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Determination of lycopene in various cultivars of fresh tomatoes by means of photoacoustic and optothermal window spectroscopies

The use of photoacoustic spectroscopy and optothermal window as new analytical tools in quantitave analysis of lycopene in various cultivars of fresh tomatoes has been elaborated. Lycopene, a major phytochemical in tomatoes, was demonstrated to possess a higher antioxidant activity than β-carotene and lutein. In addition increased consumption of tomato-based products is associated with the reduction of prostate cancer. Owing to the health concerns the need for knowing the amount of lycopene in fresh tomatoes and processed tomato products has become an issue of relevance. The two most frequently used approaches used to quantify lycopene are HPLC and spectrophotometry (SP) – both approaches are rather elaborate because the pigment must be extracted before actual analysis. In the past, photoacoustic (PA) spectroscopy and optothermal window (OW) were shown capable of directly (no need for extraction of the analyte) quantifying the lycopene content in processed tomato products. This is mainly due i) the fact that in tomatoes lycopene is a dominant carotenoid and ii) a favourable spectral contrast between lycopene and β-carotene at 502 nm. These facts have enabled rapid and accurate detection of lycopene in tomato-products with pigment content ranging from 8 mg/100 g product weight (tomato juice) to approximately 65 mg/100 g product in the case of a triple concentrated tomato puree. It was therefore of interest to also explore the potential of PA and OW methods for detection of lower quantities of lycopene. Fresh tomatoes in which all-trans lycopene is dominant, were selected as the test sample-the tomatoes originated from different countries and had varying red appearance. Samples were crushed, homogenated and then centrifuged for 5 minutes at 10000 g. The supernatant was then separated from the red coloured pellet. This latter was then deposited into the PA cell or on the surface of the OW. The PA investigation was performed at 35 Hz using argon ion laser (10 mW) at 502 nm for selective excitation. The OW study was carried out at 12 Hz using a 1.8 mW LED as a radiation source. Control measurements on the same samples were run on HPLC and SP (at 502 nm). For majority of samples lycopene data obtained by PA and OW is a good agreement with the results of SP measurements at 502 nm. Exceptions are few samples for which the supernatant fraction was not completely colourless probably indicating unsufficient extraction. In such a case, it is necessary to correct the data by the independent absorbance measurement at 502 nm of the supernatant itself. In conclusion, both, PA and OW are able to directly quantify lycopene in fresh tomatoes. Apparently, the sensitivity of two methods is sufficiently high to allow for a direct determination of this pigment in fruits with even lower lycopene content such as papaya and red grapefruit.

lycopene; quantitation; tomatoes; photoacoustic spectroscopy; optothermal window

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