518 ◾ Fundamental Food Microbiology
Hydrostatic Pressure Processing
History
Microbial cells exposed to high hydrostatic pressure (100–800 MPa or more) in a water-filled (or a
suitable liquid-filled), closed chamber die rapidly. Bob Hite in 1890 first studied the effectiveness
of destroying microorganisms by high hydrostatic pressure to preserve foods. But it took almost 90
years to apply high hydrostatic pressure to food preservation and production of several commercial
foods in Japan. The technique, since then, has generated worldwide interest and has been referred
to as ultrahigh hydrostatic pressure processing (UHP), high hydrostatic pressure processing (HHP),
or simply hydrostatic pressure processing (HPP). Early studies were conducted with high-acid and
low-protein fruit products, such as fruit juices, jams, and jellies, where the objective was to destroy
aciduric spoilage microorganisms, mainly yeasts, molds, and lactic acid bacteria as well as the vegetative cells of some potent pathogenic bacteria. Some acid-sensitive vegetative bacteria and spores
of spoilage and pathogenic bacteria were not considered because the cells do not multiply and spores
do not germinate and outgrow in a low-pH (<4.6) environment. The current interest in HPP has
included both low-pH (<4.6; high acid) and high-pH (≥4.6; low acid) foods and foods with high
protein contents. At present, destruction of different foodborne spoilage and pathogenic bacteria,
bacterial spores, yeasts, and molds is being studied. It will also be important to study the destruction of foodborne pathogenic enteric viruses, protozoa, and other parasites, as well as bacteriophages
of lactic acid bacteria. As HPP alone cannot destroy all the important microorganisms in different
types of foods, parameters are being developed for specific food groups based on similarities in their
intrinsic characteristics. The effectiveness of other parameters, such as pressurization time, pressurization temperature, the presence of one or more antimicrobials during pressurization, pH, and A W
are also being studied. Like in heat processing, studies on pressure processing are directed toward
pressure pasteurization to destroy the vegetative cells of foodborne spoilage and pathogenic microorganisms and pressure sterilization to destroy foodborne bacterial spores. Whereas the pasteurization process can be nonthermal, the sterilization process is thermal. (Currently, the unit megapascal
[MPa] is used to indicate pressure processing. The relationship between several units is as follows:
1 atm = 1 bar = 14.5 psi = 750 torr = 100 kilopascal [kPa] = 0.1 MPa.) 5,13,14,23–25
Methods, Mechanisms of Microbial Inactivation, and Advantages 5,13,26
Hydrostatic pressure processing involves exposing a packaged solid or liquid food in water or water
containing a little oil or an unpackaged liquid food (e.g., orange juice) in a closed steel chamber and
then applying pressure by pumping more of the same liquid into the closed chamber (Figure 40.1).
The pressure is transmitted instantaneously and uniformly throughout the closed chamber (as
opposed to heat transmission). In general, the come-up and come-down times of pressure are very
short (one to five minutes). The pressurization causes the volume of the pressurized material to be
compressed (10%–15% between the range of 300–700 MPa) and a rise in temperature resulting
from adiabatic heating (~3°C/100 MPa). These changes are temporary, and during depressurization,
they return to their original states; however, both influence microbial inactivation, causing the phase
transition of lipid bilayers and high-pressure disruption of ionic bonds, hydrophobic interactions,
and hydrogen bonds of the molecules (without affecting the covalent bonds). This causes the macromolecules to unfold. When the pressure is released, the molecules refold but mostly in a manner
different from the original configuration, changing the original characteristics. Volume compression (which brings the molecules closer) and adiabatic heating (resulting from temperature-related
Hydrostatic Pressure Processing
History
Microbial cells exposed to high hydrostatic pressure (100–800 MPa or more) in a water-filled (or a
suitable liquid-filled), closed chamber die rapidly. Bob Hite in 1890 first studied the effectiveness
of destroying microorganisms by high hydrostatic pressure to preserve foods. But it took almost 90
years to apply high hydrostatic pressure to food preservation and production of several commercial
foods in Japan. The technique, since then, has generated worldwide interest and has been referred
to as ultrahigh hydrostatic pressure processing (UHP), high hydrostatic pressure processing (HHP),
or simply hydrostatic pressure processing (HPP). Early studies were conducted with high-acid and
low-protein fruit products, such as fruit juices, jams, and jellies, where the objective was to destroy
aciduric spoilage microorganisms, mainly yeasts, molds, and lactic acid bacteria as well as the vegetative cells of some potent pathogenic bacteria. Some acid-sensitive vegetative bacteria and spores
of spoilage and pathogenic bacteria were not considered because the cells do not multiply and spores
do not germinate and outgrow in a low-pH (<4.6) environment. The current interest in HPP has
included both low-pH (<4.6; high acid) and high-pH (≥4.6; low acid) foods and foods with high
protein contents. At present, destruction of different foodborne spoilage and pathogenic bacteria,
bacterial spores, yeasts, and molds is being studied. It will also be important to study the destruction of foodborne pathogenic enteric viruses, protozoa, and other parasites, as well as bacteriophages
of lactic acid bacteria. As HPP alone cannot destroy all the important microorganisms in different
types of foods, parameters are being developed for specific food groups based on similarities in their
intrinsic characteristics. The effectiveness of other parameters, such as pressurization time, pressurization temperature, the presence of one or more antimicrobials during pressurization, pH, and A W
are also being studied. Like in heat processing, studies on pressure processing are directed toward
pressure pasteurization to destroy the vegetative cells of foodborne spoilage and pathogenic microorganisms and pressure sterilization to destroy foodborne bacterial spores. Whereas the pasteurization process can be nonthermal, the sterilization process is thermal. (Currently, the unit megapascal
[MPa] is used to indicate pressure processing. The relationship between several units is as follows:
1 atm = 1 bar = 14.5 psi = 750 torr = 100 kilopascal [kPa] = 0.1 MPa.) 5,13,14,23–25
Methods, Mechanisms of Microbial Inactivation, and Advantages 5,13,26
Hydrostatic pressure processing involves exposing a packaged solid or liquid food in water or water
containing a little oil or an unpackaged liquid food (e.g., orange juice) in a closed steel chamber and
then applying pressure by pumping more of the same liquid into the closed chamber (Figure 40.1).
The pressure is transmitted instantaneously and uniformly throughout the closed chamber (as
opposed to heat transmission). In general, the come-up and come-down times of pressure are very
short (one to five minutes). The pressurization causes the volume of the pressurized material to be
compressed (10%–15% between the range of 300–700 MPa) and a rise in temperature resulting
from adiabatic heating (~3°C/100 MPa). These changes are temporary, and during depressurization,
they return to their original states; however, both influence microbial inactivation, causing the phase
transition of lipid bilayers and high-pressure disruption of ionic bonds, hydrophobic interactions,
and hydrogen bonds of the molecules (without affecting the covalent bonds). This causes the macromolecules to unfold. When the pressure is released, the molecules refold but mostly in a manner
different from the original configuration, changing the original characteristics. Volume compression (which brings the molecules closer) and adiabatic heating (resulting from temperature-related