Unopened, commercially produced mayonnaise is almost indefinitely shelf stable. Once opened and refrigerated, it may remain safe to consume for two to three months beyond its "purchase by" date. Its color and flavor, however, may degrade and become unpalatable.
The lengthy shelf life of prepackaged mayonnaise is due in large part to its acidity, pasteurized eggs and preservatives. Investigators have even found these ingredients to impede the growth of salmonella and staphylococcus bacteria in ham and chicken salad made with contaminated meat. While commercially produced mayonnaise may be safer than traditionally thought, proper food handling procedures should still be followed to maintain its fitness for consumption.