3.4. Microbial enumeration at TTD in food matrices
In impedance microbiology, the TTD refers to the amount of
time required for microbial growth to produce a series of significant
deviations from baseline impedance values and usually corresponds
to bacterial concentrations of approximately 106 to
107 cells/ml (Noble, 1999). To establish cell numbers at the TTD in a
real food matrix, the growth characteristics of a Gram positive
S. aureus and Gram negative S. enteriditis were determined in detail
in the simplest food matrix, carrot puree. In addition to representing
two diverse microbial types, these strains give two distinct
CO2 production curves in the initial growth detection experiments
(Fig. 3). The S. enteriditis strain produced a dramatic change in
impedance in carrot puree whereas the S. aureus strain produced a
very flat curve with a less obvious response to increasing cell
numbers. As noted above, most microbes produced at least 3
successive 11 mS changes in conductance but for those cultures
that produced flatter curves it was of interest to compare the corresponding
growth determined by plate count.
Total plate counts were determined for both microorganisms in
carrot puree and the results were plotted against the change in
impedance (Fig. 3). As expected, the growth of the culture only
triggers a significant change in conductance, the TTD, once the cells
have reached a critical number. Based on the results, growth was
detected at 7.7 log CFU/g for the Salmonella strain. This detection
level