Fundamental Food Microbiology, Third Edition - Fuad Fathir

510 FUNDAMENTAL FOOD MICROBIOLOGY
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. The direct effect is produced 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 both single- and doublestrand
breaks in DNA at the sugar–phosphate backbone can be produced by the
hydroxyl radical. 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.
In addition to DNA damage, ionizing radiation also 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. 1–3
A. Nature of Process
IV. INFLUENCING FACTORS
Among the several methods available (x-rays, b-rays, g-rays), g-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 emits g-rays, and thus can be lost
even when it is not used. 137 Cs can also be used in foods, but it is relatively difficult
to obtain and is also required in larger amounts. The antimicrobial efficiency of
ionizing radiation increases as the dose is increased. The antimicrobial efficiency,
however, decreases in the absence of oxygen (because of reduced oxidizing reactions)
and at low A w (because of reduced free-radical formation with less water).
Freezing also reduces the efficiency because of reduced availability of reactive water
molecules. 1–3
B. Nature of Foods
g-Rays have a penetration capability of ca. 40 cm and can penetrate through paper,
plastic, and cans. Thus, foods can be exposed to g-radiation in packages, cans,