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Where Listeria can hide
Amanda Deering, Purdue professor in the Department of Food Sciences, researches the ability of foodborne pathogens to grow and remain on or in plant tissue such as romaine lettuce. (Photo provided.)
24 | PREPARED FOOD - August 2017
ou can wash all you like but
disease two months after they consumed
you can’t get rid of all of
the contaminated product. Getting people
the foodborne pathogens on
to remember what they ate last week is
your lettuce leaves because
problematic — getting them to reliably re-
some hide within the plant
member what they ate two months ago is
tissue.
extremely difficult.
Conventional post-harvest sanitisation
In the US, romaine lettuce is the fastest
practices might not be sufficient to kill the
growing crop in terms of production, export
potentially lethal pathogen because, accord-
and consumption, and Deering, Haley Oliver,
ing to a Purdue University study, Listeria
associate professor of food science, and
monocytogenes can live inside the tissue of
Archana Shenoy, a graduate research as-
romaine lettuce.
sistant, began to investigate the persistence
Amanda Deering, clinical assistant profes-
and internalisation of L. monocytogenes in the
sor in the Department of Food Science, and
lettuce. The researchers found that exposing
her team have established that the bacteria
lettuce to the bacteria could lead to infection
can live within lettuce in every stage of the
of plant tissue with the bacterium able to
plant growth process.
gain entry into the plant through cracked
When ingested, L. monocytogenes can be
seed coats, small tears in root tissue during
deadly to those with vulnerable immune sys-
germination and damaged plant tissue. Only
tems, including pregnant women, the elderly,
30 minutes’ exposure was needed for the
infants or those with HIV. L. monocytogenes
bacteria to enter the plant.
can also cross the placental barrier in
Their research showed L. monocytogenes
pregnant women, triggering a miscarriage.
can persist up to 60 days or until the time
L. monocytogenes is killed by heat but
of harvest in romaine lettuce. They are now
this kill-step is not possible with minimally
working on detection technologies as they
processed produce that is consumed raw. In
shift their focus to what can happen to the
2011 31 people in the US died from listeriosis
seed and seedlings before planting. They
after the outside of raw rockmelons was
aim to find pre-harvest control strategies to
contaminated with L. monocytogenes.
prevent produce contamination, particularly as
The source of listeriosis can be difficult
sanitisers can only treat produce externally.
to establish as people may only develop the
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