Buttermilk is made by fermenting milk with lactic acid bacteria, which convert lactose into lactic acid. This process thickens the milk and gives it a tangy taste. Buttermilk is commonly used in baking and cooking for its tenderizing and flavor-enhancing properties. It is important to note that adding buttermilk to coffee may alter its taste and texture, so it is recommended to experiment with small amounts to determine personal preference.