Spatially resolved diffuse reflectance in the visible and near-infrared wavelength range for non-destructive quality assessment of ‘Braeburn’ apples

Abstract

Spectroscopic measurements in the visible and near infrared wavelength range have achieved success in non-destructive assessment of apple quality attributes contributed by chemical components inside the fruit such as sweetness. Nevertheless, the evaluation of quality attributes related to texture of the fruit (e.g. firmness) still remains a challenge. One of the proposed solutions is to acquire and utilize separately scattering and absorption information from spectroscopic readings for quality prediction. Since scattering is related to fruit microstructure and absorption is caused by chemical composition, construction of multivariate calibration models from these optical properties to predict corresponding quality parameters of interest could be a solution for this, and also have potential for non-destructive monitoring of fruit quality from harvest to consumption. In this research, a setup for contact spatially resolved diffuse reflectance measurements in the 500-1000 nm range based on a fibre-optics probe was elaborated for the measurement of optical properties (absorption coefficient µₐ and reduced scattering coefficient µ's) of ‘Braeburn’ apples. After calibration and validation of the setup on a set of liquid optical phantoms covering the relevant range of optical properties, thirty ‘Braeburn’ apples were measured before and after shelf-life storage (2 weeks at 18 °C in normal atmosphere) with the setup and were analyzed for the main quality attributes (firmness and soluble solids content (SSC)). The estimated µₐ spectra of the apples indicated chlorophyll degradation during shelf-life storage. PLS models were investigated for apple quality prediction by using estimated optical properties spectra or diffuse reflectance spectra. These spectra covered information on chlorophyll and some carbohydrate and water absorption. The µₐ spectra also proved better than µ's spectra for predicting SSC and firmness (R²-SSC = 0.81; RMSEP(SSC) = 0.69%; and R 2-firmness = 0.71; RMSEP(firmness) = 9.68 N). The combined µₐ and µ's spectra did not improve the prediction accuracies as compared to the µₐ spectra alone. The diffuse reflectance spectra of the detection fibres did not provide a significantly better prediction performance for SSC, but gave slightly better firmness prediction (R²-firmness = 0.73-0.83; RMSEP(firmness) = 8.91-13.70 N) than the µₐ spectra.