Commercial mayonnaise is more stable than many people assume because it is made with acidic ingredients such as vinegar or lemon juice. However, that does not mean an opened jar should routinely be stored on the kitchen counter.

An unopened jar can usually remain in a cool pantry until the date printed on its label. Once opened, mayonnaise should be refrigerated according to the manufacturer’s directions. Refrigeration protects its flavor, texture, and overall quality while reducing the chance of contamination from dirty utensils, crumbs, or other foods.

FoodSafety.gov advises discarding opened mayonnaise if it has remained above 50°F, or 10°C, for more than eight hours. Therefore, a jar that has been sitting on the counter for several days should not be eaten, even if it looks and smells normal.

Homemade mayonnaise requires even greater care because it may contain raw or lightly cooked eggs and lacks the commercial processing used in store-bought varieties. It should always be refrigerated and discarded if left at room temperature for more than two hours—or one hour in temperatures above 90°F, or 32°C.

In this situation, the safest choice is to throw away the mayonnaise, clean the jar’s storage area, and open a fresh container. When food has been stored incorrectly for days, tasting it to check whether it is safe is not worth the risk.