Irradiation - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 97

retailing. The researchers also explored the possibility of vacuum packaging prior
to radiation treatment and reported that lower doses using this regime could be used
to achieve similar extensions in the shelf life for some species of fin fish. Ostensibly,
radiation of fishery products under vacuum compared to air-packaging resulted in
survivors with either higher diminished or altered microbial activity. A subsequent
study on the benefits of packaging fish prior to radiation treatment was performed
by Vengopal et al. 113 Eviscerated Indian mackerel immersed in crushed ice was
radiated (1.5 kGy) and stored at 0°C. A sensory panel judged the product acceptable
up to 20 days. Similarly treated fish which was packaged in polyethylene pouches
and placed in crushed ice exhibited a shelf life of 25 days. It was concluded that for
practical purposes, packaging of mackerel prior to treatment was not necessary as
long as the fish was exposed to melting ice made from potable water. Overall, the
use of a 1 to 2 kGy treatment for most species of fish including Bombay duck, cod,
haddock, mullet, hake, and catfish did appear to impart off-odors and tastes especially
if treatment was performed at temperatures of 1 to 4°C. The outcome of such
treatments have resulted in shelf life extensions from 5 to 7 to about 14 to 20 days. 114-120
and were attributed to the destruction of specific spoilage microorganisms, particu-
larly psychrotrophs such as Acinetobacter , Moraxella , and Pseudomonas . 119
Since Pseudomonas is a strict aerobe, the use of MAP containing CO 2 would
be expected to halt its growth. If used in concert with radiation, the combination
treatment could result in a further extension of shelf life via increased efficacy or
synergy and or a reduction in the radiation dose. Indeed such findings have been
reported, albeit with mixed conclusions. In one study using cod fillets a combination
treatment involving MAP (60:40%; CO 2 :O 2 ) and a 1-kGy treatment resulted in a
total shelf life of 24 days. 121 In comparison, the control and samples treated only
with radiation had a total sensory shelf life of 10 and 19 days, respectively. Vacuum
packaged and radiated fillets, however, had a total shelf life of 21 days indicating
that the MAP treatment was of marginal benefit when used in combination with
radiation. In all cases, sample assessment was based on raw odor and cooked flavor
and texture. Increasing CO 2 levels to 80% also failed to have a significant impact
on the shelf life extension of catfish fillets when used in conjunction with radiation
at 0.5 to 1.0 kGy. 117 Steaming or cooking 121 and the use of chemical preservatives
such as sorbates and benzoates 120,121 to control growth of surviving yeasts during
product storage have also been assessed. Based on aerobic plate counts and chemical
analyses including tri- and dimethylamine and hypoxanthine, it appeared that the
sorbate-radiation treatment provided the longest (25 d) shelf life. In contrast, the
control and air radiated sample had a shelf life of 10 and 19 days, respectively. 121
Hammad et al. 123 reported when lightly salted (1.8%) cold-smoked (28 to 30°C)
salmon was radiated at a dose of 4 kGy, despite favorable reductions in spoilage
and pathogenic microorganisms aside from Clostridium perfringenes , a noticeable
loss in color was observed. In this regard, the sensory panel reported that the normal
cherry red color of the smoked fish faded to beige white. Only slightly lower scores
were reported for taste and texture when compared to non-irradiated controls.
Decreasing the dose to 2 kGy had no effect on color fading; however, the microbi-
ological quality was reduced substantially. Overall, the researchers concluded that
a maximum dose of 2 kGy could be used to extend the shelf life of these products