Irradiation - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 103

bacteria, which are especially resistant during logarithmic growth, and yeasts nor-
mally survive and predominate especially if vacuum-packaging is utilized. 13,162,163
Studies by Roberts and Weese 158 indicated that vacuum-packaged ground beef
patties having an initial aerobic plate count of 10 4 cfu/g had a shelf life of 14, 21,
42, and >42 days at 4°C following radiation treatment at 1, 3, 5, and 7 kGy,
respectively. In contrast, with ground beef patties having an initial aerobic plate
count of 10 2 cfu/g, microbiological acceptability was maintained even after 42 days
of storage at 4°C following a 1-kGy dose. Further, non-irradiated patties having an
initial aerobic plate count of either 10 2 or 10 4 cfu/g had a shelf life of less than
7 days and a corresponding plate count greater than 10 7 cfu/g. Microbial levels of
this magnitude are well recognized to contribute to objectionable off-odors and
flavors in meats and are indicative of product deterioration. 159 The importance of
using high microbiological quality meat, especially if shelf life extension is to be
expected using a low-dose application, cannot be over stated. Additional studies
have provided results corroborating the notable extension in shelf life of ground beef
following radurization. 159,162,164-166 In the study performed by Niemand et al., 162 it
was reported that beef patties radurized at 2.5 kGy maintained acceptable odor and
appearance for up to 9 days at 4°C. Factors such as storage temperature and product
access to oxygen were demonstrated to have a major influence on shelf life. 162 In
general reducing oxygen by vacuum-packaging and/or the use of oxygen imperme-
able films had positive effects on shelf life extension. Combining radiation treatment
at 2.5 kGy with lactic acid was reported to result in a synergistic effect which further
improved shelf life. 162 Unfortunately, inclusion of acid gave a bleached appearance
to the product. Dempster 166 also reported that following radiation at 1.03 and 1.54
kGy the surface of beef burgers appeared discolored or bright red. Although internal
color was not affected, irradiation odors described as “wet dog”, “musty”, and “burnt
popcorn” were detected by the taste panel when the burgers were treated at both
doses in the presence of oxygen; the odors dissipated somewhat after treatment.
Vacuum packaging combined with low temperature storage following treatment was
advocated to reduce the formation of free fatty acids in the burgers. Ostensibly
produced by autolytic tissue enzymes, the production of free fatty acids would
definitely contribute a shortened shelf life and prompted investigations into the use
of antioxidants. 167 Interestingly, Cohen et al. 168 reported that fat levels and the degree
of grind did not affect product acceptability following radiation. The impact of
radiation on filet américan, a raw ground beef product containing a mayonnaise-like
sauce has also been examined, but only from a safety perspective. 169 Current research
associated with elimination of pathogens in ground meat is given by various
authors. 25,169,170
Ground pork, which is an equally perishable product, can also benefit from low
dose radiation especially if processed using microbiologically sound trimmings. 172
For example, Ehioba et al., 173,174 reported that doses as low as 1 kGy could extend
shelf life, based on psychrotrophic total counts corresponding to 10 7 cfu/g, from 8
to 11.5 days if the meat was vacuum packaged and stored at 5°C. Following similar
treatment and storage conditions, however, a simulated poor quality grind containing
approximately 10 5 cfu/g exhibited a shelf life extension from 4.5 to only 6 days.