Rapid Estimation of Lycopene Concentration in Watermelon and tomato puree by Visible Reflectance Spectroscopy

Abstract

Chemometric models were developed for prediction of lycopene concentration in watermelon and tomato puree from their visible reflectance spectra acquired by a fiber optic reflectance probe. An Ocean Optics S2000 fiber optic spectrometer was used to acquire reflectance spectra from puree samples in the wavelength ranging from 500 to 750 nm. Least squares (LS) and Partial Least Squares (PLS) regression were used to correlate spectral data with lycopene concentration measured by hexane extraction and spectrophotometry. An Absorbance Index (AI) obtained by subtracting apparent absorbance at 700nm from that at 565nm showed linear correlation with lycopene concentration (R2 = 0.90 for watermelon puree and 0.62 for tomato puree). A normalized Absorbance Index (NAI) obtained by dividing the AI by the sum of absorbances at 565 nm and 700 nm, also had linear correlation with lycopene concentration (R2 =0.90 and 0.61 for watermelon and tomato respectively). The LS linear regression model for watermelon puree could predict lycopene concentration with R2 of 0.93, and standard error of prediction (SEP) of 5.1 µg/g. The LS linear regression model for tomato puree could predict lycopene concentration with R2 of 0.54 and an SEP of 5.2 µg/g. The PLS regression models developed for watermelon and tomato puree, using wavelength range 500-750, had R2 values of 0.97 and 0.93 respectively. The PLS model for watermelon puree could predict lycopene concentration with an R2 of 0.97 and an SEP of 3.4 µg/g. The PLS model for tomato puree could predict lycopene concentration with an R2 of 0.88 and an SEP of 2.5 µg/g. The results indicate that this method can be used reliably for rapid estimation of lycopene in watermelon and tomato puree samples.