Irradiation - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 95

raw and or with minimal heating. The greater variability in chemical composition,
particularly fat content and color of the various species of fish and shellfish, however,
makes radiation treatment much more discerning.
Some of the earliest research performed on fish preservation, which included
mackerel and haddock fillets, utilized high-voltage cathode rays at dosages sufficient
to result in sterilization. 103,104 Although the storage life of the products was extended,
based on bacteriological and chemical analysis, off-flavors were invariably reported.
Generally the occurrence of irradiated induced flavors and odors intensified as the
dosage was increased and therefore appeared dose-related. Subsequent research
focused on identifying specific spoilage bacteria which were involved in the pro-
duction of off odors during growth on non-irradiated refrigerated fish. 105,106 Using
low-dose (2 kGy) radiation, Maclean and Welander 106 demonstrated that many odor-
generating bacteria could be eliminated. One spoilage bacterium, Micrococcus ,
however, appeared to be very resistant to this type of treatment. 106,107 In an effort to
decrease the dose required to extend the shelf life of various fish, particularly high
fat containing types such as mackerel and herring, the addition of an antibiotic,
oxytetracycline, was investigated. 105 For low-fat fish such as sole fillets, the combi-
nation of beta radiation treatment using the lowest (0.118 kGy) or intermediate dose
levels with the antibiotic (5 ppm) had a similar effect on spoilage retardation as did
the radiation treatment alone using the highest dose (2.8 kGy). 105 With medium and
high fat containing fish, spoilage at 5.5°C was complicated by the onset of rancidity,
ostensibly non-microbial in origin. It was suggested that in order to realize a sig-
nificant extension in the refrigerated shelf life of high-fat containing types including
salmon, inclusion of an antioxidant should be considered. In addition, when higher
doses were utilized (3.0 kGy) with salmon, color bleaching was observed. Increasing
the dose to 10 kGy resulted in a brown coloration which was also reported by Groninger
et al. 108 However, low-fat white-fleshfish such as halibut was much less affected both
in regards to color loss and textural changes. Nevertheless, flavor differences in both
types of fish were detected when compared to the non-irradiated controls.
Anticipating the possible commercialization of radiation for the preservation of
fish, Miyauchi 109 screened raw fillets of Pacific cod for sensory changes. Dosages
from 2.3 to 7 kGy resulted in samples having a slight radiation odor and flavor
which was described as burnt or scorched. Increasing the dose to 9.3 kGy resulted
in samples that were borderline of acceptability. Based on scores provided by a taste
panel, the control sample had a storage life from 1 to 2 weeks. Treated samples
exposed to a maximum dose of 7 kGy were acceptable following more than 3 but
less than 6 weeks of refrigerated storage. Overall, the optimum dose for radiation
appeared to be 4.6 kGy or lower. When maintained at refrigeration temperatures,
the shelf life could be extended threefold over the controls. Total plate counts
( Table 3.2 ) and concurrent chemical analysis, which included total volatile base,
acid number, and trimethylamine concentration, were not reliable indicators of
acceptability. In part this may be due to the fact that only a certain portion of the
microbial survivors remained active during spoilage or were specifically involved
in producing off-odors. In contrast when yellow perch fillets were radiated and
maintained at approximately 1°C, microbiological and chemical analysis closely
correlated with taste panel scores. 110 Following exposure to 3 and 6 kGy, the storage