For decades, mayonnaise has been blamed for Salmonella
poisoning in
meat salads and other recipes. To the contrary, University
of Georgia food scientists have found that commercially
prepared mayonnaise
actually reduces the amount of Salmonella in foods.

Salmonella is one of the most frequent causes of U.S. food-
borne illnesses,
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 food scientists, working with their counterparts at the
University
of Wisconsin, studied Salmonella and commercially produced
mayonnaise.

“We (food scientists) knew it was an old wives’ tale that
commercial
mayonnaise causes Salmonella poisoning,” said Michael
Doyle, head of the UGA Center
for Food Safety and Quality Enhancement 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.

“The Salmonella problems in Europeans’ 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 the ingredients are emulsified,” Doyle said, “the final
product’s
pH, water activity and sodium chloride content create a hostile
environment
for harmful bacteria.”

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 mayonnaise, at room temperature for more than an
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.”

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.

“Mayonnaise 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 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 mayonnaise. 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.