I can remember my mother warning me as a kid that I could get food poisoning from a sandwich that was left out of the refrigerator that had mayonnaise on it. Was this a justified concern on my mom’s part?

Image/ Marcusyoder at English Wikipedia

The answer is no. Mayonnaise is in fact not very conducive to bacterial growth. This is due to the following factors; the ingredients of commercially–prepared mayonnaise typically include pasteurized eggs, salt and a something acidic like vinegar (acetic acid) or citric acid.

Mayonnaise manufactured in the United States is quite acidic, with a pH of 3.6 to 4.0. Not only that but the organic acids used in commercially prepared mayonnaise have other antimicrobial properties.

Studies have shown that the lethal effect of mayonnaise on foodborne pathogens has been well documented. According to Michael P. Doyle, Ph.D., Professor and Director of the University of Georgia’s Center for Food Safety and Quality Enhancement, most harmful bacteria, including E. coli O157:H7, Salmonella and Listeria monocytogenes, die off within hours in the presence of mayonnaise, due to its acidity.

This is true for traditional mayonnaise, low-cal, and mayonnaise-based salad dressings.

The antimicrobial protection also extends to potato and macaroni salads that contain mayonnaise.

The food safety issue is related to foods like ham, chicken and potatoes that are frequently prepared with mayonnaise that are susceptible to bacterial growth.

If mayonnaise is left at room temperature after opening is a food safety issue? No. The issue is more related with mayonnaise flavor quality which can deteriorate if not kept refrigerated. This is the reason why the labels on mayonnaise jars suggest refrigerating after opening.

It is believed that the “dangers of mayonnaise” reputation began when people would prepare homemade mayonnaise. Not using standardized amounts of salt and vinegar and the use of unpasteurized eggs certainly contributed to the problem with pathogenic bacterial growth.

So go ahead and enjoy that sandwich or macaroni salad, if it has mayonnaise in it, the safety is actually increased.