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122 WINES&VINES November 2015 WINEMAKING WINE EAST Thiols: From Harvest to Bottle Making the most of winemaking practices that enhance fruit aromas By Russell Moss T he old adage "great wines start in the vineyard" is indeed true. How- ever, perhaps a more accurate state- ment would be: "Great wines start in the vineyard and end on the pal- ate." The truly great wines of the world come from close attention to detail in both the vineyard and the winery. I reviewed the viticultural techniques that grapegrowers can use to enhance positive tropical fruit aromas in wines from certain cultivars in the June 2015 issue of Wines & Vines. In this article, I will explore how winemakers can further en- hance the positive varietal aromas coming from thiols through practices from harvest to bottle. It's important to point out that a "thiol" is any organic compound that contains a sulfyhy- dryl (-SH) group; therefore, an off-aroma such as methyl mercaptan is also considered a thiol. However, for the purposes of this article, "thiols" refers to three important compounds that confer specific, pleasant aromas to wines. The three positive thiols of most interest are shown in the table on page 123, along with abbreviations and corresponding descriptors. Thiols are an extremely important varietal aroma class (i.e., aromas arising from the grape itself) and are responsible for some of the pleas- ant tropical fruit aromas that are intrinsic to varieties such as Petit Manseng, Sauvignon Blanc, Cayuga White, Riesling, Sémillon and Gewürz- traminer. The threshold of detection of these aromas is in the parts per trillion range. To put that in perspective, the thiol with the highest threshold of detection, 3-mercaptohexan-1-ol (3MH), could be perceived in an Olympic-sized swimming pool if you were to dose it with a mere 0.15 grams of 3MH, according to research from Carien Coetzee and W.J. du Toit. In Petit Manseng, a grape of increasing im- portance in the southeastern United States, Takatoshi Tominaga found 3MH to be at a con- centration that was about 75 times greater than the perception threshold of this compound. To my knowledge, the development of an analytical method and a survey of wines has been the only public research conducted upon this important aromatic class in the United States. The exact origin of the volatile thiols in wine continues to elude the scientific community. They are present as non-volatile amino acid conjugates in the juice, and they also seem to be produced during fermentation. However, Farhana R. Pinu discovered there is no correlation between the concentration of amino acid conjugates and the volatile thiols produced in the resulting wine. The current theory from Nina Duhamel is that sulfonic acids in the must may act as precursors, which are then reduced during fermentation to form their corresponding volatile thiol. This area of wine science offers a wealth of possibilities for potential discovery. Regardless of the origin of thiols, research and experience has elucidated vineyard and win- ery practices that can increase their concentra- tions in the resulting wine. KEY POINTS Aromatic grape varieties should be harvested at cooler times of the day and by machine, if possible, in order to conserve volatile thiols. Cold soaking must treated with SO 2 and pressing in an anaerobic environ- ment may maximize the wine's aromatic potential and minimize the negative effect of oxidation. Specific yeast strains have been identified that produce wines with high concentrations of volatile thiols. Yeast rehydration also may help to boost the wine's aromatics. Aromatic wines do well when packaged using screwcaps or synthetic corks that restrict oxygen ingress. Research has confirmed that machine-harvested fruit has higher concentrations of thiols.