Salt is a preservative and therefore, salted butter has a longer shelf life than unsalted butter. We’re talking 3-4 months of shelf time. For fresher butter, reach for the unsalted variety. (Or heck! Make your own!) However, some brands add “natural flavor” to unsalted butter, which extends its shelf life (not quite as long as salt). This is usually lactic acid, which also helps regulate its pH.