Isomerization of carotenoids, which is often encountered in food processing under the influence of temperature and light, may play a role in the observed protective effects of this group of secondary plant products. Investigation of in vitro antioxidant activity of prominent carotenoid geometrical isomers was undertaken in light of recent reports illustrating a large percentage of carotenoid (Z)-isomers in biological fluids and tissues. Alpha-carotene, beta-carotene, lycopene, and zeaxanthin were isolated from foods or supplements and subsequently photoisomerized with iodine as a catalyst. Major Z-isomers of each carotenoid were fractionated by semipreparative C(30) HPLC. In vitro antioxidant activity of all isomers collected was measured photometrically using the Trolox equivalent antioxidant capacity (TEAC) assay. TEAC values of 17 geometrical isomers investigated ranged from 0.5 to 3.1 mmol/L. Three unidentified (Z)-isomers of lycopene showed the highest antioxidant activity, being significantly higher than the result for (all-E)-lycopene, which had approximately two times the activity of (all-E)-beta-carotene. On the other hand, (9Z)-zeaxanthin had a more than 80% lower TEAC value compared to that of (all-E)-lycopene. These results allow for the in vivo relevance of (Z)-isomers of carotenoids to be considered.

Detection and quantification of tocopherols, carotenoids, and chlorophylls in vegetable oil have been used to aid their authentication. Their importance in influencing the oxidative stability of vegetable oils and their possible health benefits have been shown in numerous studies. Therefore, the need for a rapid and reliable analysis method has become increasingly important. This study demonstrates the application of C 30 RP-HPLC with electrochemical detection for the simultaneous analysis of tocopherols, carotenoids, and chlorophylls in vegetable oils. Aliquots of vegetable oils were dissolved in appropriate solvents and injected directly without saponification, thus preventing sample loss or component degradation. Effective separation of tocopherols, carotenoids, and chlorophylls was achieved. Detection was performed using a coulometric electrochemical array detector set between 200 and 620 mV. For a 25-µL injection, the respective detection limits for carotenoids, tocopherols, and chlorophylls were 1 fmol, 0.15 pmol, and 0.5 pmol, representing 1000-, 25-, and 5-fold enhancement over the UV-vis methodology. The detector response was linear between 0.01 and 2.00 µg/mL for all compounds studied. Withinday variations (CV) were between 2.0 and 6.3%, whereas between-day variations were between 2.7 and 7.4%. This method can be applied for rapid and sensitive analysis in the study of oil quality and adulteration.

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