Potential and limitations for determining lycopene in tomatoes by optical methods

Lycopene in tomatoes and tomato products is routinely determined by extraction into organic solvents and spectrophotometric quantification. The direct determination of lycopene, by measuring the color or other optical properties of a tomato homogenate, would be an attractive alternative. We have evaluated two instruments for this purpose. The color of tomatoes and tomato products is routinely determined with a reflectance colorimeter such as a LabScan (Hunter Lab). We have examined whether the parameters determined with this instrument correlate sufficiently well with the lycopene content of raw tomato juice to allow for the prediction of lycopene levels. CIE "a" values were not linearly related to lycopene content but could be fit to a logarithmic regression line. A better, but still logarithmic, fit was Obtained when the parameter optical density at 560 nm minus the optical density at 700 nm, was plotted versus lycopene. The absorbance of light transmitted through tomato juice was also measured with a Hunter Lab UltraScan. The parameter, absorbance at 560 nm minus absorbance at 700 nm was linearly related to the lycopene content of the tomato juice. The slope of the regression line, however, was affected by the method used to homogenize the tomato. Measured values for (A560-A700) were also substantially lower in cooked versus raw tomato juice samples that contained equal amounts of lycopene. Evidently, factors other than the total lycopene content greatly affect the absorbance values measured by the UltraScan.