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N. El-Assi, D. J. Huber, and J. K. Brecht

The use of irradiation to increase longevity and quality of horticultural commodities often results in undesirable softening. The biochemical basis of irradiation-induced softening is not well understood. In this study, we investigated the role of the pectic polysaccharides in irradiation-induced textural changes of `Sunny' tomato fruit. `Sunny' mature-green and pink fruit subjected to 84 or 240 Krad experienced a dosage-dependent decrease in firmness, an increase in electrolyte leakage, and an increase in chelator-soluble pectins. Additionally, pectins prepared from 240 Krad-irradiated fruit were of markedly lower mol wt compared to those from nonirradiated fruit. Irradiation-induced downshifts in pectin mol wt were also noted for preripe fruit that lack PG activity. Mol wt decreases noted for pectins from 240 Krad-treated fruit exceeded those observed for fully ripe, nonirradiated fruit The role of other cell wall polymers in irradiation-induced textural changes is currently being addressed.

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N. El-Assi, D.J. Huber, and J.K. Brecht

The irradiation of harvested fruit is typically accompanied by excessive tissue softening, a process that is not well understood. In this study, we examined the role of specific cell wall polymers and the extent of general cell wall degradation and softening in irradiated tomato fruit. `Sunny' tomato fruit at mature-green and pink stages were subjected to X-ray radiation at 0, 83, and 156 Krad. Immediate softening was noted for both maturation classes, although some postirradiation recovery was evident in green fruit. Pectic polymers of both mature-green and pink fruit exhibited depolymerization and altered neutral sugar profiles in response to irradiation. Pectins, either as components of total ethanol-insoluble solids (EIS), purified by selective extraction, or of commercial origin were similarly affected by irradiation. Cellulose preparations were unaffected by irradiation. The data demonstrate that the effect of irradiation on the cell wall exhibits specificity, can occur nonenzymatically, and does not require initiating adducts of cytosolic origin.

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Sasivimon Chomchalow, N.M. El Assi, S.A. Sargent, and J.K. Brecht

Green tomato (Lycopersicon esculentum `Sunny') fruit were stored at 2.5, 5, 7.5, 10, or 12.5 °C (36.5, 41, 45.5, 50, or 54.5 °F) for 1, 3, 5, or 7 days to determine their sensitivity to chilling injury. In subsequent experiments, fruit were treated with ethylene at 20 °C (68 °F) until the breaker stage was reached, either before or after storage at 12.5 °C for 0, 1, 3, 5, or 7 days, or 2.5 °C for 3, 5, 7, or 9 days. Number of days to reach the breaker stage was used as an indicator of initial maturity. The chilling threshold temperature for green `Sunny' tomatoes was near 7.5 °C, with delayed ripening occurring in fruit stored for ≥5 days. Longer exposure times at chilling temperatures resulted in reduced marketable life, dull color, flaccidity, and delayed, uneven (blotchy) and nonuniform ripening. Chemical composition was generally unaffected by chilling, while loss of firmness as a result of chilling exposure time rather than chilling temperatures per se was observed. Increased storage time at either 2.5 or 12.5 °C accentuated the initial differences in fruit maturity and thus resulted in less uniform ripening, especially for tomatoes stored before ethylene treatment, but the effect was much greater following 2.5 °C storage. Exposure to 2.5 °C for as little as 3 days before ethylene treatment caused blotchy ripening and decay, and reduced the marketable life of tomatoes by half compared to storage at nonchilling temperature. Treatment with ethylene before storage prevented chilling injury for up to 5 days at 2.5 °C and prolonged the marketable life of tomatoes stored at either chilling or nonchilling temperature. Tomatoes became less responsive to poststorage ethylene treatment with increased storage time at either 2.5 or 12.5 °C. More mature tomatoes and those treated with ethylene before 12.5 °C storage lost less weight. Vitamin C content was lower in more mature tomatoes, but ethylene treatment resulted in better maintenance of vitamin C by shortening the time to reach the red stage. No other significant differences in color, firmness or chemical composition at the red stage were found between fruit with different initial maturities or fruit treated with ethylene before or after 2.5 or 12.5 °C storage. Treating green tomatoes with ethylene before storage or transport is preferable to poststorage treatment because of faster and more uniform ripening, and also increased marketable life and reduced risk of injury in the event of exposure to chilling temperatures.