Microwave-Accelerated Z-Isomerization of (all-E)-Lycopene in Tomato Oleoresin and Enhancement of the Conversion by Vegetable Oils Containing Disulfide Compounds

The effect of microwave irradiation on Z-isomerization of (all-E)-lycopene contained in tomato oleoresin is investigated, and the Z-isomerization efficiency is compared to that of the conventional oil-bath heating method. With a 4-min microwave treatment at 2450 MHz frequency and 500 W power, the total Z-isomer content of lycopene increased to approximately 70%, almost without decomposition, and the efficiency, at the same temperature, is higher than that with conventional heating. In both treatments, the (9Z)- and (13Z)-isomers are mainly formed. Moreover, the effect of addition of vegetable oils to the oleoresin on the Z-isomerization by microwave irradiation is examined. When roasted seed oils and garlic oil are added, the production of (5Z)-lycopene, an isomer having higher functionality and storage stability among the Z-isomers, is significantly increased. This work reveals that the promoting effect is probably caused by the presence of disulfide compounds existing commonly in the above oils: Thiyl radicals, produced from disulfide compounds by microwave irradiation, would act as a catalyst for the Z-isomerization reaction. Practical Applications: Z-Isomers of lycopene, which are the predominant forms in the human body, have higher bioavailability and antioxidant capacity than the all-E-isomer, which is the major configuration form in raw tomatoes. Thus, appropriate methods are required to achieve the efficient Z-isomerization of (all-E)-lycopene. This study has developed an efficient production method for lycopene Z-isomers by microwave irradiation of (all-E)-lycopene in edible vegetable oils containing disulfide compounds, in which neither organic solvents nor food additives are used. It would be practically feasible to utilize this procedure not only for food, drink, and dietary supplement manufacturing, but also for daily cooking. (all-E)-Lycopene in tomato oleoresin is efficiently isomerized to the Z-isomers by microwave irradiation and the Z-isomerization is accelerated by adding vegetable oils containing disulfide compounds.