Buttermilk is a fermente dairy product produced from cow's milk with a characteristically sour taste. The product is made in one of two ways. Originally, buttermilk was the liquid left over from churning butter from cream. Today, this is called traditional buttermilk. Buttermilk also refers to cultured buttermilk, a product where lactic acid bacteria have been added to milk. Whether traditional or cultured, the tartness of buttermilk is due to the presence of acid in the milk. The increased acidity is primarily due to lactic acid, a by-product naturally produced by lactic acid bacteria while fermenting lactose, the primary sugar found in milk. As lactic acid is produced by the bacteria, the pH of the milk decreases and casein, the primary protein in milk, precipitates causing the curdling or clabbering of milk. This process makes buttermilk thicker than plain milk. While both traditional and cultured buttermilk contain lactic acid, traditional buttermilk tends to be thinner whereas cultured buttermilk is much thicker.