Polyphenols in wine
A review of this important group of wine molecules, which includes tannins and anthocyanins

In the wine trade we talk a lot about polyphenols (also known as phenolics), a group of compounds that include tannins and anthocyanins. But I’m not sure most people really know what they are referring to with any degree of certainty. So, for the benefit of those who don’t have a solid grasp of organic chemistry, I’m going to try to unpack this subject in a way that’s understandable to normal, non-science-geek types, but without losing the important details.

Polyphenols are a large group of compounds that use a chemical structure called phenol as the basic building block. That’s where the name comes from: ‘poly’ phenols are where more than one phenol group is joined to another. They are probably the most important group of flavour chemicals in red wines, but are of much less importance in whites.

So what is phenol? It’s a chemical that consists of what is known as a benzene ring (a structure consisting of 6 carbon atoms joined in a ring with a hydrogen atom attached to each, formula C6H6) with a hydroxyl group (-OH) substituted for one of the hydrogens, and so its chemical formula is C6H5OH. Phenol is an important industrial chemical, but we’re interested here in phenolics that are naturally synthesized by plants, built up of one or more phenol groups, modified in various ways and joined together. Just to give you an idea of the complexity of this subject, there are more than 8000 different phenolic compounds produced by plants. 

An important property of phenolic compounds is that they associate spontaneously with a wide range of other chemical entities, such as proteins and other phenolic compounds, by means of non-covalent forces (chemical term alert!). A covalent bond is what we usually think of when we talk about chemical reactions, when two atoms combine by sharing electrons. A non-covalent bond is weaker and doesn’t involve sharing electrons, but is a common way of joining macromolecules together, for example through hydrogen bonding and hydrophobic (avoiding water) effects.

So let’s look at some of the phenolic compounds that are important in wine. Alas, some chemical names are inevitable here. First, though, we need to consider the structure of the grape berry, because different bits contain different compounds. So, let’s go from the outside in. The skin has two types of cells, an outer layer of clear epidermal cells, and then several layers (around 6 but it depends on the variety) of hypodermal cells. Then there are three different pulp tissues: the outer and inner mesocarp, and the vascular tissue that separates them. Then we have the seed, which has two layers of seed coat (testa) cells separated by a thin walled parenchymal layer. The outer layer is a cuticle, and then between this and the lignified inner testa cells there is layer of thin walled parenchyma cells that contains almost all of the seed phenolics. Interestingly, seed browning is now thought to be because of the tannins present in this layer getting oxidised, and is not associated with the process of lignification.  

Non-flavonoid polyphenols: the pulp phenolics

We’ll begin in the pulp of the grape, with some polyphenols that are found in both red and white grapes. The first distinction we will make is between flavonoid and non-flavonoid polyphenols, and we’re starting with the latter group. Generally speaking, these are found in grapes and wines at low concentrations, with one exception: hydroxycinnamic acids are the major phenolics in white wine, and are also found in red wine. Another group is the benzoic acids, such as gallic acid. The cinnamic and benzoic acids, also known as the ‘acid phenols’, are small molecules and they’re often present in grapes in a conjugated form (e.g. as esters or glycosides). These are easily extracted from the pulp of grapes during pressing, and occur at levels of 50–250 mg/litre. Typical levels in finished wines would be 130 mg/litre.

Other non-flavonoid phenolics found in wine include the hydrolysable tannins (such as gallic and egallic acid) which come from oak barrels, the volatile phenols (such as 4-ethyl phenol, produced by Brettanomyces) and stilbenes (important in plant disease resistance, including celebrity stilbene, the phytoalexin resveratrol).

Flavonoid compounds – the skin and seed phenolics

These are the major phenolic compounds in grapes, and most of them are found in grape skins, but they also come from seeds and stems. This is where we bump into the two most famous types of the polyphenols – anthocyanins and tannins. The flavonoid phenolic are broken down into two groups: flavan-3-ols and flavonols.


First, we have flavan-3-ol monomers. These are responsible for bitterness in wine, and they may also have an astringent taste. The major ones are (+)-catechin, (–)-epicatechin, and (–)-epicatechin-3-O-gallate. These are formed before veraison and change with ripening. They mostly come from seeds.

Then we have polymers of flavan-3-ol subunits, which are generally referred to as condensed tannins or proanthocyanidins (or procyanidins). These are responsible for astringency in red wines and come from the hypodermal layers of the skin and also the parenchymal layer in the seed coat.

Most of the subunits that make up tannins are either catechin or epicatechin, and these can be modified in various ways. The length of this chain – the polymer length – can vary from two or three subunits to over 30. The length of the chain is called the degree of polymerization (DP), so a 10 DP tannins has 10 flavan-3-ol subunits in it.

Skin tannins are usually much bigger than seed tannins, and they may contain some epigallocatechin subunits. Seed tannins are smaller and lack epigallocatechin subunits. They also have a higher proportion of epicatechin gallate, which is rarely found in the skin tannins. In the grape skins tannins will have a DP of up to 80, but the average DP is around 30. In seeds, the average DP is 10.

Tannins in berries change during the ripening process. In the skin, there’s little change in quantity of tannin from verasion to harvest, but the DP changes significantly. From green to red berries the average goes up from 7 to 11, and at harvest DP is around 30.

One of the reasons that tannins are important in wine is because of the way that they affect the mouthfeel of the wine. The way we sense them is by a mixture of taste and also touch. Tannins can have a bitter taste, especially when they are smaller (with a low DP). But the main way we sense them is by touch: they are astringent. They bind to proteins in our saliva and then the tannin–protein complexes precipitate, giving a drying sensation in the mouth.

It’s interesting to think about why tannins might be doing this in our mouths. Plants form tannins as defence molecules, both for defending against microbial attack and, because plants are extremely vulnerable to being eaten, also acting as antifeedants (a chemical that causes a pest, such as an insect, to stop eating). Plants are literally rooted in place, and so they have had to go to great lengths to make themselves unpalatable, acting as chemical factories to produce a wide range of toxic defensive secondary metabolites, as well as developing physical defences, such as thorns and stings.

One of the key roles of salivary proteins is to protect us from the harmful effects of tannins by binding to them and precipitating them before they reach the gut. This makes the plants more edible than they otherwise would be, neutralizing one of their defences. If the salivary proteins didn’t cause this precipitation, the tannins would interact with digestive enzymes (which are also proteins) in our gut and render them ineffective. This would reduce the palatability of plant components by making them much less digestible.

The aversive taste of unripe fruits is in part due to high tannin concentrations, with the plant using this as a way of keeping the fruit from being consumed before the seeds are ready for dispersal, along with colour changes and high acid/low sugar. We find the bitter taste and astringent sensation of tannin aversive and, as with such unpleasant oral sensations, the aversion can protect us from harmful consumption. Thus the salivary proteins are potentially filling two roles. They allow us to detect tannins in food and to reject the food if the concentrations might be dangerous, and also help neutralize any tannins present in food to be ingested.

One of the protein types found in our saliva is mucin, which is involved in forming a lubricated, slippery protective layer over the internal surface of the mouth. Tannins remove this lubrication, causing a sense of dryness and puckering. This is what we describe as “astringent.”

Related to astringency is the taste of bitterness. The majority of tannins are chiefly sensed as astringent, but they can also be tasted as ‘bitter’ when they are small enough to interact with bitter receptors on the tongue. Tannins seem to reach their most bitter taste at a DP of 4, and then decrease in bitterness and increase in astringency, with this astringency peaking at a DP of 7 (according to some studies, at least – others suggest it carries on increasing to DP 20), before becoming steadily less astringent as they become larger.

The astringent nature of tannins can be moderated by the presence of polysaccharides (sugars) or other wine components. It is also modified by the chemical adornments that tannins can grab, and there are many of these. In wine, tannins are continually changing their length (DP) and adding things to their structure. So, structurally, wine tannins can be incredibly complicated, and researchers are still trying to correlate mouthfeel properties with structure.

Interestingly, tannins are more astringent with lower pH (that is, wines with higher acidity taste more astringent, even with the same tannin content) and less astringent with increasing alcohol. However, the bitterness of tannins rises with alcohol level, and is unchanged by pH changes.

Fining – the addition of proteins such as egg white or gelatin to a wine – doesn’t actually remove much tannin. The popular idea is that the proteins bind to tannins which then precipitate out. What happens is that the proteins do bind to tannins but largely remain in the wine, forming colloidal or soluble complexes that likely have reduced astringency.


Anthocyanins are also falvan-3-ols, and are the main pigments in wine, responsible for the colour of red wines. They are found along with tannins in the hypodermal cells of the skin, except in teinturier (red fleshed) grape varieties, where they are also found in the pulp. (He and colleagues have published a detailed review on anthocyanins for anyone wanting to dig deeper[1].)

Over 600 anthocyanins have been identified in nature, and these are formed from six different basic anthocyanin structures, called aglycones. These six are cyanidin (Cy), pelargonidin (Pg), delphinidin (Dp), petunidin (Pt), malvidin (Mv) and peonidin (Pn). These all differ in colour slightly with some more red, and some more purple. Anthocyanins also differ in colour according to the pH, with a more red colour at low pH and a more blue colour at high pH: this can be seen when you rinse out a wine glass with a tiny bit of red wine in it with tap water: the rise in pH causes the trace of wine to go from red to blue-black.

These basic aglycones are not very stable, but may be modified by chemical processes called glycosylation and acylation, which improves their stability. The acylation can take place with the addition of acetic, p-coumaric and caffeic acids. With these sorts of modifications, there can be up to 20 different anthocyanins in red grapes depending on the variety. It’s interesting to note that Pinot Noir lacks acylated anthocyanins, which explains why Pinots are usually paler in colour than most other red wines, and in contrast Cabernet Sauvignon contains 18 of the 20 anthocyanins. Indeed, the anthocyanin profile can be used as a fingerprint of the grape variety, but this is only useful with fresh grapes: by the time a wine has been made the anthocyanins will have been modified enough for this no longer to work.   

The anthocyanins can exist in several equilibrium forms in wine. One of these forms is called the flavylium ion form and this is very important because it is red coloured. Another form is the quinoidal base, which has a blue colour. Only a small proportion of anthocyanins are in these coloured forms in wine. It follows that if more of the anthocyanins are in the flavylium or quinoidal form, the wine will have more colour. When sulfur dioxide is added to wine it has a bleaching effect because it binds some of the anthocyanin as a colourless bisulfate adduct.

Anthocyanins are unstable in wine and aren’t that important for the long-term colour of red wines. In addition to the anthocyanins there are two major fermentation-derived colour groups. The first of these is the pigmented polymers. These are formed by the chemical linkage between tannins and anthocyanins. This is a covalent (strong) linkage and is very important in forming stable colour in wines. The evidence suggests that most of the pigmented polymer formation occurs during fermentation, but according to some reports by the end of fermentation about 25% of the anthocyanins are thought to be complexed with tannins, and in barrel-aged reds this figure can rise to 40% within a year.

Barrels help because they provide a bit of exposure to oxygen which helps in forming these complexes, as well as supplying some extra tannins. Acetaldehyde, the product the oxidation of alcohol, helps to form these bonds. During fermentation, the formation of tannin–anthocyanin complexes helps retain more of both tannins and anthocyanins in the wine, as it makes them more soluble and stops them dropping out as a deposit. These polymeric pigments also help with colour intensity: around two-thirds of the anthocyanins are in a coloured form when they are complexed with tannins, as opposed to around 20% of the free anthocyanins that are in a coloured form.

Then there’s another group called the anthocyanin-derived pigments, which arise from reactions between anthocyanins and other phenolics and aldehydes. This is a massive, complicated class of pigments, and is an area of intense current research, with new members are being added all the time. These are referred to as pyroanthocyanins, and include the vitisins, portisins and oxovitisins. They are stable and are resistant to sulfur dioxide bleaching. Most of them have a yellow/orange colour with the exception of portisins which are blue.

The phenomenon of copigmentation[2] needs a mention. The coloured anthocyanins (red flavilium or blue quinoidal base) are planar structures and these can react with other planar structures (in this case these will be referred to as copigments) such that they form a molecular stack that excludes water. This protects the anthocyanins from hydration, increases the colour intensity and shifts the colour towards purple. The copigments are usually other phenolic compounds, and in particular the favonols, which we will discuss below. This is one of the reasons that red grapes are sometimes co-fermented with a small proportion of white grapes: the white grape skins provide copigments so even though you’d expect that including white grapes would reduce the colour of the wine, they don’t. In fact, the opposite occurs. The classic combination is a small proportion of Viognier together with Syrah/Shiraz. But flavonol concentration is also increased by UV light exposure, so you might expect grapes that have experienced higher sunlight exposure to show a deeper colour through increased copigmentation effects.


The final group of phenolic compounds we will look at is the flavonols. They are found in the skins of both red and white grapes and act as sunscreens against UV-A and UV-B light wavelengths. Flavonol levels increase in response to enhanced UV exposure. They have a yellow colour which can contribute to the colour of white wines, but which is masked in red wines. The most important of the flavonols is quercitin, but kaempferol, myrcetin, laricitrin, isorhamnetin and syringetin are also found. White grapes lack myricetin, laricitrin and syringetin[3]. They have high antioxidant capability, but perhaps their most important role is as acting as co-pigments with anthocyanins to increase the colour of red wines.

[1] He F, Liang NN, Mu L, Pan Q-H, Wang J, Reeves MJ, Duan C-Q 2012 Anthocyanins and Their Variation in Red Wines I. Monomeric Anthocyanins and Their Color Expression. Molecules 17:1571–160

[2] Boulton R 2001 The copigmentation of anthocyanins and its role in the color of red wine: a critical review. Am J Enol Vitic 52:67–87

[3] Mattivi F, Guzzon R, Vrhovsek U, Stefanini M, Velasco R 2006 Metabolite Profiling of Grape: Flavonols and Anthocyanins. J Agric Food Chem 54:7692–7702

See also:

Tannins in wine: a review

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