Control by Novel Processing Technologies ◾ 517
is moderately higher, so that the transmembrane potential does not exceed the critical value by
1V, pore formation occurs in the membrane; the process is reversible. (This principle is used in
electroporation of cells to introduce foreign DNA.) However, if a much higher external electric
field strength is applied so that membrane potential exceeds the critical value, the pore formation
becomes irreversible, causing the destruction of membrane functions and cell death. For destruction of microbial cells, an electric field strength of approximately 15–25 kV/cm for 2–20 milliseconds (ms) is necessary. Destruction of bacterial and fungal spores requires a higher voltage and a
longer period of time.
The lethal effect of PEF against microorganisms has generated interest for its use in nonthermal pasteurization and commercial sterilization of foods. During the process, there is little
increase in the temperature of the suspension. However, to obtain greater microbial destruction,
the temperature of the suspension can be increased to 60°C or higher. Also, by increasing the
number of pulses, greater microbial destruction can be achieved.
In the PEF processing of food, a high-voltage pulse (20–80 kV/cm) is discharged between
two electrodes in the food in a short time from a series of capacitors that act as storage. PEF can
be applied as an exponentially decaying or square wave; the square wave appears to be more uniform and lethal. Pulses can be given once or more than once to enhance lethality. PEF treatment
in the presence of other antimicrobial compounds (such as bacteriocins), at higher temperature
(≥50°C), and at lower pH is more lethal to microbes. A process called Elsterile has been developed
in Germany for microbial destruction by PEF in liquid food. The liquid food, in a treatment
chamber that has two carbon electrodes, is subjected to high-voltage electric pulses. A 4-log
reduction was obtained for Lactobacillus brevis in milk by treating with 20 pulses at 20 kV/cm for
20 ms. A similar reduction was also observed by treating Saccharomyces cerevisiae in orange juice
with five pulses at 4.7 kV/cm for 20 ms. The increased reduction of the yeast cells, as compared
with bacterial cells, was thought to be a result of the low pH of orange juice and larger cell size
of yeasts.
In the United States, the antimicrobial efficiency of PEF has been studied in several fluid
foods, such as dairy products, fruit juices, and liquid egg products. Apple juice treated with 10
pulses of PEF at 50 kV/cm for 2 ms each at 45°C (113°F) retained its acceptance qualities and
vitamins for 28 days at 4°C (compared with 21 days of fresh juice). Aerobic plate counts (APCs/
mL) in orange juice were reduced by three to four log cycles by applying 32 kV/cm at ambient
temperature, and the product had a 90-day shelf life at 4°C. The population of Listeria innocua was
reduced by 2.5 log cycles in skim milk by PEF treatment at 50 kV/cm. PEF treatments in the presence of nisin reduced the Listeria population by 3.4 log. Escherichia coli and Listeria monocytogenes
populations in milk were reduced respectively by 3 log and 2.5 log at ambient temperature. In
liquid egg treated at 36 kV/cm at ambient temperature, APCs reduced by 2.7 log and the product
had a longer shelf life than untreated products. Inactivation of microbial cells by PEF in particulate food has been studied. However, to achieve a comparable reduction (as with liquid foods), a
much higher voltage was necessary.
Limited studies showed that, at ambient temperature, PEF treatment had very little lethal effect
on spores. However, in a combination of higher temperatures (~70°C–80°C or 158°F–176°F), low
pH, and the presence of EDTA and lysozyme, some spore inactivation can be achieved. A combination of 80°C (176°F) heat shock and lysozyme, followed by PEF at 60°C (140°F) reduced spores
of a Bacillus subtilis strain by three to four log cycles.
These studies showed that PEF, as a nonthermal processing technique, can be used in liquid
food to reduce mainly vegetative cells of spoilage and pathogenic microorganisms to extend its
shelf life and ensure safety.
is moderately higher, so that the transmembrane potential does not exceed the critical value by
1V, pore formation occurs in the membrane; the process is reversible. (This principle is used in
electroporation of cells to introduce foreign DNA.) However, if a much higher external electric
field strength is applied so that membrane potential exceeds the critical value, the pore formation
becomes irreversible, causing the destruction of membrane functions and cell death. For destruction of microbial cells, an electric field strength of approximately 15–25 kV/cm for 2–20 milliseconds (ms) is necessary. Destruction of bacterial and fungal spores requires a higher voltage and a
longer period of time.
The lethal effect of PEF against microorganisms has generated interest for its use in nonthermal pasteurization and commercial sterilization of foods. During the process, there is little
increase in the temperature of the suspension. However, to obtain greater microbial destruction,
the temperature of the suspension can be increased to 60°C or higher. Also, by increasing the
number of pulses, greater microbial destruction can be achieved.
In the PEF processing of food, a high-voltage pulse (20–80 kV/cm) is discharged between
two electrodes in the food in a short time from a series of capacitors that act as storage. PEF can
be applied as an exponentially decaying or square wave; the square wave appears to be more uniform and lethal. Pulses can be given once or more than once to enhance lethality. PEF treatment
in the presence of other antimicrobial compounds (such as bacteriocins), at higher temperature
(≥50°C), and at lower pH is more lethal to microbes. A process called Elsterile has been developed
in Germany for microbial destruction by PEF in liquid food. The liquid food, in a treatment
chamber that has two carbon electrodes, is subjected to high-voltage electric pulses. A 4-log
reduction was obtained for Lactobacillus brevis in milk by treating with 20 pulses at 20 kV/cm for
20 ms. A similar reduction was also observed by treating Saccharomyces cerevisiae in orange juice
with five pulses at 4.7 kV/cm for 20 ms. The increased reduction of the yeast cells, as compared
with bacterial cells, was thought to be a result of the low pH of orange juice and larger cell size
of yeasts.
In the United States, the antimicrobial efficiency of PEF has been studied in several fluid
foods, such as dairy products, fruit juices, and liquid egg products. Apple juice treated with 10
pulses of PEF at 50 kV/cm for 2 ms each at 45°C (113°F) retained its acceptance qualities and
vitamins for 28 days at 4°C (compared with 21 days of fresh juice). Aerobic plate counts (APCs/
mL) in orange juice were reduced by three to four log cycles by applying 32 kV/cm at ambient
temperature, and the product had a 90-day shelf life at 4°C. The population of Listeria innocua was
reduced by 2.5 log cycles in skim milk by PEF treatment at 50 kV/cm. PEF treatments in the presence of nisin reduced the Listeria population by 3.4 log. Escherichia coli and Listeria monocytogenes
populations in milk were reduced respectively by 3 log and 2.5 log at ambient temperature. In
liquid egg treated at 36 kV/cm at ambient temperature, APCs reduced by 2.7 log and the product
had a longer shelf life than untreated products. Inactivation of microbial cells by PEF in particulate food has been studied. However, to achieve a comparable reduction (as with liquid foods), a
much higher voltage was necessary.
Limited studies showed that, at ambient temperature, PEF treatment had very little lethal effect
on spores. However, in a combination of higher temperatures (~70°C–80°C or 158°F–176°F), low
pH, and the presence of EDTA and lysozyme, some spore inactivation can be achieved. A combination of 80°C (176°F) heat shock and lysozyme, followed by PEF at 60°C (140°F) reduced spores
of a Bacillus subtilis strain by three to four log cycles.
These studies showed that PEF, as a nonthermal processing technique, can be used in liquid
food to reduce mainly vegetative cells of spoilage and pathogenic microorganisms to extend its
shelf life and ensure safety.