Non-Destructive Assessment of Kiwifruit Flesh Firmness by a Contactless Waveguide Device and Multivariate Regression Analyses

Non-destructive and cheap methods to evaluate the slow ripening process with possible on-line applications are highly required by the industry to enhance critical post-harvest management. After a brief review of the literature, we present the potentiality of an electronic contactless device for the non-destructive assessment of the Magness-Taylor flesh firmness (Mtf) of Hayward kiwifruits. The technique combines spectral information acquired in the microwave range by an open-ended aluminum waveguide containing TX and RX antennas, placed above the sample, with the features of the multivariate analysis. The electronic controller comprises a VCO, a low noise amplifier, a gain-phase comparator, and a serial interface governed by an MCU. Partial Least Squares regression analysis (PLS) was used to build predictive models starting from gain and phase waveforms raw data in the 947-1900 MHz frequency range. The main results evidenced that explored spectra variability is related to changes occurring in the fruit during the maturity process and particularly to the cell wall degradation. PLS regression models show, in prediction, R-2 values of 0.726 (RMSE = 5 N) for the estimation of the Mtf starting from gain waveforms. A lower accuracy was observed for the model setup by considering phase waveforms. These results demonstrate that the proposed non-invasive solution combined with the PLS is a grounded starting point for estimating kiwifruit firmness with an acceptable level of accuracy.