A taste of the chemistry of wine

And while each wine contains about ninety-seven percent of wine and ethanol, each bottle also contains thousands, if not tens of thousands, of different molecules, which include everything from acids and sugars to phenolic compounds as well as low concentrations of aroma compounds

[Translation by Dr. Nachmani Moshe]

"Wine is a rare example of a common consumer product for which changes are not only tolerated, but even required," explains Gavin Sacks, a chemist at Cornell University in New York who specializes in wine. More than four hundred thousand types of wine are currently available for sale in the US, and consumers expect each of them to have its own distinct aroma, appearance and taste. Although they don't think about it, consumers actually expect that the chemistry, that is, the content of the ingredients, in each of the bottles will also be distinct and distinct. Surprisingly, wine is a complex chemical mixture. It is true that each wine contains about ninety-seven percent of wine and ethanol, but each bottle also contains thousands, if not tens of thousands, of different molecules, which include everything from acids and sugars to phenolic compounds, as well as low concentrations of compounds in mole [aroma, the natural odor component of the grape variety in Tirosh] . The role of a wine chemist, such as Sacks, is to identify which of these compounds, or even which families of them, dominate each wine variety, and then advise the winemakers on methods that will increase or decrease their concentrations, as required according to the wine brands. This consultation may include introducing changes in terms of vineyard management, the manner and location of the grape harvest, the wine production process and even the wine storage method.

Identifying the important taste and smell molecules inside the wine is a challenging task. The challenge stems in part from the large number of different molecules in the wine, but also from the fact that these volatile compounds are found in it at extremely low levels, even down to parts per trillion (ppt). Once identified, determining the origin of the molecule can be equally challenging. It is rare that the flavor molecules in wine originate from the grapes themselves, and on the contrary, they are created during the fermentation and storage phase. "It's as if someone came to a chemical store, emptied all the bottles and then asked those present what reactions are happening now," says the researcher. "These are not responses that are difficult to decipher, but there are simply many, many possibilities." "Wine is a very different mixture from any organic chemical mixture." The researcher describes the wine production as a single-vessel synthesis that occurs under mildly acidic conditions at room temperature and in the presence of a weak reducing agent. "If we look at textbooks, under these conditions no particularly interesting reactions should occur, but in wine these reactions occur for long periods of time," explains the researcher. And in addition, these reactions do not stop when the ionization process itself stops - even after bottling the wine, the molecules inside it continue to change. This is why wine is often described as a living, breathing entity.

The secrets of Sauvignon

Despite these challenges, there are a growing number of examples of flavor molecules that have been identified and that winemakers have subsequently used to improve their finished product. The combination of the two similar thiols 3-mercaptohexanol as well as 3-mercaptohexyl acetate, which give Sauvignon Blanc wines from New Zealand the distinct aroma of the fruit of the plant 'eaten clock', is one such example. These molecules consist of chains of six carbon atoms at the end of which is an alcohol or acetate group, plus a sulfur atom. "Almost any molecule that carries a thiol group and is also volatile will have the potential to smell in light of the fact that our olfactory receptors effectively recognize thiol groups (SH)," explains one of the chemists. These molecules are found in Sauvignon Blanc from New Zealand, usually in higher concentrations than the concentrations of the same variety produced elsewhere, and when the scientists tried to find the answer to this, they found a particularly complex answer. The reason lies in happy coincidences that arose in the unique environment of New Zealand in the field of growing and processing Sauvignon Blanc grapes, explains the researcher. First, the starting material of the molecules, linolenic acid, is more common in Sauvignon Blanc grapes in New Zealand than in regions with a warmer climate. "Under the conditions of a cold climate, the cell membranes have a tendency to contain a larger amount of unsaturated fats, this in order to maintain the integrity of the fatty layer," explains the chief researcher. Secondly, a lack of working hands in New Zealand means that the grapes are normally harvested by machine. The grapes are picked from the vines with a plastic ax handle and then transferred to a moving conveyor belt. "This process is quite aggressive and therefore causes the grapes to split. As soon as the fruit is exposed to the open air, the fatty starting material oxidizes quickly (within about a minute) and turns into an unsaturated aldehyde with six carbon atoms. In the next step, the carbon-carbon double bond in this molecule reacts with a sulfur group, introducing a sulfur atom into position 3 in the molecule. The researchers are still investigating exactly what this sulfur group is, but they are pretty sure that it is either the glutathione group, an antioxidant that is normally found in grapes, or the sulfite substance, a preservative that is normally sprayed on the ripened grapes with a machine right after they are picked to prevent their oxidation. After that, the grapes are transferred to the winery where they go through the usual process for making white wine - crushing, maceration and fermentation with yeast. The yeast converts the aldehyde to alcohol and the sulfur group to thiol, explains the researcher. "The yeast also converts some of the alcohol produced to its acetate ester, so that the final composition contains the compounds 3-mercaptohexanol and 3-mercaptohexyl acetate, which give the wine its distinct taste."

Exposing the science behind their wine has allowed New Zealand winemakers to have tight control over the taste of the Sauvignon Blanc they produce. The storage method of the finished wine has also changed. During the aging phase, the acetate group tends to undergo hydrolysis, a result that disrupts the perfect balance between the flavor molecules present in the wine at the time of bottling, explains the researcher. "As soon as the New Zealand wine producers realized this, they started storing their wines in extremely cold conditions until the time of their sale, and thus they manage to slow down the hydrolysis process by lowering the temperature."

Cracking the pepper mystery

Another distinct flavor in the wine, which has been the focus of much research, is the ground black pepper characteristic of the Shiraz variety that grows in areas with cold climates. "Until recently, we didn't know which chemical was responsible for the black pepper flavor in Shiraz grapes grown in cold climates," explains a wine researcher from Australia. In 2008, using advanced analytical equipment of gas chromatography coupled to mass spectrometry, researchers were able to identify the chemical responsible as the sesquiterpene (a terpene composed of three isoprene units) rotundone. "We found that the chemical was known in the past to be found in a herb called 'Guma the tubers', but it was not identified as a flavoring agent at the time," explains the researcher. Additional studies have shown that the substance is also common in black peppercorns as well as in many other herbs and spices. The substance itself, although this is not common for a flavor compound in wine - is also found in the grapes themselves. Further studies revealed that the concentrations of the substance increase as the grapes ripen. Following these findings, the wine producers can control the levels of peppery flavor they wish to have in their wine, and accordingly determine the start date of the grape harvest. The scientists also began to examine how the levels of the substance vary among the different vineyards. "It is known that different vineyards are not necessarily uniform in nature," explains the researcher. "Even if they visually look the same, contain the same seedlings, and have been cultivated in the same way, different areas may grow wine of a darker color or with slightly different flavors," the researcher notes. The researchers work in collaboration with a number of winemakers in order to teach them how to improve the wine production process, by mapping the concentration of the substance rotundone in Shiraz grapes in the vineyards. The researchers found that in a certain vineyard the change in the concentration of the substance has a strong correlation with the lighting levels - the more the grapes are in shaded areas, the higher the concentration of the substance will be. Shiraz and Sauvignon Blanc wines from grapes grown in cold climates may contain only one or two dominant aroma compounds, but this is quite rare. Normally, it is a combination of several compounds that gives the different varieties of wine their unique smell, explains the researcher. The research team is currently investigating the source of apricot and peach aromas in white wines, including Chardonnay. Previous experiments done using gas chromatography equipment combined with human smell did not result in finding a single odor compound responsible for this characteristic fruity smell - instead, the researchers believe it is a mixture of several odorants. In 2018, researchers reported that they were able to determine that a mixture of monoterpenes and ketones is responsible for the smell. Another approach to finding the mixture of compounds responsible for the smell is to remove a certain molecule from the mixture of artificial wine and check how its smell changes. "Understanding the interrelationships between these odor components and the way they affect each other within a mixture has become a significant area of ​​research in the wine sector, and more so in the field of sensory science."

The compounds responsible for the different aromas of wines are often not those originally found in the harvested grapes, but are formed during the fermentation and storage stages [Courtesy: Lindsay Springer, Sacks Lab, Cornell University]