Photo.protective effects of furocoumarins on terpenes in lime, lemon and bergamot essential oils upon UV light irradiation

The impact of naturally occurring furocoumarins on essential agrumen oils, namely bergamot, lime and, lemon, was investigated upon exposure to UV-A light. For this purpose, the oils were initially freed from furocoumarins and coumarins by precipitation in cold hexane. Such pretreated oils, as well as samples of these oils spiked with separated furocoumarins were irradiated for up to 10 days. All essential oils devoid of furocoumarins showed a massive degradation of the predominant terpenes R-(+)-limonene and ?-terpinene. For lime and lemon essential oils 10 percent and 7.5 percent of the initial R-(+)-limonene amount was degraded within 10 days, respectively. In addition, a noticeable hydroperoxide formation was observed. For ?-terpinene, this effect was even more pronounced and in both, lime and lemon essential oil samples, the terpene was entirely converted into p-cymene after 6 days. In comparison, addition of 5 percent furocoumarins to the essential oils decelerated the photo degradation of R-(+)-limonene and ?-terpinene by up to one order of magnitude. The protective effect of furocoumarins, presumably due to bathochromic shifts of emitted light to less harmful longer wavelengths, also improved the olfactory quality. The results demonstrate that blends of non-volatile furocoumarins and volatile terpenes extend the shelf-life of light-sensitive agrumen oils.