Buttermilk

Buttermilk is a dairy drink made by adding lactic acid bacteria to milk to produce a fermented dairy drink. Traditionally, it was made from the nearly fat-free milk remaining after churning butter from cream, which was cultured with natural bacteria prior to and during churning, giving a slight sour taste to the buttermilk and preventing spoilage. However, with the ubiquity of refrigeration in ind... (Wikipedia)