Ξ August 17th, 2009 | → 18 Comments | ∇ A Day at a Time, Technology, Wine News |
What follows is a brief overview on the matter of smoke taint. Recent fires in California have again made urgent the discussion of the topic, especially with the Crush looming. What follows is meant to be a simple primer for the more casual wine industry follower. It covers the matter of vineyards alone. With respect to minimizing smoke taint during vinification and associated ’scalping’ technologies recently developed, those matters will not be discussed.
Smoke taint, the contamination of grapes owing to exposure to fires, whether wild or controlled, is of considerable significance to winemakers and grape growers. Here in the Santa Cruz Mountains the latest blaze, the Lockheed Fire, though 65% contained, with sufficient resources on site to finish the job, the possible effects of smoke on vineyards remain. As with last year’s fires in Northern California and, again, in the Santa Cruz Mountains, there is some evidence, anecdotal in the main, of smoke taint. But the facts are difficult to establish, not only because of the potentially enormous financial impact to growers and contracted wineries and negociants, but also because of the varied tasting thresholds, whether it is a professional or a consumer performing the evaluation.
Smoke taint flavors and odors have been variously described as ‘ashy’, ‘burnt bacon’ and ‘wet ashtray’. Anyone who has wandered through the aftermath of a house or forest fire can readily understand these terms. The effect is all-inclusive, permeating and, in the case of a house fire, often resulting in the total loss of all possessions, clothes and furniture.
Not surprisingly, some research suggests the disagreeable qualities of the smoke taint may depend on the kinds of fuel the fire consumed. Bush fires, conifer forests, chaparral etc. may well have differing chemical signatures. Indeed, the research following upon the chemistry of smoke taint offers potentially interesting insights into discussions of terroir. If smoke may be readily absorbed by the vine and grapes, then perhaps the thriving flora of a given wine region may also leave an imprint not only on the grapes but on the finished wine as well. But that is another topic.
It is also important to mention that the toasting level of a wine barrel, the flavor and odor intensity of which is selectively pursued by winemakers, such toasting levels impart some of the same compounds to a finished wine as does fire smoke, obviously in more modest percentages and chemical combinations.
Though research is on-going, with the Australian Wine Research Institute (AWRI) taking the lead, a few scientific parameters are now fairly well established.
1) Smoke taint is the result of the absorption of principally guaiacol and 4 methyl guaiacol, (there are other compounds) into the grape skins, vines and leaves. Research indicates it is the grape skin absorption itself that is of the greatest concern. The first efforts to systematically understand smoke taint were undertaken by AWRI after winegrowers reported off-flavors after devastating bush fires in Australia in 2003. It was then discovered that smoke taint was not simply the consequence of ash and smaller fire particulates falling on the grapes, but was more systemic. Simply washing the grapes, or more aggressively, removing the waxy surface bloom of the grape, had no effect on the presence of smoke taint. The penetration of fire-generated compounds into the skin, leaves and vines was then established.
2) Absorption levels are grape-specific. For reasons not yet understood, there are specific grape varieties with a stronger affinity for guaiacol and 4 methyl guaiacol absorption. From the Department of Primary Industries (DPI) paper:
Anecdotal information gathered at the industry meetings suggested that there was variation in smoke taint between grape varieties. Varieties classed as having high susceptibility by industry were Pinot Noir, Sangiovese and
Cabernet Sauvignon, and lower susceptibility was found with Merlot and Shiraz.
3) Smoke exposure duration is of special significance. From a second DPI paper:
Although research is ongoing in this area, it is interesting to note that a single heavy exposure of smoke (30% obscuration/m) to grapevines for 30 min is sufficient to result in smoke taint in wine (assuming that the exposure occurs during a period of sensitivity).
4) The timing of smoke exposure is critical, with 7 days post veraison to harvest showing the greatest susceptibility. Again, from the second DPI paper:
There are three distinct periods of grapevine sensitivity to smoke exposure. The first period (P1) is characterized by a low potential for smoke uptake early in the growing season when shoots are 10 cm in length and at flowering. The potential for smoke uptake is variable during the second period (P2) from when berries are pea size through to 3 days post veraison. Grapevines show a low to medium sensitivity to smoke uptake during P2. During the third identified period (P3) from 7 days post veraison through to harvest grapevines have a high potential for the uptake of smoke compounds.
5) Harvesting, whether by hand or by mechanical means. This is hardly surprising, given that absorption of smoke occurs in the grape skin itself. From the first quoted DPI paper:
The concentration of ‘total guaiacol’ in the machine harvested grapes ranged up to 4.5 times that of the hand harvested grapes in the press fractions. Based on the original concentration in the grapes and the concentrations and
volumes of juice collected, around 40% of the ‘total guaiacol’ in the grapes came through in the press fractions and lees with hand harvesting and around 98% of the ‘total guaiacol’ in the grapes came through in the press fractions and lees in the machine harvested sample.
6) There does not appear to be a ‘carry-over’ effect. Vines that have absorbed smoke do not then have the that smoke expressed in the fruit the following year.
(Update! The links have been repaired.)