Prediction of tomato firmness using spatially-resolved spectroscopy

This paper reports on evaluating the firmness of tomato fruit using a newly developed spatially-resolved spectroscopy (SRS) system with an illumination optic fiber and 30 detection optic fibers arranged at the source-detector distances of 1.5-36 mm. Spatially-resolved (SR) spectra of 550-1650 nm were acquired for 600 'Sun Bright' tomatoes at six maturity stages. The firmness of tomatoes was measured using acoustic/impact, compression and puncture tests. Partial least squares (PLS) models for individual SR spectra and their combinations were developed to determine optimal prediction models for the firmness parameters. The results indicated that firmness predictions varied with the light source-detector distance or SR spectra, and the optimal single spectrum was different for prediction of different firmness parameters. Those spectra acquired for the light source-detector distances of 6-24 mm resulted in better prediction results. Combinations of SR spectra gave consistently better predictions for the multiple firmness parameters than the optimal single SR spectra, with the correlation coefficients (rp) of 0.760 and 0.911 for acoustic and impact measurement, rp = 0.935 for compression, and rp = 0.917, 0.948 and 0.859 for puncture maximum force, slope and flesh firmness. Overall, the SRS technique gave excellent predictions of firmness parameters for impact, compression and puncture tests that measured the local properties of tomato tissues, and combinations of SR spectra improved prediction results.