Cultured buttermilk is made by adding bacterial culture to organic dairy milk. The tartness of buttermilk is due to acid in the milk. The increased acidity is primarily due to lactic acid produced by lactic acid bacteria while fermenting lactose, the primary sugar in milk. This is a 3.8% M.F. buttermilk that is non-homogenized and slow-pasteurized. Separation is completely natural, so shake well before drinking, and enjoy! Expires within a minimum of 5 days.