A taste of wine chemistry

‘Wine is a rare example of a consumer packaged good where variation is not just tolerated, it is celebrated,’ explains Gavin Sacks, wine chemist at Cornell University in New York state, US. Over 400,000 different wines are currently available for sale in the US and drinkers expect each of these to have a distinct smell, look and taste. While they may not all realise it, they therefore expect the chemistry in each of these bottles to be different too.

Wine is a surprisingly complex chemical mixture. It is 97% water and ethanol, but each bottle also contains thousands, if not tens of thousands, of different molecules, ranging from acids and sugars to phenolic compounds and vanishingly low concentration aroma compounds.

The job of wine chemists such as Sacks is to identify which of these compounds, or even classes of compounds, is the most dominant in each wine variety and then direct winemakers towards practices that increase or decrease their concentrations as desired for the wines they are making. This could mean changes to vineyard management, how and when grapes are picked, the wine making process or even how the wine is stored.

They’re not hard reactions to figure out but there are so many possibilities

Identifying the important flavour and aroma molecules in a wine is challenging. This is partly due to the vast number of different molecules it contains, but also because these volatile compounds tend to be present at very low levels. Some can be detected by discerning drinkers at parts per trillion (ppt) concentrations.

Once identified, determining where a molecule came from can be equally difficult. It’s rare for wine flavour molecules to be found in the grapes themselves, and instead they form during fermentation and wine storage. ‘It is like someone went to a chemical store, knocked all the bottles over and then pointed to the mess and asked what reactions have happened,’ says Sacks. ‘They’re not hard reactions to figure out but there are so many possibilities.’

The reactions are also pretty obscure, at least to the eyes of a traditional chemist. ‘Wine is so different to a typical organic chemistry matrix,’ he explains. He describes wine making as a one-pot synthesis taking place under mildly acidic aqueous conditions, at room temperature and in the presence of a mild reducing agent. ‘If you look at textbooks nothing interesting happens under these conditions, but what’s unique in wine is that it’s happening for long periods of time,’ Sacks says. And these reactions don’t stop when the winemaking process does – the molecules in wine continue to evolve once bottled. This is why wine is often described as a living, breathing thing.

The secrets of Sauvignon

Despite these challenges, there are a growing number of examples of flavour molecules being identified and winemakers then using this knowledge to improve their product. The combination of the related thiols 3-mercaptohexanol and 3-mercaptohexyl acetate that give New Zealand Sauvignon blanc its distinctive passionfruit aroma is one such example. These molecules are straight chains of six carbon atoms with either an alcohol or an acetate group on one end and a thiol three carbons along the chain, explains Andrew Waterhouse, a US wine chemist at the University of California, Davis. ‘Almost anything that has a thiol on it that is also volatile will have a potent aroma because our olfactory detectors just love SH groups,’ he says.

These molecules are typically found in Sauvignon blanc from New Zealand at several-fold higher concentrations that in the same variety produced elsewhere, and when scientists went looking for the reason they found an unexpectedly complicated story. It is due to a series of happy accidents in New Zealand’s unique environment for growing and processing Sauvignon blanc grapes, explains Waterhouse.

Firstly, their precursor – a linolenic acid – is more prevalent in Sauvignon blanc grapes grown in New Zealand than those in warmer climates. ‘Under cool climate conditions, cell membranes tend to have more unsaturated lipids in order to maintain the integrity of their lipid bilayer,’ Waterhouse says. Secondly, a lack of manual labour in New Zealand means grapes are normally harvested by machine. The grape berries are bashed off the vine with a plastic beater bar and drop down onto a moving conveyer belt below. ‘It’s a rather violent process,’ says Waterhouse, which tends to break open the grapes. Once open to the air, the lipid precursor very rapidly oxidises (within a minute or so) to form the six-carbon α,β-unsaturated aldehyde.

The carbon–carbon double bond in this molecule then reacts with a sulfur-containing nucleophile, inserting a sulfur-containing group in the C3 position. According to Waterhouse the identity of this nucleophile is still being determined but it is probably glutathione, a natural antioxidant in grapes, or sulfite, which is routinely sprayed onto machine-harvested grapes before they leave vineyards to prevent oxidation (see box Oxidation fundamentals below). When Waterhouse started exploring this passionfruit flavour in New Zealand Sauvignon blanc he had no idea that some oxidation was needed.

The grapes are then taken into the winery where they undergo the standard white wine-making process – crushing, decanting and then fermenting with yeast. The yeast transforms the aldehyde to an alcohol and the sulfur-containing group into a thiol, Waterhouse explains. ‘The yeast also converts some of the alcohol to the acetate ester, so you end up with 3-mercaptohexanol and the 3-mercaptohexyl acetate that give the wine its distinctive flavour.’

Uncovering the science behind their wine has allowed New Zealand producers tight control over the flavour of the Sauvignon blanc they produce. Hand-harvesting can be avoided, the length of skin to juice contact during pressing boosted and yeast strains that are particularly good at putting on acetate groups selected, for example. The storage of the finished wine has also changed. During ageing the acetate group tends to hydrolyse, upsetting the perfect balance of flavour molecules present in the wine at bottling, says Waterhouse. ‘Once that was understood, New Zealand winemakers started to keep their wines very cold until they were sold, because you can slow the process of hydrolysis by lowering the temperature.’

Cracking the pepper mystery

Another distinctive wine flavour that has been under the microscope is the cracked black pepper character in Shiraz grown in cool climates. ‘Until recently we didn’t know what the chemical was that causes the black pepper flavour in cool climate Shiraz,’ explains Leigh Francis, a wine chemist at the Australian Wine Research Institute in Adelaide. The quest was made harder by the molecule’s high bioactivity in the wine – some people can detect it at just 16ppt.

In 2008, Francis’s colleagues used gas chromatography–mass spectrometry (GC–MS) paired with a panel of expert human sniffers to identify the molecule as the sesquiterpene rotundone. ‘We discovered that it had been reported previously in the nut grass Cyperus rotundus but it hadn’t previously been recognised as a flavour compound,’ he says. Later research found that rotundone is prevalent in black peppercorns and many other herbs and spices. It is also – unusually for a wine flavour compound – also present in grapes.

Further work by Francis’s team found that rotundone concentrations increase as grapes mature. Shiraz producers are therefore starting to consider how peppery they want their wine when deciding when to start harvesting grapes.

The scientists have also been looking at how rotundone levels can vary across individual vineyards. ‘It’s well known that vineyards are not necessarily uniform,’ explains Francis. ‘Even if it looks visually identical, is planted with the same material and grown in the same way, certain areas can produce a darker-coloured or different-flavoured wine.’ This flavour variation means that some winemakers harvest their vineyards in parcels; grouping together rows of vines that ripen at similar times.

Francis’s team have been working with some wineries to better inform this so-called batching process, by mapping the concentration of rotundone in Shiraz grape berries across their vineyards. The Mount Langi Ghiran vineyard in neighbouring Australian state Victoria has very high levels of rotundone, and they found a 50-fold variation in concentrations in a six hectare area. The team found that the level variations strongly correlated with light levels. ‘Vines with more shading had high levels of this pepper compound,’ explains Francis.

Find the perfect blend

Damien Sheehan, viticulturist and general manager at Mount Langi Ghiran, explains that before working with Francis they picked the six hectares used to produce their flagship wine in just two or three separate batches. ‘We would start at one end of the row and go all the way to the other end of the row and then keep working along. It was fairly crude. Now we pick as many as 13 separate parcels of fruit out of that block and do separate ferments with each of those parcels.’ The result, he explains, is a higher quality wine. His team are now picking fruit that contains the concentration of rotundone that best suits their buyer’s tastes. Some wineries have also started to change canopy management strategies to manipulate rotundone levels.

Cool climate Shiraz and Sauvignon blanc may have one or two dominant flavour compounds, but this is unusual. It’s normally combinations of compounds that give wine varieties their unique flavours, Francis explains. His team is currently exploring the origin of the apricot and peach flavours in Chardonnay and some other white wines. Earlier gas chromatography paired with human sniffing work didn’t find a single common aroma compound responsible for this so-called stone-fruit character – a mixture was suspected instead. In 2018, Francis’s team reported that using a sensory reconstitution study they had determined that it is a mixture of monoterpenes and lactones that gives rise to this stone-fruit flavour. ‘Once you think you’ve identified what the compounds are, they are added to a neutral medium to see if the character can be reproduced,’ explains Francis. ‘This reconstitution step hasn’t been done very much in wine research so far.’

Another approach being taken to unpick mixtures of flavour compounds in wine is to ‘delete’ individual molecules from an artificial wine matrix and see how the flavour changes. ‘The method involves removing an individual component from a known mixture of aroma chemicals and see how it contributes to the mix,’ explains Waterhouse. ‘Understanding the interaction of all these aroma components and how they affect each other in a mixture has become a major area of study in wine, and more broadly in sensory science.’

AOC, IGP, GC–MS?

Today, most winemakers embrace the use of science to improve their wine – although many still tend to hide it behind the scenes of a very traditionally presented product. Most wineries carry out simple chemical analyses in-house, such as sugar and acid levels (see box above). Larger wineries often have the tools for more in-depth chemical analyses. For any other necessary tests there are specialist contract laboratories. But there is currently no rapid way for low-level aroma molecules or their precursors to be detected. Sacks is working to develop one. He envisages this being used in large commercial wineries, to help determine which grapes should be fermented together. ‘At large wineries during harvest, tons of grapes arrive from different vineyards to go into tanks every 15–30 minutes. One of the most important decisions a winemaker has to make is to figure out which of these grapes are going to ferment with which other grapes,’ he explains.

This job has enhanced my enjoyment of wine

The current gold standard for low level volatile measurements in other industries is GC–MS, which typically takes 30–60 minutes per sample. ‘We would like to drop that down to 30–60 seconds per analysis,’ Sacks explains. His tool of choice is ambient ionisation mass spectrometry (Aims), which ionises samples outside the machine and then sucks them inside for analysis. It can sniff molecules coming off whole grapes or a few millilitres of grape juice or wine. But because flavour molecules tend to be at low levels, their signals are often lost in the noise of those from wine’s much larger chemical constituents. Sacks’s lab is developing a simple sample preparation step to overcome this.

‘We load up our samples in a multi-well plate and position a sorbent polymer film over the top,’ he says. This creates a volatile ‘image’ of the samples on the film that is then passed in front of the Aims detector to enable the amount of each odorant to be quantified. Sack’s can sniff out six aroma molecules in less than 30 seconds in his lab – but plans to increase this number to around a dozen. His approach will shortly be tested at the E & J Gallo winery in California, one of the largest in the US. ‘Hopefully by the end of next harvest [2019], we will have a clear idea about whether or not this is an appropriate technique for routine industry use,’ he says.

The wine chemists interviewed for this article all agreed that theirs was a dream job, allowing them to combine a love of chemical research with their favourite tipple. ‘This job has enhanced my enjoyment of wine because when I taste different wines I can understand what I taste on a different level,’ explains Waterhouse. ‘If there are other chemists around, we’ll talk about a wine’s flavour in terms of chemical components. But if I’m not with chemists, I’m not going to do that, because I want to be polite!’

Nina Notman is a science writer based in Salisbury, UK