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II- Evaluation of ripeness – Analytic tools |
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Wine quality is determined by an array of factors with grape quality is the fundamental element. Thus, the assessment of grape quality potentiel is one of the priorities of the vine grower and the oenologist interested in producing quality wines adapted to the market : the maturation has to be controlled, the optimum harvesting date set, the plots selected, the grapes paid according to their quality, the vinification protocol determined and planned, the traceability guaranteed from the vine to the consumer… In the vineyard, this assessment is traditionally carried out by visual controls, samplings (samples of 200 berries and cluster fractions) and destructive laboratory analyses. The main difficulty of these controls lies in the large variability of the berries from one cluster and among the clusters from one plot.The grape sampling technique is essential and results strongly depend on the expertise of the sampler. Upon delivery at the winery, the harvest quality can also be assessed with must. Sampling conditions and sample preparation (temperature, sample handling…) can have a considerable influence on the values to be measured, especially with regards to acidity and polyphenols. Near Infrared technology (NIRs) allows to measure the entire fruit directly, by simple contact and at high speed (a few hundred milliseconds). In fact, in this spectral band light penetrates the product to depths, wich can reach several centimetres. In other agricultural industries (meat, cereals, dairy products…), near infrared product assessment and selection methods are already widely implemented. From 2002 to 2004, ITV, Cemagref, Inra , CTIFL and ACTA (who financed the project)have worked on the development and validation of a portable sensor, usable in the field and allowing to characterize grape quality regarding « sugar », « acidity », and « polyphenols » by simple contact. The measure is based on near infrared spectrophotometry. The feasibility of predicting sugar levels of berries ang clusters by simple contact with a near infrared sensor has been established. The method performance and specifically the robustness are good since the quality of the output was shown to be independent from the variety, the terroir and the vintage. For the prediction of the two other essential grape quality parameters, i.e. acidity and grape colours (anthycyanins), the feasibility was also demonstrated with clusters. The prototype « tongs » sensor developed for berries could be applied in research centres or service laboratories, where ripening heterogeneity remains a poorly studied parameter because of the absence of rapid methods. Regarding the utilisation of the « trompblon » sensor with grape clusters, the overall results show that it is possible to develop a portable device capable of mesasuring the average quality of a vineyard by simple and non-destructive contact allowing to assess the sugar, total acidity and anthocyanin contents. * SPIR : Spectroscopie Proche Infra Rouge ** NIR s : Near Infra Red Spectroscopy |
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