The Association of Dressing and Sauces, representing a large number of major US companies, says “more than 60 years of research has proven that commercially prepared mayonnaise does not cause foodbourne illness.” Strict standards are used in creating commercial mayonnaise and mayonnaise-type salad dressings using pasteurized eggs and a careful balance vinegar, lemon juice and salt, which slows and even stops the growth of bacteria. Foods added to mayonnaise, like chicken, ham or potatoes, are the cause of bacterial growth in dishes. See www.dressings-sauces.org/mayonnaise.html .
Page 2 of the pamphlet “Make Mine Mayonnaise,” located on this website, states in the Q&A section that commercial mayonnaise and mayonnaise-type dressings are safe to store at room temperature after they are opened. Labels suggest refrigerating these foods after opening only to ensure high quality and freshness.
This means that in an emergency without electricity, continued use of commercial mayonnaise and mayonnaise-type dressings is a safe possibility. If you decide to do this, consider purchasing these products packaged in squeeze bottles for extra safety, minimizing introduction of bacteria into the contents as they are used. Store these products in the coolest, darkest place possible.