Quantity of Samples

The number of samples can be tricky and must be based on a risk assessment. One of the key
factors is facility size. Industry recommendations include 30–60 sampling sites per 50,000
square feet. Another consideration is the product risk. If a facility manufactures a ready-to-eat product, then more sampling sites are required than if a facility makes a product that will
be sent to another manufacturer to be further processed.

In addition, age and previous test results can influence the number of samples. If a facility or
equipment is older, then there is a higher chance of cracks and crevices and repairs that have
been damaged with use and time. Wear and tear on equipment and infrastructure will require
additional swabs. A new piece of equipment or a new facility may not require as many swabs.

Sample Collection Instruments

It is important to use a wide variety of tools or sampling instruments to manage the many
different situations and materials in a food processing facility. The old adage, “if you only have
a hammer, then every problem is a nail,” can be used. If only one tool is available for sampling,
then every location cannot be sampled.

Check with third-party testing labs and production/lab equipment suppliers to procure
several types of sampling instruments. Also remember the purpose of the swab and what
types of chemical components the product comprises. Use those factors to determine what
type of swab is needed—e.g., a dry swab, a swab enriched with a broth, a swab with a
chemical neutralizer, etc.

Testing for Microorganisms

A PEM program should test for a mix of indicator organisms and a mix of pathogens based on
product susceptibility and risk. An indicator organism is a mark of overall hygiene conditions.
An indicator organism indicates that an area or a piece of equipment is not cleaned to an
acceptable microbiological condition. It implies that suitable growth conditions exist for a host
of microbiological contaminants, including pathogens. Frequently used indicator organism
tests are coliforms, standard plate count, or Enterobacteriaceae.

As for pathogens, a facility must analyse its product to determine the pathogens of concern
based on the product’s chemical and physical properties. What is the product’s pH, moisture,
and water activity level? At what temperature is the product stored? Common pathogens that
are routinely tested for in a PEM program are Listeria, Salmonella, Cronobacter,
Staphylococcus species, etc. A strong PEM program will test for a multitude of pathogens at
some frequency, rather than only one pathogen. For example, if the product is wet, Listeria is
a common pathogen of concern. Ten months of the year, the facility will test swabs for Listeria.

To verify that Salmonella is also not present, twice a year the facility will send
swabs for Salmonella testing, as well. If a facility processes flour products, then the pathogens
of concern are Salmonella and possibly Escherichia coli. An infant formula plant will test
for Cronobacter and Salmonella. The pathogens being tested for in the PEM program must
match the pathogens of concern for the product(s) manufactured and processed at the

Different areas of the plant may require different pathogen testing. For example, a dry facility
may test for Salmonella routinely in the processing area. If the facility has a wet area,
then Listeria tests will be performed in that area. As stated previously, an effective PEM
program looks at the risk level of the facility and the product(s) to determine the appropriate
indicator organism and the appropriate pathogen testing.


Any presence of a pathogen is cause for concern and is considered out of specification. For
an indicator organism, the value that is considered out of specification depends on the
sampled zone.

If the results show a positive or out-of-specification indicator organism, then a root-cause
analysis must be performed according to the corrective and preventive action program. To
find the root cause of the issue, a facility must conduct a series of vector swabs. Vectoring
starts with the positive sample location and fans out looking for the root cause of the issue.
For example, if the positive test result is the floor surface at the bottom of a set of stairs, then
the floor is probably not the root cause. However, the equipment near that area of the floor
could be harboring the microbiological contaminant. A trained analyst must start at the
positive location and determine likely locations that could be causing the contamination. The
analyst will need to swab a series of locations to find the actual location that is allowing the
microorganism to proliferate. Think of the floor as the center and start fanning out to find the
actual source.

Even if the facility does not have any positive results, do not ignore the negatives—trend the
data. Many commercially available software packages are designed to help trend
environmental results. An Excel spreadsheet also can be used to trend data. The important
thing is to look at sampling sites and make decisions. PEM programs are expensive and should
be part of a continuous improvement program. If a sampling location is routinely negative,
then the testing frequency may be able to be reduced on that site, and areas that require
more scrutiny can be given greater attention and expense. For example, if a site is on the
weekly list but has not been out of specification for the past six months, then the sampling
frequency for that site could be moved to monthly, and another location that needs more
attention can be given more time and testing.


PEM programs are a key prerequisite program to a sanitation program and to any facility’s
overall food safety program. Regulators, customers, and third-party auditors all require some
sort of PEM program. There is no one-size-fits-all PEM program for facilities; rather, a PEM
program is based on a facility’s risk factors and what product(s) the facility manufactures.
Good documentation and records are important to prove to customers and regulators that
the PEM program is functioning and actively looking for microbiological contaminants in the

Results from a PEM program should be used to make data-based decisions. The key to any
good program is to catch the microbiological contaminant in the environment before product
is impacted. Do not be afraid to routinely sample random sites, and ensure that analysts are
trained to look for any potential problem areas. Getting a positive is not necessarily bad, and
is usually expected. If a facility never has a positive test, then the PEM program is likely not
effective and needs additional scrutiny. Good luck, and find those microbiological