Food Protection Trends - May/June 2019 - 233

(log CFU/g)

FIGURE 3. Boxplot illustrating the mean L. monocytogenes growth (log CFU/g) in RTE ham at three different domestic
storage temperatures. Recommended refrigeration temperature (2.5°C ± 2.2) (21 days); refrigeration temperature
exceeding recommendations (7.8°C ± 0.4) (21 days); ambient temperature (19.5°C ± 1.2) (10 days).

products can reduce the growth rate of L. monocytogenes
(41). The RTE ham used for the re-enactment contained
sodium nitrite (NaNO2) for preservation purposes. Sodium
nitrite was traditionally used to control Clostridium botulinum
in processed meat products; however, more recently it has
been reported that the growth of L. monocytogenes is also
reduced in the presence of nitrite in meat products (63).
It is reported that NaNO2 in RTE meat products can affect
the detection and recovery of L. monocytogenes as a result of
nitrite-induced injury (60). Furthermore, sodium nitrite has
been determined to reduce the growth rate and increase the
lag time of L. monocytogenes in sliced cooked meat (18), as
nitrite concentration, along with salt content, aw and pH, can
contribute to microbiological stability in meat products (7).
Combined treatment by listericidal antimicrobials can be an
effective tool for control of L. monocytogenes (65).
The pH of RTE ham was decreased at the end of maximum
storage; research suggests that the growth of lactic acid bacteria during refrigerated storage can decrease the pH of cooked
meat products from pH 6.5 to 5.3, although the initial pH of
cooked meat products will not restrict microbial growth (7).
However, this is not the case with L. monocytogenes, as only
food products with pH ≤ 4.4 (or pH ≤ 5.0 if the aw ≤ 0.94)
are reportedly unable to support L. monocytogenes growth
(10) as defined in the Commission Regulation (EC) No
2073/2005 on Microbiological Criteria for Foodstuffs (21).
Additionally, in research that detected pH changes in cooked
meats similar to those found in this study, with a decrease in
pH from 6.9 to 5.9, an increase in the lag time and a reduced
growth rate of L. monocytogenes at 5°C was also observed

(18). In the present study, the lag time was not considered, as
the microbial change over long-term storage was the focus of
the research.
The aw of RTE ham by the end the maximum storage
time increased; however, previous research has reported the
opposite occurring, with aw in cooked meats decreasing from
0.993 to 0.960 (18). Additionally, different bacterial growth
rates under the same environmental conditions can result
from the food matrix (43), which can also have an effect on
growth and inactivation of Listeria spp. (46).
The kinetics of growth of L. monocytogenes are dependent
on storage temperature, packaging and level of inoculum
(34). As discussed by Membré et al., understanding the
effects of different temperatures on growth of L. monocytogenes is important, particularly in the case of post-processing contamination of RTE foods (52), as up to a quarter of
RTE foods have been determined to be contaminated with
L. monocytogenes (32, 33, 53). Because L. monocytogenes is
a psychrotrophic pathogen, it has the ability to grow slowly
at refrigeration temperatures, and if prolonged storage is
provided, the population of L. monocytogenes can reach a
high level, although the food remains unspoiled and appears
acceptable for consumption (8). It is reported that up to 106
CFU/g L. monocytogenes can be present without obvious
signs of spoilage (5) in the foods, which the consumer may
consequently consume.
Given the increased risk of foodborne infection to vulnerable consumer groups such as older adults, there is a need to
effectively communicate through educational interventions
the importance of safe domestic refrigeration temperatures

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