516 ◾ Fundamental Food Microbiology
High-Voltage Arc Discharge 5,16
In this method, stored high-voltage electrical energy is rapidly discharged in the form of an arc
through the gap of electrodes placed in a liquid food. The process not only raises the temperature
of the liquid to some extent but also produces electrohydraulic shock. Antimicrobial effect is produced mainly because of membrane damage by the electrohydraulic shock. The major disadvantages of the method are the generation of electrolytic products and disintegration of food particles.
Improved techniques are now being developed to overcome the problems.
Pulsed X-Rays 5,19
Irradiation of food by γ-rays (E-beam) using 60 Co (and 137 Cs) has been discussed in Chapter 39.
One of the disadvantages of this method is that the radionuclides emit radiation in all directions
and all the time; it is thus uneconomical and also hazardous. In contrast, X-rays can be generated only when they are needed and in the desired linear direction. It was first recognized in
1895 and soon after that it became a routine diagnostic tool in medicine. A pulsed X-ray, which
delivers high-energy pulses in a very short time, is being studied to inactivate microorganisms in
foods only recently. The equipment can produce high energy with a high penetrating power and
the 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 (68°F). The microbial inactivation occurs, like in γ-radiation, from the damages in the DNA. X-ray irradiation has
been shown to enhance the safety and shelf life of many heat-sensitive products, such as lettuce,
spinach, tomatoes, and raw almonds.
Plasma technology 20–22
Moreau et al. defined “plasma” as a neutral ionized gas generated by excitation of gas by electric current, 22 which consists of atoms, ions, free-radicals and excited and nonexcited molecules,
photons and electrons. The free-radicals and ionic species are highly toxic to microbial vegetative
cells or endospores, including gram-positive (Bacillus, Clostridium, Staphylococcus, Listeria), gramnegative (Escherichia, Salmonella, Vibrio) bacteria, yeast (Saccharomyces) and mold (Aspergillus).
There are two types of plasma, thermal and nonthermal, based on the conditions used for their
generation. High-temperature plasma is used mostly for industrial processes, such as the automative industry and in microelectronics, and low-temperature plasma is used for food preservation
and disinfection of food contact surfaces, and the latter one is receiving increasing interest as a
novel food processing technology. The chemical composition of low-temperature plasmas of gas
mixtures of nitrogen, oxygen, and carbon dioxide consisted of O – , O 2
−
, N + , N 2
+
, O + , O 2
+
, NO + , and
CO 2
+
, which are highly toxic to microbial cells or endospores.
Pulsed electric Field 5,10,16
The antimicrobial effect of high electric field pulses is not a result of the electric heat or electrolytic products, but rather 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
High-Voltage Arc Discharge 5,16
In this method, stored high-voltage electrical energy is rapidly discharged in the form of an arc
through the gap of electrodes placed in a liquid food. The process not only raises the temperature
of the liquid to some extent but also produces electrohydraulic shock. Antimicrobial effect is produced mainly because of membrane damage by the electrohydraulic shock. The major disadvantages of the method are the generation of electrolytic products and disintegration of food particles.
Improved techniques are now being developed to overcome the problems.
Pulsed X-Rays 5,19
Irradiation of food by γ-rays (E-beam) using 60 Co (and 137 Cs) has been discussed in Chapter 39.
One of the disadvantages of this method is that the radionuclides emit radiation in all directions
and all the time; it is thus uneconomical and also hazardous. In contrast, X-rays can be generated only when they are needed and in the desired linear direction. It was first recognized in
1895 and soon after that it became a routine diagnostic tool in medicine. A pulsed X-ray, which
delivers high-energy pulses in a very short time, is being studied to inactivate microorganisms in
foods only recently. The equipment can produce high energy with a high penetrating power and
the 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 (68°F). The microbial inactivation occurs, like in γ-radiation, from the damages in the DNA. X-ray irradiation has
been shown to enhance the safety and shelf life of many heat-sensitive products, such as lettuce,
spinach, tomatoes, and raw almonds.
Plasma technology 20–22
Moreau et al. defined “plasma” as a neutral ionized gas generated by excitation of gas by electric current, 22 which consists of atoms, ions, free-radicals and excited and nonexcited molecules,
photons and electrons. The free-radicals and ionic species are highly toxic to microbial vegetative
cells or endospores, including gram-positive (Bacillus, Clostridium, Staphylococcus, Listeria), gramnegative (Escherichia, Salmonella, Vibrio) bacteria, yeast (Saccharomyces) and mold (Aspergillus).
There are two types of plasma, thermal and nonthermal, based on the conditions used for their
generation. High-temperature plasma is used mostly for industrial processes, such as the automative industry and in microelectronics, and low-temperature plasma is used for food preservation
and disinfection of food contact surfaces, and the latter one is receiving increasing interest as a
novel food processing technology. The chemical composition of low-temperature plasmas of gas
mixtures of nitrogen, oxygen, and carbon dioxide consisted of O – , O 2
−
, N + , N 2
+
, O + , O 2
+
, NO + , and
CO 2
+
, which are highly toxic to microbial cells or endospores.
Pulsed electric Field 5,10,16
The antimicrobial effect of high electric field pulses is not a result of the electric heat or electrolytic products, but rather 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