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When wine has a thirst for research. © FOTOLIA, Fotolia

When wine has a thirst for research

It's in the bouquet!

Wine is a complex substance that contains nearly 1500 different volatile or non-volatile compounds, a lot of water, sugars and obviously ethanol.  There is also glycerol (the role of which is still little understood), minerals and polysaccharides (or complex sugars).  As for the category of polyphenols1, this contains anthocyanins (pigments in red grapes) and tannins, which play a crucial role in our perception.  Last but not least of the components in wine are the aromas.  They may be fruity, floral, suggestive of animals or spices... they appear and disappear during a wine's development and have a major organoleptic impact.  For the pleasure of the nose and palate, the bouquet of a wine evolves, diversifies and becomes more complex over time.  So while INRA researchers analysing how our senses react, they are at the same time trying to determine the compounds found in wine.

Published on 11/15/2013

The softness of dry wines

molecular representation of Quercotriterpenosides. © INRA, Axel Marchal
molecular representation of Quercotriterpenosides © INRA, Axel Marchal
Most fine dry wines are devoid of sugar in the chemical sense of the term.  But paradoxically they nevertheless develop a softness - or sweetness - that remains a mystery.  Past observations suggested that during the winemaking process, yeasts and oak wood release compounds with a sugary flavour that are responsible for the sweetness of these wines.  This has now been proved.  A team from the Institute of Vine and Wine Sciences in Bordeaux (ISVV) and INRA has, for the first time, evidenced the involvement of a yeast protein (called Hsp12) in enhancing the sweetness of wine.  The scientists also discovered the influence on this sweet flavour of oak wood during wine ageing.  Their studies revealed the existence of a family of hitherto unknown chemical compounds that are released by oak wood, the quercotriterpenosides, whose sweetening potential in wine proved to be nearly 1000 times greater than that of sucrose!

Understanding astringency

When you taste an under-ripe redcurrant, tea or a glass of wine, you experience the same sensation, that of astringency.  The interaction between salivary proteins and tannins (molecules produced by plants) participates in reducing lubrication of the oral mucous membranes, causing the dry mouth sensation that is typical of astringency.  These interactions between tannins and certain salivary proteins - called PRP (or proline-rich proteins, proline being an amino acid) and whose only known function is to interact with tannins - lie at the heart of research carried out by INRA teams.   Scientists have thus shown how PRP fold themselves around tannins to entrap them, and how the resulting complexes are at the origin of astringency.  More recently, and using an original far-ultraviolet radiation technique, the scientists were able to localise and very precisely identify the site of interaction between tannins and proteins.

1Polyphenols are a family of molecules produced by plants.  The great majority of the red and blue pigments in plants are polyphenols.  They intervene in defence mechanisms against UV rays, fungal attack, etc.  Polyphenols also have beneficial effects on health and are thus used as additives by the food, pharmaceutical and cosmetics industries.

focus on aromas

Wine aromas

Of the multitude of components present in wine, aromas are amongst those found in the smallest quantities, although their organoleptic properties are unique.  To date, more than 700 aromatic compounds have been assayed in wines, belonging to three main families of aromas (primary, secondary and tertiary) that are released during different stages of wine development.  In the glass, or even in the mouth, the life of a wine continues.  When they come into contact with air, aromas can evolve; some may dissipate within a few minutes and others may appear.  An important characteristic of dry white wines is the finesse their aromas and fruitiness.  These wines are a balance between two main components: acidity and softness.  As for red wines, they are described as a function of the balance between three components: acidity, richness and astringency.