Buttermilk can also be purchased in a powder form to which water is added to liquidify it. Instead of water, low-fat milk can be substituted to fortify the powdered form with added calcium. Since the bacteria that may be harmful has been removed during the heating and fermentation process and since lactic acid has been added which helps to prevent bacteria from growing, the Buttermilk can be kept for longer periods of time in the refrigerator. As the Buttermilk ages, the lactic acid increases and begins to eventually sour the milk which may cause it to curdle. As it sours, it becomes inedible and indicates it is time to discard any remaining Buttermilk.