The Effect of Vineyard Proximity to Eucalyptus Trees on Sensory Characteristics of Red Wine

Can proximity to a grove of Eucalyptus trees result in wines with flavors or aromas associated with that of Eucalyptus?  In certain regions of Australia, and other locations with populations of Eucalyptus trees, it has been noted that consumers can taste flavors and aromas in wine that was made from grapes planted near these trees.  Most species of Eucalyptus possess volatile compounds in their leaves, with the most common compound found in those species used for Eucalyptus oil being 1,8-cineole (a.k.a. “eucalyptol”).  In red wine, eucalyptol shows flavors and aromas of “fresh”, “cool”, “medicinal”, and “camphoraceous”.

 

Many studies have tried to answer the question of where does the eucalyptol in red wine come from.  Is it a result of the proximity of some vineyards to Eucalyptus groves?  Do some precursors to 1,8-cineole occur naturally in red wine, only to develop into eucalyptol during the winemaking process?  Could both mechanisms exist in wine?  Several studies have found evidence for both pathways, though certain details are unknown.

 

The study presented today, published in 2012, aimed to examine the relationship between grape chemical composition and proximity to Eucalyptus trees, as well as the influence added Eucalyptus leaves have on the chemical composition of the finished wine.  The latter focus was chosen since leaves and other debris often “contaminate” the must due to them being well mixed in with the grapes; it’s possible these leaves when pressed with the grapes have an effect on the chemical and aromatic composition of the finished wine.

 

Methods

(I have more details on the methods if you have specific questions….just ask).

For part of this study, wines made from grapes in three areas of Australia: Western Australia, Victoria, and Coonawarra.

 

For grape and fermentation analysis, Shiraz grapes were harvested 1 day prior to commercial harvest in the Padthaway region of South Australia during three vintages: 2008, 2009, and 2011.  Grapes were sampled from 4 distances from a Eucalyptus grove; the first row located 5m away from the Eucalyptus trees, and the further row located 125m from the trees.

 

Leaves from Eucalyptus trees were collected for chemical analysis, as well as grapes, grape stems, and grape leaves.

 

Polyethylene traps were placed at each of the experimental distances to see if any of the compounds in question are airborne in nature.

 

For winemaking, some of the grapes were used to make rosé wine, while the rest were made in different batches exposed to varying treatments: 1) treated with grape leaves and stems; 2) treated with Eucalyptus leaves and bark.  Control (no leaves, stems, or bark treatment) batches were also made.  Those Eucalyptus leaves used for these

treatments were leaves that were found mixed into the grapevines themselves (i.e. they fell from the tree and landed in the canopy of the grapevines at some point).  All wines (except the rosé) went through malolactic fermentation and were eventually bottled and sealed with a screw cap closure.

 

Leaves, grapes, stems, bark, and wine were all analyzed using GC-MS for 1,8-cineole levels.

 

Results

In the vineyard:

• The species of Eucalyptus located next to the experimental vineyard was Eucalyptus leucoxylon subsp. pruinosa (South Australian Blue Gum).
• The highest levels of 1,8-cineole were found in wines made from grapes that were located closest (5m) to the Eucalyptus trees at most sites.
  • Those wines from Western Australia grapes had the highest levels of 1,8-cineole present in grapes located within 20m from the Eucalyptus trees.
  • 1,8-cineole levels were negligible in wines made from grapes at the furthest distances from the trees (230-430m away).
  • This pattern was also noted in the experimental vineyards in Victoria.
• To keep 1,8-cineole levels in wine at acceptable levels for consumers (based on past research on consumer preference for the flavors and aromas derived from eucalyptol), the results of this study indicated that harvesting grapes at a distance of about 50m from Eucalyptus trees would minimize the aromatic effects of eucalyptol on wine.
• The skins of grapes had about 4 times more 1,8-cineole per kg compared with the pulp.
  • In terms of extraction during winemaking, about 80% of the 1,8-cineole originated from the skins of the grapes.
• Similar to wine, levels of 1,8-cineole were highest in grapes located closest to the Eucalyptus trees and decreased with increasing distance from the trees.
  • Despite this trend, there was large variation within rows in terms of 1,8-cineole levels in grapes.  This may be due to the thickness of the canopy, how exposed the grapes are to air, as well as the size of the berries themselves.
• 1,8-cineole levels were much higher in grapes leaves and stems than the berries themselves.
  • This may partially explain the large variation between rows discussed above—if the 1,8-cineole is a volatile compound, the leaves and stems could provide some sort of protection of the grapes themselves, thus are the first things the compound hits as it travels through the air.  This could explain why concentrations of the compound are higher in grape leaves and stems than the grapes themselves, and perhaps those grapes that are more exposed would have higher levels of the compound than those grapes that are shaded by leaves (needs to be tested).
• 1,8-cineole was found to be present on the polyethylene traps, indicating that the mechanism for transfer is, in fact, airborne (potentially as an aerosol).

Wine/Winemaking:

• The levels of 1,8-cineole in rosé wines remained constant throughout the winemaking process, indicating that skin contact (i.e. maceration in red winemaking) is needed to increase 1,8-cineole levels in red wine.
  • As a confirmation, 1,8-cineole was not found in those white wines that were purchased and tested.
• Control wines (i.e. wines without leaves or stems added) showed a slight increase in 1,8-cineole levels during the winemaking process, indicating that the skins alone are enough to marginally increase this compound in red wine.
• Wines with the grape leaves and stems treatment showed a significant increase in 1,8-cineole levels, which makes sense considering this compound was found to be significantly higher in grape leaves and stems than the berries to begin with.

 

Conclusions

 

Overall, the results of this study indicate that proximity to Eucalyptus trees does increased 1,8-cineole levels (i.e. eucalyptol) in grapes and wine, and that the further away from the trees you go, the lower the levels of this compound.  According to the results of this study, 50m seemed to be the “sweet spot” in terms of minimizing the influence of eucalyptol in the finished wine while still retaining some of the desired characteristics of the compound.

 

It was also clear from this study that 1,8-cineole is mostly present in the grape skins (not the pulp), thus why the red wines in the study had significantly higher levels of the compound than the rosé wines which had minimal skin contact, and why the white wines in the study had no 1,8-cineole present (no skin contact).

 

It was interesting, to me, to see the results of the wine made from grapes treated with grape leaves and stems, as this is often a situation that is seen in the wine industry during harvest, particularly when much of the harvest is mechanized and fewer workers are employed to carefully sort the leaf and stem debris from the grapes prior to pressing.  1,8-cineole levels were found to be significantly higher in the leaves and stems of grapes, so when wines were created from grapes that were pressed along with leaves and stems, the levels of 1,8-cineole in the finished wine were markedly higher than the control wines without the leaves and stems treatment.

 

Why do the leaves and stems contain more 1,8-cineole than the grape berries themselves?  Is it due to the type of material found in each of these parts?  Or is it a result of a shielding of the grapes by these structures against inbound aerosol or volatile contamination?  One theory expressed by the authors, that of which I think is very possible, is that the leaves and stems act as a sort of protector of the grapes again any aerosols flying about through the air, much like they are protecting the grapes against sunlight.  I think a good follow-up study would be to take a look at this phenomenon more directly—compare the 1,8-cineole levels of grapes when exposed to the air to grapes that are covered under varying levels of canopy protection.

 

Overall, I think this study provides very useful information for those dealing with vineyards in close proximity to Eucalyptus groves, though there are certainly many more questions that were raised regarding physiochemical mechanisms of eucalyptol absorption and transport through the grape vines and wine ultimately created from those grapes.

 

What do you all think of this study?  Do you have any experience with this topic?  Feel free to share any comments you have!

 

Source: Capone, D.L., Jeffery, D.W., and Sefton, M.A. 2012. Vineyard and Fermentation Studies to Elucidate the Origin of 1,8-Cineole in Australian Red Wine. Journal of Agricultural and Food Chemistry 60: 2281-2287.