Non-destructive internal quality assessment of “Hayward” kiwifruit by waveguide spectroscopy

► The research aims to non-destructively assess the kiwifruit quality by waveguide spectroscopy. ► The 2–20GHz frequency range was explored. ► The assessment of the SSC and MTf were conducted with PLS regression analysis. ► SSC and MTf were predicted with R2 values up to 0.804 and 0.806, respectively. Soluble solids content (SSC) and Magness–Taylor flesh firmness (MTf) of “Hayward” kiwifruits were non-destructively assessed by means of a waveguide, that houses the fruit, connected to a sweeper oscillator and a spectrum analyzer. A preliminary test was conducted with a plastic fruit filled with solutions with different SSC values in the frequency range from 2 to 20GHz (with a step of 1GHz). The best linear correlations (R2 up to 0.987) between electric signals and SSC solutions in the above described test were found in the 2–3GHz and 15–16GHz steps. These steps were used for the dielectric measurements on kiwifruit samples during storage of 28days at 14°C. Partial least squares (PLS) regression were then used to predict SSC and MTf from these acquired spectra. In “test set” validation, PLS models showed R2 values up to 0.804 (RMSE=0.98°Brix) and 0.806 (RMSE=8.9N) for the prediction of SSC and MTf, respectively.