Wildfires and Wine: Unified Session Looks at Loss Prevention, Mitigation and Management Related to Fire and Smoke Taint
February 15, 2018
With the risk of wildfire impacts to vines and wines still fresh in people’s minds after the 2017 harvest season fires in California’s North Coast, in addition to fires in Washington state and in Spain, the 2018 Unified Wine and Grape Symposium in Sacramento presented a session by two authorities on “Wildfires and Wine.”
Dr. Mark Krstic of the Australian Wine Research Institute (AWRI) has extensive experience with wildfire and smoke taint research and outreach efforts in Australia. 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. The two speakers previously presented information at the 2017 National Conference of the American Society for Enology and Viticulture (ASEV) in Bellevue, WA, covered in the Wine Business Monthly December 2017 article, “Smoke Taint in Wine.”
“Climate change is increasing the risk of fires, in both frequency and intensity, and smoke taint from wildfires has been an issue in wine grape growing regions in the U.S., Canada, Chile, Europe, South Africa and Australia,” Krstic said.
Australia has had the longest history of dealing with wildfires and smoke taint issues, dating to 2003. Many of Australia’s wine grape growing regions were exposed to smoke from wildfires during the country’s prolonged drought, in addition to smoke from controlled burns by government agencies.
Krstic provided an overview of what is known about smoke taint and risk factors based on 15 years of research in Australia. He said grapes exposed to smoke for as short a period as 30 minutes can develop problems, although timing of exposure is a critical factor. The period from seven days post-veraison until harvest is when smoke exposure poses the highest risk of smoke taint. Smoke taint risk is a function of smoke concentration and exposure duration, but assessing risk is complicated and much is still unknown.
The major pathway for smoke taint is through direct chemical absorption into the berries via the waxy cuticle of the berry skin. Krstic said a potential minor pathway, perhaps 5 percent, can be via vine leaf exposure, if smoke chemicals are translocated from the leaf to the fruit. However, there is no carryover of taint chemicals between seasons in perennial vine parts.
There have been attempts to determine differences between varieties in sensitivity to smoke exposure, but Krstic said studies of Sangiovese, Shiraz and Cabernet Sauvignon have had inconsistent results.
“Smoke contains thousands of different compounds,” Krstic said. Australia has begun measuring smoke particulate concentration with nephelometers, and the wine industry is working with atmospheric and air pollution chemists to better understand smoke chemistry.
Guaiacol and 4-methyl guaiacol (4-MG) are the two main compounds associated with smoke taint, and these are the most common markers used to date 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.
Smoke-related compounds 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 perceived as smoke aromas in the early stages of grape and wine processing and sensory evaluation can confirm smoke taint. However, absence of smoke aromas does not mean taint compounds are not present. Lab analysis for free compounds and bound glycosides is recommended.
Bound glycosides are odorless, but they can hydrolyze in wine over time to release free volatile compounds and play a role in flavor/aftertaste. Krstic said these bound glycoside precursors, “are like the unseen part of an iceberg below the water.” Unwanted volatiles can be released from glycosides during fermentation. During bottle aging, acid in wine can hydrolyze glycosides to release volatile phenols with off-flavors.
Collins said research indicates that enzymatic activity in saliva can hydrolyze mono- and di-saccharides and be perceived as a smoky aftertaste. Bacterial activity in the mouth can also hydrolyze glycosides that can be perceived as smoke flavor.
Research has reported smoke-related glycosides of syringol, 4-methylsyringol, phenol, and isomers of cresol. Other types of glycosides include glucosyl glucosides, glucopyranosides and disaccharides.
Collins has analyzed 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 identified 23 glycosides 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 said, “We probably ought to look at a broader set of chemical markers which may give us a better ability to predict risk.”
Smoke Taint Character in Wine
Common sensory descriptors associated with smoke taint are “smoky,” “burnt,” and “ashtray.” “Ashy” and “drying,” are often associated with aftertaste, and these can linger on the palate a long time. Taint aroma perceptions can differ based on wine variety. Other descriptors include: ”band-aid,” “medicinal,” “cigarette smoke,” “barnyard,” “campfire,” “fecal,” “bitter,” “burnt sulfur,” “burnt sawdust,” “extremely dry tannins,” “bacon,” and “plastic.”
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 occurs in free and bound forms in grape skins of Merlot, Shiraz, Tempranillo and Grenache. The concentration of compounds, combinations of compounds, grape variety, and wine processing methods can all influence the presence, perception and intensity of smoke taint aromas and flavors. Smoke taint also masks positive aromas, flavors and varietal character.
Managing Smoke Taint
Krstic listed potential techniques to mitigate and manage smoke taint:
--Hand harvest grapes.
--Exclude/remove all leaves.
--Process smoke exposed grapes separately from other grapes.
--Press whole clusters to minimize skin contact and extraction. This is easier with white varieties than with red wine varieties.
--Separate press fractions.
--Reverse osmosis (RO) can provide short-term reduction of smoke-related free compounds, but cannot remove bound glycosides the way these systems are operated. Flash détente can remove volatile free compounds but not bound glycosides.
--Market wine for immediate consumption.
--Initial experiments with fining agents have been ineffective or had mixed results. Activated carbon has shown promise in removing smoke-related compounds in some trials, but Krstic said it was not very effective in removing volatile phenols in Pinot Noir. It can also remove desired sensory compounds and characteristics.
Krstic was asked how smoke taint is most commonly handled in Australia. He explained: “In the early days, people thought they could dilute taint by blending. Now, a lot of grape contracts have language requiring grape assessment at harvest, and if taint compounds are found, the fruit is not harvested. This is happening in many premium wine producing areas, and in some years, some producers don’t produce certain wines.” The industry prefers to err on the side of caution. Krstic summarized, “Consumers can be confident what’s going out in the market will not affect brand perception and reputation.”
Recent and Ongoing Research
Collins began research trials in Washington in 2016 with hoop-houses placed over vine rows within a vineyard with controlled and measured applications of smoke blown into the hoop-house to expose grapes for specific time periods. This method allows for an analytical comparison with non-exposed vines and grapes in the same vineyard. Within the hoop-houses, smoke intensity and composition is monitored, including the size of smoke particles.
Wines made from 2016 trial fruit included Riesling and Cabernet Sauvignon. In 2017, trials were expanded to include multi-day smoke exposures for Chardonnay, Merlot and Cabernet Sauvignon. Smoke was produced from several Washington plant fuels, including sage, rabbitbrush, cheatgrass and conifer bark mulch. A small scale RO system is used to test different RO membranes as mitigation treatments. Mitigation trials will also focus on hydrolysis of glycosides.
Collins said, “We’re trying to tease out differences in smoke characters from different fuels, and determine what these differences may be in the smoke, in the fruit, and in the wine.”
Collins collected samples of California wines from the 2017 vintage from North Coast wineries that harvested grapes after smoke exposure during the October fires. These will be analyzed as part of ongoing research to identify and evaluate glycosides as markers for smoke taint.
Fire Damage in Vineyards
If a fire burns into a vineyard, physical vine damage must be assessed. Krstic said there can be varying levels of leaf scorch, heat damage, or burned vine wood. Fire tends to burn along PVC irrigation drip lines that will melt, and also cause potential vine damage. Damaged vineyard infrastructure, such as irrigation lines and wooden posts, will require replacement.
Krstic said, “Heat can kill vine phloem tissue, and you can end up with vines that eventually die.” However, he said it can be challenging to assess long-term damage to vascular tissue, and it may only be observable based on growth and productivity over one or more growing seasons.
In Australia, fire is now taken into account in vineyard design, the use and placement of firebreaks, and in clearing and managing nearby vegetation. Growers are more cautious about the annual fire season, look at long-range weather forecasts, and temperatures and moisture levels in relation to fire risk. Growers monitor wind conditions and direction related to potential smoke drift and duration of vine exposure to smoke. Australia is expanding its network of air pollution sensors to monitor smoke concentration and chemistry for risk assessment.