508 ◾ Fundamental Food Microbiology
emits γ-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.
Nature of Foods
γ-rays (E-beam) have a penetration capability of approximately 40 cm and can penetrate through
paper, plastic, and cans. Thus, foods can be exposed to γ-radiation in packages, cans, 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. 9 Food compositions
(thickness and particle size) also determine the efficiency of irradiation in reducing microbial numbers.
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 spore formers, 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, a possible increase in neither pathogenicity nor an induction of a gene that transcribes
for a toxin is expected to occur as observed from the many studies conducted in these areas.
Methods
Doses
A radiation dose was originally designated as a 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
emits γ-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.
Nature of Foods
γ-rays (E-beam) have a penetration capability of approximately 40 cm and can penetrate through
paper, plastic, and cans. Thus, foods can be exposed to γ-radiation in packages, cans, 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. 9 Food compositions
(thickness and particle size) also determine the efficiency of irradiation in reducing microbial numbers.
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 spore formers, 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, a possible increase in neither pathogenicity nor an induction of a gene that transcribes
for a toxin is expected to occur as observed from the many studies conducted in these areas.
Methods
Doses
A radiation dose was originally designated as a 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