Characterisation of the coloured thermal degradation products of bixin from annatto and a revised mechanism for their formation

The main thermal degradation product of 9′-cis-bixin, the principal liposoluble colouring component of annatto food colouring, has been prepared, isolated and purified. Its structure has been confirmed as the trans-monomethyl ester of 4,8-dimethyltetradecahexaenedioic acid using HPLC with photodiode array detection, 1H NMR spectroscopy and mass spectrometry. The compound has been shown to exist in different stereoisomeric configurations in solution and to be susceptible to hydrolysis, forming a range of compounds analogous to bixin and norbixin. The results lead to a revised mechanism for the thermal degradation of 9′-cis-bixin.