More technical questions regarding volatile phenols management have been asked as the last grapes from 2017 get harvested in Northern California. In this blog post we discuss the limitations attached to current analytical methods and how to make the best of it.
Why is it difficult to assess the risk for smoke tainted aromas?
Most smoke taint wine analysis use gas chromatography. It makes sense since winemakers are precisely interested in volatile phenols (ie. those that are gases at wine drinking low temperature). However, the big issue is that most of these phenols compounds are not stored within grapes as volatile phenols. Rather, they are stored primarily as glycosides. Consequently, assessing only the volatile phenols does not give you the whole picture because phenols can be masked in the fruit.
What can we do today to assess the risk for smoke tainted aromas?
Sugar-linked phenols bond may be quickly broken (i.e. “hydrolyzed”) by yeast enzyme… or later possibly by enzymes in your own mouth when tasting wine. The problem with the method of gas chromatography is that it can’t detect phenols as volatile prior to their hydrolysis. According to Dr. Zandberg, from the University of British Columbia and co-author of last month’s article on analyzing volatile phenols in smoke exposed grape berries prior to fermentation, two main analytical options are available in the wine industry for early assessment:
- Hunt for specific sugars-linked volatiles phenols. The problem here is that we do not exactly know at this point what those sugars are.
- Analyze small amount of fruit after fermentation. This is a really good method but is not necessarily practical. Grape producers do not always have time to do a batch fermentation. Wineries buying grapes need reassurance that grapes are not tainted before the trucks show up.
In this context, developing a method to predict the potential for tainted smoke in wine from fresh fruit and before fermentation is still a topic of research today. Considering those limitations, what is the best we can do?
BERRY SUGAR EFFECTS
Amount of phenols bound to sugars: varietal sensitivity is not clear after veraison.
Sensitivity differences among cultivars have been reported. However, 2 weeks after veraison, Kelly et al. (2014) have shown that if smoke exposure occurs for a same stage of berry ripening, cultivar has no effect on the accumulation of total phenols bound to sugars. However, the level of vine vigor seems to have an effect on fruit sensitivity. According to Kelly et al. (2012) the primary mode of entry of volatile phenols into berries is directly via the berry cuticle rather than via leaves. For that reason, dense canopies contribute to protect berries from higher exposure to smoke particles. This may be something to remember after the end of fermentation. Wine susceptibility to display smoke tainted aromas may be connected with canopy porosity.
Influence of vine ripening stage on smoke uptake
Australian and Canadian reports have revealed that vine sensitivity to smoke is increasing as grapes mature. The period when smoke exposure occurs between veraison + 7 days and harvest is correlated with the most intense perception of smoke tainted aromas. However, since the ripening cycle can be divided into 2 periods of sugars accumulation, many readers have asked whether berries were more or less sensitive to smoke exposure while active sugar accumulation is going on. To the best of our knowledge the question whether more phenols get chemically bound to sugars if smoke exposure occurs during the active phase of sugar accumulation remains unsolved.
SKIN MACERATION AND FERMENTATION EFFECTS
Influence of winemaking practices on volatile phenol uptake
Regardless of plant phenological stage, canopy density or cultivar sensitivity to smoke taint, winemaking will have a profound effect on smoke tainted aromas. In California, levels of compounds related to smoke aroma measured in young wines can be 5 to 10 times higher than those found in corresponding berries. As demonstrated by Australian researchers, a higher proportion of sugars bound to phenols is sequestered within the skin. Consequently skin maceration will modulate how much compounds will be extracted into wine.
Assessing the risk early
The absence of free volatile phenols does not ensure that grapes are unaffected by smoke taint. In fact, the concentration of volatile phenols in grapes can be poorly correlated with measurement of their corresponding sugars bound phenols (here and here). Thus, having no volatile phenols early does not mean no smoke tainted flavors later. Even if your grapes contained non-detectable levels of volatiles phenols, you should not rest. Significant quantities of phenols can appear during fermentation. Yeast, enzyme and acids reactions will trigger the apparition of volatile phenols by breaking their bound to sugars (source). In practical terms, if the smoke exposed grapes show undetectable levels of volatile phenols, you should analyze more carefully sugar bound phenols from a small fermented batch to get a better indication.
#1: Have a reference from a smoke-free growing environment
High temperatures naturally trigger the production of volatiles phenols in plants (here and here). As such, it is necessary to compare the level of volatile phenols found in your sample with the level naturally found in grapes non exposed to atmospheric smoke. Typical levels from smoke free grapes have been reported here for comparison purposes.
#2: Be extra careful with extended skin contact and enzymes
The positive effects of enzymes and maceration time on tannins quality may be overridden by the negative effects of breaking the bounds between phenols and sugars.
Credit picture : Debbie Wolfe