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An attempt to record the concentration profile of lycopene pigment in the skin of raw tomatoes by means of photoacoustic spectroscopy

Due to its role as a natural antioxidant and the effects that benefit human health, lycopene, the red pigment present in some fruits is currently attracting considerable public attention. Tomato is the major source of dietary lycopene. It is known that the bioavailability of lycopene in unprocessed fresh tomatoes is lower than that of processed tomato products such as juices, purees and pastes. Spatial distribution of lycopene pigment throughout the tissue of the raw tomato is not uniform. According to several reports, a dominant portion of lycopene is found in the approximately 120 microns thick outer skin, while the bulk itself contains the remaining quantity. Data found in the literature refers to the out come of tedious HPLC studies performed on thin (microtome) tomato slices. In the work described here, photoacoustic (PA) spectroscopy at 502 nm excitation wavelength of the argon laser (10 mW) was used in an attempt to assess the information about the depth dependent concentration of lycopene in the skin of the whole ripe raw tomatoes. Initially, PA measurements were conducted on a two layers model sample and the correctness of the inversion approach checked repeatedly before proceeding with the investigation of realistic raw tomato samples. The thickness of the tomato skin was estimated using the commercially available equipment. Overall, the analysis of PA data collected over a wide range of modulation frequencies suggests the presence of the lycopene containing layer in the skin of raw tomato. Unlike what is encountered in the HPLC, the PA studies measurements are completed quickly and are also practically non-destructive. The PA data is currently being validated by the HPLC method and two other established advanced methods.

lycopene; HPLC; PA; tomato

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