Spectral characterisation of Z-isomers of lycopene formed during heat treatment and solvent effects on the E/Z isomerisation process

The geometric isomerisation of (all-E)-lycopene, purified from tomato paste, was investigated in various organic solvents. Isomerisation ratios to the Z-isomers of lycopene in CH2Cl2 and CHCl3 over 24 h were calculated to be 19.7% and 11.4% at 4 °C and 77.8% and 48.4% at 50 °C, respectively. In CH2Br2, more than 60% was attained in the first several hours, independent of temperature. The predominant Z-isomers obtained thermally, (9Z)-lycopene and (13Z)-lycopene, were purified and their absorption maxima and molar extinction coefficients in hexane were determined for the first time. Absorption values at 460 nm were also measured for both Z-isomers along with (all-E)-lycopene to accurately evaluate their concentrations by HPLC analysis. This approach successfully revealed that (13Z)-lycopene formed predominantly in benzene or CHCl3 at 50 °C; in contrast, the 5Z-isomer was preferentially obtained in CH2Cl2 or CH2Br2.