Studies on classification models to discriminate ‘Braeburn’ apples affected by internal browning using the optical properties measured by time-resolved reflectance spectroscopy

This work aimed at studying the feasibility of time-resolved reflectance spectroscopy (TRS) to nondestructively detect internal browning (IB) in ‘Braeburn’ apples through the development of classification models based on absorption (μ_a) and scattering (μ_s′) properties of the pulp.This research was carried out in two seasons: in 2009, apples were measured by TRS at 670 nm and in the 740–1040 nm spectral range on four equidistant points around the equator, whereas in 2010 apples were measured by TRS at 670 nm and at 780 nm on eight equidistant points.The values of the absorption coefficients measured in the 670–940 nm range increased with IB development. On the contrary, μ_s′780 was higher in healthy fruit than in IB ones. The μ_a780 also significantly increased with IB severity, showing high values when IB affected the pulp tissues compared to the core ones. Also μ_a670 changed with IB development, but it was not able to clearly discriminate healthy fruit from IB ones because its value was also affected by the chlorophyll content of the pulp.The absorption and scattering coefficients were used as explanatory variables in the linear discriminant analysis in order to classify each apple tissue as healthy or IB; then the models obtained were used for fruit classification. The best classification performance was obtained in 2010 using μ_a780 and μ_s′780 and considering the IB position within the fruit: 90% of healthy fruit and 71% of IB fruit were correctly classified. By using all the μ_a measured in the 670–1040 nm range plus the μ_s′780, IB fruit classification was slightly better while healthy fruit classification was worse. The better result of 2010 was due to the increased number of TRS measurement points that allowed better exploration of the fruit tissues. However, the asymmetric nature of this disorder makes detection difficult, especially when the disorder is localized in the inner part of the fruit (core) or when it occurs in spots. A different TRS set-up (position and distance of fibers, time resolution) should be studied in order to reach the deeper tissue within the fruit in order to improve browning detection.