During this time and on the strength of the above-mentioned preliminary indications, more dedicated but still informal investigations were launched at Nietvoorbij and experimental wines were made from grapes harvested at various distances from blue gum trees. The organoleptic evaluation of these wines indisputably showed that the proximity of blue gum trees was indeed the cause of blue gum characteristics in red wine. The mechanism was still unknown. There were some opinions that the blue gum characteristic was transferred to vines through the roots of neighbouring trees, which were known to penetrate over long distances into vineyards. However, anyone who has parked a car beneath a blue gum tree during flowering will have experienced that the car gets covered in fine globules of nectar or gum. It, therefore, follows logically that it is probably this nectar/gum globules or similar compounds that are carried by wind and end up amongst others on bunches of vines. The flowering stage of blue gums varies according to cultivar but is generally spread over a long period during summer, also during the harvesting time of wine grapes. Naturally, the type of blue gums and the strength, direction and duration of reigning winds will affect the prominence of blue gum characteristics in wine from exposed vineyards.
Why does the blue gum/eucalyptus characteristic occur only in red wines Again, logic indicated that in the making of red wine the substance concerned is more extracted because of skin contact than in the case of white wine, where skin contact is usually only brief. The substance concerned is also probably more soluble in alcohol, in which case fermenting red wine on the skins should induce still higher concentrations thereof in the resulting wine.
During Kobus Louw’s term as head of the Soil Science Section at Nietvoorbij, experimental wines were again informally made in a similar fashion as above. The conclusions were the same – the presence and distance of blue gum trees from grape vines is directly related to the intensity of a blue gum or eucalyptus characteristic in red wines (K. Louw, 2013, personal communication).
During 2000/1, experimental wines were made at Bergkelder with grapes obtained from two Cabernet Sauvignon vineyards at Durbanville and Devon Valley, both with a reputation of having blue gum characteristics in the wines made from them. Naturally, there were blue gum trees bordering these vineyards. Distances of experimental plots from these trees were 30, 120 and 230 m in the case of the Devon Valley vineyard and 15, 85 and 155 m in the case of the Durbanville vineyard. The active compound of the eucalyptus characteristic was not known at that stage and analytical facilities were in any case not available, therefore, the experimental wines were only organoleptically evaluated. The results were again quite straightforward – the closer the blue gum trees, the more pronounced the blue gum/eucalyptus characteristics showed in the wines concerned, which in certain circles were described as having a ‘mint’ character.
The cause then of the ‘mint’/blue gum/eucalyptus characteristic in some red wines was therefore known in informed circles in South Africa for quite a while, but because of aforementioned reasons, never formally made known.
In Australia, where blue gum trees are indigenous and often used as windbreaks, the so-called ‘mint’ characteristic was certainly also known for quite a while, but until recently not mentioned, probably because it was considered unwanted in some cases or because of some uncertainty whether this characteristic is acceptable for the consumers and to what extent. In 2008 viticulture consultant, John Whiting, wrote that the Bendigo wine region was inter alia searching for the better management of what he plainly described as the ‘minty’/eucalypt’ characteristic in their red wines (Whiting, 2008). Descriptive terms for this were later given as: eucalyptus, fresh, cool, medicinal, camphoraceous (Lindh, 2009), very much like the properties of mother’s feared ointment. In reaction to findings of the ETS Laboratory in the USA, namely that 1,8-cineole or eucalyptol relates to the blue gum/eucalyptus characteristic in wine, research on this in Australia by the AWRI (Australian Wine Research Institute) was announced in 2009 (Lindh, 2009). A survey showed that out of 146 commercially available red wines from all over Australia, 40% contained more eucalyptol than the sensory threshold value of 1.1 µg/ℓ. No eucalyptol concentrations above 0.9 µg/ℓ were recorded in any of the 40 white wines that were tested.
In spite of the now already convincing results from field trials that the proximity of blue gum trees is linked to the eucalyptol concentration in wine (Van Leeuwen et al., 2007; Capone et al., 2009; Lindh, 2009), there were still two other theories, namely that eucalyptol develops from chemical forerunners during the wine-making and bottle ageing processes, or that grapes produce the compound naturally (Saliba, 2009). However, not one of these theories could be verified. AWRI research showed the forerunner compounds in grapes are not able to generate 1,8-cineole concentrations higher than sensory threshold levels (Capone et al., 2012).
Field investigations aimed at the mechanism of transfer of eucalyptus characteristics from blue gum trees to grapes undeniably proved that the eucalyptus (eucalypt) characteristic is carried over through the air in various forms like volatile aroma compounds, but also as leaves and pieces of bark. Vine leaves and stems had even higher eucalyptol concentrations than berries (Capone et al., 2012). The contribution of blue gum leaves and bark, but also vine leaves and stems to eucalyptus characteristics, will naturally be higher in cases where grapes are machine harvested.
Now that the real cause of eucalyptus characteristics in red wines was scientifically confirmed, there are investigations into methods to manage the potential impact thereof and to determine the levels of eucalyptol in wine still acceptable to consumers. Investigations indicated that the ‘consumer rejection threshold’ or CRT, is about 27.5 µg/ℓ (ppb) (Saliba, 2009). Results also indicated that a moderate eucalyptus characteristic should not be regarded as contamination and that some consumers even have a preference for it. It was also found that the threshold value of eucalyptol varies according to the specific wine and grape cultivar.
During the OIV (Office Internationale de la Vigne et du Vin) congress in Avignon (2002), it was announced that research was conducted on flavour compounds in wines that may possibly be carried over from the garriques vegetation, which commonly occur in the Gard district (Languedoc environment) and from which a distinctive honey is obtained. The garriques is a mountain shrub type of biome, which reminds of our fynbos, in which some highly aromatic plants occur, like wild thyme. It seems that no results on this have yet been published, possibly because they were negative, or if positive, the French have discreetly decided that such compounds are not natural grape components and are therefore not acceptable.
Here in South Africa, references to fynbos characteristics in certain wines by over enthusiastic wine evaluators are also sometimes experienced. Even from some producers and winemakers, comments were heard that inter plantings in vineyards of indigenous aromatic plants such as buchu, may have a market value if the specific character is transferred to the wine. The claims of fynbos characteristics in wines and the exposure of grapes to neighbouring or interplanted aromatic plants, are highly controversial. The latter is artificial and if flavour compounds can be carried over in this way, they still are not natural grape flavour compounds. Where to draw the line Surely it is easier to add fynbos flavour as an extract out of a bottle. Then we are back to the embarrassing situation of a while ago concerning the artificial addition of methoxy-pyrazines in some Sauvignon blanc wines and the more recent phenomenon of very prominent and highly suspicious coffee characteristics in some red wines.
The natural factors that determine terroir are climate, topography and soil, together with biological factors such as human intervention by means of cultivar choice, cultivation and wine-making practices that bring the natural terroir factors to maximum expression in order to create a unique agricultural product, not necessarily wine. Neighbouring aromatic plants are no part of this. The only non-grape flavour compounds which, because of a centuries old tradition, is accepted by the OIV in natural wines, are that derived from oak. Nevertheless one should grant the Australians their natural environment of blue gum trees and follow with interest their efforts to manage the eucalyptus character in wines and to determine what levels are acceptable. In South Africa blue gum trees are aliens and in many situations quite prolific or unwanted invaders. However, the eucalyptus characteristic in some South African wines is a reality and can surely not be summary declared as unwanted. Therefore, as in Australia, the management of the eucalyptus characteristic and the levels up to which it is acceptable merit attention (analysis facilities to support this management are available). Furthermore an increasingly better informed consumer market will most probably in any case eventually determine to what extent eucalyptus characteristics will be acceptable, if at all.
Capone, D.L., Francis, I.L., Herderich, M.J. & Johnson, D.L., 2012. Managing eucalyptus aromas. Wine & Viticulture Journal, July/Aug. 2012, 22 – 27.
Lindh, K., 2009. Are wines affected by the proximity of vineyards to Eucalypt trees Aust. & New Zealand Grapegrower & Winemaker, Feb. 2009 (541), 56 – 57.
Saliba, A., 2009. How much eucalyptus flavour is too much The great debate. Aust. & New Zealand Grapegrower & Winemaker, Aug. 2009 (547), 50 – 52.
Van Leeuwen, K., Pardon, G., Elsey, G., Sefton, M. & Capone, D., 2007. Are Australian wines affected by the proximity of vineyards to eucalypt trees Determination of 1,8-cineole (eucalyptol) in red wines. In: Proceedings of the Thirteenth Australian Wine Industry Technical Conference, Adelaide, Australia.
Whiting, J., 2008. Putting richness into Ripe Rich Reds. Aust. & New Zealand Grapegrower & Winemaker, Aug. 2008 (535), 27.
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