Tannin Structure in Red Wine

Washington State University's enology primer (Jim Harbertson, originally published 2008) describes tannin as the common name for several classes of phenolic compounds, divided into condensed and hydrolyzable tannins. Condensed tannins, or proanthocyanidins, are the most abundant class of phenolics in grapes and wine; they are polymers of flavan-3-ol subunits and are found in the skin and seed of the grape. The hydrolyzable tannins found in wine are the non-flavonoid ellagitannins derived from oak barrels. The AWRI adds that once grape tannins are extracted they begin to chemically rearrange into wine tannins, which can be significantly different in structure from the original grape tannins.

• WSU (2008) gives the estimated mean length of condensed tannin polymers at harvest as 27 in grape skins and 6 in seeds
• The flavan-3-ols catechin and its epimer epicatechin are found in the seeds, and WSU identifies them as the main source of flavonoid bitterness in wine
• According to the AWRI, tannins in grape skins and seeds begin to be extracted into the must when grapes are crushed, and skin tannins tend to be more easily extracted than seed tannins
• The AWRI (2012) notes that Pinot Noir seeds and stalks contain high concentrations of tannin, but extracting them can introduce green flavours and astringent and bitter characters

WSET explains that tannins increase the friction in the mouth as they interact with the proteins in saliva, preventing them from lubricating, which leads to the characteristic drying sensation. The AWRI likewise links the sensation of astringency to strong associations between tannins and salivary proteins. WSU's primer states that tannins are responsible for the astringency of red wines, while flavonoid bitterness is primarily due to the flavan-3-ols catechin and epicatechin.

• WSET describes tannins as grippy or even astringent, like a cup of stewed tea, as fine and smooth, especially in a mature wine, or as sandpapery, gravelly or cotton woolly
• AWRI research reported in 2014 found that larger, more water-soluble wine tannins are more astringent, while smaller, less water-soluble, more coloured tannins were perceived as hotter, more bitter and less astringent
• WSU reports that in model wine systems, larger tannin polymers have harsher astringency descriptors
• WSET notes that serving a red wine too cold may make its tannins feel firmer or more astringent, and that with less or no alcohol, tannins are experienced as more astringent
• The AWRI observes that wines of lower pH are often reported as more astringent

The WSET Level 3 Systematic Approach to Tasting Wine places tannin in the palate assessment, alongside sweetness, acidity, alcohol and body, and rates it low, medium(-), medium, medium(+) or high. On lines where the entries are separated by a hyphen, students must select one and only one option. WSET suggests learning the structural sensations side by side: acidity makes the mouth water, while tannins have a mouth-drying effect.

• WSET's suggested red comparison sets a light-bodied Pinot Noir or Beaujolais against a full-bodied, high-tannin Cabernet Sauvignon or Malbec, with an aged Reserva or Gran Reserva Rioja falling somewhere in between for tannin
• In that comparison, WSET says Cabernet Sauvignon may have quite a high level of tannin, feeling drying and grippy, whereas Pinot Noir generally has less tannin and may feel softer and more delicate
• WSET counts tannin, with alcohol and acidity, among the big three elements of mouthfeel, and notes that tannins can also be found in skin-contact white wines and may be contributed by oak
• The WSET Level 3 specification lists tannin among the components of the grape, alongside water, sugar, acids, colour and flavours

The AWRI reported in 2014 that its analysis of vertical series of red wines showed aged wines can have tannin concentrations similar to young wines, dispelling the commonly held belief that changes in astringency with ageing come from the loss of tannin through precipitation. The changes are instead due to compositional changes in the tannins. Tannins isolated from older wines associated only weakly with saliva-like proteins compared with tannins from younger wines, consistent with the lower astringency of old wines. WSU's primer (originally published 2008) had likewise concluded that the precise mechanism for the loss of astringency during aging had yet to be proven.

• Tannins combine with anthocyanins to form polymeric pigments, which WSU describes as the source of stable colour in red wine because they are more resistant to bisulfite bleaching and their colour is less pH dependent
• In AWRI's analysis of 30 and 50 year vertical series of Cabernet Sauvignon and Shiraz (reported 2014), colour density after the first two years post-bottling was more strongly associated with pigmented tannins than with anthocyanins; the same AWRI article states that, in general, no coloured anthocyanins were left in red wine by four years
• The AWRI found a large proportion of aged wine tannin cannot be broken apart, unlike grape tannin, and this non-hydrolysable tannin associates very weakly with saliva-like proteins, which may partly explain why red wines soften with age
• In AWRI trials reported in 2014, injecting air into red wine fermentations reduced the concentration and size of tannins, increased stable colour formation and reduced astringency

WSET explains that thinner-skinned varieties such as Pinot Noir have less colour and tannin to give to the wine, while thick-skinned Cabernet Sauvignon can create deep-coloured wines with plenty of mouth-gripping tannins. The AWRI reported in 2012 that although Pinot Noir grapes have high tannin concentrations, Pinot Noir wines tend to be low in tannin, most likely because of Pinot's low ratio of skin-to-seed tannin compared with other varieties. WSU notes that wines made from different cultivars show different average amounts of tannin.

• The AWRI (2012) gives a typical Pinot Noir grape must as 39% juice, 28% pulp, 24% skin, 5% stalk and 4% seeds on a wet weight basis
• In AWRI research reported in 2014, riper grapes were associated with increased total skin tannin concentration and higher anthocyanin, which appears to enable the extraction of tannin
• An AWRI trial reported in 2014 harvested Cabernet Sauvignon from one vineyard at five maturity stages; the lower-alcohol (12%) wines had lower tannin and a lower proportion of skin-to-seed tannin than the higher-alcohol (15%) wines, and astringency increases correlated with higher tannin in later-harvested wines
• In the same trial, consumers preferred wines of 13% alcohol or higher, but liking did not increase further between 13% and 15% alcohol despite increases in tannin
• The AWRI reported in 2014 that quality gradings of young red wine were positively correlated with tannin concentration, tannin size, a higher proportion of skin-derived tannins and overall wine colour

WSET compares tannin extraction to brewing tea: temperature and time both play a role in how deeply coloured and tannic a red wine becomes during fermentation. The AWRI describes the length of skin and seed contact, during fermentation and any post-fermentation maceration, as probably the most critical factor in defining the phenolic profile and sensory properties of the wine, and one of the easiest to control. The WSET Level 3 specification lists pre-fermentation extraction, alcoholic fermentation and extraction, and post-fermentation extraction among the stages of red winemaking.

• Extended post-fermentation maceration leaves skins, seeds and any stems in contact with the wine after fermentation to optimise colour, flavour and tannin structure; the AWRI, which trialled 21 and 60 day treatments on Cabernet in 2018, says the length in the literature can vary from a few days to two months or more
• The AWRI lists the development of bitterness and overly astringent tannins among the most commonly reported risks of extended post-fermentation maceration, and notes the technique is traditional in Burgundy with Pinot Noir, Bordeaux with Cabernet-family varieties and Piedmont with Nebbiolo
• In an AWRI Pinot Noir trial reported in 2012, total and pigmented tannin were dramatically higher in wines given 45 days of extended post-ferment maceration, and wines with stems added were also high in tannin
• WSU (2008) reports the oak ellagitannins vescalagin and castalagin at a maximum of 7 and 21 mg/L in wine, and says any direct organoleptic effect of oak tannins remains uncertain
• Grape and seed tannin extracts, called oenotannins, can be legally added to wine; the AWRI's oenotannin treatment page gives addition rates of 50 to 1,000 mg/L (its 2018 Cabernet trial used 300 mg/L) and cites a 2013 review finding they had little impact on colour stabilisation

WSET notes that wines with more tannin can be smoothed out by serving them with a savoury, high-protein, often meaty dish. WSET's food and wine pairing guidance describes a magic triangle of salt, acidity and fat: having these on the palate before an acidic, astringent and tannic wine helps soften it. The WSET Level 3 specification lists tannins (oak) among the wine considerations for food and wine pairing.

• WSET's classic example pairs a salty rib eye steak with a tannic and acidic Barolo, where the salt and fat on the steak smooth the tannins and make the wine softer
• WSET suggests tasting a young Chianti Classico after lemon juice and salt, then after salty mature cheddar, where the fat in the cheese makes the wine seem even smoother
• For a good food and wine match, WSET says most people want the food to make the wine seem softer, sweeter and fruitier

Primary sources: https://www.wsetglobal.com/media/3119/wset_l3_wines_sat_en_jun-2016.pdf, https://www.wsetglobal.com/media/11731/wset_l3wines_specification_en_highres_may2022_issue2.pdf, https://www.wsetglobal.com/knowledge-centre/blog/2026/the-secret-language-of-wine-what-is-mouthfeel, https://www.wsetglobal.com/knowledge-centre/blog/2021/september/23/what-determines-the-colour-of-wine, https://www.wsetglobal.com/knowledge-centre/blog/2019/november/how-to-build-your-wine-tasting-skills/, https://www.wsetglobal.com/knowledge-centre/blog/2023/july/13/four-rules-to-masterful-food-and-wine-pairing, https://wine.wsu.edu/extension/grape-and-wine-phenolics/, https://www.awri.com.au/industry_support/winemaking_resources/winemaking-practices/winemaking-treatment-oenotannin-addition/, https://www.awri.com.au/industry_support/winemaking_resources/winemaking-practices/winemaking-treatment-extended-post-fermentation-maceration/, https://www.awri.com.au/wp-content/uploads/2014/04/1602-smith-et-al-WVJ-28-2-2014.pdf, https://www.awri.com.au/wp-content/uploads/May-June-2012-AWRI-Report.pdf. Last verified 2026-09-29