Most store-bought mayonnaise is safer than many assume, but only if it’s handled properly.
Made with pasteurized eggs and acidic ingredients, it’s formulated to resist bacterial growth—though that protection has its limits. Once opened, each use of a spoon or knife can introduce new microbes. If the jar is left out for more than two hours, particularly in a warm kitchen, those microbes can multiply to unsafe levels. Keeping it consistently below 40°F (4°C) is key to slowing that process.
Homemade mayonnaise requires even greater care. Without preservatives and often made with raw or lightly cooked eggs, it’s far more susceptible to contamination and should be refrigerated immediately and consumed within a few days.
Always use clean utensils, seal the container tightly, avoid temperature fluctuations, and if there’s any doubt about smell, texture, or color, discard it. No sandwich is worth the risk of food poisoning.