Irradiation - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 99

microbial counts on treated samples increased by 2.0 to 2.5 and 1 log, respectively;
Lactobacillus spp. constituted a major portion of the microflora and a D 10 value of
0.59 kGy indicated that these bacteria were of moderate resistance when compared
to other spoilage bacteria. For example, Pseudomonas spp., which normally consti-
tute the major portion of the spoilage microflora of refrigerated prawn, 130 have D 10
values ranging from 0.10 to 0.23 kGy . 93,131 Pathogens including Salmonella , Vibrio ,
Listeria , and Shigella , which can also contribute to shellfish spoilage, have similar
D 10 values. 132-135 Attempts to lower the treatment dose and consequently the produc-
tion of radiation-induced odor and taste problems have been a primary objective of
various investigators. In part this has been achieved by combining ionizing energy
and freezing. Obviously differences among shrimp species and processing conditions
must be considered. From a sensory perspective Hau et al. 136 reported that the
threshold treatment dose, relative to off-flavor development, for frozen (-10°C)
headless prawn, packaged in polyethylene pouches was 4.5 kGy. This value is higher
compared to previously reported data; however, it was reasoned that treatment using
a frozen product decreased free radical generation and mobility thereby reducing
the production of off-odors and flavors. 57,137 Treatment doses used in the study ranged
from 2.5 to 7.5 kGy, with the highest dose reducing the initial population to about
10 2 cfu/g. In contrast, the population in non-irradiated samples maintained at -10°C
surpassed 10 6 cfu/g after 48 h of storage. At this level the product was considered
borderline in regards to acceptability.
Shucked clam meats and soft shelled clams, gamma treated at doses up to 8 kGy
and stored at 6°C, were periodically deep fried and evaluated by a taste panel; at
40 days of storage product acceptability was still maintained. 138 By lowering the
storage temperature to 0.6 to 1.7°C, Ronsivalli et al. 139 demonstrated that the treat-
ment dose could be reduced to 4.5 kGy, and the chowder prepared from treated clam
meats was acceptable even after 15 days. Acceptability of clam meat was also
reported by Conners and Steinberg 140 after treatment using 2.5 to 5.5 kGy. In contrast,
a maximum dose of 2.0 kGy was reported by Novak 141 for treatment of Gulf Coast
oysters. When stored in crushed ice the oysters maintained high acceptability as
judged by a sensory panel even after 21 days of storage. Non-irradiated controls
were off-flavored by 7 days. The extended shelf life of the treated oysters was due
in part to a 99% reduction in the initial microbial population and was accompanied
by decreases in methylamine and ammonia levels suggesting that chemical changes
were closely correlated with microbial numbers. Overall, radiation treatment of
oysters is perhaps more important and/or urgent than previously mentioned products
for two main reasons: they do not freeze well in terms of texture and taste and
therefore are rarely preserved using this method, and they are frequently consumed
raw which can constitute a potential health hazard. In regards to the latter point,
although many pathogenic bacteria associated with shellfish including oysters are
killed by low dose radiation treatment thereby allowing pathogen-free fresh raw
seafood in the market place, viruses including hepatitis A and rotavirus tend to be
more resistant. Therefore, higher doses, usually greater than 2.0 kGy, are required. 22,102
When examining vacuum-packaged king crab meat following radiation, a panel
consistently noted sensory changes when the microbial population approached
10 8 cfu/g. In contrast, controls were judged unsatisfactory when the total bacterial