Originally, buttermilk was the milky liquid that remained after churning cream into butter. But the product that we use today is made by adding cultures to fresh milk, causing the milk to sour and thicken. This buttermilk is more akin to yogurt or sour cream. Its slight acidity helps make pancakes fluffy, biscuits rich and flaky, and muffins, scones, and shortcakes moist and tender. Beyond baking, it's the main ingredient in ranch and other creamy dressings that want a little tanginess, and when used in as a marinade or batter for fried chicken, it makes the meat extra tender.
Powdered buttermilk can work in a pinch, (follow package instructions to substitute) as can plain yogurt, thinned with a litte water or milk to achieve the same consistency. You can also make your own sour milk by adding 1 Tbs. lemon juice or white vinegar to 1 cup whole, low-fat, or nonfat milk and letting it stand at room temperature for 10 minutes.
Across the U.S., buttermilk is readily available in low-fat and nonfat versions. A whole-milk variety is also easy to find in the South (but not so common elsewhere). All three versions are interchangeable in baking, though a higher-fat buttermilk will make your baked goods slightly richer.
Always shake the carton before using, because the milk solids tend to settle.
Because of its acidity, which inhibits bacterial growth, buttermilk lasts longer then fresh milk—up to three or four weeks in the refrigerator. Buttermilk make also be frozen for up to six months. However, it will separate when it's thawed, and even after remixing, the texture won't be quite the same.