Internal quality assessment of kiwifruit by bulk optical properties and online transmission spectra

The purpose of this study was to compare the accuracy and robustness of the detection models based on bulk optical properties (BOP) with that based on conventional spectroscopy in kiwifruit quality evaluation. 81 kiwifruit were selected as experimental samples in this study. A single integrating sphere system was built to estimate the bulk absorption coefficient ( μ a ) and bulk reduced scattering coefficient ( μ s ′ ) of samples and a self-designed online system was used to obtain transmission spectra. The relationship of μ a and μ s ′ with SSC and flesh firmness was analyzed, and detection models were established using partial least squares regression (PLSR). Competitive adaptive reweighted sampling (CARS) was also used to eliminate the variables in the original spectra that do not contribute to the improvement of the model performance. Results showed that μ a at 670 nm decreased with the increase of SSC, μ a at 720–900 nm and 950–1000 nm increased with the increase of SSC, and spectra of μ s ′ decreased with the decreasing firmness. CARS-PLSR models were developed, based on μ a , μ s ′ , μ a × μ s ′ , μ e f f , μ t ′ , and transmission spectra. The accuracy of the model based on BOP in predicting internal quality was better than that based on transmission spectra. The model based on μ a was the best for SSC ( R p 2  = 0.97, RMSEP = 0.25%), and the model based on μ a × μ s ′ was the best for flesh firmness ( R p 2  = 0.97, RMSEP = 0.02 N). 20 kiwifruit that differed from the experimental samples in planting orchard and harvest time were used to compare the robustness and portability of the models. Results showed that all SSC models and the firmness model based on μ a × μ s ′ had good robustness and portability. However, the model based on transmission spectra had a poor performance in predicting the firmness of samples from the external validation set. This study provides an effective reference for the prediction of firmness and SSC based on BOP and transmission spectra of kiwifruit.