Directional versus total reflectance spectroscopy for the in situ determination of lycopene in tomato fruits

Non-destructive tools for evaluating the lycopene content in tomatoes are of great interest to the entire fruit chain because of an increasing demand for beneficial health products. With the aim of developing compact low-cost reflectance sensors for lycopene determination, we compared Partial Least Squares (PLS) prediction models by using either directional or total reflectance in the 500-750 nm range. Directional reflectance at 45 degrees with respect to the LED lighting direction was acquired by means of a compact spectrometer sensor. Total reflectance was acquired through a 50-mm integrating sphere connected to a spectrometer. The analysis was conducted on two hydroponic greenhouse cultivated red tomato varieties, namely the large round 'Dometica' (average diameter: 57 mm) and the small cherry 'Juanita' (average diameter: 26 mm). For both varieties, the spectral variance of directional reflectance was well correlated to that of total reflectance. The performances of the PLS prediction models were also similar, with R-2 of cross-validation between 0.73 and 0.81. The prediction error, relative to the mean lycopene content of full ripe tomatoes, was similar: i.e. around 16-17% for both varieties and sensors. Our results showed that directional reflectance measured by means of portable, low-cost and compact LED-based sensors can be used with an adequate precision for the non-destructive assessment of lycopene in tomatoes.