commercial test kits, are based on the detec-
tion of the production of an enzyme (glu-
curonidase) through E. coli(Russell, 2003).
Cell Mass
The quantifying of cell mass has been
used to estimate microbial populations in
certain research applications but is not often
employed for routine analysis because it can
be more time-consuming and less practical
than other methods. The fluid that is meas-
ured is centrifuged to pack the cells, with
subsequent decanting and discarding of the
supernatant, or it may be filtered through a
bored asbestos or cellulose membrane, which
is then weighed.
Turbidity
Turbidity is an arbitrary determinant of
the number of microorganisms in a liquid.
This technique lacks utility and is rarely used
because the food particles in suspension con-
tribute to turbidity and inaccurate results.
Radiometric Method
In this technique, a sample is introduced
into a medium containing a^14 C-labeled sub-
strate, such as glucose. The amount of^14 CO 2
produced is measured and related to micro-
bial load. Because some microorganisms will
not metabolize glucose,^14 C-glutamate, and
(^14) C-formate media have been devised. This
technique is limited to applications where
data acquisition is required within 8 hours
and/or technician labor must be reduced.
Impedance Measurement
Impedance measurements determine the
microbial load of a sample by monitoring
microbial metabolism rather than biomass.
Impedance is the total electrical resistance to
the flow of an alternating current being
passed through a given medium. Microbial
colonies on media produce changes in
impedance that can be measured by the con-
tinuous passage of a small electrical current
in as soon as 1 hour. This technique offers
potential as a rapid method of determining
microbial load. Previous research has revealed
a correlation of 0.96 between impedance-
detecting time and bacterial counts. Imped-
ance may be used to enumerate aerobic plate
count (APC) coliforms,E. coli, psychrotrophs,
andSalmonellaorganisms to predict shelf life
and to do sterility testing.
Endotoxin Detection
The Limulus Amoebocyte Lysate (LAL)
assay is for the detection of endotoxins pro-
duced by gram-negative bacteria (including
psychrotrophs and coliforms). Amoebocyte
lysate from the blood of the horseshoe crab
forms a gel in the presence of minute
amounts of endotoxin. Due to heat stability,
both viable and nonviable bacteria are
detected, making the test useful in tracing the
history of the food supply. The LAL assay
involves placing a sample into a prepared
tube of lysate reagent, incubating 1 hour at
37ºC, and evaluating the degree of gelation.
Bioluminescence
This biochemical method, which has been
simplified for easy use, measures the pres-
ence of adenosine triphosphate (ATP) by its
reaction with the luciferin-luciferase com-
plex. It can be incorporated in the estimation
of microbial load of a food sample. The bio-
luminescent reaction requires ATP, luciferin,
and firefly luciferase—an enzyme that pro-
duces light in the tail of the firefly. During
the reaction, luciferin is oxidized and emits
light. A luminometer measures the light pro-
duced, which is proportional to the amount
of ATP present in the sample. The ATP con-
tent of the sample can be correlated with the
number of microorganisms present because
all microbial cells have a specific amount of