KINETICS AND MECHANISM OF THE LIGHT-INDUCED DETERIORATION OF LEMON OIL

The kinetics and mechanisms involved in the light-induced photodegradation of lemon oil (LO) have been investigated using singlet molecular oxygen [O2(1-DELTA(g))] time-resolved phosphorescence detection and steady-state irradiation, in organic and micellar aqueous solutions. A parallel study on the kinetics of limonene (L) photooxidation was carried out, for comparative purposes. An O2(1-DELTA(g))-mediated photooxidative process takes place in LO both in the absence and in the presence of external sensitizers. In the former case the pigments present in LO itself efficiently sensitize the generation Of O2(1-DELTA(g)) with a quantum yield of 0.14. L and other minor componets of LO [especially the terpinenes, efficient O2(1-DELTA-(g)) quenchers] are responsible for the photodegradation. Upon direct irradiation of LO in the wavelength range 300-350 nm an additional photochemical process operates: the decomposition proceeds from electronically excited states of LO susceptible components, which prevails over the competitive O2(1-DELTA(g)) mechanism. The results should be carefully considered in a practical sense, when LO or LO-containing foods are stored under daylight conditions. Direct and sensitized light exposures cause irreversible transformations of LO.