Buttermilk originally comes from the liquid left behind after cream is churned into butter. When cream is agitated during churning, the fat globules clump together to form solid butter, and a thin, slightly acidic liquid separates out. That liquid is traditional buttermilk. Most buttermilk sold in grocery stores today, however, is not a byproduct of butter-making at all. It’s manufactured by adding bacteria to low-fat milk and letting it ferment.
Traditional Buttermilk: The Original Version
For thousands of years, buttermilk was simply what you had left in the churn after making butter. Households couldn’t afford to waste it, so it became a cooking ingredient and a drink in its own right. Because raw cream naturally picks up bacteria from the environment, the leftover liquid would begin to ferment on its own, developing a tangy flavor and thicker texture over time.
This kind of buttermilk varied depending on the cream it came from. If the cream was fresh and sweet when churned, the buttermilk tasted mild. If the cream had already begun to sour (a common practice in warmer climates before refrigeration), the buttermilk came out noticeably tangy and thicker. In India, traditional buttermilk, called chaas, is still made this way by churning yogurt (dahi) with water. It remains a staple beverage, valued for digestion and as a way to cool down in hot weather.
How Commercial Buttermilk Is Made
Almost all buttermilk on store shelves is “cultured buttermilk,” meaning it never went near a butter churn. Instead, manufacturers start with pasteurized low-fat or skim milk and add specific bacterial cultures. These bacteria do two jobs: one group produces lactic acid, which gives buttermilk its sour taste and drops the pH to around 4.4 to 4.5. A second type of bacteria produces a compound called diacetyl, which is responsible for the rich, buttery aroma.
The inoculated milk sits at roughly room temperature (around 70°F) for 14 to 16 hours while the bacteria work. During that time, the lactic acid thickens the milk’s proteins and transforms its flavor. The result looks and tastes quite different from the thin, watery byproduct of butter churning. It’s thicker, creamier, and more consistently tangy, which is why it became the standard in commercial production.
Why Buttermilk Matters in Baking
Buttermilk’s acidity is the reason so many baking recipes call for it. When buttermilk meets baking soda (sodium bicarbonate), a straightforward acid-base reaction releases carbon dioxide gas. Those tiny bubbles get trapped in batter and expand in the oven, giving pancakes, biscuits, and cakes their lift. Without an acidic ingredient like buttermilk, baking soda can’t do its job and may leave a metallic, soapy taste behind.
The acid also tenderizes gluten, which is why buttermilk biscuits and pancakes have a softer, more delicate crumb than versions made with regular milk. And the lactic acid contributes a subtle tang that balances sweetness in cakes and quick breads.
Nutrition Compared to Regular Milk
Cultured buttermilk has a nutritional profile similar to skim milk, with one notable difference: its fat content tends to run higher than you might expect, typically between 1% and 2% depending on the brand. It delivers comparable amounts of calcium, protein, and B vitamins as regular milk.
If you’re lactose-intolerant, buttermilk is only slightly easier to digest than regular milk. A 150 ml serving of buttermilk contains about 6 grams of lactose, compared to 7 grams in the same amount of whole cow’s milk. The fermentation process converts some lactose into lactic acid, but not enough to make a dramatic difference for most people with sensitivity. It’s worth noting that lactic acid itself contains no lactose, even though the names sound similar.
Buttermilk Drinks Around the World
The idea of drinking thinned, tangy dairy is far from an American invention. In South India, neer more (Tamil for “water buttermilk”) blends yogurt or buttermilk with water, salt, and spices like asafoetida, fresh chiles, cilantro, and mint. Some versions include a “temper,” where spices are briefly toasted in oil and poured over the drink just before serving. In the Middle East, ayran and doogh serve a similar role, combining yogurt with water and salt, sometimes with dried mint. Across Scandinavia, naturally fermented buttermilk (kjernemelk in Norwegian) has been a common household drink for centuries.
What all these drinks share is the same principle: lactic acid bacteria transform dairy into something tangy, lighter, and easier to consume in large quantities than straight milk or yogurt.
Storage and Shelf Life
Buttermilk keeps well if refrigerated below 40°F (4.4°C). An unopened carton typically lasts 7 to 14 days past the printed expiration date. Once opened, plan to use it within about two weeks. If you won’t finish it in time, buttermilk freezes well for up to three months. The texture may separate slightly after thawing, but a good shake brings it back together, and it works perfectly fine in baked goods even if slightly grainy.
Stick with pasteurized buttermilk. Unpasteurized versions carry a higher risk of foodborne illness and offer no meaningful nutritional advantage.