You know those people who are telling you not to pack a tuna sandwich for that hike? Well, they're the ones who can take a hike, because mayo may actually be keeping us safe.

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Of all the anxieties surrounding taking work to lunch—and there are plenty, from what to carry said lunch in, to making sure said lunch isn't stolen—perhaps none is as nagging as the anxiety around mayonnaise.


In short, people seem convinced that, if left at room temperature for even a short period of time, the mayonnaise on their sandwich will sprout all sorts of bacterial growths—growths that will cause illness, financial ruin, and all sorts of other tragedies.

A seminal study from 2000 took a look at the fragility of mayonnaise and set the record straight: "Salmonella, E. coli O157:H7, E. coli, L. monocytogenes, Staphylococcus aureus, and Yersinia enterocolitica die when inoculated into mayonnaise and dressings."

To put it just as plainly, but in layman's terms, store-bought mayonnaise contains enough acid (from vinegar or lemon juice) to not only kill food-borne pathogens, but also to prevent them from forming.

What this means is that the angst around an egg salad sandwich—that is, the fear of letting the sandwich sit out for an hour or two at room temperature because the mayo might spoil—is actually backwards. If anything, the mayonnaise is preventing microbial growth. The eggs (and turkey, and sliced ham) would be more dangerous without it.

Karen, the government's cartoon food safety expert (yes, you read that correctly), concurs with the results of the study. People often finger mayonnaise as the culprit for food-borne illness, but according to Karen, "usually it's the meat, poultry, fish or eggs in a sandwich kept out of the refrigerator for more than two hours that is the medium for bacteria to grow." (For a cartoon, Karen is really quite informed about these things.)

Some important notes: This is only true with commercially-made, store-bought mayonnaise. Homemade mayonnaise, which uses raw eggs, is a different matter altogether, and should be kept cold. And because other components of a sandwich are susceptible to spoilage, your lunch should probably not sit out at room temp—and certainly not in the sun (on, say, the beach)—for hours and hours on end.

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But when it comes to your commute, or leaving a sandwich on your desk for an hour or two, you can relax. Focus on other lunch anxieties instead, like who stole your dessert from the fridge.