Issue link: http://winesandvines.uberflip.com/i/513570
June 2015 WINES&VINES 71 WINE EAST GRAPEGROWING passion fruit, grapefruit and guava aromas that come from volatile thiols. Sauvignon Blanc, specifically, represents an excellent opportunity for the "savvy" (no pun intended) vintner to capitalize on an international grape that can produce relatively high yields and has potential for high grower returns. However, the choice to grow Sauvignon Blanc will be determined by one's terroir. If the site has a propensity for high vine vigor, cold damage and bunch rots, then one might con- sider planting Petit Manseng. This variety is relatively hardy in the Virginia climate and can consistently ripen, while Sauvignon Blanc may have trouble ripening in some years. A matter of style High-quality Sauvignon Blanc is typically de- scribed as having "fresh" or "crisp" acidity with flavors and aromas of passion fruit, grapefruit, guava, gooseberry and green pepper. In the eastern United States, the acid profile of Sau- vignon Blanc presents a significant challenge, as the ripening season is characteristically hot and grapes tend to lose a lot of acidity when compared to cooler regions such as Sancerre, Pouilly Fume in France and Marlborough, New Zealand. However, the clever vintner can com- pensate for the loss of acidity by making two picks (an acid pick and a flavor pick) and/or the careful addition of tartaric acid in the win- ery, followed by blending. Petit Manseng main- tains its acidity in hot climates and is typically made into sweet and semi-sweet styles, as one often has to leave the fruit on the vine until ~29° Brix before the sugar and acid levels reach an optimal balance. If acidity is not a problem, what about fla- vors and aromas? One of the characteristics that has made Sauvignon Blanc so successful is its relatively simple aromatic profile. The aromas of this variety are not subtle or hidden behind layers; they are straightforward, and the comparison to their corresponding descrip- tors is clear to even the most novice wine drinker. A survey of Sauvignon Blancs from around the world determined that the only clearly distinguishable Sauvignon Blanc wines, by country, came from New Zealand. This was due to their high concentration of passion fruit, grapefruit, gooseberry and guava aromas, which arose from the volatile thiols (Benkwitz, et al., 2012). New Zealand Sauvignon Blanc has rapidly been gaining market share in the United States. It now represents approximately 29% of all the Sauvignon Blanc sold in the United States (New Zealand Trade and Enterprise, 2014) and has had an average annual growth of 17% by volume over the past five years (New Zealand Winegrowers, 2013). This competition from New Zealand may present a qualitative chal- lenge for American vintners, but it may be curbed as winemakers develop an understand- ing of how to increase the varietal intensity of Sauvignon Blanc wines. Vineyard practices It is understood that moderate water stress of grapevines after fruit set produces wines of a high quality (van Leeuwen et al., 2004). Mod- erate water stress of Sauvignon Blanc after fruit set has been demonstrated to increase the con- centration of the conjugated thiol precursors (Peyrot des Gachons et al., 2005). However, these researchers only evaluated aroma poten- tial and did not vinify the fruit and analyze the volatile thiols in wines. As it has been shown that thiols may be formed during fermentation from other precursors, the effect of water status on the volatile thiols in wine is not fully un- derstood and requires further research. Peyrot des Gachons et al. also found a cor- relation between the thiol conjugates and ni- trogen. The vines that were not in a nitrogen deficit had fruit with significantly higher con- centrations of conjugated thiols compared to vines that were deficient in nitrogen. Chone et al. (2006) made a soil application of 60 kg of nitrogen per hectare (53.5 pounds per acre) of ammonium nitrate at berry set to Sauvignon Blanc that was historically low in nitrogen. The fruit of the control had a yeast assimilable nitrogen (YAN) concentration of 29 ppm at harvest, whereas the fruit from the treated vines had a YAN concentration of 174 ppm. All of the cys-conjugated thiols increased significantly with nitrogen application. A 30% THIOL ORIGINS Thiols are any organic compound that contain an –SH (sulfhydryl) group. Much of the eno- logical research on thiols has focused upon those responsible for sulfur off-aromas such as H 2 S and dimethyl sulfide. The volatile thiols responsible for the characteristic tropical fruit aromas of Sauvignon Blanc were not discovered until the mid-1990s by a group of scien- tists in Bordeaux (Tominaga et al., 1996; Darriet et al.,1995). Since then, much has been learned about their synthesis within grapes and in their volatile form within wine. However, further research into varietal thiols is still needed, as there are many unanswered questions. There are three main positive volatile thiols found in wines. Their aromatic descriptors can be found in the table "Olfactory Descriptors of Thiols" (below). Two of these thiols—4-mer- capto-4-methylpentan-2-one (4MMP) and 3-mercapto-hexan-1-ol (3MH)—are present in the grape in their non-volatile form as cysteinylated and glutathionylated conjugates. These conjugates are taken up by yeast during fermentation via amino acid uptake pathways. The carbon-sulfur bond between the cysteine or glutathione and the thiol is cleaved by an enzyme that is present in certain wine yeast (Subileau et al., 2008; Coetzee & du Toit, 2012). This cleaving liberates the thiols into their aromatic form. After liberation, 3MH can then be- come enzymatically esterified with acetic acid to become 3-mercapto-hexylacetate (3MHA) (Swiegers et al., 2007). The concentration of conjugated thiols found in the grape have little correlation with the concentration of volatile thiols found in the resulting wine (Capone et al., 2011; Pinu et al., 2012). This suggests that there are other means by which volatile thiols are formed. Schneider et al. (2006) demonstrated that volatile 3MH and 4MMP could be formed from other carbonyl compounds during fermentation. They hypothesized that 3MH and 4MMP can be formed indirectly by the bonding of the carbonyl with a cysteine molecule, followed by a conversion to the volatile thiol by yeast. They further theorized that thiols may be formed directly by a 1, 4 addition of H 2 S to conjugated carbonyls, followed by a reduction step to form the volatile thiol. Regardless of whether or not the conjugated precursors in the grape correlate to the volatile thiols in wine, it has been demonstrated through previous research and application that vineyard practice can influence the volatile thiol concentrations in the resulting wine. How- ever, the exact mechanisms of the viticultural influence are less understood and may be an area for further research. OLFACTORY DESCRIPTORS OF THIOLS Volatile thiol Aroma description 4-mercapto-4-methylpentan-2-one Box tree, broom, black currant and passion fruit 3-mercapto-hexan-1-ol Passion fruit, gooseberry, grapefruit and guava 3-mercapto-hexylacetate Passion fruit, box tree, grapefruit, guava, gooseberry Source: Coetzee & du Toit, 2012