Susan Ebeler, University of California, Davis, US, reveals the science behind the flavour in everyone's favourite tipple

As you sit down to quaff a glass of wine after work, do you ever pause to notice the faint aroma of boxwood, violets or paraffin? Your perception of wine flavour mainly involves four of the five senses - vision, touch, taste and smell. And it is our nose that gives the greatest contribution to the flavour by detecting the aroma of hundreds of volatile chemical compounds in the wine. The compounds responsible for the smells above are - at low concentrations - typically present in popular grape varieties such as Sauvignon blanc, Pinot noir and Riesling. 

As you reach the end of the bottle do you ever start to ponder about the people who make your wine? It may surprise you to learn that science - including analytical chemistry, sensory science, genetics and molecular biology - plays a large part in providing much of the information needed by wineries to make their wine as appealing as possible to our senses. This includes knowledge of the chemical compounds that contribute to wine flavour, how this flavour is formed during grape growth and the winemaking process, how humans perceive flavour compounds and how human genetics can influence individual perception of smells. 

Over 1000 different aroma compounds in grapes and wines have now been identified using various analytical tools. Gas chromatography, one of the most powerful tools for separating complex mixtures into individual components, has been widely used for this purpose since the 1950s.  However, sensitivity to aromas varies significantly from person to person, so not all volatile compounds that are identified using analytical tools will actually contribute to what you smell.  Conversely, sometimes our sensitivity far exceeds that of the analytical chemist's tools.  The aroma of the compound that contributes to bell pepper aroma in many Sauvignon blanc and Cabernet Sauvignon wines for example can be sensed by humans at concentrations as low as 2 ng per litre - that's equivalent to two drops mixed into 20 Olympic size swimming pools - far below what any analytical technique can currently sense without special sampling or extraction techniques. New methods for preparing and extracting samples are now allowing scientists to push the limits of analytical detection without requiring litres and litres of sample. Additionally by combining gas chromatography with the human sense of smell, in a technique called gas chromatography-olfactometry, scientists can now isolate the 15-20 chemical compounds that contribute the most to wine aroma from the hundreds or thousands of compounds identified in a specific wine. 

Wine aroma is further complicated by interactions between volatile aroma, and non-aroma, compounds.  The interactions mean that two aroma compounds may smell differently when mixed together. Additionally a strong fruity aroma can also mask a weaker vegetable aroma in a wine.

Using a combination of sensory and analytical tools, and an improved understanding of the genetics, physiology and neurobiology of how humans perceive smells, scientists are ever improving our knowledge of the factors that contribute to overall wine aroma perception.

Molecular biologists are also studying how plant and yeast genes impact aroma formation in the grape and during fermentation, improving our understanding of the biochemical processes involved in flavour formation. 
Armed with this knowledge, viticulturists and winemakers can study how vineyard practices - such as soil conditions, irrigation and fertilisation - and winemaking processes - such as choice of yeast strain, fermentation temperature, yeast nutrients, oxygen exposure - impact the chemical composition and sensory properties of their wine.  All this information allows winemakers to identify grape varieties, yeast strains, and fermentation conditions that will yield desired flavour properties for specific wine styles and consumer preferences.

I suspect you didn't realise the chemistry in a glass of wine is quite so complex.  So, the next time you enjoy a glass of fine wine take time to toast the chemists, plant scientists, viticulturists and winemakers who've used the latest scientific tools to ensure that it tastes and smells the way you know and love.

Read Pavla Polásková, Julian Herszage and Susan Ebeler's critical review 'Wine flavor: chemistry in a glass' in issue 11, 2008 of Chemical Society Reviews