Impact of varying ethanol and carbonation levels on the odor threshold of 1,1,6-trimethyl-1,2-dihydronaphtalene (petrol off-flavor) and role of berry size and Riesling clones

Abstract

1,1,6-trimethyl-1,2-dihydronaphtelene (TDN) evokes the odor of “petrol” in wine, especially in the variety Riesling. Increasing UV-radiation due to climate change intensifies formation of carotenoids in the berry skins and an increase of TDN-precursors1. Exploring new viticultural and oenological strategies to limit TDN formation in the future requires precise knowledge of TDN thresholds in different matrices. Thresholds reported in the literature vary substantially between 2 µg/L up to 20 µg/L2,3,4 due to the use of different methods. As Riesling grapes are used for very different wine styles such as dry, sweet or sparkling wines, it is essential to study the impact of varying ethanol and carbonation levels. Therefore we determined the odor detection threshold (DT) with a three alternatives forced choice (3-AFC) test and calculated the best estimate threshold (BET) for each panelist, followed by the calculation of the BET for the whole panel. Matrices varied from water, a model wine, a dry Riesling wine and sparkling wine; the latter two exceptionally low in TDN. Carbonation in water, model wine and Riesling wine ranged from no addition to 2.5 and 6 bar pressure. Ethanol altered from 8 to 14% alc. in 2% alc. increments. Carbonation yielded an inconsistent effect due to a better volatilization leading to lower thresholds and masking of the TDN perception leading to slightly higher thresholds. Increasing ethanol levels however showed a clear tendency to raise the TDN thresholds, presumably due to better solubility and masking by its own pungent odor. Absolute thresholds varied in water between 2.6 and 4.0 µg/L and in wine between 8.5 and 15.2 µg/L. Since TDN is formed by a breakdown of carotenes, its occurrence correlates positively with the degree of sun exposure. This is partially governed by berry size and cluster density given by clonal differences. Thus we studied free and bound TDN in grapes and wines from 8 different Riesling clones. Furthermore, berries were divided in a two fractions according to a diameter smaller and larger than 13 mm. The berry fractions were separately crushed, pressed and fermented. Preliminary results indicate that the clonal influence on the formation of free TDN in the wines was substantially larger than the impact of berry size.

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