Though buttermilk and soured milk are often used interchangeably, buttermilk is actually a type of soured milk. Both buttermilk and soured milk products are popular ingredients in many recipes.
A type of soured milk, the term buttermilk originally applied to the watery byproduct of churning butter. Today, buttermilk in the United States is mostly known as cow's milk that has added bacteria cultures. The bacteria produces lactic acid as it ferments the milk.
Sour milk is milk that has acid or bacteria added. Some common acids added are acetic acid (found in vinegar) and citric acid (common in lemon juice and other citrus fruits). The acid or bacteria causes the milk to curdle and become thick.
In the American South, buttermilk was once a popular drink in the early 20th century. Today, it's a popular ingredient in baking and cooking.
Though buttermilk and soured milk are not necessarily the same, when a recipe calls for buttermilk, you can use soured milk in its place. For example, 1 cup of buttermilk can be substituted with 1 tbsp of vinegar and enough milk to make 1 cup. Let the milk curdle for five minutes before using.
Spoiled milk, not to be confused with soured milk, is just ordinary milk that has gone beyond its expiration date or has been left out to spoil. Unlike soured milk or buttermilk, drinking spoiled milk can make you sick.