Microwave heating of tomato puree in the presence of onion and EVOO: The effect on lycopene isomerization and transfer into oil

Tomatoes, the major sources of lycopene in human diet, are often processed into multi-ingredient products that contain vegetable oil and onion. Microwave heating (250 W for 20 min) tomato-based purees in the presence of both onion and extra virgin olive oil (EVOO) clearly promoted Z-isomerization of lycopene and increased transfer of lycopene into oil (expressed as partition factor (PF) between tomato-based puree and oil), while only PF increased with either one of both ingredients. The proportion of Z-lycopene and PF increased with the addition of onion (10-50%) and EVOO (1-10%). This increase could be fitted with a linear equation, with R-2 over 0.807. Microwave heating time (0-30 min at 250 W) also promoted Z-isomerization of lycopene and PF (R-2 over 0.910), and among different microwave heating combinations (power, time), the combinations of high power and short time had greater effects. The PF of total-Z-lycopene was higher than that of all-E-lycopene. The proportion of total-Z-lycopene and 5-Z-lycopene present in the tomato-based purees after the same microwave treatment was positively correlated with the PF of total lycopene, with R-2 higher than 0.690 (p = 0.0008), which illustrated that the increased proportion of Z-lycopene with onion and EVOO could contribute to enhanced transfer of lycopene.