505
Chapter 39
Control by irradiation
introduction
Irradiation (Radiation) and Radioactivity
In the electromagnetic spectrum, energy exists as waves, and the intensity of the energy increases
as the waves get shorter. On either side of visible rays (~400–800 nm) are invisible long waves
(>800 nm; IR and radio waves for radio, TV, microwave, and radar) and invisible short waves
(<300 nm; UV rays, X-rays, β-rays, γ-rays, and cosmic rays). Exposure to long waves, visible light
waves, and UV rays does not cause any change in the atomic structures. In contrast, exposure to
X-rays, β-rays, and γ-rays can remove electrons from the outer shell of an atom and thus form an
ion pair (negatively charged and positively charged). Ion formation or ionization does not make
an atom radioactive. To induce radioactivity, the nucleus of an atom has to be disrupted by much
higher energies, such as by neutrons; X-rays, β-rays, and γ-rays do not have that much energy.
For application in food preservation, X-rays, β-rays, and γ-rays were studied for their ability
to penetrate inside foods and kill microorganisms, their efficiency, and their effect on food quality. β-rays (actually electrons, similar to cathode rays) have very little penetration power; they
cannot penetrate inside metal cans and are thought to be ineffective in food preservation. X-rays,
although they have good penetration power, cannot be effectively focused on foods and, because
of this low efficiency, are not considered favorably for application in food. In contrast, γ-rays (photons) have high penetration power (~40 cm thick) and may be considered effective and economical
for use in foods. Cobalt-60 ( 60 Co) and cesium-137 ( 137 Cs) are considered to be good sources of
γ-rays. 60 Co, an artificially induced radioactive isotope, is produced for use in nuclear medicine.
When its energy level becomes so low that it cannot be further used in medicine, it is used to irradiate foods. Because of the easy availability of 60 Co over 137 Cs, it is preferred for food irradiation.
Thus, foods irradiated with 60 Co do not become radioactive. Instead, the atoms (and molecules) in
a food and in microbial cells form ion pairs and other components that inhibit the multiplication
of microorganisms (and food cells, such as the inhibition of sprouting in potatoes). 1–3
Chapter 39
Control by irradiation
introduction
Irradiation (Radiation) and Radioactivity
In the electromagnetic spectrum, energy exists as waves, and the intensity of the energy increases
as the waves get shorter. On either side of visible rays (~400–800 nm) are invisible long waves
(>800 nm; IR and radio waves for radio, TV, microwave, and radar) and invisible short waves
(<300 nm; UV rays, X-rays, β-rays, γ-rays, and cosmic rays). Exposure to long waves, visible light
waves, and UV rays does not cause any change in the atomic structures. In contrast, exposure to
X-rays, β-rays, and γ-rays can remove electrons from the outer shell of an atom and thus form an
ion pair (negatively charged and positively charged). Ion formation or ionization does not make
an atom radioactive. To induce radioactivity, the nucleus of an atom has to be disrupted by much
higher energies, such as by neutrons; X-rays, β-rays, and γ-rays do not have that much energy.
For application in food preservation, X-rays, β-rays, and γ-rays were studied for their ability
to penetrate inside foods and kill microorganisms, their efficiency, and their effect on food quality. β-rays (actually electrons, similar to cathode rays) have very little penetration power; they
cannot penetrate inside metal cans and are thought to be ineffective in food preservation. X-rays,
although they have good penetration power, cannot be effectively focused on foods and, because
of this low efficiency, are not considered favorably for application in food. In contrast, γ-rays (photons) have high penetration power (~40 cm thick) and may be considered effective and economical
for use in foods. Cobalt-60 ( 60 Co) and cesium-137 ( 137 Cs) are considered to be good sources of
γ-rays. 60 Co, an artificially induced radioactive isotope, is produced for use in nuclear medicine.
When its energy level becomes so low that it cannot be further used in medicine, it is used to irradiate foods. Because of the easy availability of 60 Co over 137 Cs, it is preferred for food irradiation.
Thus, foods irradiated with 60 Co do not become radioactive. Instead, the atoms (and molecules) in
a food and in microbial cells form ion pairs and other components that inhibit the multiplication
of microorganisms (and food cells, such as the inhibition of sprouting in potatoes). 1–3