The main ingredients in mayonnaise are eggs, oil and water. The presence of eggs is usually a sure sign a food is perishable and needs to be refrigerated. This is not the case with store-bought mayonnaise. If you rush a jar of mayonnaise back to the refrigerator once the seal is broken, you should know you don't need to rush it back. The mayonnaise will not spoil if you leave it on the counter. Homemade mayonnaise is another story.
Mayonnaise has gotten a bad rap over the years, but the rumors surrounding this tasty dressing are simply not true. Mayonnaise is best when refrigerated after opening, but the reasons for refrigeration have nothing to do with bacteria or food poisoning. The mayonnaise from the supermarket undergoes strict quality tests. Also, using mayonnaise in a sandwich does not cause the sandwich to spoil more quickly. If anything, mayonnaise slows the growth of the bacteria associated with food-borne illnesses.
Store-bought mayonnaise is made with pasteurized eggs and contains lemon juice and vinegar -- both acids that make the mayonnaise an unfavorable place for bacteria to grow. Refrigerating commercial mayonnaise after opening has more to do with quality and less to do with spoilage. If you leave commercial mayonnaise unrefrigerated, taste suffers and out in the warm sun, it turns brown. A refrigerator set at 40 degrees Fahrenheit maintains the quality of an open jar of mayonnaise for up to two months.
If you make your own mayonnaise, you do need to refrigerate it. Unlike commercially made mayonnaise, homemade versions often lack the amount of acid and salt necessary for preservation. In regards to homemade mayonnaise, refrigeration is just as important for preventing bacterial growth as it is for maintaining quality. A fresh batch of homemade mayonnaise will last approximately five days in the refrigerator. Once the five-day period is up, throw the mayonnaise out.
Just as temperatures above 40 F affect the taste of mayonnaise and sometimes cause it to turn brown once you open the jar, freezing temperatures affect the product. Temperatures of 0 F or lower change the texture, thickness and the consistency of the mayonnaise. Because it affects the quality of the mayonnaise, you shouldn't freeze it, even for a short time.