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

Abstract

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 Ximenez 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.