Spoiled mayonnaise typically gives itself away through changes in smell, color, texture, or taste. Because mayo is an emulsion of oil, egg, and acid, any breakdown in that mixture produces noticeable signs you can catch before taking a bite. Here’s what to look for and how long you can expect an open jar to last.
The Four Clearest Signs of Spoiled Mayo
Start with your nose. Fresh mayonnaise has a mild, slightly tangy smell. Spoiled mayo develops an acidic or putrid odor that’s been compared to cheese left out in the sun. If you open the jar and get hit with anything sharp or sour beyond the normal vinegar tang, that’s your answer.
Next, look at the surface. Fresh commercial mayo is uniformly white or off-white. Any darkening or yellowing means the oils are breaking down. Actual mold, even a small spot, means the entire jar should go in the trash, not just the moldy portion. Mold sends invisible threads deep into soft foods like mayo, so scooping off the top isn’t safe.
Texture is the third check. A thin layer of separated liquid on top of mayo can happen naturally, especially if the jar has been sitting for a while, and a quick stir usually fixes it. But if the mayo looks watery throughout, curdled, or lumpy, that separation has gone too far and the emulsion has broken down. At that point, toss it.
Finally, if everything looks and smells fine but you’re still unsure, a tiny taste will tell you. Rancid mayo has a distinctly stale, bitter, or “off” flavor that’s nothing like the creamy mildness you’d expect. Trust your instincts on this one.
How Long Opened Mayo Lasts
The USDA recommends using opened commercial mayonnaise within 2 months of refrigerating it. That timeline assumes you’ve kept it at or below 40°F and haven’t left it sitting on the counter for extended periods. Most commercial brands are shelf-stable before opening because they’re made with pasteurized eggs and have a low pH (high acidity) that inhibits bacterial growth. Once you break the seal, though, you introduce air, moisture, and bacteria from utensils every time you dip a knife in.
For unopened jars, the printed date on the label is your guide. A best-by date offers some flexibility, roughly a week past that date is a reasonable window. An expiration date is stricter. If the jar carries an actual expiration date and it’s passed, discard it regardless of how it looks or smells.
Homemade Mayo Spoils Much Faster
Homemade mayonnaise uses raw egg yolks, which carry a slight risk of salmonella and lack the preservatives found in store-bought versions. The USDA says homemade mayo made with pasteurized eggs can be refrigerated for up to 4 days in a covered container. If you used regular (unpasteurized) eggs, the window is even shorter.
Some home cooks report keeping homemade mayo made with pasteurized eggs for a month or longer, and food scientist Harold McGee has noted that the vinegar or lemon juice in the recipe does provide some protection. But homemade batches don’t go through the same rigorous pH testing that commercial products do. Commercial mayonnaise is manufactured at a target pH of 4.4 or below, a level where the acetic acid from vinegar actively destroys common foodborne bacteria. Your kitchen batch may or may not hit that mark, so erring on the side of using it quickly is the safer bet.
Why Commercial Mayo Is Safer Than You’d Think
Mayo has a reputation as a picnic hazard, but commercially produced mayonnaise is actually one of the more hostile environments for bacteria. The combination of vinegar, low pH, and pasteurized eggs makes it difficult for pathogens to survive. Research published in the Journal of Food Protection found that pH adjusted with acetic acid (vinegar) is the single most important factor in destroying harmful bacteria in mayonnaise and similar dressings.
The real risk at a picnic isn’t the mayo itself. It’s the foods mayo gets mixed into, like chicken salad or potato salad, where other ingredients raise the pH and provide nutrients that bacteria thrive on. Those mixed dishes need to stay cold far more urgently than a sealed jar of Hellmann’s does.
Storage Tips That Extend Shelf Life
Keep your jar in the main body of the refrigerator, not the door. The door is the warmest spot and experiences the most temperature fluctuation every time you open the fridge. Use clean utensils each time you scoop from the jar. Double-dipping with a knife that touched bread, meat, or other foods introduces new bacteria and speeds up spoilage.
If you buy mayo in squeeze bottles rather than jars, you’ll generally get a longer usable life because the bottle limits air exposure and cross-contamination. There’s no spoon or knife going in, so fewer opportunities for outside bacteria to hitch a ride. Either way, once you notice any of the signs above, no amount of refrigeration will reverse the process. When in doubt, a fresh jar costs less than a bout of food poisoning.