Wines & Vines

62 WINES&VINES February 2018 GRAPEGROWING PRACTICAL WINERY & VINEYARD W ildfires have become an increas- i n g p r o b l e m i n t h e U n i t e d States in recent years. During the first half of 2017, 58,583 wildfires burned roughly 8.5 million acres, according to the National Interagency Fire Center. 1 Compared to the 10-year average of approximately 6 million acres burned per year by wildfires, the 2017 land loss exceeds this by far. The effects are hazardous, with millions of homes threatened and financial losses that add up to billions of dollars over the past 10 years. 2 Smoke taint in wine Unfortunately, more and more fires also strike in famous grapegrowing areas, such as the October 2017 firestorm in Sonoma, Napa and Mendocino counties in California. Apart from the direct danger of losing a vineyard to the fire, there is also an indirect risk of smoke exposure. When smoke from a wildfire is passing over vineyards, vinification of the affected berries can lead to unpalatable wines. These wines convey strong "ashy," "burnt" and "smoky" aromas, with "an excessively drying" back- palate and retro-nasal "ash" perception. 3,4 Recent studies report on the occurrence of this "smoke taint" in wines from California and Canada, but also from Australia, South Africa and Greece. 5-8 The consequence is a significantly reduced market value, as con- sumers have been shown to reject smoke- tainted wines. 9 Smoke-derived volatiles in grapevines Smoke from wood fire contains small vola- tile phenols such as guaiacol, 4-methyl- guaiacol, syringol, m-cresol, o-cresol and p-cresol (see "Smoke-Derived Volatile Phe- nols and Resveratrol" below) that contribute to the characteristic "burnt" odor. 10,11 Of these compounds, guaiacol, m-cresol and 4-methylguaiacol have the lowest aroma thresholds of 23 µg/L, 20 µg/L and 30 µg/L (BET, or best estimate threshold), respec- tively, emphasizing why they are considered the primary chemicals associated with smoke taint. 11 The U.S. wine industry has accepted the quantification of guaiacol and 4-methylguaiacol as indicators for smoke taint potential in grape juice and/or wine. It is suggested that they enter grapes mainly through the skins and the waxy cuticle rather than through the leaves. 12 However, even when the level of indica- tors was low and the grape juice did not show a specific ashy note, the corresponding wines exhibited smoke taint. 13,14 This is pointing toward the existence of a native grapevine-regulation system that processes the volatile phenols and masks their unpleas- ant smoky aroma. Binding of volatile phenols to grape sugars Juice from grapes affected by smoke was studied in laboratory analyses to discover what occurred with the volatile phenols. 15 The experiments demonstrated that the plant had bound the smoky compounds to grape sugars (see "Uptake and Release of Smoke Taint in Grapevines" on page 63). The com- pounds lost their volatile properties, being now odorless and hidden within the juice. Why is the grapevine processing the smoky volatiles in such a way? Recently, one cause was discovered by researchers of the Technical University of Munich, Germany. 3 Scientists found enzymes native to the grapevines that convey the bind- ing of grape sugars to the smoky volatiles. These enzymes, called glycosyltransferases (GTs), transfer sugar molecules. In a labora- tory environment, the activity of three GTs toward the most important smoke indicator compounds was tested. It was confirmed that the enzyme UGT72B27 processed all indicator compounds with high efficiency. Natural glycosyltransferase activity This leads to the question: Why is an enzyme native to the grapevine active on foreign smoky molecules? Here, the researchers found an explanation. They suggested that, under normal circumstances, the enzyme is binding glucose moieties to trans-resvera- trol, a naturally occurring metabolite in grapevines. 16 It has been detected in single and sugar-bound form in whole berries and in resulting wines and is usually released by the plant upon pathogen-infection or in- Smoke Taint in Wine How smoke-derived volatiles accumulate in grapevines By Katja Härtl and Wilfried Schwab Chemical structures of characteristic smoky aroma volatiles found in smoke from wood fire (guaiacol, 4-methylguaiacol, syringol, o-cresol, m-cresol, and p-cresol), and the naturally occurring grapevine metabo- lite trans-resveratrol. OH OCH 3 OH OCH 3 CH 3 OH CH 3 OH CH 3 OH CH 3 OH OCH 3 H 3 CO OH OH HO guaiacol 4-methylguaiacol syringol o-cresol m-cresol trans-resveratol SMOKE-DERIVED VOLATILE PHENOLS AND RESVERATROL

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