The E/Z isomer ratio of lycopene in foods and effect of heating with edible oils and fats on isomerization of (all-E)-lycopene

Since Z-isomers of lycopene are more bioavailable and show a higher antioxidant capacity than the (all-E)-isomer, it is important to investigate foods containing high amount of the Z-isomer and develop practically feasible method for Z-isomerization of (all-E)-lycopene. First, we investigated the E/Z isomer ratio of lycopene in raw and commercially available processed tomato products using an improved normal-phase HPLC method. The tomato products contained 4.6–33.4% of Z-isomers to the total lycopene, (5Z)-lycopene being the most abundant Z-isomer. The oil-containing products like tomato sauce and tomato soup suffered heat processing contained a higher percentage of Z-isomers of lycopene (27.4–33.4%). Subsequently, the impact of the amount and types of oils added on thermal Z-isomerization of (all-E)-lycopene contained in tomato puree was investigated. Increased addition of olive oil to tomato puree increased the production of lycopene Z-isomers upon heating at 120°C. (all-E)-Lycopene contained in tomato puree was converted to Z-isomers in the range of 39.2–50.7%, when 5% of vegetable oil (linseed, soybean, corn, sesame, rapeseed, rice bran, safflower seed, olive, sunflower seed, or coconut oils) or animal fat (beef tallow and pork lard) was added before heating at 120°C for 30 min. When sesame oil was employed, the total Z-isomerization ratio and (5Z)-lycopene content were significantly increased. Practical applications: The dietary intake of lycopene offers many health benefits such as decreased risk of cancer and arteriosclerosis. Lycopene has a large number of geometric isomers caused by E/Z isomerization at arbitrary sites within the eleven conjugated double bonds, and the functionalities such as antioxidant capacity and bioavailability of the Z-isomers are higher than those of the all-E-isomer. This study clarified the foods richly containing Z-isomers of lycopene and demonstrated thermal Z-isomerization of (all-E)-lycopene contained in tomato puree with edible oils and fats. These findings will contribute to effective intake of lycopene and the development of facile isomerization of (all-E)-lycopene to Z-isomers in the fields of food, drink, and dietary supplement manufacturing, as well as for daily cooking at home. The Z-isomers of lycopene contents in processed tomato products and thermal isomerization of (all-E)-lycopene to the Z-isomers with edible oils and fats were investigated. The tomato products cooked with oils contained higher levels of the Z-isomers, and sesame oil had excellent promoting effect of thermal Z-isomerization of (all-E)-lycopene.