Fundamental Food Microbiology, Third Edition - Fuad Fathir

CONTROL BY NOVEL PROCESSING TECHNOLOGIES 519
direction they are needed. Pulsed x-ray, which delivers high-energy pulses in a very
short time, is being studied to inactivate microorganisms in foods. The equipment
can produce high energy with high penetrating power and least variation in dose
uniformity. Limited studies have revealed that when used properly, the method can
destroy five to six log cycles of microbial cells and spores at 20�C. The microbial
inactivation occurs, like in g-radiation, from the damages in the DNA.
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III. PULSED ELECTRIC FIELD
The antimicrobial effect of high electric field pulses is not due to the electric heat
or electrolytic products, but rather to the ability to cause damage to the cell membrane.
When microbial cells in a suspension are exposed to pulses of high-voltage
electric fields, a potential difference occurs between the outside and inside of the
membrane. When the external electric field strength is moderately higher, so that
the transmembrane potential does not exceed the critical value by 1 V, 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
ca. 15 to 25 kV/cm for 2 to 20 ms is necessary. Destruction of bacterial and fungal
spores requires a higher voltage and a longer period of time. 3,4
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 a greater microbial destruction, the temperature of the suspension can be
increased to 60ºC 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 to 80 kV/cm) is discharged
between two electrodes in the food in a short time from a series of capacitors
that acts as a 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
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 due to the low pH of orange juice and larger cell size of
yeasts.
In the U.S., the antimicrobial efficiency of PEF has been studied in several fluid
foods, such as dairy products, fruit juices, and liquid egg products. Apple juice