Irradiation - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 92

conditions. One of the main drawbacks when using these relatively high-dose treat-
ments, however, is the production of flavor defects which can be minimized some-
what only if the carcasses were frozen during treatment. Using a storage temperature
of about 1°C, McGill et al. 88 reported that when fresh whole carcasses were packaged
in polyethylene pouches, spoilage was observed after 11 days. This coincided with
a total plate count of about 10 6 cfu/cm 2 . In contrast, carcasses that were radiated
using pasteurization doses of about 0.93 kGy reached total plate counts of only
10 2 cfu/cm 2 after 11 days, and indications of spoilage were not detected until 20 days
of storage; overall, the shelf life was extended by at least 5 days compared to the
controls. Noticeable off-odors at this dose were not detected by the taste panel.
Higher treatment doses were even more effective in retarding microbial growth;
however, under these conditions rancid-like odors were detected by the taste panel
especially after 16 days of storage. It was concluded that a proper storage temper-
ature, regardless of treatment dose, was paramount if optimum shelf life extension
was to be achieved. Additional studies using higher application doses confirmed that
shelf life extension and survivor levels were dose related. 62,89,90 For example, irradi-
ation of whole carcasses using 5 and 7 kGy extended the shelf life at 5°C to about
16 to 20 and 19 to 24 days, respectively 90 ( Table 3.1 ). In a recent study, Abu-
Tarboush 63 reported that when whole carcasses were packaged in polyethylene bags
and irradiated using doses from 2.5 to 10 kGy in plastic boxes containing crushed
ice, the lag phase of the total aerobic count could be extended to 6 days at 4°C.
From a microbiological perspective this represented a 12-day extension in shelf life.
In comparison, no lag phase was observed in the controls. Extending the lag phase
for psychrotrophic microorganisms was far less favorable except at the highest dose.
No objectionable odors were detected by the trained panel for either the raw or
cooked product. Following cooking, which consisted of boiling samples of breast
and thigh for 12 min, the panel further failed to detect noticeable changes in appear-
ance, texture, and taste. It was recommended that a dose higher than 2.5 kGy had
little benefit in regards to shelf life extension and pathogen elimination. At a dose
of 2.5 kGy, Basker 91 reported that whole chicken carcasses maintained at 1°C had
a shelf life, based on odor development, ranging from 11 to 16 days.
Treatment dose and storage temperature are known to have a profound influence
on the nature or types of survivors; therefore, differences in spoilage patterns includ-
ing off-odors can be expected. Generally following low-dose (

2.5 kGy) treatment
Moraxella/Acinetobacter , Serratia , yeasts, and lactic acid bacteria including Lacto-
bacillus and Micrococcus tend to survive in varying levels. 90,92-95 In treated foods
stored at refrigeration temperatures under vacuum or 100% CO 2 (anaerobic modified
atmosphere packaging), however, lactic acid bacteria normally predominate. 95,96
Additional Gram positive bacteria including Brochothrix thermosphacta and Micro-
bacterium spp. may become problematic especially during extended storage. 94 Inter-
estingly, Lactobacillus spp. and the radiation resistant bacterium Moraxella phe-
nylpyruvica were more easily destroyed if treated under vacuum or in 100% CO 2 . 36
Although yeasts are generally more radioresistant compared to vegetative bacteria,
they make up only a minor fraction of the spoilage microflora of poultry and other
meat products; however, following radiation Candida and Saccharomyces have been
reported to contribute to surface discoloration. 11,63

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