Fundamental Food Microbiology, Third Edition - Fuad Fathir

526 FUNDAMENTAL FOOD MICROBIOLOGY
Table 39.3 Viability Loss Determination of Lis. monocytogenes Strains by
Pressurization in Inoculated Frankfurter (High-Level Inoculation)
Treatment a Log CFU/g Log Viability Loss/g
Control 7.6 —
PN 6.1 1.5
HP 3.4 4.2
HP + PN ND b ~7.6
a Cell suspension of a five-strain mixture of pressure-resistant Lis. monocytogenes was
inoculated in hotdog (sterile) packages (50g/package at 7.6 log CFU/g). Some of the
packages were inoculated with a mixture of pediocin AcH and nisin (PN; 1:1; 3000 AU/g).
Packages were pressurized at 345 MPa for 5 min at 50�C. Enumeration was done by
surface plating on prepoured plates of tryptic soy agar. Each result is the average of four
packages. Control, without any treatment; PN, with pediocin and nisin only; HP, only
pressurized; HP + PN, pressurized in the presence of PN.
b No CFU was detected in 0.4 g sample/package.
absence of the pathogen because the technique cannot detect a very low level of
survivors (theoretically @1 CFU/package). However, even from very few survivors
in the package, a psychrotrophic pathogen can multiply during refrigerated storage
to reach a detectable level (Table 39.4). In contrast, the isolation technique can be
effectively used to detect any survivor in a package. Thus, for a highly potent
pathogen for which a zero tolerance is desired, the processing parameters should be
evaluated by the isolation technique. The results also showed that pressurization in
the presence of a suitable antimicrobial (such as a bacteriocin mixture) is more
effective in reducing foodborne bacteria.
Recently, an effective pulse pressurization technique has been developed to
produce sterile foods. 12 In this, a food inoculated with the spores of Bacillus and
Clostridium spp. is preheated to 90�C and given a first pressurization treatment at
690 MPa for 1 min; following depressurization and a pause for 1 min, it is pressurized
again for 1 min at 690 MPa. This technique has produced sterile macaroni with
Table 39.4 Viability Determination of Lis. monocytogenes Strains in Inoculated
Frankfurters (Low-Level Inoculation)
Enumeration (log/g) after Isolation (+/–) after Storage at
Treatment
Storage at 4����C
4����C
1 d 7 d 1 4d 1 d 7 d 14 d
Control (2) 3.2 5.6 6.3 2/2 2/2 2/2
BP (3) NDb 0.5 3.6 3/3 3/3 3/3
HP (6) ND 0.4 0.7 1/6 5/6 6/6
HP + BP (6) ND ND ND 0/6 0/6 0/6
a See footnote of Table 39.3 for explanations. Number in parentheses indicates number of
sample tested. Enumeration was done by plating on tryptic soy agar and isolation was
done on moxalactam agar. Each package was opened and mixed with a selective
enrichment broth. A portion was used for enumeration and the remaining materials in the
package were incubated at 37�C for 1 to 2 d. A loopful of material was then streaked on
a moxalactam agar plate, which after incubation for 48 h was examined for black
characteristic colonies for Listeria survivors.