Light sensitivity of carotenoids used as food colours

Abstract

The quantum yield for the photobleaching of the carotenoid pigments lutein undβ-carotene solubilized in aqueous solutions has been determined for monochromatic light at different wavelengths to provide an objective measure of the light sensitivity of these natural food colours. The quantum yields are proportional to the square root of the partial pressure of oxygen, indicative of a complex photo-oxidation mechanism in carotenoids. In UV light, lutein is more sensitive thanβ-carotene. Quantum yields for air-saturated solutions at 20° C are 7.8–10−3 mol einstein−1 at 313 nm and 5.1·10−4 at 366 nm for lutein compared with 4.7·10−3 and 3.9·10−4 forβ-carotene. For visible light, photo-oxidation is less severe and quantum yields at 436 nm are less than 3·10−5 for lutein and 3.7·10−5 forβ-carotene, respectively. The photobleaching of an alfalfa extract used as colorant in beverages, and in which the pigment is a complex mixture of geometrical isomers of mainly lutein palmitate, is also significant, although the quantum yield is generally lower than that for lutein. Sensitized photodegradation with initial light absorption by other pigments could be important to such extracts, as has been demonstrated in a model system with rose bengal as a light absorber. The dependence of the quantum yield on the oxygen partial pressure shows that at least two reaction paths are of importance to the sensitized photo-oxidation and a large deuterium isotope effect provides evidence for the involvement of singlet-oxygen in the sensitized photodegradation.