A field-deployable spectroscopic approach for optimizing kiwifruit harvest timing and postharvest quality management

Extensive research has focused on using visible-near-infrared (Vis-NIR) spectroscopy for laboratory-based quality assessment of postharvest agricultural products. Recently, field-deployable solutions are becoming increasing available, however, assessing fruit firmness remains challenging. Along with sugar content, firmness is a critical factor because of its strong influence on fruit taste and postharvest shelf life. This study designed a portable Vis–NIR spatially resolved spectroscopic (SRS) system by integrating three modules: (1) a 36-channel fiber array was arranged in six radial distances to acquire spatially resolved diffuse reflectance profiles of 108 kiwifruit samples from each of the two kiwifruit species ‘Cuixiang’ and ‘Ruiyu’ with different forchlorfenuron (CPPU) treatments, (2) a cost-effective hyperspectral imaging camera within a wavelength range of 600–1100 nm, with a resolution of 4.5 nm combined with halogen illumination was employed to reduce the measurement time of spectral data, and (3) partial least squares (PLS) regression analysis coupled with the competitive adaptive reweighted sampling technique were developed to optimize scattering-enhanced firmness prediction and scattering-suppressed soluble solids content (SSC) quantification. Field validation using the ‘Cuixiang’ species demonstrated the operational robustness of the system, with $R_{\mathrm{val}}^2$ = 0.65 and RMSE_val = 1.23 N for firmness and $R_{\mathrm{val}}^2$ = 0.65 and RMSE_val = 1 % for SSC. In contrast, for ‘Ruiyu’ species, $R_{\mathrm{val}}^2$ and RMSE_val were 0.14 and 2.18 N for firmness, $R_{\mathrm{val}}^2$ and RMSE_val values of 0.39 and 1.45 % for SSC. This could be because ‘Ruiyu’ have thicker skin and more fine hairs, which may have interfered the light reflectance measurements of the designed device. These results highlight the potential of contact SRS as an useful tool for assessing the ripeness of ‘Cuixiang’ kiwifruit, particularly in terms of firmness.