Kinetics and yields for the formation of coloured and aromatic thermal degradation products of annatto in foods

Abstract

The thermal stability of annatto food colouring has been studied in model systems and foods. High-performance liquid chromatography with photodiode-array detection has been used to monitor the isomerization products of 9′-cis-bixin, the principal colouring component of annatto, and its major coloured C17 degradation product. Headspace gas chromatography–mass spectrometry has been used to determine the degradation products toluene and m-xylene in the head space of model systems and foods containing annatto heated in situ. Fish and cheese spiked with 9′-cis-norbixin produced m-xylene whereas control samples did not. Low levels (ca. 10–50 μg/kg) of m-xylene were detected in the headspace of annatto-coloured retail samples of custard powder, extruded snacks, margarine and bread crumbs. Higher levels of m-xylene were detected in the headspace of kippers (ca. 150–200 μg/kg) and observed in the headspace of Red Leicester type cheese (not quantified). The findings indicated that annatto is readily degraded to form both coloured degradation products and the aromatics m-xylene and, to a lesser extent, toluene. In practice however, degradation is slow under heating conditions normal for foods. Neither aromatic species was detected in the headspace of any food studied above 200 μg/kg.