Control by Novel Processing Technologies ◾ 523
techniques. Results of high-level inoculation studies showed that pressurization in the presence of
a bacteriocin mixture is more lethal than pressurization alone (Table 40.3). The results of low-level
inoculation studies revealed that by the enumeration technique alone, a package might indicate an
apparent 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 40.4).
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
table 40.3 Viability Loss Determination of Lis. monocytogenes Strains by
Pressurization in inoculated Frankfurters (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 (50 g/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 five minutes 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.
table 40.4 Viability Determination of Listeria monocytogenes Strains in
inoculated Frankfurters (Low-Level inoculation)
Treatment a
Enumeration (log/g) After
Storage at 4°C
Isolation (+/–) After Storage at 4°C
1 day
7 days
14 days
1 day
7 days
14 days
Control (2)
3.2
5.6
6.3
2/2
2/2
2/2
BP (3)
ND b
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 40.3 for explanations. Number in parentheses indicates number
of samples 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 one to two days. A loopful of material was then
streaked on a moxalactam agar plate, which, after incubation for 48 hours, was examined for black characteristic colonies for Listeria survivors.
b No CFU was detected in 0.4 g sample/package.
techniques. Results of high-level inoculation studies showed that pressurization in the presence of
a bacteriocin mixture is more lethal than pressurization alone (Table 40.3). The results of low-level
inoculation studies revealed that by the enumeration technique alone, a package might indicate an
apparent 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 40.4).
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
table 40.3 Viability Loss Determination of Lis. monocytogenes Strains by
Pressurization in inoculated Frankfurters (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 (50 g/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 five minutes 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.
table 40.4 Viability Determination of Listeria monocytogenes Strains in
inoculated Frankfurters (Low-Level inoculation)
Treatment a
Enumeration (log/g) After
Storage at 4°C
Isolation (+/–) After Storage at 4°C
1 day
7 days
14 days
1 day
7 days
14 days
Control (2)
3.2
5.6
6.3
2/2
2/2
2/2
BP (3)
ND b
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 40.3 for explanations. Number in parentheses indicates number
of samples 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 one to two days. A loopful of material was then
streaked on a moxalactam agar plate, which, after incubation for 48 hours, was examined for black characteristic colonies for Listeria survivors.
b No CFU was detected in 0.4 g sample/package.