Hold the Mayo

A discussion board forum recently dipped into the topic of mayonnaise and food safety. One of the contributors posted, “I always throw stuff out if it has mayo on it and is the slightest bit old.” But should the presence of mayo be the sole determining factor?

Many consumers, knowing that mayonnaise contains eggs, consider it a high-risk for food poisoning. This prejudice has been reinforced for years by the association of potato salad and other mayonnaise-containing dishes with food-borne illness. But there’s more to the story than meets the eye.

Commercial mayonnaise is a very unwelcoming environment for bacteria such as Salmonella, Listeria monocytogenes, Staphylococcus aureus and E. coli. In contrast to many other commercial products, there has never been a food-poisoning outbreak in the United States that was tied directly to commercial mayonnaise.

The safety of commercial mayonnaise rests on its formula, of which the main ingredients are pasteurized egg yolks and acetic acid (vinegar). Acetic acid and its relatives, citric acid and lactic acid, are known inhibitors of bacterial growth. The antibacterial action of these organic acids rests not only with the acidity that they produce in the mayonnaise, which is considerable, but also with their biological properties. Inorganic acids, such as hydrochloric acid or sulfuric acid, are not nearly as effective as the organic acids at preventing the growth of bacteria. Mayonnaise manufacturers take great care to adjust the acidity of their products to a consistent level to ensure its safety and stability.

Several studies of mayonnaise safety have been carried out - some independent, and some funded by food companies. Consistently, these studies have reached the following conclusions:

• Bacterial pathogens such as Salmonella, Listeria monocytogenes and E. coli O157:H7 do not grow in commercial mayonnaise.
• These same pathogens can survive for many weeks if they are introduced into mayonnaise that is then stored in the refrigerator, but die off in just a few days if the mayonnaise is stored at room temperature.

Homemade mayonnaise is a much riskier product. It’s usually made from raw eggs yolks, oil and vinegar or lemon juice. The acidity of homemade mayo varies from cook to cook and from batch to batch. All that’s needed is a single contaminated egg (most likely with Salmonella enteritidis) and a slight reduction in the acidity of the mixture to produce a dangerous situation.

In 1999, researchers in England developed a safer formula for home-made mayonnaise. They used 20-35 mL (4-7 tsp.) of pure lemon juice for each egg yolk. To improve the odds that any pathogens would die off, the researchers also recommended holding the home-made mayonnaise at room temperature (72ºF or 22ºC) for 72 hours (if only 4 tsp. of lemon juice was used) or 48 hours (if 7 tsp. of lemon juice was used) before using or refrigerating the batch of mayonnaise.

Commercial mayonnaise is a very safe product, as long as it’s in the jar. But, when mixed with other food - such as potatoes, vegetables, fish, or other sandwich fixings, it’s acidity is diluted to the point that it will no longer prevent microbes from growing. So handle all foods that contain mayonnaise with kid gloves - but not just because of the mayo.