Can Mayo Last Unrefrigerated After Being Opened?


Mayonnaise often gets the blame when picnic-goers develop upset stomachs or other gastrointestinal discomforts. However, this long-held belief is a myth, according to the U.S. Department of Health and Human Services food safety website. Unless mayonnaise sits at room temperature for an extended time, it rarely harbors bacterial that causes foodborne illness.

Commercial Mayonnaise

  • Mayonnaise manufacturers must follow strict safety requirements. Most importantly, commercial mayonnaise contains high-acid ingredients such as vinegar and lemon juice that create an unfriendly environment for growth of bacteria. Additionally, eggs used in commercial mayonnaise are pasteurized, a process that uses heat to kill bacteria and viruses.

Storing Mayonnaise

  • The U.S. Department of Health and Human Services food safety website reports that mayonnaise is safe at room temperature for a short period of time, but the mayonnaise should be discarded if it is left at temperatures of 50 degrees Fahrenheit for more than eight hours. Prompt refrigeration ensures that mayonnaise retains its texture and flavor. The University of Wisconsin Extension Nutrition Education Program reports that an open, refrigerated jar of mayonnaise retains its quality for up to six months, as long as the refrigerator cools at a safe food storage temperature between 32 and 40 F.

Homemade Mayonnaise

  • All mayonnaise is not created equal, and homemade mayonnaise should never remain at room temperature for any length of time. Unlike commercial mayonnaise, homemade mayonnaise is usually made with raw eggs, which are not pasteurized. Unpasteurized eggs are sometimes contaminated by salmonella, a dangerous bacteria that causes symptoms such as abdominal cramps and diarrhea. Salmonella is especially dangerous for infants, the elderly, pregnant women and people with weakened immune systems. Homemade mayonnaise doesn't always contain enough high-acid lemon juice or vinegar to kill harmful bacteria.

Mayonaisse and Other Foods

  • Although mayonnaise is safe to eat, adding mayonnaise to other foods does not protect those foods from developing dangerous bacteria. For example, potato salad made with mayonnaise and eggs should never be left at room temperature for more than two hours, and no more than one hour on a hot day. The same holds true for foods that contain meat or dairy products, which develop bacteria quickly at temperatures between 40 and 140 F. As a precaution, keep bowls of salad that contain mayonnaise and other ingredients on ice.

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