Current knowledge, research needs and directions
As climate change and periods of prolonged drought affect vineyard regions worldwide, increasing the potential for wildfires during the growing season, smoke taint has become a global concern in winemaking. Smoke taint in grapes and wine has been reported and studied in Australia for more than 15 years in response to wildfires and controlled burns exposing vineyards and grapes to smoke for prolonged time periods. In 2008, wildfires in California’s North Coast brought attention to smoke taint issues for California vintners. Fires in California and Pacific Northwest vineyard regions in subsequent years, including 2017, have increased the need to understand and manage smoke taint in wine.
A smoke taint outreach seminar presented by two authorities at the 2017 National Conference of the American Society for Enology and Viticulture (ASEV) on June 29 in Bellevue, Washington provided an update on current knowledge and ongoing research activities to better understand and mitigate smoke taint issues. Dr. Tom Collins, assistant professor at Washington State University (WSU), is conducting smoke taint research at the WSU Wine Science Center at the Tri Cities campus with support from the Washington State Wine Commission. Dr. Mark Krstic of the Australian Wine Research Institute (AWRI) is extensively involved with smoke taint research and outreach efforts in Australia.
Overview and Chemistry
Smoke density, duration of smoke exposure and the timing of smoke exposure during the growing season are factors that affect smoke taint in grapes in wine. Smoke taint compounds are absorbed within the skins of grape berries, and grapes are most susceptible from the period after veraison and until harvest.
Research in Australia showed that guaiacol and 4-methyl guaiacol (4-MG) are the two main compounds associated with smoke taint, and these are common markers used in lab analysis for grapes and wine. Other compounds identified in wines made with smoke-affected grapes include 4-ethyl guaiacol, 4-ethyl phenol, eugenol and furfural.
Although a number of smoke-related compounds have been identified, an important issue is that they can be present in two forms—as free volatile phenols, and as bound glycoside compounds that can later be released. Free volatile phenols are sometimes detected as smoke aromas in the early stages of grape and wine processing.
Bound glycosides are odorless. Glycosides play a role in flavor/aftertaste and they can hydrolyze over time to release more free volatile compounds. Unwanted volatiles can be released from glycosides during fermentation. During bottle aging, acid in wine can hydrolyze glycosides to release volatile phenols with potential off-flavors. Research also indicates that enzymatic activity in saliva and bacterial activity in the mouth can hydrolyze glycosides that can be detected as smoke taint flavor.
Collins cited research that reported glycosides of syringol, 4-methylsy- ringol, phenol and isomers of cresol. There are also different types of glyco- sides, including glucosyl glucosides, glucopyranosides and disaccharides.
Collins has analyzed 2015 vintage wines from Washington and California made from smoke-affected grapes of Cabernet Sauvignon, Cabernet Franc and Pinot Noir. Free volatile phenols identified were guaiacol, 4-MG and syringol. The analysis also identified 23 glycosides, including several not previously reported, to be associated with smoke taint. Glycosides (bound forms) found in the samples included: guaiacol, 4-MG, cresols, syringol, glucosides, di-glucosides, di-pentosides and pentosyl-glucosides.
Collins has analyzed smoke-affected grapes and wines for both free and bound compounds at different stages of wine processing, and in wines of different ages after bottling. At almost every step in the process, he said, “The vast majority of compounds present were present as glycosides.” Further research is needed to determine the potential contribution of individual glycosides to smoke taint in wine. Additional undiscovered glycosides may also exist that contribute to smoke taint.
Other complicating factors are that some taint compounds, and associated aromas/flavors, can also be derived from Brettanomyces or toasted oak. Some compounds naturally occur in some grape varieties. Guaiacol has been found in both free and bound forms in grape skins of Merlot, Shiraz, Tempranillo and Grenache. In addition, the concentration of compounds, combinations of compounds, grape variety, wine processing methods and styles can all influence the presence, perception and intensity of smoke taint aromas and flavors.
Smoke Taint Aromas and Flavors
Collins provided ASEV seminar attendees samples of smoke-affected research wines for sensory evaluations: a 2016 Riesling from Washington, 2015 Pinot Noir and Cabernet Franc from Washington, and a 2015 Cabernet Sauvignon from California. None of the wines received remediation treat- ments to reduce smoke taint and none had oak contact. The red wines were aged in stainless steel until bottling and bottle aged for one year prior to the ASEV tasting.
About half the tasters detected smoke taint character in the Riesling as an “ashy” aftertaste. Most tasters found off-characters in the Pinot Noir, and they listed these descriptors: ”band-aid,” “medicinal,” “ashy,” “cigarette smoke,” “barnyard,” “campfire,” “fecal,” “bitter,” “burnt sulfur,” and a “dry” and “ashy” aftertaste.
The Cabernet Sauvignon had fewer noticeable negatives than the Pinot Noir. Descriptors included: “cigarette,” “sawdust” and “burnt sawdust,” “extremely dry tannins,” and “plastic.” The Cabernet Franc’s descriptors included: “dull metallic,” “drying,” “smoky” and “ashtray.”
Collins and Krstic said two of the most common characters associated with smoke taint are “ashy” and “drying,” particularly with aftertaste, and these can linger on the palate a long time. As with other types of defects, smoke taint also masks positive aromas, flavors and varietal character; and its intensity is affected by wine variety and the matrix effects of the wine’s overall chemistry.
Collins observed, “At this point we haven’t identified all the compounds and factors that play a role in smoke-related off-characters, let alone the combinations of compounds. The ‘plastic,’ character for example, we don’t know what it is related to chemically.” Krstic said “plastic” character can show up later during bottle aging.
By Ted Rieger
December 16, 2017
Source and Full Article at Winebusiness.com
Ted Rieger, CSW, is a writer and photographer based in Sacramento, CA and has been a writer for wine industry media since 1988.
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