodborne illness.