Food Protection Trends - September/October 2022 - 393

is known to be contaminated (e.g., Listeria in a wall or
coving). It is also recognized, however, that the potential
detection of Listeria from a known source or vector will
have higher incidence than for general environmental
samples. Agreement of the number of samples to be taken
at each potential source or vector to detect the presence of
Listeria is, thus, required.
Stage 3: Determining the efficacy of source and vector
controls. For every source and vector detected, a control
action should be established, e.g., if a crevice between a drain
and the surrounding flooring material has been found to
harbor Listeria, it should be sprayed with an oxidizing biocide
at a suitable time period, e.g., every 8 h. An environmental
sample to assess the disinfectant control action would be
to sample the floor around the crevice 7 h, 55 min after
disinfectant application. If the sample is negative for Listeria,
the 8-h disinfectant application is working. If it is positive, the
application time should be reduced. Again, it is necessary to
be conscious of the number of samples that might need to be
taken to verify that the control is working adequately.
Stage 4: Maximize the detection of Listeria. Once controls
have been ensured to be working for high hygiene barriers,
the control of identified sources and vectors, and end-ofproduction
cleaning and disinfection, the final focus of the
environmental sampling plan is to try to actively detect Listeria
in the high hygiene area. This is undertaken by refocusing
on the choice of collector samples in relation to known or
potential sources and vectors.
Environmental sample numbers
The number of product and environmental samples taken for
Listeria is dependent on a number of factors, primarily defined
by the nature of the RTE food product. RTE products that are
fully pasteurized (e.g., cooked meat products), because of the
nature of this pathogen reduction step, tend to have fewer raw
material, product intermediate, and finished product samples.
More samples may be taken, however, dependent on the degree of
postpasteurizing product processing (e.g., slicing, dicing, freezing)
and the complexity of the process. Conversely, RTE products that
are a mixture of pasteurized and decontaminated ingredients (e.g.,
sandwiches) will have more (risk assessed) raw material, intermediate,
and finished product samples. The number of product lines
or stock-keeping units will also be important, with many retail
customers suggesting finished product sampling frequencies such
as one sample per stock-keeping unit per week.
The number of environmental samples taken for Listeria
will depend on the stage of the environmental journey and the
RTE product. With the adoption of the Listeria 5-point system,
the sample numbers may increase (dotted lines in Fig. 4) as the
hunt for sources and vectors increases and will then decrease
as the plan matures toward a predictive state. Sample numbers
will also be higher in the environments of more complex fully
pasteurized RTE and pasteurized or decontaminated ingredient
RTE product sites.
The likely incidence of L. monocytogenes in the product or
environment also influences sample numbers. For example,
if the typical Listeria isolation rate is 10% of environmental
samples, more than one detection of Listeria in 10 samples
from the same site might be indicative of a contaminated
source or vector. If, however, the isolation rate is typically
1% of all environmental samples, more than one sample
in a hundred taken at the same site might be needed to
identify a contaminated source or vector. In the UK, the
author has observed little change in the estimation of average
L. monocytogenes incidence in finished product since the
suggestion that levels of <1% of samples should be positive
for L. monocytogenes in the early 1990s (14). Best practice
remains approximately 0.1% detection of L. monocytogenes in
fully pasteurized RTE products and 0.2 to 0.3% detection in
products containing decontaminated ingredients. Detection
of L. monocytogenes in the environment is a factor of 1 to 3
times this product level, dependent on whether pasteurized or
decontaminated ingredients are used in the high hygiene area.
In summary, it is not unusual for small- to medium-size RTE
food manufacturers to undertake 100 to 200 product samples
and, similarly, 100 to 200 environmental samples, for L. monocytogenes,
per month. Larger RTE manufacturers may take 300
to 500 product samples and 300 to 500 environmental samples
per month.
The type of sample may also change during the evolution of
the Listeria sampling journey, and the percentages of samples
that should be taken in the investigational and predictive stages
are suggested in Table 2. Whereas the percentages may change,
the actual number of samples may be the same, e.g., for barrier
samples, the percentage has gone from 20 to 30% between the
investigational and predictive stage, but this may be reflective
of the fewer samples taken at the predictive stage.
The endpoint of the environmental sampling journey asks
these questions:
* Are my high hygiene area barriers working-are they
stopping Listeria from getting in? This is particularly
useful for high hygiene areas in which fully pasteurized
RTE products are manufactured.
* Are my potential Listeria source and vector controls, and
cleaning and disinfection programs, working effectively?
* Is there any evidence of Listeria in my high hygiene area?
If no Listeria is found in the collectors, and source, vector,
and equipment cleaning controls are effective, there is a
reasonable probability that Listeria is not present in the
high hygiene area.
Finally, environmental sampling plans should be reviewed
at least annually, usually in conjunction with reviews of
other food safety programs such as HACCP. In addition,
environmental sampling plans should be reviewed if there are
any changes in the following:
* Raw materials (particularly in high hygiene areas where
equipment and environmental areas may harbor any
Listeria related to the raw materials)
September/October Food Protection Trends 393
Food Protection Trends - September/October 2022

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