Lots to learn about volatile flavour compounds

It is something of an understatement to say that Dr Leigh Francis and his team at the Australian Wine Research Institute (AWRI) will be doing a lot of number crunching between now and the middle of next year.

Data are still arriving from the different stages of the numerous component parts of a project involving complex chemistry, international collaboration and high-level input from both winemakers and wine sensory specialists. Clear trends are emerging, but there are still a lot of questions requiring clarification.

The four-year project, funded by Wine Australia, is seeking to better understand what causes a number of specific sensory attributes in wine – among them ‘greenness’ in red wine, ‘stone fruit’ in white wine and ‘tropical fruit’ in Chardonnay – and in particular what role is played by volatile compounds.

Each area of study is throwing up some interesting findings, but the ‘greenness’ question has involved the most legwork.

‘The first stage was to determine what winemakers actually mean when they refer not just to green flavours and aromas but also to green tannins, textures and mouthfeel; we had a reasonable idea in relation to Cabernet but not Shiraz’, Dr Francis said.

‘Once we work out what the chemical compound is we can advise on what manipulation can happen in the vineyard or winery to avoid greenness, but you can’t link a compound to a nebulous concept.’

The solution was to ask a trained sensory panel to define and rate the characteristics of 20 ‘green’ wines then compare their detailed assessment with those of 20 winemakers, who grouped each wine using terminology that came naturally to them. In very simple terms, greenness correlated most closely with green capsicum or vegetal aroma and also with bitterness (a word winemakers tend not to use in relation to finished wine).

The researchers then began exploring the compounds linked to this greenness, first in Cabernet Sauvignon and then in Shiraz. Of great interest is the likelihood that methoxpyrazines, which are important in Cabernet, are also found in Shiraz if grape stalks are part of the winemaking process. That goes against conventional wisdom, and has implications for whole bunch fermentation of Shiraz and Pinot Noir.

A separate but related study is looking at the influence on wine flavour of trees surrounding vineyards or specifically planted as wind breaks. Trials have been conducted looking at Pinus radiata in the Adelaide Hills, native Sheoaks in Langhorne Creek and Monterey Cypress in the Yarra Valley. Preliminary data suggests the cypress is having the greatest effect.

‘We’re looking at the impact of even just having a few leaves or fine needles find their way into the picking bin; I’m sure many people wouldn’t even notice they were there’, Dr Francis said.

The AWRI team is also working with the University of Bordeaux to investigate what causes the stone fruit characteristics often found in both Chardonnay and Viognier, and again there appears to be a number of parts to the story.

‘There are compounds called lactones that are important to apricots and peach flavour,’ Dr Francis said. ‘They can be found in wines but usually at well below the level that would cause a flavour impact, so we are investigating that quite closely.

‘At the same time it appears that compounds that we know are important in Riesling and other floral varieties also come into play. You can create a distinct stone fruit character by adding several compounds together and we’ve got quite a good understanding now of what happens if you take away one or add one.’

A fourth stream of the project is investigating whether a group of thiols known to be important in Sauvignon Blanc is behind the grapefruit or tropical fruit characteristics in some Chardonnays, particularly from cool regions, and whether they are involved in other Chardonnay flavour profiles.

Two studies – one of mainly premium Australian wines and the other of wines made under controlled conditions from grape juices sourced from vineyards across Australia – both found thiols in quantities well above the level needed to have a sensory effect. The question, then, is whether other compounds in Chardonnay are suppressing the tropical notes.

As Dr Francis notes: ‘It’s all really interesting stuff.’