technology may appear to be less expensive
than bioluminescence, a cost-analysis study
demonstrated that rapid microbial testing
offers a savings of approximately 40% over
traditional testing methods (Le Coque,
1996/1997). A limitation to this test is that
cleaning compound residues can quench the
light reaction to prevent proper response
from the assay system. Many commercial
bioluminescence detection kits contain neu-
tralizers to combat the effect of
detergents/sanitizers. ATP bioluminescence
is ineffective in powder plants when milk
powder or flour residues exist. Furthermore,
organisms that are naturally luminescent
exist in seafood plants. This increases the
incidence of false-positive results on the sur-
faces tested. Furthermore, yeasts have up to
20 times as much ATP as bacteria, to com-
plicate enumeration. A major advantage of
this test is that ATP from tissue exudates can
be detected, whereas other tests do not offer
this feature. Furthermore, this test identifies
dirty equipment.
Research has been conducted on increas-
ing the sensitivity of bioluminescence reac-
tions through identification of the adenylate
kinase enzyme that produces ATP. This
approach permits the counting of lower
numbers of microorganisms present.
Valdivieso-Garcia et al. (2003) determined
that a bioluminescent enzyme immunoassay
(BEIA), using salmonella-specific mono-
clonal antibody M183 for capture and
biotinylated monoclonal antibody M183 for
detection, offers another alternative for the
detection of salmonella, with the additional
advantage of providing a 24-hour test for
detecting salmonella in chicken carcass
rinses. However, these scientists indicated
that tests are still needed for the isolation
and detection that will establish the true
prevalence ofSalmonellain chicken samples.
Catalase
This enzyme may be found in foods and
aerobic bacteria. Because catalase activity
increases with the bacterial population, its
measurement can estimate bacterial load. A
Catalasemeter utilizes the disc flotation prin-
ciple to quantitatively measure catalase
activity in foods and can detect 10,000 bacte-
ria/mL within minutes. This unit, which
incorporates the biochemical method of
detection and enumeration, may be used as
an on-line monitoring device to detect con-
tamination problems in raw materials and
finished products, to control vegetable
blanching and milk quality, and to detect
subclinical mastitis in cows. The catalase test
is applicable to fluid products.Direct Epifluorescence Filter
Technique (DEFT)
This biophysical technique is a rapid,
direct method for counting microorganisms
in a sample. This method was developed in
England to monitor milk samples and has
been applied to other foods, even though it is
not used routinely in the food industry. Both
membrane filtration and epifluorescence
microscopy are used with this technique.
Microorganisms are captured from a sample
on a polycarbonate membrane. The cells are
stained with acridine orange, which causes
the viable bacteria to fluoresce orange and
the dead bacteria to fluoresce green under
the blue portion of the ultraviolet spectrum.
The fluorescing bacteria are counted, using
an epifluorescence microscope, which illumi-
nates the sample with incident light.
This technique has been used to evaluate
dairy and muscle foods, beverages, water,
and wastewater. The keeping quality of pas-
teurized milk stored at 5ºC and 11ºC can be
predicted within 24 hours by preincubating