Radiation Protection of Foods, and Nature of Microbial Radiation Resistance
373
either byproducts of atomic fission or atomic waste products. Gamma rays have excellent penetration
power, as opposed to beta rays.
60 Co has a half-life of about 5 years; and the half-life for
137 Cs is about
30 years.
X-Rays
These rays are produced by the bombardment of heavy-metal targets with high-velocity electrons
(cathode rays) within an evacuated tube. They are essentially the same as gamma rays in other respects.
Microwaves
Microwave energy may be illustrated in the following way.
24 When electrically neutral foods are
placed in an electromagnetic field, the charged asymmetric molecules are driven first one-way and
then another. During this process, each asymmetric molecule attempts to align itself with the rapidly
changing alternating-current field. As the molecules oscillate about their axes while attempting to go
to the proper positive and negative poles, intermolecular friction is created and manifested as a heating
effect. This is microwave energy. Most food research has been carried out at two frequencies: 915
and 2,450 megacycles. At the microwave frequency of 915 megacycles, the molecules oscillate back
and forth 915 million times per second.
24 Microwaves lie between the infrared and radio frequency
portions of the electromagnetic spectrum (Figure 15–1). The problem associated with the microwave
destruction of trichina larvae in pork products is discussed in Chapter 29.
PRINCIPLES UNDERLYING THE DESTRUCTION OF MICROORGANISMS
BY IRRADIATION
Several factors should be considered when the effects of radiation on microorganisms are considered,
and these are discussed in the following subsections.
Types of Organisms
Gram-positive bacteria are more resistant to irradiation than Gram negatives. In general, spore
formers are more resistant than non-spore-formers (with the exception of species discussed later in
this chapter). Among spore formers, Paenibacillus larvae seem to possess a higher degree of resistance
than most other aerobic spore formers. Spores of Clostridium botulinum type A appear to be the most
resistant of all clostridial spores. Apart from the extremely resistant species, Enterococcus faecium
R53, micrococci, and the homofermentative lactobacilli are among the most resistant of non-sporeforming bacteria. Most sensitive to radiations are the pseudomonads and flavobacters (including the
new genera created by the delimiting of these genera as noted in Chapter 2), with other Gram-negative
bacteria being intermediate. A general spectrum of radiation sensitivity from enzymes to higher animals
is illustrated in Figure 15–2. Possible mechanisms of radioresistance are discussed below.
With the exception of endospores and the extremely resistant species already noted, radioresistance
generally parallels heat resistance among bacteria.
With respect to the radiosensitivity of molds and yeasts, the latter have been reported to be more
resistant than the former, with both groups in general being less sensitive than Gram-positive bacteria.
373
either byproducts of atomic fission or atomic waste products. Gamma rays have excellent penetration
power, as opposed to beta rays.
60 Co has a half-life of about 5 years; and the half-life for
137 Cs is about
30 years.
X-Rays
These rays are produced by the bombardment of heavy-metal targets with high-velocity electrons
(cathode rays) within an evacuated tube. They are essentially the same as gamma rays in other respects.
Microwaves
Microwave energy may be illustrated in the following way.
24 When electrically neutral foods are
placed in an electromagnetic field, the charged asymmetric molecules are driven first one-way and
then another. During this process, each asymmetric molecule attempts to align itself with the rapidly
changing alternating-current field. As the molecules oscillate about their axes while attempting to go
to the proper positive and negative poles, intermolecular friction is created and manifested as a heating
effect. This is microwave energy. Most food research has been carried out at two frequencies: 915
and 2,450 megacycles. At the microwave frequency of 915 megacycles, the molecules oscillate back
and forth 915 million times per second.
24 Microwaves lie between the infrared and radio frequency
portions of the electromagnetic spectrum (Figure 15–1). The problem associated with the microwave
destruction of trichina larvae in pork products is discussed in Chapter 29.
PRINCIPLES UNDERLYING THE DESTRUCTION OF MICROORGANISMS
BY IRRADIATION
Several factors should be considered when the effects of radiation on microorganisms are considered,
and these are discussed in the following subsections.
Types of Organisms
Gram-positive bacteria are more resistant to irradiation than Gram negatives. In general, spore
formers are more resistant than non-spore-formers (with the exception of species discussed later in
this chapter). Among spore formers, Paenibacillus larvae seem to possess a higher degree of resistance
than most other aerobic spore formers. Spores of Clostridium botulinum type A appear to be the most
resistant of all clostridial spores. Apart from the extremely resistant species, Enterococcus faecium
R53, micrococci, and the homofermentative lactobacilli are among the most resistant of non-sporeforming bacteria. Most sensitive to radiations are the pseudomonads and flavobacters (including the
new genera created by the delimiting of these genera as noted in Chapter 2), with other Gram-negative
bacteria being intermediate. A general spectrum of radiation sensitivity from enzymes to higher animals
is illustrated in Figure 15–2. Possible mechanisms of radioresistance are discussed below.
With the exception of endospores and the extremely resistant species already noted, radioresistance
generally parallels heat resistance among bacteria.
With respect to the radiosensitivity of molds and yeasts, the latter have been reported to be more
resistant than the former, with both groups in general being less sensitive than Gram-positive bacteria.