460 Modern Food Microbiology
minutes after the MPa treatments at 375 and 400.
31 D MPa values for two salmonellae were reported by
Metrick et al.
31 at 340 MPa and 23
◦ C in buffer and chicken. For Salmonella serotype Typhimurium
in buffer and chicken, D MPa values were 7.40 and 7.63, respectively, while for Salmonella serotype
Senftenberg the respective values were 4.20 and 7.13. These investigators were able to calculate D
values in spite of the “tailing” of survivor curves. In a more recent study, the D value for Salmonella
serotype Typhimurium in fresh pork loin samples at 25
◦ C and 414 MPa was 1.48.
3 In the same study,
the D MPa = 414 for L. monocytogenes was 2.17. In another study, the D MPa = 350 was 8.52 minutes for
strain Scott A of L. monocytogenes on pork chops.
33 The latter investigators found this organism to
be more resistant than the indigenous biota of pork chops.
In a study of S. cerevisiae in orange juice, Zook et al.
64 found the following D values: 10.81 minutes
with 300 MPa; 0.97 with 400 MPa; and 0.18 minute with 500 MPa. The z value was around 117 MPa,
and results were similar for apple juice.
64
The effect of a w and potassium sorbate on the inactivation of Z. bailii at 21
◦ C and pH 3.5 in a
laboratory system has been studied,
38 and the times required to inactivate (detection limit <10 cfu/ml)
under the test conditions are indicated below:
a w = 0.98 + potassium sorbate ≥ 345 MPa =< 2 minutes
a w = 0.98 (no sorbate)
517 MPa = ≥ 4 minutes
a w = 0.95 + potassium sorbate ≥ 517 MPa = 4 minutes
a w = 0.95 (no sorbate)
≥ 517 MPa = 10 minutes
These findings demonstrate the antagonistic effect of low a w on HHP and the potentiating effect of
potassium sorbate. These investigators concluded from this study that approximately 10
5 cells of Z.
bailii could be inactivated at 689 MPa regardless of a w , 1,000 ppm potassium sorbate, or duration of
treatment.
38 In another study, 304 MPa for 10 minutes at 25
◦ C in citrate buffer at pH 3.0 produced the
total inactivation of 10
8 cfu/ml of Z. bailii.
40 No effect was observed in the same menstruum when
152 MPa was applied for 30 minutes.
The effect of pH on HHP inactivation of E. coli 0157:H7 was assessed by using orange juice
inoculated with 10
8 cfu/ml and adjusting the pH of samples from 3.4 to 5.0.
26 The conditions that
allowed for a 6-log reduction of the organism were as follows: 550 MPa for 5 minutes in orange juice
with pH 3.4, 3.6, 3.9, or 4.5 but not pH 5.0. A similar reduction was achieved at pH 5.0 by combining
HHP treatment with mild heat (30
◦ C).
26
The combined effect of nisin and HHP on Listeria innocua and E. coli in liquid whole egg (pH 8.0)
was studied, and with nisin at 5 mg/l and 450 MPa at 20
◦ C for 10 minutes, a 5-log reduction of E.
coli and a 6-log reduction of L. innocua were obtained.
44 In an earlier study, nisin and the bacteriocin
Pediocin AcH were shown to increase the lethality of HHP treatments.
23 These and a number of
other food additives increase the effectiveness of HHP by lowering the baroresistance of bacteria.
45
When mechanically recovered poultry meat containing 100 ppm nisin and 1% glucono-delta-lactone
was exposed to about 350 MPa, shelf life was extended during the 36-day storage at refrigerator
temperatures.
63
The relative sensitivity of six foodborne pathogens in buffer, milk, and poultry meat was investigated by Patterson et al.,
42 who found Yersinia enterocolitica to be the most sensitive with >10
5
reduction in numbers occurring in pH 7.0 buffer with MPa as indicated below for 15 minutes. To
achieve a similar reduction for the other five under similar conditions, the following HHPs were
required:
minutes after the MPa treatments at 375 and 400.
31 D MPa values for two salmonellae were reported by
Metrick et al.
31 at 340 MPa and 23
◦ C in buffer and chicken. For Salmonella serotype Typhimurium
in buffer and chicken, D MPa values were 7.40 and 7.63, respectively, while for Salmonella serotype
Senftenberg the respective values were 4.20 and 7.13. These investigators were able to calculate D
values in spite of the “tailing” of survivor curves. In a more recent study, the D value for Salmonella
serotype Typhimurium in fresh pork loin samples at 25
◦ C and 414 MPa was 1.48.
3 In the same study,
the D MPa = 414 for L. monocytogenes was 2.17. In another study, the D MPa = 350 was 8.52 minutes for
strain Scott A of L. monocytogenes on pork chops.
33 The latter investigators found this organism to
be more resistant than the indigenous biota of pork chops.
In a study of S. cerevisiae in orange juice, Zook et al.
64 found the following D values: 10.81 minutes
with 300 MPa; 0.97 with 400 MPa; and 0.18 minute with 500 MPa. The z value was around 117 MPa,
and results were similar for apple juice.
64
The effect of a w and potassium sorbate on the inactivation of Z. bailii at 21
◦ C and pH 3.5 in a
laboratory system has been studied,
38 and the times required to inactivate (detection limit <10 cfu/ml)
under the test conditions are indicated below:
a w = 0.98 + potassium sorbate ≥ 345 MPa =< 2 minutes
a w = 0.98 (no sorbate)
517 MPa = ≥ 4 minutes
a w = 0.95 + potassium sorbate ≥ 517 MPa = 4 minutes
a w = 0.95 (no sorbate)
≥ 517 MPa = 10 minutes
These findings demonstrate the antagonistic effect of low a w on HHP and the potentiating effect of
potassium sorbate. These investigators concluded from this study that approximately 10
5 cells of Z.
bailii could be inactivated at 689 MPa regardless of a w , 1,000 ppm potassium sorbate, or duration of
treatment.
38 In another study, 304 MPa for 10 minutes at 25
◦ C in citrate buffer at pH 3.0 produced the
total inactivation of 10
8 cfu/ml of Z. bailii.
40 No effect was observed in the same menstruum when
152 MPa was applied for 30 minutes.
The effect of pH on HHP inactivation of E. coli 0157:H7 was assessed by using orange juice
inoculated with 10
8 cfu/ml and adjusting the pH of samples from 3.4 to 5.0.
26 The conditions that
allowed for a 6-log reduction of the organism were as follows: 550 MPa for 5 minutes in orange juice
with pH 3.4, 3.6, 3.9, or 4.5 but not pH 5.0. A similar reduction was achieved at pH 5.0 by combining
HHP treatment with mild heat (30
◦ C).
26
The combined effect of nisin and HHP on Listeria innocua and E. coli in liquid whole egg (pH 8.0)
was studied, and with nisin at 5 mg/l and 450 MPa at 20
◦ C for 10 minutes, a 5-log reduction of E.
coli and a 6-log reduction of L. innocua were obtained.
44 In an earlier study, nisin and the bacteriocin
Pediocin AcH were shown to increase the lethality of HHP treatments.
23 These and a number of
other food additives increase the effectiveness of HHP by lowering the baroresistance of bacteria.
45
When mechanically recovered poultry meat containing 100 ppm nisin and 1% glucono-delta-lactone
was exposed to about 350 MPa, shelf life was extended during the 36-day storage at refrigerator
temperatures.
63
The relative sensitivity of six foodborne pathogens in buffer, milk, and poultry meat was investigated by Patterson et al.,
42 who found Yersinia enterocolitica to be the most sensitive with >10
5
reduction in numbers occurring in pH 7.0 buffer with MPa as indicated below for 15 minutes. To
achieve a similar reduction for the other five under similar conditions, the following HHPs were
required: