On-Vine Monitoring of Grape Ripening Using Near-Infrared Spectroscopy

This study evaluated the ability of near-infrared (NIR) spectroscopy to characterise the behaviour of white and red grapes during on-vine ripening, as a function of grape position in the bunch (high, middle and low) and bunch orientation (north, south, east and west) and to distinguish between different ripening stages with a view to optimising harvesting times depending on the grape variety and the type of wine to be made. A total of 24 bunches of two wine-grape varieties (cv. Pedro Xim,nez and cv. Cabernet Sauvignon) were labelled and analysed directly on the vine using a commercially available handheld micro-electro-mechanical system spectrophotometer (1,600-2,400 nm). Principal component analysis was performed to study relationships between the various configurations (grape position and bunch orientation), ripening stages and spectral data. Results for the white-grape variety showed that grapes high on the bunch behaved differently during ripening from those in central or low positions and that east-facing bunches behaved differently from the rest. For both varieties, analysis of bunch spectral characteristics enabled three stages of ripening to be distinguished: early, middle and late. Subsequently, the ability of NIR technology to classify wine grapes as a function of reducing-sugar content, with a view to optimising harvest timing, was evaluated by partial least squares discriminant analysis: 88 % of white grapes and 88 % of red grapes were correctly classified while over 79 % of samples were correctly assigned to representative groups. These results confirmed that NIR technology in the spectral range 1,600-2,400 nm is an appropriate technique for on-vine monitoring of the ripening process, enabling selective harvesting depending on the type of wine to be made.