Biomedical Engineering Reference
In-Depth Information
The effect of irradiation on microbial cells is mainly due to the damage of nucleic acids
and generation of reactive oxygen species (mainly hydrogen peroxide), which result in
oxidative damage to cell membrane (Abn and Lee, 2006). The moisture content of the
product, temperature during irradiation, presence or absence of oxygen, the fresh or frozen
state (increasing resistance in frozen state) influence the microbial resistance to radiation,
particularly in the case of vegetative cells (Farkas, 2006).
Salmonella
serotypes seem to be
the most radioresistant bacteria and the dose to eliminate salmonellae in food would also
be sufficient to inactivate all non-sporeforming pathogens (Farkas, 1998). Recommended
dose of radiation varies with the type of food to be irradiated, for example: frozen and
chilled poultry, 3-5 and 1.5-2.5 kGy, respectively; 4 kGy to control pathogens in frozen
fish, shrimp, prawn and frog-legs and
Vibrio vulnificus
in oysters; 4-6 kGy to kill food-
borne parasites and 3-10 kGy to guarantee an adequate microbial safety level in spices,
dried vegetables and herbal teas (Farkas, 1998; O'Bryan
et al
., 2008 ; Arvanitoyannis
et al
.,
2009a, 2009b). Irradiation does not inactivate viruses, enzymes and microbial toxins
within the doses recommended for foods (Farkas, 1998). Proteins, fats and carbohydrates
are not notably altered by irradiation although higher doses may cause slight aroma and
color changes in beef, pork and poultry (Wood and Bruhn, 2000; Abn and Lee, 2006).
Lipid oxidation is accelerated by irradiation by generating hydroxyl radicals and off-odors.
The presence of oxygen plays an important role in increasing lipid oxidation whereas in
frozen foods free radical mobility is compromised, lowering the rate of lipid oxidation. In
addition, sulfur compounds that are part of protein structure may generate off-odors during
aerobic storage of irradiated foods. Undesirable changes in the final product can be
minimized by using double packaging and antioxidant addition (Abn and Lee, 2006). New
advances in lowering the irradiation dose that is needed to inactivate pathogenic bacteria,
such as minimizing overdose zones or inducing sensitivity to microorganisms, are
necessary to abate the adverse effects of irradiation on sensory properties of some products
(Borsa, 2006 ).
At present there are approximately 60 commercial irradiation facilities operating in
the United States and in over 40 countries irradiation is allowed (Sommers, 2006).
Irradiation has been approved by the FDA to eliminate insects and bacteria from wheat,
flour, spices, and fruits, to control sprouting of potatoes and onions and ripening of fruits
and vegetables, control trichinosis in pork and inactivate pathogenic bacteria in red and
poultry meat (Abn and Lee, 2006; Sommers, 2006). The FDA has now approved
irradiation to eliminate harmful bacteria in lettuce and spinach leaves (FDA, 2008).
Several companies across the United States supply irradiated ground beef and poultry
meat and irradiation has been successfully used for shipping horticultural products from
Hawaii to the United States and from Australia to New Zealand. In addition, authorities
in Brazil have approved irradiation of all foods as a quarantine method (Borsa, 2006;
Sommers, 2006). More than 200 gamma-ray facilities are being used for food radiation.
Cobalt-60 (approved by the FDA) is the most widely used radiation source due to its
ability to treat large pallet loads (30 tonnes per hour) of low density packages (0.3 g/cm
3
)
with a minimum dose of 2 kGy. More than 1000 industrial electron beam accelerators are
available for irradiation with energies up to 10 MeV. However, only few are used for food
irradiation (Cleland, 2006). X-rays with energies up to 7.5 MeV are also approved by the
FDA. X-ray processing facilities that are capable of treating 50 tonnes/h of food in pallets
with package density varying from 0.5 to 0.8 g/cm
3
and using a minimum dose of 2 kGy
(Cleland, 2006 ).
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