Most “buttermilk” in stores has never been near a butter churn. It’s skim milk with added cultures and often thickeners, so it ends up closer to a drinkable yogurt. Real buttermilk is what our grandmothers had: the thin, mild liquid that drains off when butter comes together.
Keep refrigerated. Best enjoyed within 7–10 days. Like all raw dairy, it will naturally sour over time (which is great for baking).