Control by Irradiation ◾ 507
and larvae in food. It also prevents the sprouting of some foods, such as potatoes and onions.
However, irradiation cannot destroy toxins or undesirable enzymes in a food; in this respect, it
differs from heat treatment (heat also does not destroy heat-stable toxins and enzymes). Irradiation
is a cold sterilization process inasmuch as the temperature of a food does not increase during treatment, and thus irradiated foods do not show some of the damaging effects of heat on food quality.
However, irradiation can cause oxidation of lipids and denaturation of food proteins, especially
when used at higher doses. 1,7,8
Mechanisms of Antimicrobial Action
When an object (food or microorganism) is exposed to high-energy γ-rays (10 –1 to 10 –2 nm), the
energy is absorbed by thousands of atoms and molecules in a fraction of a second, which strips
electrons from them. This produces negative and positive ion pairs. The released electrons can be
highly energized and thus can remove electrons from other atoms and convert them into ions.
This energization and ionization can adversely affect the normal characteristics of biological
systems.
Ionizing radiation produces both direct and indirect effects on microorganisms. The direct
effect is produced from the removal of electrons from the DNA, thereby damaging them and
from the ionization of water molecules present in the cell. The hydrogen and hydroxyl radicals
formed in this process are highly reactive and cause oxidation, reduction, and the breakdown of
C–C bonds of other molecules, including DNA. Studies have shown that a hydroxyl radical can
break both single and double strands in DNA at the sugar-phosphate backbone. In addition, the
radicals can change the bases, such as thymine, to dihydroxydihydrothymine. The consequence
of these damages is the inability of microorganisms to replicate DNA and reproduce, resulting
in death.
Indirectly, ionizing radiation causes damage in the membrane and other structures, causing
sublethal injury. Some microorganisms can repair the damage to the DNA strands (especially
single-strand breaks) and in the bases and are designated as radiation-resistant microorganisms.
Microbial death by ionizing radiation, as in heat treatment, occurs at a predictable rate, which,
like heat, is dependent on dose (strength and exposure time), microbial species, and environmental factors. Because of this, the D value (minutes to reduce cell viability by 1 log of a species in a
population at a given exposure) can be derived. This, in turn, can be used to determine the time
necessary to reduce the population to a desirable level under a specific condition of treatment.
When microorganisms are exposed to UV radiation (~260 nm), the energy is absorbed by the
nucleotide bases in the DNA. The bases can react with each other to form dimers (e.g., thymine
dimers) and cause breaks in the DNA strand. Microbial death and injury are associated mainly
with DNA damage. 1
influencing Factors
Nature of Process
Among the several methods available (X-rays, β-rays, and γ-rays), γ-rays have a higher potential for
effective and economical use in food preservation. 60 Co is predominantly used in food irradiation
because it is more readily and economically available. It has a half-life of ~5.3 years. It continuously
and larvae in food. It also prevents the sprouting of some foods, such as potatoes and onions.
However, irradiation cannot destroy toxins or undesirable enzymes in a food; in this respect, it
differs from heat treatment (heat also does not destroy heat-stable toxins and enzymes). Irradiation
is a cold sterilization process inasmuch as the temperature of a food does not increase during treatment, and thus irradiated foods do not show some of the damaging effects of heat on food quality.
However, irradiation can cause oxidation of lipids and denaturation of food proteins, especially
when used at higher doses. 1,7,8
Mechanisms of Antimicrobial Action
When an object (food or microorganism) is exposed to high-energy γ-rays (10 –1 to 10 –2 nm), the
energy is absorbed by thousands of atoms and molecules in a fraction of a second, which strips
electrons from them. This produces negative and positive ion pairs. The released electrons can be
highly energized and thus can remove electrons from other atoms and convert them into ions.
This energization and ionization can adversely affect the normal characteristics of biological
systems.
Ionizing radiation produces both direct and indirect effects on microorganisms. The direct
effect is produced from the removal of electrons from the DNA, thereby damaging them and
from the ionization of water molecules present in the cell. The hydrogen and hydroxyl radicals
formed in this process are highly reactive and cause oxidation, reduction, and the breakdown of
C–C bonds of other molecules, including DNA. Studies have shown that a hydroxyl radical can
break both single and double strands in DNA at the sugar-phosphate backbone. In addition, the
radicals can change the bases, such as thymine, to dihydroxydihydrothymine. The consequence
of these damages is the inability of microorganisms to replicate DNA and reproduce, resulting
in death.
Indirectly, ionizing radiation causes damage in the membrane and other structures, causing
sublethal injury. Some microorganisms can repair the damage to the DNA strands (especially
single-strand breaks) and in the bases and are designated as radiation-resistant microorganisms.
Microbial death by ionizing radiation, as in heat treatment, occurs at a predictable rate, which,
like heat, is dependent on dose (strength and exposure time), microbial species, and environmental factors. Because of this, the D value (minutes to reduce cell viability by 1 log of a species in a
population at a given exposure) can be derived. This, in turn, can be used to determine the time
necessary to reduce the population to a desirable level under a specific condition of treatment.
When microorganisms are exposed to UV radiation (~260 nm), the energy is absorbed by the
nucleotide bases in the DNA. The bases can react with each other to form dimers (e.g., thymine
dimers) and cause breaks in the DNA strand. Microbial death and injury are associated mainly
with DNA damage. 1
influencing Factors
Nature of Process
Among the several methods available (X-rays, β-rays, and γ-rays), γ-rays have a higher potential for
effective and economical use in food preservation. 60 Co is predominantly used in food irradiation
because it is more readily and economically available. It has a half-life of ~5.3 years. It continuously