Effect of Low-dose Gamma Irradiation on the Postharvest Quality of Seedless Kishu Mandarins (Citrus kinokuni mukakukishu)

Tephritid insects are commonly found on packaged citrus and postharvest treatments such as fumigation and cold treatment are used to mitigate the risk from these pests.  In 2006, the USDA approved irradiation at a generic dose of 150 Gy for any tephritid fruit fly and 400 Gy for all insects except pupae and adult of Lepidoptera.  In this work, seedless Kishu mandarins were treated with 0, 150, 400 and 1000 Gy. The fruit was stored at 6 °C and evaluated for quality after two days, three weeks and four weeks (3 weeks at 6 °C + 1 week at 20 °C) to determine the impact on quality shortly after treatment, after three weeks of sea shipment, and after three weeks of shipment plus one week under retail conditions. The immediate and dose-dependent impact of irradiation was seen in reduction of firmness of the pulp and an increase in titratable acidity.  The concentrations of individual organic acids also increased following treatment. The fruit treated with 400 Gy always showed the lowest content of total soluble solids among irradiated fruit, with fruit treated with 150 and 1000 Gy showing similar values. This trend was also observed for individual sugars but only immediately after irradiation application. Hesperidin and narirutin were the most abundant of the seven tested phenolic compounds. The content of the majority of tested phenolic compounds increased immediately after irradiation application but their content tended to decrease in a dose-dependent fashion during the rest of the experiment.  Irradiation did not significantly alter the tristimulus color of the flesh.  Consumer tests of mandarin segments using nine point hedonic scales revealed insignificant differences in overall liking, taste, texture, color, and juiciness of the mandarin segments.  The greatest impact of 400 and 1000 Gy treatment was development of rind damage a few days following irradiation which manifested as dark coloration on the fruit peel.  No internal damage was observed.  The fruit treated with 400 and 1000 Gy developed severe fungal infections during the storage at 20 °C and therefore it was discarded before the completion of the experiment. In the 150 Gy samples, some rind damage was evident after three weeks of treatment but exposure to room temperature enhanced damage making the fruit unacceptable to consumers.