Irradiation - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 109

resulted in a similar extension of shelf life for fresh strawberries. Marcotte 225 and
Pszczola 226 reported earlier on a successful test market introduction of irradiated
strawberries in the U.S. and Brecht et al. 227 has reported on the successful use of
irradiation in conjunction with modified atmosphere storage. Miller et al. 228-230 report
on the use of low dose electron beam irradiation to treat two blueberry varieties and
gamma irradiation for another two. In the electron beam treated blueberries, firmness,
flavor, and texture decreased as irradiation dose increased from 0.25 to 1.0 kGy, but
skin color, decay, soluble solids content, and pH remained unchanged. In the case
of gamma irradiation, there was minimal changes in these quality factors at the
dosage levels required for commercial applications for quarantine disinfestation (i.e.,
0.5 to 1.0 kGy). Some recent work by Cumming 224 using gamma irradiation suggests
blueberries will tolerate doses up to 4.0 kGy, which significantly decreases mold
growth by approximately 50% after 2 weeks of storage.
Gamma irradiation also has been successfully used on cherries as a replacement
for methyl bromide fumigation. 231 The cherries were irradiated at doses from 0.1 to
1.0 kGy, then stored for 14 to 21 days at 10°C before quality was determined. No
variation of fruit or stem color, soluble solids content, acidity, or sensory quality
was noted. However, there was some loss of firmness in the irradiated fruit treated
at doses above 0.4 kGy. Later work by Warner 232,233 confirmed that irradiation was
preferrable to fumigation for preservation of quality in cherries.
Vegetables
Irradiation at low doses (50 to 150 Gy) has been commercially used in many coun-
tries to suppress sprouting in many products such as potatoes, yams, turnips, beets,
Jerusalem artichoke, onions, carrots, garlic, sweet potatoes, and ginger. 215 The use
of ionizing radiation at these doses to inhibit sprouting of white potatoes was
approved in the U.S. in 1964. 234 The applications of ionizing radiation for sprout
inhibition and its effect on quality of potatoes has been reviewed by Matsuyama and
Umeda. 235 A minimum dose of 20 to 30 Gy for onions and 30 Gy for potato tubers
is required for sprout inhibition and smaller doses may actually stimulate sprouting.
Doses greater than 150 Gy may cause detrimental effects in both tubers and bulbs.
At sprout inhibiting doses, approximately 15% of the initial ascorbic acid was lost;
however, after prolonged storage the differences between irradiated and unirradiated
potatoes were not significant. There were some changes noted in the free amino
acids of potatoes irradiated at 150 Gy, but these changes disappeared during storage.
Earlier in an excellent review, Thomas 236 reported that a dose of 0.1 kGy inhibited
sprouting irreversibly regardless of variety and storage temperature. Best results
were obtained when good quality tubers harvested with minimal injuries and cured
to heal bruises were irradiated. However, irradiation of potatoes, even at antisprouting
doses, induces or enhances three types of discoloration: black spot, vascular brown-
ing, and darkening after cooking.
Although most of the work reported on irradiation of potatoes was at low doses
for antisprouting, Cumming 237 has reported on experiments conducted at higher
doses up to 10 kGy. He found that at a dose of 2 kGy, polyphenol oxidase activity
is reduced, resulting in less darkening on slicing. There was some softening of texture