Buttermilk improves the taste and texture of baked goods by adding moisture and tangy richness without a lot of fat.
The acids in buttermilk have a relaxing effect on gluten. That's why biscuits made with buttermilk are more tender than biscuits made with regular milk.
Buttermilk also has a lightening effect. The chemical reaction between buttermilk and baking soda produces plentiful bubbles of carbon dioxide, which lift baked goods to great heights.
If you'd like to substitute buttermilk for milk in a favorite recipe, take care to adjust the leavening ingredients to take into account buttermilk's acidity. For each cup of buttermilk you use in place of regular milk, reduce the amount of baking powder in the recipe by 2 teaspoons and add ½ teaspoon of baking soda.
Keep in mind that baking soda, unlike baking powder, loses its lifting power shortly after it is mixed with liquid ingredients, so it is best to mix your recipe quickly and get it into the oven right away.
Buttermilk has a much longer shelf life than regular milk. It will keep in the refrigerator for at least two weeks and probably well beyond its sell-by date. Just be sure to shake the carton vigorously before pouring, as buttermilk will thicken and get a little lumpy after a couple of weeks.
If you don't have any buttermilk on hand for spur-of-the-moment baking, it is easy to make a substitution. Simply mix 1 cup of low-fat milk with 1 tablespoon of vinegar or lemon juice and let it stand for 10 minutes. Or thin ¾ cup of plain low-fat yogurt with ¼ cup of milk.