Kinetic studies of lycopene stability and isomerization during processing and storage

Lycopene has numerous geometrical isomers, but in nature it is almost exclusively in the all-trans (ATL) form. Recent studies suggest that the cis (CL) isomers may be more bioavailable and/or bioactive than ATL, so increasing the CLs concentration in foods may improve their biological value. The stability of lycopene isomers was investigated in various systems, but there are few kinetic studies and none used purified lycopene isomers in defined model systems.;The objective of this work was to compare the stability of cis and trans lycopene isomers under processing and storage conditions typical for commercial food processing, and generate a kinetic model able to predict the extent of the reversible isomerization of ATL to 13-CL as function of the processing temperature.;ATL was extracted from tomato products. A published RP-HPLC method was modified and adapted to discriminate and quantify at least 3 isomers. A new semi-preparative HPLC method for CL preparation was developed, using thermally-treated ATL preparation as starting material. The isolated and purified isomer was identified as 13-CL. While the method is very time-consuming (4--5 weeks to prepare 1 mg), about 4 mg of 13-CL were prepared overall.;Kinetic studies were performed on solutions of ATL and 13-CL in tributyrin, subjected to selected temperatures between 4 and 85°C or emulsified at pH 1.5 and 5.5. The system was successfully described using a simplified set of chemical reactions assumed to follow first order kinetics. In all cases, ATL was found more stable than 13-CL, although the difference decreases at lower temperatures. The ATL isomerization to 13-CL is reversible, and aims towards reaching an equilibrium, whose isomeric distribution is temperature-dependent. Arrhenius activation energy and collision factor values were calculated for the reversible ATL isomerization to 13-CL.;The study of ATL-fortified tomato homogenate heated at 65°C found that 'native' lycopene is stable at this temperature, unless fats are added. Fortification was proved possible, but the 'added' lycopene is significantly less stable than the 'native'. The prediction of 'added' lycopene behavior using the kinetic data developed above is discussed.;This study advances the understanding of lycopene isomerization and/or breakdown that may occur during processing and storage.