Efficient E/Z conversion of (all-E)-lycopene to Z-isomers with a high proportion of (5Z)-lycopene by metal salts

Abstract

In this paper, the effects of eleven metal salts on the isomerization and stability of lycopene were screened. The results showed that AlCl3, Al2(SO4)3, FeCl3, Fe2(SO4)3, and CuSO4 had significant catalytic effects (p < 0.05) on lycopene isomerization. The total content of lycopene Z-isomers were in the order of FeCl3 (70.8%) > Fe2(SO4)3 (54.0%) > AlCl3 (50.2%) > CuSO4 (40.0%) > Al2(SO4)3 (29.3%) after reacting with 10 mmol/L metal salts at 40 °C for 60 min. When the concentration was greater than 10 mmol/L, lycopene was seriously degraded by FeCl3 and Fe2(SO4)3, but the retention rate of lycopene remained above 70% by AlCl3. Then, the effects of these five metal salts on lycopene isomerization and degradation were analyzed by kinetic tools. All five metal salts reduced the activation energy of lycopene isomerization and degradation reaction. The activation energy of lycopene degradation catalyzed by AlCl3 was 23.9 kJ/mol, higher than that of induced by FeCl3 and Fe2(SO4)3. The estimated equilibrium values of total-Z-lycopene and most stable isomer (5Z)-lycopene reached up 81.1% and 33.0% in the presence of AlCl3. Therefore, AlCl3 is an ideal lycopene catalyst based on the comprehensive retention rate, catalytic efficiency, and high production of (5Z)-lycopene.