Determination of Aroma Compounds in Grape Mash under Conditions of Tasting by On-line Near-Infrared Spectroscopy

Abstract

The production of high-quality wines requires the use of high-quality grapes. Some compounds originating from grapes may negatively influence the odour and flavour of the resulting wine in their original form or as precursors for off-odours and –flavours. Therefore, a rapid evaluation of the grapes directly upon receival at the winery is advantageous. Up to now, grape aroma is mainly evaluated by tasting, however, this leads to subjective results. The use of near-infrared (NIR) spectroscopy allows a rapid, objective and destruction-free analysis without previous sample preparation. Moreover, the measurement can be integrated into an existing process without additional sampling.

The aroma compounds in grape mash samples were analysed by gas chromatography-mass spectrometry (GC-MS) under highly comparable conditions to those in the mouth (temperature, pH value, enzymes etc.). This way, only compounds were detected, which are released during tasting. Furthermore, the destruction of compounds and possible artefact formation should be avoided. The determined concentrations of the detected aroma compounds were related to the corresponding NIR spectra of the samples and calibration models were developed for determination by NIR spectroscopy.

Regarding the presence and the concentrations of the detected aroma compounds, differences occurred between samples with varying phytosanitary status of the grapes and for several grape varieties. The addition of artificial saliva solution before measurement did not significantly influence the concentrations of the aroma compounds. Global calibration models for the single aroma compounds showed varying results (0,365 ≤ R2 ≤ 0,976). Separate calibration models for cultivation region and/or grape colour improved the prediction accuracy.

For the majority of the detected aroma compounds at least a semi-quantitative determination by NIR spectroscopy is possible to evaluate the aroma quality of grapes. However, more samples need to be analysed to extend the models and to assure sufficient prediction accuracy.