Both 2-nitrophenol and 3-fluoroaniline were generated by
L. monocytogenes as a result of b-glucosidase and hippuricase
activity, respectively, indicating that they were potentially useful
markers for L. monocytogenes contaminated milk. The potential for
a rapid method of L. monocytogenes identification was demonstrated
by the time of 9 h that was required to generate both VOCs.
However, the initial inoculum used here was high at 1–1.5 107
CFUml1 of whole milk; this would be significantly higher than
would be found in contaminated food samples. Since L. monocytogenes
liberated both 2-nitrophenol and 3-fluoroaniline in 100%
of inoculated milk samples, any sample where VOCs are not
produced can be considered negative for L. monocytogenes.
However, method sensitivity data indicated that this only applies
at initial inocula of at least 1–1.5 102 CFUml1 after overnight
incubation. Approved culture-based methods for the detection of
L. monocytogenes are highly sensitive; the limit of detection of the
ISO 11290 method is 5–100 CFU per 25 g of food [18,19]. However,
enrichment procedures are necessary prior to isolation via
culturing to allow growth of L. monocytogenes to levels sufficient
to enable detection; 5–6 days may be required to obtain results [5].