It might seem harmless — a jar of mayonnaise sitting on the kitchen counter for days, just like many grandparents have done for years. But when it comes to food safety, mayonnaise is one item you don’t want to take chances with. Even though store-bought mayo contains preservatives and acidic ingredients like vinegar or lemon juice, leaving it out at room temperature for too long can still create the perfect environment for harmful bacteria to grow.

Once opened, commercial mayonnaise should be refrigerated and kept at or below 40°F (4°C). If it’s left out at room temperature, the general food safety rule applies: it should not sit unrefrigerated for more than 2 hours — or 1 hour if the room temperature is above 90°F (32°C). After that window, bacteria such as Salmonella or Staphylococcus can multiply quickly, even if the mayo still looks and smells normal.

Homemade mayonnaise is even more sensitive. Because it often contains raw eggs and lacks the stabilizers found in store-bought versions, it should never be left out for extended periods and must always be refrigerated immediately. Unlike commercial mayo, homemade versions can spoil faster and carry a higher risk of foodborne illness if not handled properly.

One reason many people think mayo is safe on the counter is because the original unopened jar can be stored at room temperature. But once opened — and especially once a knife or spoon goes in and out of the jar — contamination becomes possible. Every dip introduces new bacteria, and without refrigeration, those microbes multiply much faster.

If mayonnaise has been sitting out for several days, the safest choice is simple: don’t risk it — throw it away. Food poisoning isn’t worth saving a jar, especially for older adults, children, or anyone with a weaker immune system. When it comes to mayo, refrigeration isn’t just a recommendation — it’s protection.

So while Nana’s kitchen habits may come from another time, modern food safety guidelines are clear. After opening, mayonnaise belongs in the fridge — and if it’s been sitting out for days, it’s better to replace it than regret it later.