Mayonnaise is a common kitchen staple, used as both a condiment and a recipe ingredient. Yet many people still wonder whether it needs refrigeration or can safely be left out. The answer matters, as improper storage can increase the risk of foodborne illness and reduce shelf life. Mayonnaise is an emulsion made from oil, egg yolks, and an acid such as vinegar or lemon juice. The acid helps slow bacterial growth, but it does not eliminate risk completely.

Commercial mayonnaise is produced with pasteurized eggs and often contains preservatives, making it far more stable than homemade versions. Homemade mayonnaise, which usually uses raw egg yolks and no preservatives, spoils much faster and requires extra care. Once opened, store-bought mayonnaise should always be refrigerated.

While it can tolerate brief periods at room temperature—such as during a meal or picnic—it should not be left out for hours, especially in warm conditions. Heat accelerates bacterial growth and increases spoilage risk. Homemade mayonnaise must be refrigerated at all times and is best used within three to five days. Because it lacks preservatives, it is far more vulnerable to contamination.

Despite its reputation, mayonnaise itself is rarely the direct cause of food poisoning. Illness more often results from cross-contamination, such as using the same utensil for raw foods and then returning it to the jar, or leaving mayonnaise exposed to heat for extended periods. Bottom line: refrigerate mayonnaise after opening, keep homemade versions cold, avoid prolonged exposure to warm temperatures, and always use clean utensils. Proper storage ensures both safety and quality.