Efficacy of Orange Essential Oil and Citral after Exposure to UV-C Irradiation to Inhibit Penicillium digitatum in Navel Oranges

The effect of UV-C irradiation on antifungal properties of orange essential oil (EO) against Penicillium digitatum in inoculated Navel oranges was examined. The UV-C irradiation of orange EO resulted in a 20% loss of the major constituent, limonene, and the generation of three hydroperoxide oxidation products, (2S,4R)-p-mentha-6,8-diene-2-hydroperoxide,(1S,4R)-p-mentha-2,8-diene-1-hydroperoxide, and (1R,4R)-p-mentha-2,8-diene-1-hydroperoxide. The P. digitatum growth in oranges dipped in non-irradiated orange EO at 1000–4000 µL L−1 was not significantly different to control the fruit. Dipping in UV-C treated orange EO inhibited the growth of P. digitatum with 4000 µL L−1 having the greatest effect. No phytotoxic injury to the rind was observed at any concentration. Citral, as a known antifungal chemical, was included for comparison. The non-irradiated citral (1000 µL L−1) was more effective than irradiated orange EO, but elicited rind phytotoxicity. The irradiated citral was less effective in inhibiting P. digitatum growth with the loss of citral, but not hydroperoxide formation. These results suggest UV-C irradiated orange EO as a potential alternative to synthetic fungicides to inhibit P. digitatum decay. The source of orange EO could be waste flavedo generated by the orange juice processing industry.