Measuring tropical and citrus aromas in wine

Sauvignon Blanc grapevines growing in the Barossa Valley, South Australia

Sauvignon Blanc grapevines growing in the Barossa Valley, South Australia. Credit: Amanda Slater (CC BY-SA 2.0)

By David Jeffery

Wine is enjoyed by people the world over in a global market worth many billions of dollars. But what is it about wine that we enjoy? Well, the aromas and flavours that we perceive when drinking wine are certainly part of it. We experience these things through the interaction of our senses with volatile molecules in the wine, just as we perceive the odours of freshly cut grass or perfume and the flavours of roast beef and orange juice.

The molecules in wine come from the grapes themselves, the yeasts used for winemaking, from storage in oak vessels and even from reactions once the wine is bottled. Overall, this leads to a complex combination of hundreds of volatile compounds which have varying impacts on the aromas of different wines.

In various wines you may smell roses, berries, honey, vanilla, chocolate, or any number of other aromas. Of particular interest to us are specific molecules known as ‘varietal thiols’ that contribute characteristic tropical and citrus aromas to Sauvignon Blanc wines (and other varieties). These molecules are some of the most potent food odorants known, which is why they are so interesting, but that also makes them very challenging to measure.

Being able to determine the levels of these aroma compounds is important, because after all, “you can’t manage what you can’t measure”. We know that people enjoy the tropical and citrus aromas of Sauvignon Blanc and buy this wine for that reason. Although, sometimes there can be too much of a good thing, especially for these very potent aroma compounds. Therefore, it is extremely useful to be able compare winemaking or grape growing effects on the levels of varietal thiols so wine producers can have greater control over their presence in Sauvignon Blanc wines.

It is much easier to say we want to measure varietal thiols in wine than it is to actually do it. Our team worked on a method for many months to come up with something completely new and exciting (well, exciting for us as researchers, knowing how difficult the task was). Volatile molecules such as these thiols are typically analysed in the gas phase using gas chromatography. This technique separates the components in a vaporised mixture so they can be identified and quantified, often using a mass spectrometer that detects the mass-to-charge ratio of molecules. Because of the very low concentrations of varietal thiols in wine, it takes a lot of effort to prepare the samples when using the usual instrumentation, causing the analytical process to be inefficient. So, rather than prepare the varietal thiols for analysis as a gas, we did something a little tricky and modified the thiols within the wine so they could be analysed as a liquid. This greatly simplified the sample preparation process and we then used high performance liquid chromatography coupled to mass spectrometry to detect and quantify the compounds.

When our method was ready we tested a range of different Australian wines to check the concentrations of varietal thiols, making sure the method worked well. Now we are able to use this new method to conduct further studies, with the aim of providing greater insight to producers so they can continue to provide the kinds of aromas and flavours that wine drinkers love.

This research was a collaboration between the University of Adelaide and the Australian Wine Research Institute (AWRI), as members of the Wine Innovation Cluster at the Waite Campus. The work was performed by David Jeffery and Renata Ristic from the School of Agriculture, Food and Wine at the University of Adelaide and Dimitra Capone and Kevin Pardon at AWRI. We are grateful for the financial support of the University of Adelaide Wine Future, and Australia’s grapegrowers and winemakers through their investment body, the Australian Grape and Wine Authority, with matching funds from the Australian government.

This research was originally reported in Analytical Chemistry. For the full research paper and to reproduce please refer to: http://dx.doi.org/10.1021/ac503883s

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