MSU Extension

It’s summertime, and that often brings to mind images of picnics and summer salads- potato salad, macaroni salad and just plain lettuce salad. For some, it also conjures up not-so-pleasant memories of food poisoning. Summertime incidents of food poisoning are often blamed on salads and casseroles, especially those containing mayonnaise, but is it true that these unassuming foods are the culprit of all our summer ills? Lets take a closer look.

First of all, things like potato salad are often blamed for causing food-borne illness due to the mayonnaise, and the fact it contains eggs. The truth is, mayonnaise (manufactured) does not cause illness, bacteria does, and bacteria just happen to love to grow in many of the foods we combine with mayonnaise- potatoes, pasta, eggs and chicken. Bacteria need six things to grow, and they can be remembered by the acronym FAT-TOM. The F stands for food. Bacteria need protein to grow, just like us, and we’re essentially competing against them for food. Bacteria are trying to eat our food right out from under us. The A is for acidity. Most bacteria do not like an acidic environment, so they tend to grow on foods that have a higher pH, like potatoes, pasta, and meat. The first T is for temperature. Although bacteria can grow in extreme conditions, they grow best between 40 degrees Fahrenheit and 140 degrees Fahrenheit, also known as the temperature danger zone. Keeping foods out of this zone will reduce the likelihood that bacteria are growing on your food. The second T is for time. Bacteria need time to grow, but probably not as much as you imagine. They double their population about every 20 minutes. That means if you started with one bacteria on your plate, you would have two in 20 minutes, over 4000 in four hours, and about 60 billion if left out for 12 hours! The O is for oxygen. This one is tricky because some bacteria need oxygen to grow, but others don’t, like the bacteria that cause botulism poisoning. Finally, the M stands for moisture. Bacteria like a moist or damp environment. If a food item is too dry, they won’t grow there, and that’s why dehydrated foods last a relatively long time.

So, let’s bring it all back to the potato salad at the picnic and see if bacteria have all the things they need to grow and potentially make us sick. Food, lots of proteins and starches to consume. Acidity- while the actual mayo itself is a little on the acidic side, it’s not enough to stop bacteria from growing. Plus all the foods it’s mixed with are low-acid foods that are perfect for bacterial growth. Temperature and time- this one is often a likely culprit of creating a bad situation at your picnic. Leaving your salad or any food in the temperature danger zone for more than two hours is an invitation for bacteria. If the temperature is above 90 degrees Fahrenheit, that time is cut to one hour. Most of the bacteria that would inhabit your salad like oxygen, which there would be plenty of. However, if you’re using home-canned potatoes, botulism toxin, which is produced by bacteria in no-oxygen conditions, can be a concern. And moisture, well there’s plenty of that in most salads, provided by either moist vegetables, mayonnaise, or other dressings. As you can see, if conditions are right, bacteria can grow in any summer salad, not just ones with mayonnaise.

Michigan State University Extension recommends handling summer salads and other foods properly to reduce the risk of bacteria growth and food-borne illness. The simplest thing you can do to keep food safe is to keep it out of the temperature danger zone. Store food in the refrigerator and transport foods in a cooler with plenty of ice. Make sure dishes don’t sit out for more than two hours, less than an hour in high temperatures. Also keep dishes covered so that flies, and other dirt and debris don’t land in the bowl. Properly wash hands before preparing and serving food. At picnics or for camping, make a portable handwashing station to ensure dirt isn’t transferred to food from dirty hands. Following these guidelines will help reduce the opportunity for illness-causing bacteria to grow in your food.