Biomedical Engineering Reference
In-Depth Information
plate count approached 10 6 cfu / g. 142 Ostensibly the levels of specific spoilage micro-
organisms are more important compared to total or spoilage associated numbers. 143
In addition, although trimethylamine and total volatile base levels increased with storage
time in both radiated and control samples, only values in excess of 0.9 and 12%,
respectively, could be related to product acceptability. Following treatment at 2 kGy the
shelf life was extended to 3 weeks storage at 1°C. Doubling the dose concomitantly
doubled the shelf life under similar storage conditions. In a previous study, 128 a treat-
ment using 2.5 kGy was demonstrated to extend the shelf life of crabmeat to 3 to
4 weeks when stored at about 3.5°C compared with 1 week for control samples.
Chilled saucer scallops ( Amusium balloti ) when treated with 0.5, 1.5, and 3 kGy
exhibited a shelf life of 18, 23, and 42 days, respectively. 144 The initial total plate
count levels, ranging from 10 5 to 10 7 cfu/g, were reduced by a 2-, 3-, and 4-log
reduction following radiation at 0.5, 1.5, and 3 kGy, respectively. The initial psy-
chrotrophic count (PC), which is perhaps a better index for potential spoilage, was
about 10 6 cfu/g and was reduced by 1, 2, and 3 logs, respectively. Improvements in
the shelf life of the product were partly based on the observation that during chilled
storage PC levels in the treated samples did not reach 10 7 cfu/g until 4, 20, and
28 days of storage, respectively. In contrast, control samples reached this level after
only 7 days. When the samples were baked and presented to a taste panel, no
differences in odor or texture were noted between the treated and control samples,
at least prior to 8 days of storage. In contrast, scallops ( Plactopecten magellanicus)
treated with 1.5 and 3 kGy received no practical benefit in regards to shelf life
extension because of development of off-odors and flavors described as burned. 145
In 1997 both the U.S. Food and Drug Administration (FDA) and the Department of
Agriculture (USDA) approved a petition for the use of safe irradiation dosage levels
with fresh and frozen beef, lamb, and pork products to control foodborne illnesses;
however, packaged and processed products such as frankfurters were excluded.
Approvals for irradiation include maximum doses of 4.5 and 7.0 kGy for refrigerated
and frozen meat, respectively. 146
Fresh beef has a limited shelf life even when refrigerated and as such long-
distance transport especially by ship can be unsatisfactory. At temperatures of 0 to
2°C putrefactive spoilage occurs in 6 to 7 days and results principally from the
growth of Gram-negative psychrotrophs. Typically spoilage occurs when the aerobic
plate count reaches 10 7 cfu/cm 2 and is accompanied by the formation of surface
slime, green discoloration, and off-odors. 147,148 The major microbial participants
include the Pseudomonas-Acinetobacter-Moraxella group and members of the
Enterobacteriaceae . 147 Although Pseudomonas is the most important bacterium with
regards to spoilage of fresh refrigerated meat, it is, as previously mentioned, easily
destroyed by relatively low doses of radiation. In this regard, Wolin et al. 149 more
than 40 years ago demonstrated that beef steaks which were radiated at dosages less
than 1 kGy had a shelf life at 2°C which was 4 to 5 times greater than non-irradiated
controls. Furthermore, at this level of application irradiation-induced sensory
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