Buttermilk derives its name from being the leftovers of making butter. Since almost all of the liquid's fat goes to the butter, the milk that's left is actually quite lean. Its rich thickness is a result of the slight “souring” from the formation of lactic acid due to bacteria in the milk. The acidity of buttermilk plays a vital role in many recipes, especially quickbreads like biscuits and pancakes, as it balances the alkalinity of baking soda. The chemical reaction between acid and alkaline causes quickbreads to rise. If you don't have any buttermilk on hand, however, you can easily make a substitute from cream and vinegar.
Measure out the amount of buttermilk called for in your recipe into a small bowl or glass measuring container.
Add 1 tbsp. vinegar or lemon juice for every cup of milk or cream needed. For example, if your recipe calls for 1 ½ cups buttermilk, combine 1 ½ cups milk or cream and 1 ½ tbsp. vinegar or lemon juice.
Stir the mixture thoroughly.
Allow the mixture to sit for about five minutes before adding it to your recipe. The milk will begin to look curdled, which is normal.