By Lauren Chattman

Newsday

Despite its name, buttermilk is not buttery at all. The name refers to its origin as a byproduct of making butter.

In olden times, the liquid left over after butter was churned was allowed to stand, becoming thick and sour as airborne bacteria consumed its sugars and produced tasty lactic acids.

Today, commercial buttermilk is made by adding a bacterial culture to low- or nonfat milk to produce a similarly thick and tart liquid.

I've always known that buttermilk was low in fat because my Dad used to drink it as a satisfying snack when he was on a diet, which was about every other month of my entire childhood. He also insisted that it settled his stomach and was the reason his own mother lived to be almost 100.

What I didn't know until I grew up to become a baker is that buttermilk improves the taste and texture of baked goods. Buttermilk adds 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 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.

You may worry that if you buy a quart of buttermilk to make a batch of biscuits or a quick bread, you will wind up throwing out 3 cups of it before you have a chance to bake again. Let me reassure you that this won't happen. Because it is highly acidic, buttermilk has a much longer shelf life than regular milk.

It will keep in the refrigerator for at least 2 weeks and probably well beyond its sell-by date. I'll admit that I've used month-old buttermilk in waffles and biscuits, and it tasted great.

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 cup of plain low-fat yogurt with cup of milk.