Buttermilk
Buttermilk is the liquid remaining after cream is processed into butter. In the past, buttermilk was a natural fermentation product, as the cream was left overnight to allow bacteria to convert lactose into lactic acid. This process gave it its distinctive sour taste and thicker texture.
Today, most buttermilk sold on the market is cultured buttermilk. The process involves adding lactic acid bacteria to skim milk. The result remains the same: a versatile and beneficial acidic liquid.