The Chemistry Behind Wine's Aromas

Wine aromas are the result of a complex interplay of factors, from the grape variety used to the winemaking process and storage conditions. The human nose, with its ability to differentiate thousands of unique scents, plays a crucial role in wine tasting. By swirling the wine in a glass, we release aroma compounds, allowing our olfactory receptors to detect a range of familiar scents. These can be broadly categorised into fruit/floral/herbal, spice, and earth aromas. The art of perceiving and describing these aromas is a skill honed by experts through practice and a deep understanding of oenology.

Grape variety

The aroma of wine is influenced by a multitude of factors, one of which is the grape variety used in its production. The diverse spectrum of aromas associated with different grape varieties is a reflection of the vine's adaptation to its ecological conditions and competition with other plants.

Genetic variation in aroma biosynthesis genes causes differences in aroma between grapevine varieties. For example, Muscat and Gewurztraminer grapes accumulate terpenoids due to an allelic variant of 1-deoxy-D-xylulose-5-phosphate synthase, a terpenoid biosynthetic gene. Terpenoids give these wines a distinct floral aroma. Similarly, Sauvignon wines have higher levels of methoxypyrazines, compounds that evoke the typical capsicum aroma, due to differential expression of an O-methyl transferase gene.

The grape aroma itself is synthesized in grape berries by enzymes such as terpene synthases, O-methyl transferases, and carotenoid cleavage dioxygenases. The terpene synthase gene family has expanded in the grapevine, emphasizing the importance of terpenoids in this species. Terpenoids, along with other volatile compounds, contribute to the varietal aroma of wines.

Additionally, grape berries store most of the volatiles they produce as glycosides, which do not directly contribute to wine aroma but affect it indirectly. During fermentation, vinification, and wine aging, these glycosides can be converted into volatile aglycones, releasing aromatic compounds. The majority of volatile compounds responsible for aroma combine with sugars in the wine to form odourless glycosides, which can later revert to an aromatic form through the process of hydrolysis, catalyzed by enzymes or acids in the wine.

The concentration of monoterpenols in grapes and wines also varies significantly between grapevine varieties, further influencing the aroma profile. Aldehydes, while mostly eliminated during fermentation, can contribute to the herbaceous notes in wines, particularly when unripe grapes are used. Overall, the interplay of these various genetic, enzymatic, and chemical factors results in the unique aroma characteristics associated with different grape varieties used in winemaking.

Fermentation

The type of yeast used during fermentation can also have a significant impact on the aroma of the wine. Wild yeasts, which are naturally present in vineyards and on grapes, can produce high-quality, unique-flavoured wines but are often unpredictable and may introduce undesirable traits or contribute to spoilage. On the other hand, cultured yeasts, which are specifically isolated and inoculated for winemaking, offer more control over the results and make it easier to predict the outcome. The use of different strains of yeasts is a major contributor to the diversity of wine, even among the same grape varieties.

In addition to yeast selection, other factors that influence fermentation include sugar content, temperature, and the type of fermentation vessel used. Sugar content is essential for successful fermentation, as yeast converts sugar into alcohol. However, if there is too much sugar, the yeast has to work harder, which can result in "stressed yeasts" that produce off-flavours.

Temperature control is also crucial during fermentation. Lower temperatures preserve fragile, volatile aromatics in the wine, retaining a more "fruity" character. However, if the temperature is too low, yeast works slowly or may have difficulty fermenting all of the sugar. Higher temperatures allow for better grape skin tannin extraction but tend to drive off fruity flavours and aromas. Finding the right fermentation temperature involves balancing these various factors.

The choice of fermentation vessel can also influence the aroma of the wine. For example, fermenting wine in oak barrels allows for gradual oxidation, resulting in a smoother, creamier flavour with notes of vanilla and oak. In contrast, stainless steel fermentation vessels are highly non-reactive, resulting in bright, clean, and crisp wines.

Ageing

The Chemistry of Ageing

Esters and Aromas

Esters are compounds formed when the alcohol in wine reacts with acids. This process, known as esterification, is influenced by the type of yeast used during fermentation. Hydrogen, which is more abundant in wines with higher acidity, promotes esterification. Paradoxically, hydrogen can also reverse the reaction, converting esters back into alcohol and acid. This dynamic nature of esterification contributes to the constant evolution of wine flavours. Esters are particularly aromatic right after fermentation, often smelling like apples, bananas, red berries, or tropical fruits. Over time, they can develop into distinct flavours, such as the buttery aroma found in Chardonnay after five years of ageing.

The Role of Tannins

Tannins, a type of phenolic compound, play a crucial role in ageing wine. Found in the stems, seeds, and skins of grapes, tannins give wine its dry texture and astringent qualities. In red wines, which have significantly higher tannin content than white wines, tannins bind to proteins in the saliva, causing the mouth to pucker. During ageing, tannins undergo polymerisation, forming long chains that settle as sediment at the bottom of the bottle. This process reduces the astringency of the wine, resulting in a smoother and mellower mouthfeel. Additionally, tannins help preserve wine by preventing oxidation.

Oxygen's Dual Role

Oxidation is a double-edged sword when it comes to ageing wine. On the one hand, oxygen helps fruity flavours fade and blend with woodsy flavours from the barrel, contributing to the development of earthy and nutty notes. Decanting wines encourages oxidation and can improve the flavour profile of young wines. On the other hand, excessive oxidation can turn wine sour and prematurely undrinkable. Therefore, maintaining consistent humidity levels and storing wine in a cool, dark place are crucial to striking the right balance and allowing the wine to age gracefully.

The Evolution of Wine Aroma

The evolution of wine aroma during ageing can be understood through its progression from youth to maturation, apogee, and decline. In its youth, a wine's aroma is dominated by fruity and flowery notes, with acidity and tannins taking centre stage. As the wine matures, these youthful aromas gracefully transition to more savory scents. At its apogee, the wine's aromas reach their richest and most complex state, revealing a harmonious interplay of flavours. However, if aged too long, the wine enters a decline, losing its qualities and becoming fruitless, brown, and dominated by acidity.

Factors Influencing Ageing

Several factors influence the ageing process and, consequently, the development of wine aromas. These include the tannin content, acidity, and intensity of aromas in the wine. Wines with higher tannin content, such as Cabernet Sauvignon, Syrah, and Nebbiolo, tend to have better ageing potential. Acidity also plays a crucial role, as acidic wines generally age better. Additionally, intense and aromatic wines, rich in flavours and aromas, are more likely to gain complexity and depth over the years. The vintage and terroir of the grapes, winemaking techniques, and storage conditions are other critical factors that can enhance or hinder the ageing process, ultimately shaping the wine's aroma profile.

Volatile compounds

The majority of volatile compounds responsible for aroma combine with sugars in the wine to form odourless glycosides. Through the process of hydrolysis, caused by enzymes or acids in the wine, they revert to an aromatic form. The act of tasting wine is essentially the act of smelling these vaporized aroma compounds.

During fermentation and for the first few months of a wine's existence, chemical reactions among volatile compounds occur frequently, and a wine's aroma will change more rapidly during this period than at any other point. As a wine ages and matures, changes and developments in aroma will continue to take place but at a slower and more gradual pace.

The technique of microoxygenation and the temperature at which a wine is served can also affect the aromatic bouquet of a wine.

Human perception

The human perception of wine aroma is a complex and highly individual experience. The sense of smell is critical to our ability to discern the wide variety of flavours in wine. The human nose can differentiate between thousands of unique scents, which, in combination with the basic tastes perceived by the tongue, allows us to identify the multitude of flavours in wine.

The act of tasting wine involves smelling the wine to detect its aroma compounds, which are then interpreted by the brain. The human nose has olfactory receptor cells, each sensitive to a different aroma, which pick up these compounds and transfer the information to the brain via the olfactory bulb. The average human can be trained to distinguish thousands of smells, but can usually only identify a handful at a time. This phenomenon is known as the "tip of the nose phenomenon". It can be countered by providing a list of possible choices, which can help individuals positively identify the aroma.

The technique of wine tasting involves first swirling the glass to volatilize the alcohol and release the wine's innate scents. The taster then sticks their nose into the glass and inhales, identifying any initial scents, such as flowers or fruits, and then drilling down to identify more specific notes, such as the type of fruit or floral characteristics.

The wine's aroma can be further analysed by taking a sip, which warms the wine in the mouth and mixes it with saliva to vaporize the volatile aroma compounds. These compounds are then inhaled retro-nasally through the back of the mouth, where they are received by millions of nerve cells.

The human perception of wine aroma is influenced by a variety of factors, including the grape variety, winemaking process, and storage conditions. The ageing process, in particular, has a significant impact on a wine's aromatics, with older wines developing more complex and subtle bouquets.

Additionally, individual differences in sensitivity and recognition thresholds among humans can lead to variations in the perception of certain aromatic compounds. This results in different individuals describing different aromas and flavours from the same wine.

The art of perceiving and describing wine aromas is a highly skilled and nuanced process, requiring focus, concentration, and a well-developed olfactory memory.

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