TESTING
By Margaret D. Hardin, Ph. D.
Microbial Detection –
Taking It to the Limits
As the food industry increases the de- mand for more and more testing, whether it is for customers, regulators or
for self-monitoring, the concept of detection becomes more and more important. Testing often includes food product
testing and environmental monitoring for pathogens, indicators and/
or spoilage organisms. During a recent visit to a small processor, the
owner of the company asked several questions about a laboratory
report they had just received from their third-party testing laboratory.
The questions included the following: What does less than 10 coliforms mean
and why is it less than 10 coliforms in one sample and less than three coliforms in
another? What is the difference between a result of less than 10 and a negative result? Why are some results (for pathogens) negative in 25 g and other results negative in 375 g? The concerns this processor expressed all lead back to the products
(matrix) being tested, how much product was in the sample ( 10, 25 or 375 g), how
many organisms were present in the sample, as well as what detection method was
used, what was it detecting and how much could it detect? For the sake of discussion here, microbial detection will be defined as the ability to recover and discover
or determine the existence or presence of microorganisms.
Detection Methodology
Since microorganisms are so very small, the detection of their presence in a
food sample is challenging at best, even when present at levels in the thousands to
millions of cells per gram or milliliter of a food product or rinsate or on an equip-
ment surface. The detection and quantification of microorganisms in food and
food processing begins with the ability of the method to move (or remove) the mi-
crobes from the food (or surface) to the detection system. The methods by which
microorganisms are removed or detached from a food or surface are quite varied.
Taking a sample from a food product or surface may be accomplished by swabbing
or sponging a food or food equipment
surface, excising a thin layer of food
product surface or rinsing the food
product or piece of equipment in or
with a buffered diluent. Frequently used
methods for removing (detaching) mi-
crobes from food product or swab sam-
ples include blending, stomaching (or
macerating) and rinsing. Whichever
method is used for sampling and detach-
ing microorganisms, it is important to
remember that this generally represents
only a fraction of the actual level pres-
ent. Besides the ability of any method to
fully detach or remove microbes from a
sample, additional questions arise as to
what happens to the organisms as the
sample is shaken, stomached or
blended. Do the microbes reattach to
other portions of the food sample or are
they driven more deeply into the sample
or into a sampling sponge or swab dur-
ing this mechanical action of shaking,
blending or stomaching?
