The following article is a discussion between The Academic Wino and leaders in the wine treatment business in the United States. Interviews were conducted in November 2011, so any new research since then will not be included in the discussion.
*Edit: For an update on the most recent research that was published after these interviews were conducted, please read the first comment after this article from Bob Kreisher, who explains in excellent detail the current state of this research within the last month or so.
Since the Mendocino, California wildfires of 2008, researchers have been attempting to find ways to mitigate the negative sensory characteristics of which smoke taint in wine is associated. Even before these fires, Australian vineyards have been coping with the same problem, as wildfires in that region are even more frequent than they are in California or other places in the United States.
What is Smoke Taint?
Smoke taint is a condition created when grapevines are exposed to large quantities of smoke at a key point in the maturation of the grapes. Smoke exposure from veraison to harvest results in noticeable smoke taint properties such as “smoky” and “ash” in the final wine.
Smoke volatiles are absorbed by the leaves of the vines and are bound to sugars within the plant. These glycolated volatiles are moved throughout the plant via the xylem, ultimately accumulating in the berries. According to Eric Herve from ETS Laboratories (locations in CA, OR, and WA), while in these conjugated forms, smoke taint volatiles are not easily detectable, and could result in false negatives. In other words, juice samples will test negative for guaiacol and 4-methylguaiacol, currently the only known indicators of smoke taint in wine, when they are bound by sugars. However, during fermentation, guaiacol and 4-methylguaiacol are released from these conjugated forms and are detectable in the finished wine.
Herve has since established an assay by which the free fraction of guaiacol is measured in whole berry samples instead of the juice, which tends to have much lower levels of guaiacol compared to the whole berry fraction, resulting in a higher likelihood of identifying smoke tainted samples.
What is complicated about smoke taint?
Guaiacol and 4-methylguaiacol are currently the only two compounds in smoke taint that have been identified. What complicates the issue, however, is that there are actually many compounds that contribute to smoke taint in wines, and no others have been identified.
According to Eric Dalhberg, president of Winesecrets (locations in CA, OR, TX, VA, British Columbia and Ontario), only measuring guaiacol and 4-methylguaiacol to identify smoke tainted wines is problematic. Some wines can have increased levels of guaiacol with no flavor impact, while other wines that are low in guaiacol can have a negative flavor impact. Simply measuring the levels of guaiacol and 4-methylguaiacol in wines doesn’t necessarily mean that it is suffering from smoke taint, and that sensory analysis appears to be the only way to determine if smoke taint is present. Sensory analysis of smoke tainted wines could become problematic if palette fatigue set in and created a situation where a wine is scored negative for smoke taint when in fact it was there.
Bob Kreisher, president of Mavrik North America (MNA, Santa Rosa, CA), stated that there are many compounds involved that produce a wine with the characteristic flavor of smoke taint.
Kreisher believes there is likely a synergistic effect occurring between all of the volatile compounds that contribute to smoke taint. For example, Kreisher noted that if you take two wines below the sensory threshold for smoke taint and combine them, you might create a wine that is above the sensory threshold. Though the identity of these compounds is unknown, the team at MNA has successfully identified the chemical characteristics of the volatiles and has applied this knowledge to their amelioration techniques.
The Australian Study
Smoke taint research is not novel, and there have been many papers published on the subject. Recently, a team of researchers in Australia published an article claiming that they are the first to demonstrate the amelioration of smoke taint from wines using the techniques of reverse phase osmosis and solid phase adsorption to selectively remove guaiacol and 4-methylguaiacol.
The authors used a proprietary spiral wound nanofiltration membrane with a molecular weight cut-off of 150-200amu and a polystyrene based adsorbent resin. Smoke tainted wine was pumped through under high pressure to attain flow rates of 50mL/min and 600mL/min. Samples were collected at various times throughout the experiment, and once the process was complete, wine was bottled and stored at 15oC until sensory analysis could be performed.
However, after revisiting the ameliorated wines 6, 12, and 30 months after the treatment, the authors noted that the smoke taint characteristics gradually returned over time.
What has been done in the US?
Representatives at both Winesecrets and MNA claimed that none of the wines they treated from the 2008 wildfires suffered recurrence of smoke taint, unlike the results of the Australian study. At Winesecrets, activated carbon is used in the solid phase adsorption step, instead of the resin that used in the Australian study. According to Dahlberg, using activated carbon is much more effective than using resin for the removal of smoke taint from wine.
At MNA, instead of only focusing on guaiacol and 4-methylguaiacol, they targeted a range of volatile compounds by creating membranes that selectively remove compounds based on several variables. One problem with the Australian study, according to Kreisher, is that they used membranes that only selected compounds based on molecular weight.
The problem is that there are many ways in which a compound can pass through a membrane, which are not solely dependent upon molecular weights. By removing a range of volatile compounds, and by manipulating other variables such as electron attraction and flow rate in the solid phase adsorption step, MNA is successfully able to remove the smoke taint from the wines, without dramatically reducing the desired volatiles, and none have returned with a recurrence of smoke taint.
What caused the recurrence of smoke taint in the Australian study?
The Australian researchers claimed there was a recurrence of smoke taint as a result of 4-ethylphenol and 4-ethylguaiacol being released into the wine over time. The problem with this claim, Kreisher says, relates to how glycobound phenols are released. Simply put, guaiacol, 4-ethyguaiacol, and other phenols, are released from their glycolated forms during the fermentation process. Once fermentation is complete, there is not enough sugar to release guaiacol and 4-ethylguaiacol at the levels found in the Australian study.
Kreisher believes the Australians found a recurrence of smoke taint in their wines due to the devolution of other volatile phenols into guaiacol and 4-methylguaiacol. If they had targeted the entire range of volatile compounds to begin with, he says, they would not have found a recurrence of smoke taint in their wines.
Another mechanism for this recurrence, Kreisher elaborated, is the possible synergistic effect of the smoke taint volatile phenols that were not removed and a Brettanomyces infection. There is a possibility that a treated wine with smoke taint volatile levels below threshold that becomes infected with Brettanomyces, an infection by which 4-ethylphenol and 4-ethylguaiacol are increased; could have a synergistic effect that results in the wine showing smoke taint above sensory threshold levels.
Even though the Australian study is the first study demonstrating the amelioration of smoke taint from wines using the aforementioned methods, work has been occurring simultaneously in the US. Companies in the United States appear to have established smoke taint amelioration methods that are similar to those used in the Australian study, but with key differences that eliminate the problem of smoke taint recurrence in wine.
An independent review of all known methods may be the next step toward finding the most effective smoke taint amelioration technique.
The Academic Wino would like to thank Eric Herve, Eric Dahlberg, and Bob Kreisher for taking the time to speak with us and provide us with important information for this post.