BehindTheMedspeak: Who knew? Mayonnaise protects against food poisoning

Wait a minute.

Everyone knows that you're likely to get sick after eating salads containing mayonnaise that have been left out too long at a picnic.

That may still hold true — but according to New York Times debunker-of-urban-legends-in-chief (DOULIC) Anahad O'Connor, writing in his "Really?" column in the July 1, 2008 New York Times Science section, mayonnaise actually retards salmonella and staphylococcus bacterial growth in chicken or ham salad.

Here's the revelatory Times piece.

The Claim: Mayonnaise Can Increase Risk of Food Poisoning

The Facts: This is the time of year when food poisoning typically spikes, and one popular picnic ingredient that always attracts suspicion is mayonnaise.

But studies cast doubt on that.

Most commercial brands of mayonnaise contain vinegar and other ingredients that make them acidic — and therefore very likely to protect against spoilage. When problems occur, they usually result from other contaminated or low-acid ingredients (like chicken and seafood), improper storage and handling, or homemade versions that contain unpasteurized eggs.

One prominent study published in The Journal of Food Protection found, for example, that in the presence of commercial mayonnaise, the growth of salmonella and staphylococcus bacteria in contaminated chicken and ham salad either slowed or stopped altogether. As the amount of mayonnaise increased, the rate of growth decreased. When temperatures rose to those of a hot summer day, the growth increased, but not as much as in samples that did not contain mayonnaise.

For backyard chefs, some high-risk foods in summer are raw shellfish, bulk ground beef (health officials say a single hamburger can contain meat from hundreds of animals) and unwashed fruits and vegetables.

The Bottom Line: Despite its reputation, mayonnaise can reduce food spoilage.

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Here is the abstract of the Journal of Food Protection study cited above.

Microbiological Safety of Mayonnaise, Salad Dressings, and Sauces Produced in the United States

The literature on the death and survival of foodborne pathogens in commercial mayonnaise, dressing, and sauces was reviewed and statistically analyzed with emphasis on Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes. The absence of reports of foodborne illness associated directly with the consumption of commercially prepared acidic dressings and sauces is evidence of their safety. Salmonella, E. coli 0157: H7, E. coli, L. monocytogenes, Staphylococcus aureus, and Yersinia enterocolitica die when inoculated into mayonnaise and dressings. Historically, mayonnaise and dressings have been exempt from the acidified food regulations and have justly deserved this status due primarily to the toxic effect of acetic and to a lesser extent lactic and citric acids. These organic acids are inimical to pathogenic bacteria and are effective natural preservatives with acetic being the most effective in killing pathogenic bacteria at the pH values encountered in these products. Statistical analysis on data reported in the literature shows that the most important and significant factor in destroying pathogenic bacteria is pH as adjusted with acetic acid followed by the concentration of acetic acid in the water phase. The reported highest manufacturing target pH for dressings and sauces is 4.4, which is below the 4.75 pKa of acetic acid and below the reported inhibitory pH of 4.5 for foodborne pathogens in the presence of acetic acid. The overall conclusion is that these products are very safe. They should remain exempt from the acidified food regulations providing adequate research has been done to validate their safety, and the predominant acid is acetic and reasonable manufacturing precautions are taken.