520 ◾ Fundamental Food Microbiology
three or more units in which while one is being filled, the second one is pressurized or depressurized and the third one is emptied. With the availability of units that can handle larger volumes,
this becomes a commercial reality. The process has three parameters: pressure, temperature of
pressurization, and time of pressurization. By adding suitable antimicrobials during pressurization, it can be made a four-dimensional unique process.
Destruction of Microbial Cells 5,13,14,26
Bacterial Cells
There have been many studies on the pressure-induced viability loss and sublethal injury of foodborne bacterial cells, with more studies being reported for the pathogens. Some of these include
pathogens, such as Salmonella serovars, Campylobacter, Lis. monocytogenes, Staphylococcus aureus,
Yersinia enterocolitica, Vibrio parahaemolyticus, and Esc. coli O157:H7, and spoilage bacteria, such
as Pseudomonas spp., Serratia liquefaciens, Leuconostoc mesenteroides, and Lactobacillus sake. The
antibacterial effect of hydrostatic pressure can be summarized as follows:
◾ Gram-negative bacteria are relatively more susceptible than gram-positive bacteria, and rods
are more sensitive than cocci.
◾ Species and strains of a species differ in sensitivity.
◾ A strain can be sensitive to thermal treatment but resistant to pressure treatment and vice
versa.
◾ Pressure resistance increases with higher consistency, lower A W , higher pH, and higher lipid
content of the suspending media.
◾ Viability loss increases directly with the increase in pressure, temperature, and time with
time having the least effect.
◾ Pressurization also induces sublethal injury in viable cells.
◾ Suitable antimicrobial agents (physical and chemical) can be included during pressurization
to enhance viability loss.
Bacterial cell death in a given set of pressurization parameters occurs in a predictable way.
Like in thermal treatment, the death rate follows first-order kinetics, especially at a pressure above
300 MPa and a pressurization temperature of 45°C (113°F) and above. D values of eight different
foodborne bacterial cell suspensions in peptone solution following pressurization at 345 MPa and
50°C (122°F) were found to be <0.6 minutes. Thus, by a suitable combination of pressure and
temperature, a 5- to 6-log reduction can be achieved within five minutes; it is increased further by
1–3 log by incorporating an antimicrobial agent (e.g., a bacteriocin) during pressurization.
Bacterial Endospores
Bacterial spores are fairly resistant to pressures below 700 MPa at ambient temperatures. However,
pressures above 700 MPa at temperatures above 70°C (158°F) can be detrimental to them. The
information available from reported studies can be generalized as follows:
◾ Spore inactivation is directly proportional to the levels of pressurization (at ≥700 MPa), pressurization temperature (at ≥70°C), and pressurization time (at ≥30 minutes).
◾ Spores of species and strains of species vary greatly in pressure resistance.
three or more units in which while one is being filled, the second one is pressurized or depressurized and the third one is emptied. With the availability of units that can handle larger volumes,
this becomes a commercial reality. The process has three parameters: pressure, temperature of
pressurization, and time of pressurization. By adding suitable antimicrobials during pressurization, it can be made a four-dimensional unique process.
Destruction of Microbial Cells 5,13,14,26
Bacterial Cells
There have been many studies on the pressure-induced viability loss and sublethal injury of foodborne bacterial cells, with more studies being reported for the pathogens. Some of these include
pathogens, such as Salmonella serovars, Campylobacter, Lis. monocytogenes, Staphylococcus aureus,
Yersinia enterocolitica, Vibrio parahaemolyticus, and Esc. coli O157:H7, and spoilage bacteria, such
as Pseudomonas spp., Serratia liquefaciens, Leuconostoc mesenteroides, and Lactobacillus sake. The
antibacterial effect of hydrostatic pressure can be summarized as follows:
◾ Gram-negative bacteria are relatively more susceptible than gram-positive bacteria, and rods
are more sensitive than cocci.
◾ Species and strains of a species differ in sensitivity.
◾ A strain can be sensitive to thermal treatment but resistant to pressure treatment and vice
versa.
◾ Pressure resistance increases with higher consistency, lower A W , higher pH, and higher lipid
content of the suspending media.
◾ Viability loss increases directly with the increase in pressure, temperature, and time with
time having the least effect.
◾ Pressurization also induces sublethal injury in viable cells.
◾ Suitable antimicrobial agents (physical and chemical) can be included during pressurization
to enhance viability loss.
Bacterial cell death in a given set of pressurization parameters occurs in a predictable way.
Like in thermal treatment, the death rate follows first-order kinetics, especially at a pressure above
300 MPa and a pressurization temperature of 45°C (113°F) and above. D values of eight different
foodborne bacterial cell suspensions in peptone solution following pressurization at 345 MPa and
50°C (122°F) were found to be <0.6 minutes. Thus, by a suitable combination of pressure and
temperature, a 5- to 6-log reduction can be achieved within five minutes; it is increased further by
1–3 log by incorporating an antimicrobial agent (e.g., a bacteriocin) during pressurization.
Bacterial Endospores
Bacterial spores are fairly resistant to pressures below 700 MPa at ambient temperatures. However,
pressures above 700 MPa at temperatures above 70°C (158°F) can be detrimental to them. The
information available from reported studies can be generalized as follows:
◾ Spore inactivation is directly proportional to the levels of pressurization (at ≥700 MPa), pressurization temperature (at ≥70°C), and pressurization time (at ≥30 minutes).
◾ Spores of species and strains of species vary greatly in pressure resistance.