Buttermilk is classified as a dairy product; therefore, it does expire. Buttermilk is a processed food and its expiration varies by type and storage methods.
An unopened carton of buttermilk can be safely stored in the refrigerator for seven to 14 days past the printed date. This product should be stored at temperatures of 40 degrees Fahrenheit or lower.
If buttermilk has a foul and sour smell, changes in color or contains lumps, discard it immediately. These are signs of spoilage, and consuming it poses a risk of food poisoning.
Do not freeze buttermilk. Freezing this dairy product changes its consistency and results in clumps when thawed.