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 Salad juices increased the growth of Salmonella bacteria by 110
percent over normal levels, researchers found.
“Salad leaves pose a particular infection risk because they are
usually minimally processed after harvesting and consumed raw,” said
senior study author Primrose Freestone, a clinical microbiology
lecturer at the University of Leicester.
Researchers are paying more attention to salad produce contamination
after 100 people in the United States contracted Salmonella
infections from bean sprouts in 2014. Salmonella causes 1.4 million
cases of foodborne illness and 400 deaths annually in the United
States, according to the Centers for Disease Control and Prevention.
“Our project does not indicate any increased risk for eating leafy
salads, but it does provide a better understanding of the factors
contributing to food poisoning risks,” Freestone told Reuters Health
by email. “It also highlights the need for continued good practice
in salad leaf production and preparation.”
Freestone and colleagues measured growth of Salmonella enterica, the
strain commonly found in foodborne outbreaks in recent years. They
crushed several salad leaf types - such as spinach, red chard and
red romaine lettuce - to obtain leaf juice.
During a five-day refrigeration period, which is typical storage
time for bagged salad, 100 Salmonella bacteria multiplied to more
than 100,000. Salad leaf juice also enhanced the bacteria’s ability
to attach to the sides of the plastic bags and containers, as well
as to the leaves themselves.
“Most concerning was that we found exposure to the juices released
from the salad leaves appeared to enhance the Salmonella’s capacity
to establish an infection in the consumer,” Freestone said. “Salad
leaves are an important part of a healthy diet but have been
associated in recent years with a growing risk of food poisoning.”
Leafy salads carry a 3 percent risk for food poisoning due to
pathogens such as Salmonella, the study authors write in the Journal
of Applied and Environmental Microbiology. The European Food Safety
Authority has classified leafy green salads as one of the top
sources of foodborne infections, with salmonellosis accounting for
more than 30 percent of outbreaks.
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“Consumers seem to be more preoccupied with nutritional facts, but
they should not forget that foodborne pathogens can be deadly,” said
Kimon Karatzas, an assistant professor of food microbiology at the
University of Reading in the UK who was not involved with the study.
“Avoiding fresh produce is not a solution, but if possible,
consumers should buy fresh uncut produce over chopped.”
Future studies should investigate how Salmonella survives in
different kinds of fresh produce, Karatzas told Reuters Health by
email. His research team is developing disinfectants that eliminate
microorganisms from fresh produce.
“The fact that bacteria growth is enhanced by the presence of
nutrients from a food is not very surprising,” said Martin Adams, a
food microbiology professor at the University of Surrey in the UK
who was not involved with the study. “What did concern me was that
the particular strain of Salmonella was able to grow at 4 degrees
Celsius (39.2 degrees F), or refrigeration temperature.”
Salmonella strains typically don’t grow below 7 degrees Celsius,
Adams said, and the accepted absolute minimum growth temperature is
5 degrees Celsius (41 degrees F).
“It is very important that salad vegetables are washed thoroughly
before consumption, which is good advice that goes back many years,”
Adams told Reuters Health by email. “Although prepared bagged salads
have already been washed, another washing before use would give an
added level of reassurance.”
SOURCE: http://bit.ly/2ggMkjS Journal of Applied and Environmental
Microbiology, online November 18, 2016.
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