Unlocking the secrets of eucalyptol
Australian winemakers can now better control the levels of minty or Eucalyptus flavours in their wine thanks to an almost six-year-long project funded by GWRDC at the Australian Wine Research Institute.
AWRI senior scientist, and lead researcher on this project, Dimitra Capone said the project began in 2008, when the AWRI’s problem solving service was asked to assess wines made from fruit from rows at different distances to Eucalyptus trees.
The first step for the research team was to develop a rapid analytical method to identifying the compound 1,8 cineole, otherwise known as eucalyptol and responsible for the flavour and aroma characteristics most often described as ‘Eucalyptus’, ‘mint’ and ‘camphoraceous’.
‘At the same time, we started a survey of commercial red and white wines – assessing 146 red wines and 44 whites from all around Australia’, Dr Capone said.
The survey showed about 40 per cent of the red wines contained eucalyptol above the standard sensory threshold (1.1 µg/L). For the 44 white wines analysed, only very small amounts (below 0.8 µg/L) were detected.
‘We subsequently found that the low concentration of eucalyptol in white wines examined was because the compound accumulates in the skins and is only extracted during winemaking through time on skins’, she said.
‘We sourced the wine from all across Australia, and the survey showed that this Eucalyptus/mint flavour isn’t confined to any particular region – in fact it occurs all over the world wherever Eucalyptus trees are planted, including California, Spain, Portugal and South America.
‘It can however be a little more common in some regions more than others, such as areas around Clare Valley, Margaret River, Langhorne Creek, Coonawarra, Padthaway and central Victoria where some vineyards can be in quite close proximity to Eucalyptus trees’, Dr Capone said.
In 2009–10 the project progressed on a number of research fronts. It followed red fermentations to assess eucalyptol levels during skin contact; measured eucalyptol levels in leaves, skin, stem and pulp as a function of proximity to Eucalyptus trees; conducted investigations regarding the transformation of grape metabolites, the effect of closures and time in bottle; and consumer preference for minty/Eucalyptus aromas in wine.
In this time, they confirmed that the greatest primary source of the 1,8 cineole compound came from grapes with the closest proximity to Eucalyptus trees.
The consumer preference research also revealed that the majority of consumers tested preferred wines with the minty flavour.
But the research and discoveries didn’t end there.
In 2011, fermentation studies were completed assessing the effect of Eucalyptus leaves and grape leaves and, more recently, surveys of specific wine types were completed. Also, studies were run to find if the compound can be translocated from the soil or leaves to the berry.
As a result, Dr Capone said the AWRI confirmed one or two of their hypotheses but also turned up some unexpected findings.
‘We found that absorption of the compound by grape berries, while important, is much less a factor than the presence of Eucalyptus leaves or bark in the grape bins during harvest’, she said.
‘Even a small number of Eucalyptus leaves in a harvest bin can have a very large effect on eucalyptol levels in wines.’
Another unexpected result showed grape leaves or grape stems were also a major source of the compound.
The discoveries led to further work being conducted by the AWRI, which saw the team working with producers to assess practical methods to adjust the levels of the compound.
‘The knowledge generated from this project has given producers the practical means for reducing this flavour, if desired’, Dr Capone said.
‘By adjusting their practices to remove matter other than grapes (MOG) in rows closest to Eucalyptus trees, either through careful hand-harvesting, sorting, or adjustment of mechanical harvesting – a real difference can be attained.’
Though the research on this particular compound, 1,8 cineole, is coming to an end, Dr Capone said a next stage is already being planned.
‘We plan to address the observation that grape leaves can act as a source of flavour derived from outside sources, by assessing the role of grape leaves and stems, and other flavour components, from the local vineyard environment’, she said.
The team have also started to investigate whether other minty compounds, apart from eucalyptol, might be involved.
IMAGE: The greatest primary source of the 1,8 cineole compound, also known as eucalyptol, came from grapes with the closest proximity to Eucalyptus trees.
For more information contact Dr Liz Waters, GWRDC R&D Program Manager, firstname.lastname@example.org