Does adding mayo to picnic foods increase my risk of food poisoning?

Mayonnaise is an emulsification of egg yolks, oil, and acids such as vinegar or lemon juice. Tales of people getting sick from it probably stem from the era when it was always made from scratch, and home cooks could have unknowingly used raw eggs contaminated with salmonella. But these days, if you’re bringing potato salad that’s made with commercially prepared mayo to a picnic, there’s not much to worry about.

The eggs used in commercial mayo are pasteurized, pretty much eliminating the risk of salmonella, says Thomas Schwarz, an independent food safety consultant. He says that the acids bring the pH of commercial mayo to about 4.2 to 4.5, which “isn’t very inviting to microorganisms,” and the emulsification of the product makes the Aw (water activity, which is a measure of free water available for organisms to grow) “quite low.” The addition of salt also makes mayonnaise a less inviting medium for bacterial growth.

In the 1980s, the Food Research Institute of the University of Wisconsin conducted studies showing that, in the presence of mayo, harmful bacteria slow in growth, or die. Because the commercial manufacturing of mayonnaise follows standards defined by the FDA, the product “contains sufficient amounts of acid to kill salmonella and some other harmful food-borne bacteria,” says Michael P. Doyle, PhD, regents professor and director of the Center for Food Safety at the University of Georgia.

Food-borne illnesses associated with mayo are probably due to contaminated foods being mixed with it. “Many low-acid foods, like chicken, ham, or potatoes, are susceptible to the growth of bacteria and are often mixed with mayonnaise,” says the Association for Dressings & Sauces (ADS). “Despite the microbiological safety of commercial mayonnaise, mixing mayonnaise with contaminated ingredients will not assure the safety of these combined mixtures.”

Schwarz says that “homemade mayo is very different,” because the home cook is probably making it from whole, fresh egg yolks, the pH isn’t being measured (to determine how much acid is needed), and it may not be emulsified to sufficiently lower the Aw.

To be safe at your next picnic, the ADS suggests following food safety procedures such as avoiding cross contamination of surfaces during prep, keeping perishable foods in a cooler until you want to eat them, and refrigerating leftovers within a few hours.

CHOW’s Nagging Question column appears every Friday.