Non-destructive Vis/NIR time-series to model apple fruit maturation on the tree

Apple growers make harvest decisions based on a few simple destructive tests e.g. soluble solids and starch content, and fruit firmness. However, non-destructive technologies also offer possibilities to monitor fruit maturation and ripening and to build prediction models for an optimum harvest date (OHD) for fruit intended for long-term storage. Technological developments now enable point spectroscopy Vis/NIR spectral scans to be made on fruit in the orchard using portable hand-held devices. In fruit crops like mango, prediction models process spectral information in real-time and provide information about when to pick fruit. A standard type hand-held spectrophotometer (F750, Felix Instruments, USA) was used to collect spectral time-series data from 2016 to 2018 for 'Braeburn' apples during fruit development, growing at the Kompetenzzentrum Obstbau Bodensee in Southwest Germany. Decreasing chlorophyll levels were closely associated with apple maturation, in particular the parameter rededge was consistently close to 689 nm at the OHD for 'Braeburn' over all three study years and was less variable than a normalized difference vegetation index (NDVI). Predictions of an OHD using partial least squares regression models based on all available spectral information were not robust enough to define an OHD with a narrow harvest window. Future efforts to model an OHD for a bicolored apple cultivar like 'Braeburn' should try to untangle changes in the visible spectrum from the carotenoid, anthocyanin and chlorophyll pigments.