First steps towards the development of a non-destructive technique for the quality control of wine grapes during on-vine ripening and on arrival at the winery

NIR spectroscopy was used as a non-destructive technique for the assessment of changes in certain internal quality properties of wine grapes (Vitis vinifera L.) during on-vine ripening and at harvest. A total of 108 different wine grape samples were used to construct calibration models based on reference data and NIR spectral data, obtained using a commercially-available diode-array spectrophotometer (380-1700 nm). The feasibility of testing bunches of intact grapes was investigated and compared with more traditional methods of presentation, such as berries or must. Predictive models were constructed to quantify changes in soluble solid content (SSC, °Brix), reducing-sugar content (g/l), pH-value, titrable acidity (g/l tartaric acid), tartaric acid (g/l) and malic acid (g/l), these being the major parameters used to chart ripening. NIRS technology provided good precision for the bunch analysis mode assayed for SSC (r² = 0.89; SECV = 1.41 °Brix), for reducing-sugar content (r² = 0.87; SECV = 17.13 g/l) and for pH-value (r² = 0.69; SECV = 0.19). Models developed for testing other fruit acidity parameters yielded results sufficient to provide a screening tool to distinguish between low and high acidity values in intact grapes. Significantly, the results obtained with bunch presentation were similar to those obtained with berries and must, thus justifying further implementation of NIRS technology for the non-destructive analysis of quality properties both during on-vine ripening and on arrival at the winery. This method allows musts to be processed separately depending on initial grape quality, assessed with a single spectrum measurement and in a matter of seconds.