Postharvest Biology and Technology 58 (2010) 21–28 Contents lists available at ScienceDirect Postharvest Biology and Technology journal homepage: www.elsevier.com/locate/postharvbio The concentration of trans-lycopene in postharvest watermelon: An evaluation of analytical data obtained by direct methods Darko Dimitrovski a , Dane Bicanic b,c,∗ , Svjetlana Luterotti d , Charlotte van Twisk e , Josephus Gerardus Buijnsters f , Otto Dóka g a Faculty of Technology and Metallurgy, Ss. Cyril and Methodius University, Rugjer Boˇ skovi´ c 16, 1000 Skopje, Macedonia b Laboratory of Biophysics, Wageningen University, Dreijenlaan 3-Transitorium, 6703 HA Wageningen, The Netherlands c Department of Food Quality and Nutrition, Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottieva 6, 10000 Zagreb, Croatia d Faculty of Pharmacy and Biochemistry, University of Zagreb, Ante Kovaˇ ci´ ca 1, 10000 Zagreb, Croatia e Product Quality Development, Agrotechnology and Food Sciences, Wageningen University, Bomenweg 2-Biotechnion, 6703 HA Wageningen, The Netherlands f Institute for Molecules and Materials, Radboud University Nijmegen, Heijendaalseweg 135, 6525 AJ Nijmegen, The Netherlands g Department of Physics and Mathematics, University of West Hungary, Deak F. sq. 1, 9200 Mosonmagyaróvar, Hungary article info Article history: Received 5 August 2009 Accepted 7 May 2010 Keywords: Watermelon trans-Lycopene assay Optothermal window Colorimetry abstract The performance of the newly proposed laser-based optothermal window (OW) method and colorimetry for quantification of trans-lycopene in 10 watermelon homogenates has been evaluated. Reverse phase HPLC served as an established reference method. Both, OW and colorimetry are direct methods as they, contrary to the HPLC, obviate the need for extraction, which leaves homogenization of the sample as the only preparatory step prior to the analysis itself. The evaluation of analytical performance of each method leads to the conclusion that the OW method and colorimetry are both suitable for quick screening of the trans-lycopene concentration of red-fleshed watermelon homogenates. Linear correlation is highest (R = 0.917) for the laser-based OW method. This detection concept offers an additional but very unique advantage. By virtue of the operational principle of the OW method, it is possible to avoid the effect of saturation, a phenomenon known to cause difficulties when interpreting data collected by other analytical methods. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Watermelon (Citrullus lanatus, family Cucurbitaceae) is a fruit characterized by a smooth external rind (green or yellow) and a juicy, sweet, red coloured flesh (Wikimedia Foundation, 2008). Nowadays a few hundred watermelon cultivars, both seeded and seedless are used commercially. The red colour imparted to water- melon is due to carotenoid lycopene, an unsaturated hydrocarbon (11 conjugated and 2 non-conjugated double bonds) with an iso- prenoid polyene chain structure made up of 40 carbons and 56 hydrogen atoms (Ronen et al., 1999). Because of the established beneficiary effects of lycopene (free-radical scavenger) on human health, lycopene is often used as a food supplement as well as a natural food colorant (CondéNet, Inc., 2008). Plant matrix is an important factor for the digestion and absorption of lycopene from the gastrointestinal tract. For example, the consumption of watermelon juice has been shown to cause significant increase in ∗ Corresponding author at: Laboratory of Biophysics, Wageningen University, Dreijenlaan 3-Transitorium, 6703 HA Wageningen, The Netherlands. Tel.: +31 317 484954; fax: +31 317 482725. E-mail address: Dane.Bicanic@wur.nl (D. Bicanic). lycopene concentration in human plasma (Alison et al., 2003). The health benefits resulting from a lycopene rich diet (Gerster, 1997) include reduced incidence of cardiovascular diseases, lower risk for contracting some types of cancer (for example prostate cancer), improvement of bone mineral density and of sperm mobility, bet- ter protection of skin exposed to the UV radiation, etc. (Chen et al., 2001; Hadley et al., 2002, 2003; Andreassi et al., 2004; Stahl et al., 2001; Wattanapenpaiboon et al., 2003; Gupta and Kumar, 2002). Profiles and the concentrations of carotenoids of red-fleshed watermelons have been studied extensively (Collins and Perkins- Veazie, 2006; Perkins-Veazie et al., 2001; Perkins-Veazie and Collins, 2006a). The outcome of the study conducted on 50 water- melon cultivars (Perkins-Veazie et al., 2006b) showed that: (i) the total concentration of carotenoids in watermelon varies between 37 and 122 mg/kg and (ii) that lycopene is the most abundant (84–97%) carotenoid in watermelon. It is present predominantly in the trans-form; the major isomers are 13-cis- and 5-cis-lycopene. 1.1. Carotenoid analysis High performance liquid chromatography (HPLC), UV–vis spec- trophotometry (SP), and the evaluation of colour by colorimetry 0925-5214/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2010.05.002