An unopened jar of commercial mayonnaise can usually sit safely in the pantry until its best-by date because it is made to be shelf-stable before opening. Once the jar has been opened, however, it should normally be refrigerated, especially when the label specifically says “refrigerate after opening.”
Commercial mayonnaise is fairly acidic, which makes it less hospitable to many harmful bacteria than people sometimes assume. But that doesn’t mean an opened jar should sit on the kitchen counter for several days. Every time a knife or spoon goes into the jar, crumbs and other food particles can introduce contamination. Warm temperatures can also cause the oils to deteriorate and affect the flavor, smell, and texture.
If mayonnaise has been left at room temperature for only a short period while preparing a meal, it generally isn’t automatically ruined. But an opened jar that has remained unrefrigerated for days is safest to throw away, particularly if the kitchen is warm. Don’t rely only on smell or appearance, because food can sometimes contain harmful bacteria without looking obviously spoiled.
Homemade mayonnaise deserves even more caution because recipes often contain raw or lightly cooked eggs and don’t have the same acidity and preservatives as commercial products. It should be kept refrigerated and used relatively quickly.
So if Nana has been keeping an opened jar of mayonnaise on the counter for several days, eating it isn’t worth the gamble. Replace it with a fresh jar and store the opened container in the refrigerator according to the instructions on the label.
When it comes to mayonnaise, the simple rule is: unopened in the pantry, opened in the fridge.
