Food Protection Trends - May/June 2019 - 231

(log CFU/g)
FIGURE 2. Mean L. monocytogenes growth (log CFU/g) in soft cheese at three different domestic storage
temperatures. Recommended refrigeration temperature (2.5°C ± 2.2) (continuous line); refrigeration temperature
exceeding recommendations (7.8°C ± 0.4) (broken line); ambient temperature (19.5°C ± 1.2) (dotted line).

2.7 log CFU/g when stored at 2.5°C. Maximum populations
achieved in soft cheese stored at 7.8°C and 19.5°C were not
achieved in soft cheese stored at 2.5°C. Data presented in Fig.
2 indicate that no L. monocytogenes was isolated on day zero
following inoculation. Very little difference is observed in
populations of L. monocytogenes from day seven on in the soft
cheese, whether stored at 7.8°C or 19.5°C; this suggests that
maximum population densities that have been reached varied
according to storage temperature.
A two-way between-groups analysis of variance was conducted to explore the impact of storage temperature and
the type of food products on the population increase of
L. monocytogenes (log CFU/g) per day. Both variables were
determined to have a statistically significant interaction effect
on L. monocytogenes populations (F (2, 97) = 4.602, P = 0.012).
A statistically significant difference (P < 0.001) was observed for L. monocytogenes populations at the three different
temperatures (F (2, 97) = 96.99, P = 0.000). The effect size
(partial eta squared = 0.67) indicated a large effect in the
strength of the association of temperature on population size.
Post-hoc comparison using the Tukey HSD test indicated
that the mean log CFU/g L. monocytogenes was significantly
greater at higher temperatures, as indicated in Table 3. The
distribution of mean L. monocytogenes populations according
to storage temperature is illustrated in Fig. 3.

Furthermore, a significant difference was observed for
L. monocytogenes between the two food products (F (1, 97) =
32.430, P = 0.000) with greater mean values for L. monocytogenes in soft cheese than in RTE ham, as indicated in Table 3.
Growth rates of L. monocytogenes in food products
stored at different domestic storage temperatures
As indicated in Table 4, the growth rate (µ) was greatest
in RTE ham stored at 19.5°C. In both products, the growth
rates at temperatures exceeding recommendations (7.8°C and
19.5°C) were greater than those at 2.5°C. The greater the storage
temperature, the greater the growth rate of L. monocytogenes in
both foods. The generation time of L. monocytogenes at 2.5°C
was 1.3 - 2.5 days (58.9 hours t-1 for RTE ham and 32.1 hours
t-1 for soft cheese) whereas at 7.8°C, generation times were
less than one day (23.7 - 22.0 hours t-1). The generation time
of L. monocytogenes at 19.5°C was 11 hours t-1. The higher the
temperature, the shorter the generation time.
DISCUSSION
As seen in previous research (34), a period of inactivation
(lag phase) was observed at the start of the re-enactment
sampling following initial inoculation of food products with
L. monocytogenes. Survival and growth of L. monocytogenes
was determined in both food products (soft cheese and

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