Irradiation - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 88

or keeping quality. Foods receiving these dosages are usually unfrozen. From
a standpoint of shelf life extension of refrigerated foods including meat and
fish, the most important and sensitive bacterium is Pseudomonas.
Gamma rays, electron beams, and X-rays are the most common sources of
ionizing radiation. Of the three, gamma rays from isotopes including 137 Cs and in
particular 60 Co are most frequently used in the food industry. 4,11 The energy that is
absorbed per unit time is referred to as the dose rate. Gamma ray sources, in
comparison to electron beam generators, provide a relatively low dose rate. For
example, average doses of 100 to 10,000 Gy/h are typical. 11 Therefore, products to
be gamma radiated generally require a longer exposure time in order to achieve a
specified absorbed dose. Electron beams, which are produced by linear accelerators
powered by electricity, require considerably shorter exposure times, typically sec-
onds or minutes. However, the effective penetration range of an electron beam
depends on its energy level. For treatment with foods, 10 MeV has been set as the
upper limit. 2,40 Compared to gamma rays the depth of penetration is usually lower,
about 5 cm if treated on one side or 10 cm if treated on both sides. 11 Successful
application of electron beam treatment is therefore limited by the thickness of the
food to be treated and would be appropriate for topical use especially with fruits.
For other whole foods, including eviscerated poultry carcasses, gamma radiation
would be more suitable since the higher penetration rays would reach the cavity
which is a known repository for both spoilage and pathogenic bacteria.
IRRADIATION TREATMENT OF FOODS
D AIRY P RODUCTS
Cheese
Extending the shelf life and/or sterilization of dairy products for immunocompromised
patients using radiation treatment is not a widely accepted practice. The main reason
for its limited use is that ionizing energy, through the formation of radiolytic products
especially in high lipid-based foods, generates unacceptable off-odors and flavors via
oxidation, polymerization, decarboxylation, and dehydration reactions even at low
doses. 22,41 In particular, polyunsaturated fatty acids are prone to oxidation by free
radicals produced during treatment. In addition, oxidation of casein and the production
of methyl radicals has been shown to result in the generation of “wet dog” off-flavors. 42
These chemical reactions, to some extent, can be reduced if the products are initially
frozen and/or treated in an environment with limited water, light, and oxygen. Yet,
despite these shortcomings, low-dose radiation for the specific purpose of extending
dairy product shelf life does hold promise. In such applications, the treatment should
be considered as supplemental or complementary with use of other preservation tech-
niques including refrigeration and/or preservatives such as sorbic acid.
Bongirwar and Kumta 43 reported that Cheddar cheese developed off-flavors when
irradiated at 0.5 kGy; however, none was detected when the dose was reduced to
0.2 kGy. A dose greater than 1.5 kGy, when applied to Turkish Kashar cheese, not