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April 2018 WINES&VINES 79 PRACTICAL WINERY & VINEYARD GRAPEGROWING Various methods using a so- phisticated gas chromatograph mass spectrometer in a commer- cial or university laboratory (with lengthy and tedious sample prepa- ration) are employed to success- fully and accurately measure volatile thiols in wine. Trace con- centrations present in wine have the ability to generate various fruity descriptors due to the very low olfactory detection threshold (lowest concentration at which 50% of the population are able to detect the aroma). Types of volatile thiols and their sensory thresholds Three main volatile thiols are responsible for the tropical fruit nuances in wines. They are 3MH (3-mercaptohexan-1-ol), 3MHA (3-mercaptohexyl acetate) and 4MMP (4-mercapto-4-methyl- pentan-2-one). The smell is quite potent (or "punchy," as the Kiwis say) at higher concentrations, and descriptors used include tropical fruit, passionfruit, grape- fruit, guava, gooseberry, box tree, tomato leaf and black currant. Perception thresholds for 4MMP, 3MH and 3MHA in model wine a r e 0 . 8 n g / L , 6 0 n g / L a n d 4.2 ng/L, respectively. Recently it was discovered that volatile thiols contribute to the "green" odors in wine such as green pepper and stalky attributes generally assumed contributed only by methoxypyrazines (an- other aroma group significantly adding to Sauvignon Blanc fla- vor). 1,2 Recently wine sensory evaluations have seen the use of the term "thiols" on the tasting sheets (unpublished data). The term has not been properly de- fined, however it is assumed to refer to the general group of at- tributes that is brought by the volatile thiols. The fact that the volatile thiols can contribute to both fruity flavors and green odors can be challenging. The concentration of these impact compounds in wine will influence the intensity of the attributes, however the concen- tration may also determine the descriptor used to describe the wine. 1,3 For example, "lower" concentrations of 3MHA will im- part passion fruit, guava and gooseberry odors while a sweaty a r o m a d e v e l o p s a t h i g h e r concentrations. Cat pee (box tree) as an aroma descriptor has been used at ele- vated volatile thiol (4MMP) con- centrations. However, during aging, the concentration of the thiols will change due to degrada- tion (such as the hydrolysis of 3MHA to 3MH) or oxidation, and with this alteration a change in aroma attributes may occur. When this happens, the wines that con- tained an initial higher concentra- tion of volatile thiols might display fruitier aromas for a lon- ger period of time compared to the wines that contained lower initial quantities. Concentrations of volatile thiols found in international wines vary significantly, with Marlborough wines generally taking first place. The effect of "minor" differ- ences in thiol concentrations be- tween wines is under review. In wines where 3MH concentration varied from 0 ng/L to 875 ng/L, the aroma of the wines did not greatly differ, 2 and it seems a higher quantity of thiols are needed to really have an impact on the wine aroma (even though concentrations greatly exceed per- ception threshold). It might be worth investigating at what specific concentration the volatile thiols will start having an influence on specific white wines from South Africa. The effect of the wine matrix and the presence of other aromatic and non-aromatic compounds on the perception of the volatile thiols should be con- sidered. Suppressing and enhanc- ing effects greatly influence how these compounds are perceived. Origin of volatile thiols The volatile thiols (4MMP and 3MH) are not present in grapes and are only released/formed from precursors to their volatile and pleasant-smelling form dur- ing alcoholic fermentation. 3MHA (an acetate ester) is formed from 3MH through yeast-driven acety- lation and does not normally ex- ceed the concentration of 3MH in the final wine. 4 During the 1990s and 2000s, research regarding participation of the cysteine (Cys-3MH) and You know how good your Zinfandel is... Promote your wines to leading Zinfandel enthusiasts and extend your winery's relationship in the market. JoinUs! We are champions of Zinfandel— America's Heritage Wine Become part of our proud legacy zinfandel.org • 530-274-4900 Zinfandel Advocates & Producers is a 501(c)(3) non-profit organization. ZAP is dedicated to advancing public knowledge of and appreciation for American Zinfandel and its unique place in our culture and history. Daily wine industry news winesandvines.com ONLINEMARKETINGSYSTEM Wines Vines Analytics Visit winesandvines.com/OMS or call 866.453.9701 to set-up your free demo. Reach Thousands of Winery Decision Makers The Wines & Vines Online Marketing System is designed to help your business: • Increase sales • Generate winery leads • Manage clients and sales territories • Improve marketing effectiveness