Automated optical grape-sorting of rotten grapes: effects of rot infections on gluconic acid concentrations and glycerol/gluconic acid ratios in must and wine

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Abstract
An increasing influence of climate change on grape and wine quality, identified by higher rot infections, could be observed in the last years in some northern wine-growing regions of Europe. Therefore, an innovative possibility of grape-sorting by an automated optical system was tested for quality control and quality improvement of wine. Different grape varieties from the Mosel wine region were sorted concerning rot, especially Botrytis cinerea, in the years 2010 and 2011. The resulting fractions were analysed for gluconic acid because higher contents are considered as an indicator for Botrytis infection. Highest concentrations were determined in the negative fractions with rotten berries and lowest concentrations in the positive fractions with healthy berries. Furthermore, glycerol was determined to assess the stage of the Botrytis infection via the glycerol/gluconic acid ratio. A lower ratio represents an advanced development of the rot and is considered as an indicator of a high degree of ‘unwanted’ grey mould. Despite the higher rot contamination in 2010, the results of the glycerol/gluconic acid ratios show that the Botrytis fungus developed towards noble rot in 2010 compared to 2011. In this study, automated optical grape-sorting was successful concerning rot as sorting factor and provides an innovative possibility for a more effective quality control and quality improvement during winemaking.

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