Gorgonzola, one of the most importa

Gorgonzola, one of the most important Italian cheeses, awarded
with the “Protected Denomination of Origin” (PDO) from the European
Commission, represents an optimal
food substrate for the growth of Listeria monocytogenes. Gorgonzola is a blue-veined, mouldripened
cheese, made with pasteurised cow's milk and characterised
by a final pH ranging from 4.5 to 6.5 and a sodium chloride
content of 2.3-7%. During ripening, biochemical changes due to the
proteolytic activity of Penicillium roqueforti, that develops as an
internal blue-green mould, cause an increase in pH, influencing the
microflora's evolution and favouring possible contamination by
ubiquitous dairy microorganisms and food-borne pathogens,
among which L. monocytogenes has raised particular concern. In 2006 Gianfranceschi et al.
described the first case of listeriosis associated with the consumption
of contaminated Gorgonzola cheese in Italy. Although the
pasteurization of milk inactivates the pathogen, it can enter the
product after contact with the environment during ripening. Contamination during processing principally
depends on the ripening and storing conditions of the cheeses, and
together with the characteristics of the L. monocytogenes and of the
product, these conditions can make pathogen survival and multiplication
possible. This contamination, if present,
consists of a small number of cells and is limited to the rind, which has been declared not edible by the Consortium
for the Protection of Gorgonzola Cheese. However,
L. monocytogenes can still be a threat to consumer health because
contamination can be transferred to the internal paste during
slicing. In fact, it is know how the bacterial transfer from surfaces to food or food to food, called “cross-contamination”, can occurs via
hands or hands contact surfaces as well as equipment like cutting
boards or knifes. Different studies on pathogen crosscontamination
of foods have been documented and, in general, it is estimated that
cross-contamination is involved in 39% of food-borne outbreaks.
Epidemiologic data have indicated that the foods involved in
listeriosis outbreaks showed a contamination significantly greater
than 100 cfu g
1. For this reason, Commission
Regulation (EC) No. 2073/2005 established, as a food safety
criteria for tolerable levels of L. monocytogenes in ready-to-eat
(RTE) foods, a maximum level of 100 cfu g
1
. This limit is allowed
at the end of production on the condition that the pathogen does
not grow during the shelf life and exceed this level at consumption.
If this criterion is not met, L. monocytogenes must not be present in
25 g samples. When the behaviour of the pathogen in a product
under particular conditions is not known, the acquisition is recommended
of experimental data concerning the implicated product. Microbiological challenge testing
(MCT), which evaluates whether an inoculated organism can grow
and at which point the growth reaches unacceptable levels in a
specific product, has become an
important tool to document the behaviour of L. monocytogenes in
RTE foods. Since the publication, in 2008, of the EURL Lm Technical
Guidance document for conducting shelf-life studies on
L. monocytogenes in ready-to-eat foods to the guidance document
of 2004, many studies have applied
challenge test to determine the ability of different foodstuffs to
support L. monocytogenes growth.
The aim of this research was to investigate whether cutting
could be responsible for the contamination of Gorgonzola cheese
slices with L. monocytogenes. Moreover, we wanted to verify
whether, in case of positive dragging, L. monocytogenes was also
able to grow in cut slices, exceeding the limit imposed by
Commission Regulation (EC) No. 2073/2005 and thus posing a risk
to the consumer during storing.
For these purposes, two different MCTs were applied. The first
MCT simulated Gorgonzola cheese cutting procedures to study the
possible transfer of L. monocytogenes from the rind, at different
levels of contamination, to the cheese paste. The second MCT
examined the behaviour of L. monocytogenes transferred to cut
slices over the shelf life of the cheese for 55 days of storage at 4 C.