For years, many of us grew up watching grandparents leave mayonnaise on the kitchen counter without a second thought. To some, it looks risky and even dangerous. But the truth about mayonnaise storage is far less alarming than most people believe, and it depends on one critical detail that almost no one pays attention to.

Store-bought mayonnaise is not the same as homemade mayo. Commercial mayonnaise is made with pasteurized eggs, vinegar, and oil, creating a highly acidic environment. That acidity is the key. It prevents harmful bacteria from growing quickly, which is why unopened jars are shelf-stable for long periods. Once opened, the mayo does not instantly become unsafe at room temperature.

In normal indoor conditions, commercially produced mayonnaise can safely sit out for several hours, even up to a day, without becoming dangerous. Leaving it out for a few days, however, is not recommended. While it may not immediately cause illness, heat, air exposure, and contamination from utensils increase the risk over time and degrade quality fast.

Homemade mayonnaise is a completely different story. Because it uses raw eggs and lacks preservatives, it should never be left out. Homemade mayo must be refrigerated at all times and used within a short window, usually just a few days.

Another factor people overlook is contamination. Double-dipping knives, crumbs, or spreading mayo on warm food introduces bacteria that speed up spoilage. Even refrigerated mayonnaise can go bad faster if it’s handled carelessly.

So why did older generations seem unaffected? Short exposure, cooler kitchens, clean utensils, and strong acidity did most of the work. Still, modern food safety standards are stricter for a reason.

The safest rule is simple. Store-bought mayonnaise can sit out briefly, but it belongs in the fridge once opened. If it’s been on the counter for days, trust your instinct and toss it. When it comes to food safety, caution beats nostalgia every time.