Identification and origin of volatile compounds responsible for important wine sensory attributes
To determine the identities, origins and levels in grapes and wines of compounds responsible for ‘greenness’ in red wine, ‘stone-fruit’ in white wine and ‘tropical fruit’ in Chardonnay and to determine the impact of botrytis and site on the levels of rotundone (‘black pepper’) in grapes and wines.
While many sensory attributes of wines have a known cause, there are major flavour characteristics where the source compounds, and/or the mechanisms that control their levels in grapes and wine, are not known.
- ‘green’: Methoxypyrazines are important to the green aromas of Cabernet Sauvignon and Sauvignon Blanc, but they are generally not found in Shiraz or Pinot Noir. C6 alcohols give grassy aroma and have been implicated in greener wine flavour, but the evidence is not strong. The possibility that cover crops may contribute to green characters builds on anecdotal reports and on the knowledge that eucalyptol can transfer from eucalyptus leaves in the harvested loads of fruit into must and wine.
- ‘stone fruit’: Several γ-lactones important to the aroma of peach and apricot fruit have been found in wines, but at low levels. It is possible that these compounds can act additively, but evidence is lacking.
- ‘tropical fruit’ in Chardonnay: The contribution of thiol compounds to tropical fruit characters of Sauvignon Blanc wines is well established and, although these compounds are also present in other white varieties, there is less information available on their importance to tropical fruit characters in these varieties, particularly Chardonnay.
- ‘black pepper’: Rotundone is the impact odorant for pepperiness in red wines. Its levels in grapes vary across regions and seasons, with the highest levels in cool climate regions in cooler years. Its levels are also variable within vineyards. Site, aspect and fungal infection may impact on its levels.
Aroma compounds will be identified through sensory analysis, gas chromatography/mass spectrometry-olfactometry and targeted quantification of lead compounds from the literature. Hedonic (liking) response of consumers will be included. The importance of compounds will be confirmed by additional studies and use of commercial wines.
The knowledge generated by this project will allow researchers to link grape and wine compounds to wine quality and style, and assess whether these compounds or proxies can be used to objectively measure grape quality. Such tools would allow grape growers and winemakers to tailor wine style through measuring and monitoring throughout the grapegrowing and winemaking process.