Irradiation - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 104

After 9 days of storage, Gram-positive bacteria, mainly Lactobacillus , predominated
in both the good and poor quality treated meats. Radiation did not result in any
change in product pH or thiobarbituric acid value (indication of lipid oxidation).
Although the microbial spoilage pattern was similar for both the treated and control
samples, the rate of spoilage for the former was much reduced. 172 At a dose of 1.91
kGy or higher, survivors were absent even after 35 days of storage at 2°C. Raw
ground pork and beef, which is commonly used in fermented sausage, was radiated
at 0.2 and 0.5 kGy. 175 Although total plate counts were only reduced by 1.3 and 2.2
logs, respectively, the real benefit arose from the reduction of coliforms and staphy-
lococci. It was also suggested that a decrease in the levels of competing microor-
ganisms would produce a more uniform product allowing better process control and
the possibility of reducing starter culture inocula.
Pork
Approval for the irradiation of pork by the U.S. FDA was granted initially in 1985. 54,176
The USDA Food Safety and Inspection Service (FSIS) granted similar approval; 177
however, since the initial purpose was for elimination of parasites, namely, Trichinella
spiralis , only absorbed doses between 0.3 and 1 kGy were sanctioned.
Despite the relatively low approval dose, Mattison et al. 178 reported that boneless
vacuum-packaged pork loins treated with 1 kGy contained significantly lower levels
of both mesophiles and psychrotrophs when compared to non-irradiated controls.
Even after 21 days of storage at 4°C neither bacterial group reached spoilage levels
of 10 6 cfu/cm 2 . From a sensory perspective, treated cooked samples were indistin-
guishable from the controls even after 14 days of storage, indicating that perhaps
any off-odors generated as a result of treatment slowly dissipated. In any event, the
lack of detectable odors is in agreement with a reported threshold dose for pork of
1.75 kGy. 57 By increasing the dose to 3 kGy, Lebepe et al. 19 reported that vacuum-
packaged boneless pork loins stored at 2 to 4°C exhibited a shelf life of approxi-
mately 90 to 91 days. The shelf life was based on microbiological spoilage levels
of 10 7 and 10 8 cfu/cm 2 for mesophiles or psychrotrophs and lactobacilli, respectively.
In contrast, non-irradiated loins spoiled after 42 days. Although a sensory evaluation
was not performed, treated samples were invariably darker compared to the controls;
pH and thiobarbituric acid values did not differ significantly from the control. Treated
loins following storage at 2 to 4°C for 41 days were further demonstrated to have
a retail shelf life of 10 days at 5 to 7°C, based on psychrotrophic counts, when cut
into pork chops and packaged using an oxygen permeable film.
Using a combination of MAP and radiation, Grant and Patterson 180 reported that
a mixture of 25% CO 2 and 75% N 2 improved the sensory and microbiological quality
of radiated pork. The presence of oxygen, however, was shown to dramatically affect
the sensory (based on color and odor) shelf life of this product. 181 Following radiation
of loins in packages containing 20% O 2 at 1 kGy, they were immediately rejected
by a sensory panel on the basis of strong off-odors and discoloration. In comparison,
the sensory shelf life of loins packaged in barrier bags without any O 2 and radiated
at 1 kGy was 26 days at 5°C. Non-irradiated controls with 20% O 2 had a sensory
shelf life of only 4 days.