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Last updated 2010-08-09
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Copyright © 2003-2010
Martin Lersch

Introduction to wine chemistry
Wine is a complex mixture of several hundred different compounds - approximately 800 for red wine, significantly less for white wine. Apart from the obvious contents of around 12% ethanol and 86% water, it is the remaining compounds that lend wine it's characteristics. The constituents of wine can be divided into non-volatile and volatile molecules as follows:

Non-volatiles / % Volatiles / %
1 Glycerol 0.045 Volatile acids (mostly acetic acid)
0.4 Organic acids (tartaric acid, lactic acid, succinic acid + more) 0.025 Esters (ethyl acetate + many more)
0.2 Carbohydrates 0.025 Nitrogen species (amino acids, proteins and others)
0.2 Minerals 0.004 Aldehydes (acetaldehyde, vanillin + many more)
0.1 Tannins/phenolics (includes pigments) 0.001 Higher alcohols (amyl, iso-amyl, butyl, iso-butyl, hexyl, propyl and methyl)
traces Vitamins
(numbers taken from this figure at UC Santa Cruz)

The non-volatiles contribute to the taste of wine: glycerol gives wine a "body" by increasing the viscosity, the acids lower the pH, the sugars moderate the acidity, and the tannins/phenolics are responsible for a wines astringency. It's important to note that all these characteristics are sensed solely by the tongue! The volatiles, which are present at much lower concentrations (totaling approximately 0.1%), are primarily sensed by the nose and contribute to a wines aroma.

Lopez and coworkers (J. Sci. Food Agri. 1999, 79, 1461 "Identification of impact odorants of young red wines made with Merlot, Cabernet Sauvignon and Grenache grape varieties: a comparative study" (free abstract, full text requires subscription)) have shown that only a fraction of the large number of volatiles present in red wine are essential for a wine to be identified as wine. The most active odorants in wine included the following compounds:

Major compounds in red wine

They also showed that none of the impact compounds were characteristic of only one variety and that the differences between the varieties studied were quantitative rather than qualitative.

Having said this, I should quickly add that the multitude of other compounds certainly do influence the aroma of wine. They lend a wine it's terroir as connoiseurs would put it.


Interactions between saliva and wine
From the table above we see that wine only contains about 0.1% tannins (also known as polyphenols). The sources of tannins found in red wine are the seeds, skin and stem of the grapes. When we drink red wine, the tannins react with proteins in our saliva to form water insoluble protein-tannin complexes. A precipitate is formed and as a result, the lubricating properties of the saliva are lost and our tongue feels rough and dry. In other words, we experience the astringency of the red wine.

An elegant experiment to show this is as follows (I learnt this at the 2004 International workshop of molecular gastronomy in Erice):
  • Take a sip of a dry red wine, preferably rich in tannin.
  • Keep the wine in your mouth for 10-20 seconds without swallowing.
  • Spit it into a empty glass and watch how a precipitate forms (this might take a minute or two). Notice how the color changes from red to light red or even pink (see picture below).
  • Rinse your mouth by chewing a piece of bread and drink some water.
  • Take a small sip of the wine that you just spit out (if you dare!). Since the tannins of this wine have already reacted with your saliva, it is as if they were removed from the wine, leaving a fad and flat wine without much taste at all.
Interactions between tannins in red wine and saliva
Top: red wine. Bottom: formation of precipitate in red wine mixed with saliva.

The saliva flow rate and the concentration of proteins varies from person to person (the latter with a factor of 20). Furthermore the flow rate and protein concentration also varies throughout the day and is also influenced by what you are eating/drinking and even by the smell of food. As a consequence, a person with a high saliva flow rate and/or a high concentration of proteins is more likely to approve of a red wine rich in tannins than someone with a low saliva flow and a lower protein concentration. Knowing this, you should not be surprised that wine preferences can be very individual.


more to come...

Binary taste interactions
Interactions between food and wine
Other influences
Sensory studies 
Conclusions


Take a look here for a list of books about wine and wine chemistry