Testing of a simplified optical system for rapid ripeness evaluation of white grape (Vitis Vinifera L.) for Franciacorta sparkling wine

The aim of the research was to design, build and test a prototype of a low-cost and user-friendly device to support small-scale growers in determining the optimal harvest date according to grape ripening degree. The acquisition of the spectral reflectance at four specific wavelengths (630, 690, 750 and 850 nm) was proposed. Light emitting diode (LED) technology was chosen as the light source, to achieve the capability to individually adjust the light emission intensity for each measurement channel. A customized optical fiber was specifically designed and employed. The prototype was tested on 95 bunches of white grapes for a rapid estimation of ripening parameters of grape for Franciacorta wine directly in field. At the same time, spectral measurements on grape berries were performed (for data comparison) with a commercial portable vis/NIR spectrophotometer operating in the wavelength range of 400-1000 nm. The percentages of correctly and misclassified grape samples were determined according to thresholds for the analyzed parameters (ripe > 17 °Brix for SSC and < 7 g dm-3 for TA) suggested by the Franciacorta Consortium. Overall PPV results deriving from the simplified system showed slightly low classification capabilities (SSC on berry = 79.0%, SSC on bunch = 77.5%, TA = 84.3%) compared to the commercial device (SSC on berry = 84.6%, SSC on bunch = 89.5%, TA = 90.5%). Only a small loss of information was noticeable between the PLS models calculated using 2048 wavelengths and the MLR models calculated employing only the four channels of the simplified device.