Thermal stability and isomerization of lycopene in tomato oleoresins from different varieties

Lycopene, a tomato carotenoid, has been associated with the inhibition of certain chronic diseases including prostate cancer. Tomato oleoresin is a lipid-rich material resulting from successive solvent extraction of the tomato fruit. Thermal stability and isomerization of lycopene in oleoresins prepared from 3 different tomato varieties, Roma, High Lycopene, and Tangerine, and tomato peel waste, were studied at 25°C, 50°C, 75°C, and 100°C in the dark. Thermally degraded lycopene compounds and isomers of lycopene were analyzed by a combination of C₃₀ reversed-phase high-performance liquid chromatograph with a photodiode array detector, UV-visible spectrometer, or mass spectrometer. Effects of antioxidants on lycopene were also studied at 50°C. As the storage temperature increased from 25°C to 100°C, the degradation of total lycopene in oleoresin from all samples increased significantly (P < 0.05). Lycopene at 25°C and 50°C may degrade mainly through oxidation without isomerization. Isomerization of lycopene in tomato oleoresins increased at 75°C and 100°C. Tetra-cis lycopene in Tangerine tomato varieties followed different degradation and isomerization pathways compared with all-trans lycopene in other tomato varieties. Addition of α-tocopherol or butylated hydroxytoluene slowed the rate of degradation of lycopene in oleoresin.