Fundamental Food Microbiology, Third Edition - Fuad Fathir

CONTROL BY IRRADIATION 511
baskets, and bags. Frozen, dry, or anaerobically packaged foods need higher doses
of treatment to obtain the desirable antimicrobial effect. In contrast, treatments such
as curing, high hydrostatic pressure, high temperature, and low pH enhance the
antimicrobial effect of radiation in food. Food compositions (thickness and particle
size) also determine the efficiency of irradiation in reducing microbial numbers.
C. Nature of Microorganisms
Microorganisms vary greatly in their sensitivity to ionizing (and UV) radiation.
Because of size differences, molds are more sensitive than yeasts, which are more
sensitive than bacterial cells; bacterial cells are more sensitive than viruses (including
phages). Among bacteria, Gram-negative cells are more sensitive than Gram-positive
bacteria, and rods are more sensitive than cocci. Species and strains of bacterial cells
vary greatly in their sensitivity to irradiation. Some strains, designated as radiation
resistant, have effective metabolic systems to repair the cellular damages (especially
single- and double-strand breaks of DNA and base damage). These include some
bacterial strains that are important in foods, such as Salmonella Typhimurium,
Escherichia coli, Enterococcus faecalis, and Staphylococcus aureus. Spores are quite
resistant to irradiation, probably because their water content is very low. Among the
sporeformers, spores of Clostridium botulinum Type A and Bacillus pumilus are
probably the most resistant to irradiation. Generally, Bacillus spores (aerobes) are
less resistant than Clostridium spores (anaerobes).
The rate of death of microorganisms by irradiation follows first-order kinetics
(i.e., straight line patterns) as the thermal destruction curve and can thus be expressed
as a D value (time to destroy 90% of viable microorganisms). Accordingly, it can
be influenced by a higher initial population, relative numbers of resistant cells in
the population, number of spores present, and age and condition of growth of a
strain. Toxins of microorganisms are not destroyed by ionizing radiation at the dose
levels recommended in foods. Although irradiation can cause mutations in some
microbial cells in a population, either a possible increase in pathogenicity or an
induction of a gene that transcribes for a toxin is not expected to occur, as observed
from the many studies conducted in these areas.
A. Doses
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V. METHODS
Radiation dose was originally designated as rad, and 1 rad was defined as the quantity
of ionizing radiation that results in the absorption of 100 ergs of energy per gram
of an irradiated material. The current unit is gray (Gy), and 1 Gy is equivalent to
100 rad. When 1 kg of food absorbs the energy of 1 joule (1 joule = 10 7 ergs), it
has received a dose of 1 Gy. According to the international health and safety
authorities, foods irradiated up to 10,000 Gy (10 kGy) are considered safe.
The relative sensitivity of microorganisms to irradiation dose is a function of
their size and water content. Approximate lethal dose levels for insects and different