1, 1, 6, -trimethyl-1,2-dihydronapthalene (TDN) is a member of the C13-norisoprenoids family, which are a minor group of chemical component of wines, but exist in all internationally important varieties. TDN is a compound that is associated with both positive and negative attributes in both white and red wines, commonly known as the kerosene or petrol aroma in Riesling wines. At low levels TDN is desirable and delicious, but at a high concentration, TDN can dominate the wine and become an off character that exerts negative sensory attributes to the wine, incurring consumer rejection (Sacks et al, 2012). TDN has a sensory threshold of 2ug/L. It exist in highest quantities in aged Riesling wines, reaching as high 50ug/L but is also widely prevalent in Chardonnay, Sauvignon Blanc, Pinot noir and Cabernet Sauvignon at levels close to its threshold, with exception of 6.4ug/L in Cabernet Franc (Sacks et al, 2012). TDN is considered to be potential degradation products from β-carotene and lutein. The first step of TDN formation is from the photochemical or enzymatic degradation of C-40 carotenoid compounds, which exist in wine at 1-2mg/L concentrations in total (Marais et al, 1990). The change in TDN over time is considered to be results of acid catalyzed hydrolysis of carotenoid derived precursors, such as Zeaxanthin, Riesling acetal and glycosylated precursors (Daniel et al, 2009).
The quantity of TDN was close to zero at harvest and increases with wine aging due to the hydrolysis and rearrangement of TDN precursors over time. Exposure to sunlight and warmer climate would increase the level of TDN precursors up to 2-4 folds. TDN concentrations in both red and white wines can be increased by sunlight exposure and leaf removal. Generally, exposure to sunlight after veraison was able to increase TDN concentration to the greatest extent. Kwasniewski et al. (2010) suggested that mid-season shading might be encouraged, while sun exposure can still be conducted at other periods to optimize berry ripeness, without significant effect on wine TDN levels. Defoliation treatments at early growth stages would contribute to more production of TDN (Schüttler et al, 2015). It was suggested that carotenoid breakdown is likely through enzymatic activity that is genetically regulated. A dramatic increase in C13-norisoprenoids during veraison was possibly related to the CCD (carotenoid cleavage dioxygenases) gene expression after veraison (Versini et al, 1996). Longer storage time, higher storage temperature and lower wine pH were discovered to favor TDN formation. The possible explanation of such effects may be that lower pH provides a more acidic environment for acid hydrolysis of glycosylated precursors of TDN, and warmer temperature increases the rate of the reaction. Selection of yeast strains may have certain effects in combination with viticultural practices, but the longevity of such effects through aging needs to be further examined. (Robinson et al, 2011) Screw caps may better preserve TDN in bottled wines while natural and synthetic corks may deplete TDN through adsorption (Capone et al, 2003)
With the impact of global warming, TDN is raising more concerns relating to wine quality and its level needs to be controlled. The concept of “TDN management” may become more important, especially in warmer Riesling growing regions. TDN is quantified through gas chromatography (GC), along with the other members of the family of C13-norisoprenoids.
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