Is Buttermilk Just Sour Milk?
Buttermilk is not just sour milk. It is a separate product made by the fermentation of milk, often with the use of lactic acid bacteria. The bacteria help to convert lactose, the sugar found in milk, into lactic acid, which gives the buttermilk its tangy flavor. Buttermilk can have a similar acidity level to sour milk, but the two products are not interchangeable in recipes.