Food poisoning: it's not the mayonnaise!

by James Channing Shaw, MD

One July, years ago, a neighbor at our summer cottage came over to borrow some mayonnaise.

"Of course," my mother said. "Help yourself." She retrieved a jar of mayonnaise from a cupboard (the refrigerator back then was too small to hold everything).

The neighbor, noticing the jar hadn’t been refrigerated and was half used, asked, "How long has this been at room temperature?"

"All summer.”

Blank stare from the neighbor.

"It never goes bad. The lemon juice is a preservative," my mother explained. She had been storing mayonnaise at room temperature every summer at the cottage for ten years. No one had ever become sick.

The neighbor couldn’t handle it. She left, dismayed, without the mayo.

So here are the questions: Would you eat mayonnaise from a jar left out all night? How about two weeks? If you got food poisoning after the company picnic, would you blame the mayo? Popular thinking says that you'd definitely blame the mayonnaise in the potato or chicken salad, and would throw out the mayo if left out overnight.

But popular thinking is often wrong. Here are some answers: Although mayonnaise gets much of the perceived blame these days for food poisoning, by far the most common culprit in food borne illness in North America is Staphylococcus aureus, a type of bacteria that comes from food handlers. Up to 50% of healthy adults are carriers of Staph. aureus. (it hides unnoticed in nasal passages and throat). There are other causes of food borne illnesses, like E. coli in poorly processed meat, but Staph. aureus ranks first. Once introduced to the food, the Staph bacteria multiply and produce a toxin that causes the illness we know and hate. The important issue is NOT the mayo; it is human contamination. Prevention comes from proper food prep, i.e. hand washing and keeping hands away from the face. Refrigeration only becomes an issue after the food is prepared. If it is contaminated, refrigeration certainly retards the bacterial growth and toxin production.

Surprisingly, mayonnaise is self-preserving. The acids from vinegar and lemon juice kill most bacteria. Studies have proven that chicken salad is preserved longer by adding mayonnaise, and (get ready for this), store-bought mayo is safe at room temperature indefinitely, even after opening.

Homemade mayo is a special case. Rarely, raw egg yolks (essential to mayonnaise) contain Salmonella, and the only way to assure the killing of Salmonella is through pasteurization. Fortunately, store-bought mayo is pasteurized, so it is not a problem. Instructions for home pasteurization can be found on the internet and in cookbooks. In essence, you heat the egg yolk/vinegar mixture to about 72º Celsius, enough to kill the bacteria but not cook the yolks.

So give mayonnaise a break. It has been unfairly misunderstood for years.

See also: PRACTICING SAFE CHICKEN, a new posting. here's the link:
http://james-channing-shaw.blogspot.ca/2013/03/normal-0-false-false-false-en-us-ja-x.html.