Where do the aromas of wines come from?
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© Apolo Photographer / Unsplash, CC BY
The smell of wines is determined by nearly a thousand “volatile” molecules and their color and taste by other “non-volatile” molecules, according to a study published by our partner The Conversation.
The soil, the climate, the vintage, the techniques used and the grape variety used induce molecular differences that distinguish one wine from another.
The analysis of this phenomenon was carried out by Jean Martin-Pernier, doctoral student at the University of Bordeaux.
During the end-of-year celebrations, everyone wonders which bottle of wine to taste with typical Christmas dishes.
A must-have is generally present on all tables: champagne.
If you are a seafood lover, you will certainly have taken the care to buy a bottle of dry white wine, a Sancerre or a Petit Chablis for example.
The syrupy white wine, very sweet, becomes more discreet.
It is generally served as an aperitif, or as an accompaniment to foie gras.
The forgotten of winter, rosé, which will resurface as summer approaches, is still sometimes found on our tables during this festive period.
The big winner of holiday meals, the one that goes easily from starter to dessert is still red wine.
Burgundy, Bordeaux, Loire, Rhône and Languedoc wines, each can go perfectly with the traditional dishes served at Christmas.
What are the secrets of these essential drinks for our holiday season?
Aromas, colors, smells - where do the subtle chemical identities of wines come from?
As a neophyte, it can sometimes be complicated to understand how experts manage to differentiate wines according to their smells, tastes and colors.
The secret lies in what exactly makes the smell, the taste and the color: the chemical composition of the wines.
There are no less than a thousand so-called “volatile” molecules, which are responsible for the smells of wines.
Color and taste are defined by other molecules, which in turn are non-volatile.
A vineyard in Vix, France © Alasdair Elmes / Unsplash
If we manage to differentiate between wines, it is because there is a difference in composition, at the molecular level, between these wines.
This difference is linked to various factors: Bordeaux grape varieties (Merlot and Cabernet Sauvignon) will give wines richer in tannins and anthocyanins than Pinot Noir, a typical Burgundy grape.
The climate, high temperatures and low hydrometry will lead to sweeter grapes (rich in glucose and fructose) and ultimately give a more alcoholic wine and therefore modify the chemical balance of the wine.
The winemaking methods will extract different amounts of compounds depending on the type of wine desired.
The combination of these factors directly influences the chemical composition of the wine, this is called the terroir: the soil, the climate, the vintage, the techniques used, the grape variety used and the people who work in the vineyard and in the vineyards. cellars.
Without being a seasoned scientist, when you compare two wines, you actually differentiate certain molecules that make up these wines: we feel the differences in the chemical nature of the molecules, but also the difference in terms of quantity.
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In this sense, the researchers of the oenology research unit of the University of Bordeaux questioned themselves on various techniques which make it possible to differentiate between wines.
These techniques, for example gas chromatography coupled with mass spectrometry, make it possible to identify and discriminate the volatile molecules characteristic of wine aromas.
Other techniques, such as high performance liquid chromatography, also make it possible to discriminate the non-volatile compounds in wine responsible for its color and structure.
Today, we can create real chemical and molecular identity cards for our grands crus.
These maps being of great complexity, it is however impossible to reconstitute a wine thanks to its chemical imprint.
It is even possible to differentiate the wines according to their vintage, their production areas, their producers, even if they are close neighbors.
Why do wines age differently?
Oxygen is the main molecule responsible for aging wine.
On a similar principle which causes iron to rust for example, wine oxidizes because oxygen reacts with the multitude of active molecules in wine.
Cellars, Bollinger champagnes, in Aÿ, France © Lomig / Unsplash
For a wine to be able to keep, evolve and reach an optimal tasting potential - 10 to 15 years or even more for some wines, it must be protected from an excessive supply of oxygen without being completely deprived of it.
To do this, cork stoppers have historically been used, but today there are different types of stoppers: plastic, agglomerate, glass, sometimes less effective, but above all less accepted by consumers for long-term conservation than cork.
Red wines have found their defenses against oxidation: antioxidant molecules, such as polyphenols (anthocyanins and tannins), responsible for the red color and perceptions in the mouth.
These capture oxygen and prevent it from combining with the different complexes responsible for visual and olfactory defects in wine - for example, acetic pitting, oxidase breakage, loss of coloring matter.
Sulfur dioxide, a major input during winemaking, also combines with the oxygen present in wine.
It is therefore thanks to sulfur dioxide and antioxidant molecules that certain grands crus can be kept for so long.
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White wines, on the other hand, do not contain polyphenols.
Sulfur dioxide will partly be enough to keep them, but the dose used does not allow them to keep an optimal state over several years.
Some other natural antioxidants will protect them, but very partially.
This does not imply that dry white wines cannot age, but that they must be more protected than red wines from oxygen, in particular by the use of a stopper that is very poorly permeable to oxygen.
For their part, sweet white wines can therefore keep much longer than dry white wines, because their chemical composition is drastically different.
Because we stop their fermentation to keep a high level of sugars with sulphites, sweet whites contain more sulphites than dry wines, which are, as in many food industries, the main protectors against oxidation.
However, for a wine to age properly with a qualitative and harmonious evolution, a moderate amount of oxygen must be combined in part with these complexes (tannins, anthocyanins) and other aromatic molecules, without completely oxidizing them. .
What to taste with sweet white wine?
During our festive meals at the end of the year, the sweet white wine is often confined to the accompaniment of foie gras.
Yet it goes perfectly with different dishes, from chocolate tasting to parsley cheese, including dishes of Asian origin.
Sweet wine and foie gras - a classic pairing. What does science say? © Jacqueline Macou - Pixabay
A study conducted in collaboration between Bordeaux and Dijon used a sensory analysis tool to identify in real time what the wine drinker perceives during the tasting of a sip of wine: the method of temporal dominance of sensations , developed by Pascal Schlich.
This technique allows a detailed understanding of the appreciation of wines, in particular the influence of dishes associated with wine tasting, and also the evolution of perceptions during tasting.
The study shows that the tasting of a sweet dish has a negative effect on the appreciation of the taster of sweet wine, the sugar masking the intensity of the wine.
In addition, there is nothing to scientifically confirm that foie gras does not adapt to sweet wines: no change in preference has been observed between wine alone or wine accompanied by foie gras.
In this sense, it is difficult to define a single sidekick for sweet white wines, as it will have to adapt to the different profiles of these wines.
The study rather reveals the strong heterogeneity of the preferences of each one, for example the preferences for the combination of liquoreux-foie gras or liquoreux-Comté, and the importance of reasoning in terms of diversity of proposal of food and wine pairing.
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This analysis was written by Jean Martin-Pernier, doctoral student at the University of Bordeaux (with the participation of Soizic Lacampagne, thesis director, research engineer; Sophie Tempère, lecturer; Michael Jourdes, lecturer; Justine Laboyrie doctor in oenology; Lucille Gendre, oenologist).
The original article was published on The Conversation website.
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