Stand-alone LED sensors for future field monitoring of grape (Vitis vinifera L.) ripeness

In the wine industry, grape ripening control is a complex process that is critical to the production of high quality wines, but currently ripening control is cumbersome and inefficient. This inefficient control of maturation is associated with reduced wine value. This work aims to develop a fully integrated stand-alone optical device for grape quality monitoring directly in the field. Therefore, during the 2019 sampling campaign, a first prototype version of a fully integrated optical device was developed by INL following a "stripe" design, where the spectrometric components were mounted on a long flexible substrate that can be placed on or inside the grape bunch. The multiple spectrometers were placed along the stripe, allowing simultaneous measurements at different parts of the bunch to provide more representative information of the entire bunch. Four optical bands associated with the evolution of grape ripening parameters, such as anthocyanin and sugar development, chlorophyll degradation and water content reduction, were integrated. Four light-emitting diodes (LEDs) in the Vis and SW-NIR range were used to illuminate the grapes. Close to these, but optically isolated by an opaque barrier, four photodiodes (with an active area of 520 × 520 µm2) equipped with spectral filters to allow intensity measurements at the desired wavelengths were used. Four PLS models were developed to predict the total soluble solids and the potential alcohol (R2 around 0.90 and RMSEP of 2.22 and 1.54 respectively). A very promising model was also obtained for the prediction of titratable acidity (R2 of 0.93 and RMSEP of 1.39), while a pH prediction model (using 4 LVs) was developed, showing a lower performance than the previous parameters (R2 of 0.76 and RMSEP of 0.15), but still with potential for use with further improvements.