Introduction
The topic of tannins is about to come up at the Annual ASVO Oenology seminar on October 6th in Adelaide. To try and prepare those attending the seminar, the following article discusses some basic concepts about tannins and their application in wine. It’s difficult to discuss tannins without getting into some serious chemistry; so the article starts as simply as possible for those who haven’t studied chemistry (and for those who did some Wine Chemistry but who forgot the details once they walked out of the exam room!).
What are tannins?
The name tannin originally comes from the leather industry. The process of producing leather from hide is called tanning. The plant materials used in tanning contained polyphenolics that reacted with the protein in the hide to produce a stable, water and heat resistant leather. The compounds that were present in these plant materials that brought about this tanning process were named tannins.
The name tannin is now used to refer to any plant derived polymeric components that have certain properties. These properties include water solubility, reactivity with proteins and a molecular weight between 500 and 3000 units.
In wine, tannins are one of the major polyphenolic groups. They exist as polymers of two main groups of phenolic compounds; the flavonoids and the non-flavonoids.
What is a phenolic compound?
A phenolic compound has phenol as its basic structural unit. Phenol itself is a 6 membered carbon (C6) aromatic benzene ring backbone with one hydroxy functional group attached.
Fig 1: phenol structure
Phenol
The two main groups of phenolics that tannins are derived from are the:
- 1. Flavonoids
- 2. Non-flavonoids
What are flavonoids?
All flavonoids (the name is derived from flavone, Latin for yellow) have a C6-C3-C6 structure. That is, the basic structure consists of two C6 aromatic rings separated by a chain of 3 carbon atoms. The C3 chain usually, but not always, forms another ring using an oxygen atom to complete the ring.
Fig 2: flavonoid structure
Flavonoid
To make things more complicated the flavonoid group can be further subdivided into sub-groups such as flavones, flavanones, isoflavones, flavonols, chalcones and anthocyanidins. These subgroups each have a distinctive structure, depending upon what other functional groups are attached to the basic C6-C3-C6 backbone
Some examples of specific flavonoids that may be familiar are: quercetin, malvidin, catechin, cyanidin.
Fig 3: couple of common flavonoids
Quercetin Catechin
Of course it gets even more complicated than this, as most flavonoids are usually bonded to sugar molecules as well. Sugars such as glucose bond to flavonoids to give rise to molecules such as malvidin-3,5-diglucoside. This compound is used as a marker for testing for American vine species as it is present in them but is not found in Vitis vinifera grapes.
Malvidin –3,5-diglucoside
What are the non-flavonoids?
The non-flavonoids are all based on a single C6 benzene ring compared to flavonoids which have two C6 rings as mentioned above. The non-flavonoids are based upon either hydroxybenzoic acid (C6-C1 backbone), or from hydroxycinnamic acid (C6-C3 backbone).
Fig 4: structures of hydroxybenzoic and hydroxycinnamic acids
With the addition of extra functional groups on the basic structure, C6-C1 compounds include gallic, vanillic, salicylic acids. The better known C6-C3 compounds include coumaric and caffeic acids.
Unfortunately it is not quite that simple. These non-flavonoid acids are not usually present unattached; they more often form esters with sugars or alcohols to give rise to a wide range of phenolic compounds.
Where do phenolics in a wine come from?
The major sources of phenolics are from fruit, oak and additives such as commercial tannins.
The phenolics that come from grapes are found in the juice, seeds and skins. A typical breakdown of the phenolic content of a grape is 65% from seeds, 30% from skins and 5% from the juice. The skins contain predominantly flavonoids, juice contains virtually only non-flavonoids. The phenolics that are present in the oak used in barrels are also effectively only non-flavonoids. The main ones of interest from oak are gallic acid, ellagic acid, vanillaldehyde and the cis-lactones.
How do phenolics differ between white and red wines?
White fermentation is not carried out with skins present unlike red wines. So, flavonoids from skins are avoided in white wines but encouraged in reds. Also any phenolics from seeds are avoided in whites as well.
Fermenting on skins extracts phenolics; the extraction rate increasing with time of contact (maceration), higher temperature and higher alcohol content. As red wines are also more likely to be barrel aged, further increases in phenolic content occurs.
What are polyphenolics and what is meant by tannin polymerisation?
Tannins are polymers of certain phenolic compounds (as described above). If two phenolic compounds bond together a polyphenolic dimer is formed, three form trimers etc. Typically up to ten phenolic molecules can bond to form tannins. This appears to be the upper limit in wine as the larger the molecule the less soluble it becomes.
Polymerisation of phenolics to form tannins continues as the wine ages, so the concentration of tannins increases. This is particularly so for red wine. During the barrel aging, further tannins are extracted from the wood, having a large effect on the wine’s aroma and taste. Aging in barrels also assists the slow polymerisation leading to softer tannins in the finished wine.
Pigment molecules such as anthocyanins can also be bonded with tannins forming coloured co-polymers.
What is meant by hydrolysable and condensed tannins?
As the chemistry of tannins is very complex simple classification systems have been used. Hydrolysable tannins refer to the polymers formed by the non-flavonoid compounds with sugars. These tannins tend to be easy to hydrolyse. Condensed tannins refer to the flavonoid polymers that usually form stronger C-C bonds and so are harder to hydrolyse. So this classification is based on the ease of hydrolysing tannins, and tends to be more of historical interest. Copolymers can be formed from flavonoids and non-flavonoids making this classification system somewhat difficult.
What effect do tannins and phenolics have on wine?
The two main effects are astringency and bitterness. Astringency is mainly due to tannins, the higher molecular weight polyphenolics. Astringency is a feeling rather than a taste. It is caused by the hydroxy (-OH) groups in tannins binding with the protein molecules in the mouth. This is why it is felt throughout the mouth. Bitterness is a taste and is caused by the lower molecular weight phenolics.
Fining can, to some degree, selectively remove phenolics by molecular weight and therefore reduce bitterness or astringency separately. Gelatine is commonly used to remove tannins and so reduce astringency in wine. Casein and PVPP react with the lower molecular phenolics and so are used to reduce bitterness in wine.
Bitterness is masked by astringency, so when tannins are removed the bitterness can become more obvious. Higher sugar content tends to mask bitterness, although it is enhanced by higher alcohol levels.
Conclusion
The above article discusses some basic concepts of tannins and their effects on wine. The latest developments in the knowledge and use of tannins will be presented at the ASVO “Advances in Tannins and Tannin Management” seminar, to be held at the Adelaide Convention Centre on 6th October 2005.
References:
1. “Concepts in Wine Chemistry, 2nd edition”, Chapter III, Margalit. Y; The Wine Appreciation Guild, San Francisco, 2004
2. Concepts in Wine Technology” Chapter III, Margalit. Y; The Wine Appreciation Guild, San Francisco, 2004
3. “Wine Analysis and Production” Zoecklein, B.W; et al, Kluwer Academic Publishers, 1999
4. “Chemistry of Wine Flavor”, ACS Symposium Series 714, Chapter 10, 1998
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