Two Million Salmonella Cases a Year

Salmonella is one of the most frequent causes of U.S. food-borne infections, with an estimated 2 million cases reported each year. As with other food-borne infections, Salmonella can cause a range of illnesses including severe diarrhea, nausea, fever, fatigue and dehydration.

UGA Center for Food Safety (CFS) food scientists, working with their counterparts at the University of Wisconsin, studied Salmonella and commercially produced mayonnaise.

Shattering an 'Old Wives Tale'

"We (food scientists) knew it was an old wives' tale that commercial mayonnaise causes Salmonella poisoning," said Michael Doyle, head of the UGA CFS in Griffin.

"But the industry needed scientific proof to back up their product," he said. "Cases of Salmonella poisoning linked to mayonnaise most often occur in Europe where homemade mayonnaise is commonly used."

Europeans often make homemade mayonnaise using eggs and oil, but not enough vinegar. The eggs are unpasteurized, and the mayonnaise lacks the important acid content that vinegar provides

Europeans: Blame the Eggs

"The Salmonella problems in Europe's homemade mayonnaise can be traced to unpasteurized eggs," Doyle said. "Also, they don't add enough vinegar because they don't like the taste."

The U.S. Food and Drug Administration regulates the amount of acid mayonnaise makers must add to their products. This acid comes from the vinegar and lemon or lime juice.

"Once these ingredients are emulsified," Doyle said, "the final product's pH, water activity and sodium chloride content create a hostile environment for harmful bacteria."

Mayonnaise Kills Salmonella Cells

Salmonella can't survive in commercial mayonnaise, Doyle said. "Once mayonnaise is blended with other foods, like the ingredients for ham or chicken salad, the meat is perishable, and that's where the bacteria grow," said Doyle.

In lab tests, the food scientists added mayonnaise to foods inoculated with Salmonella. The number of Salmonella cells declined immediately after the bacteria was added to either chicken or ham salad that contained commercial mayonnaise.

Refrigerating the salads kept Salmonella from growing, too. Neither the ham nor the chicken salad had increased numbers of Salmonella cells up to 24 hours after refrigeration.

The meat salads were also tested at room temperature. After five hours, both showed "relatively little growth" of Salmonella cells.

"However, it is best not to hold perishable foods, even those that contain commercial mayonaise, at room temperature for more than a hour," Doyle said.

Further tests showed that Salmonella growth slows as the amount of mayonnaise is increased.

"Overall, our research has shown that mayonnaise helps slow the growth of Salmonella in most meats and poultry," Doyle said. "Mayonnaise reduces the rate at which these bacteria can grow."

Low-Cal Works Too

The UGA researchers also studied reduced-calorie mayonnaise, which Doyle said contains more water and less vinegar and oil. They found it also slows Salmonella growth, but not as much as regular mayonnaise.

Despite these findings, consumers should still refrigerate meat salads made with mayonnaise, said UGA Extension food scientist Judy Harrison.

"Mayonaise will not maintain its acidity level very well over time when mixed with other less acid foods like meats, poultry, eggs or potatoes," she said. "Bacteria can begin to multiply if these foods are allowed to remain between 40 and 140 degrees F. Always keep salads such as these at refrigerator temperature."

For safety and the best quality, Harrison said, refrigerate the mayonnaise, too.

"The more times you open the jar and remove some of the product," she said, "the more chances there are for moisture, food particles or mold spores to enter the mayonaise. This could cause changes in the mayonnaise itself, especially at room temperature."

Using a clean knife or spoon each time, she said, will make food particles less likely to get into the jar.

(Article by Sharon Omahen, University of Georgia College of Agricultural and Environmental Sciences. Egg photograph courtesy of Ohio State University.)