By Dr. Aude Watrelot –
In wine, polyphenols are responsible for the difference between white and red wines as they play a major role in the color and the texture/taste. Polyphenols are a large family of complex compounds including two groups, phenolic acids and flavonoids, recognized as having antioxidant and anti-inflammatory properties. Among this latter group, anthocyanins are responsible for the red-purple grape skins, sometimes flesh, color and condensed tannins are popular for their antioxidant properties and the astringency perception.
In this research winemaking focus, tannins in grapes will be explained starting from the chemical structure of tannins to the role of grape tannins in wine.
What are Grape tannins?
Grape tannins are also called condensed tannins or proanthocyanidins based upon their chemical structure. Condensed tannins are oligomers or polymers of flavan-3-ols that are a sub-group of flavonoid. As a result of acid hydrolysis in presence of alcohol at high temperature, proanthocyanidin releases an anthocyanidin by breaking the interflavan bonds (Figure 1). Tannins are highly reactive with proteins such as collagen from animal skins that has been discovered during the fabrication of leather. This physico-chemical property is involved in the astringency perception of red wine.
Grape condensed tannins are characterized by their four main constitutive units: epicatechin, catechin, epigallocatechin and epicatechin gallate which can be found as extension and/or in terminal units (Figure 1). The mean degree of polymerization (mDP) corresponds to the number of constitutive units which are linked to each other mostly through C4-C8 interflavan bonds. Those three characteristics of condensed tannins affect their conformation in red wine and also their interaction with other macromolecules. For example, galloyl groups or a high mDP are associated with high astringency intensity.
Grape tannins are mainly found in seeds and skins but can be present in the whole berry. Depending on the grape part and variety, the quantity, structure and size determined by the mDP of tannins are different. In grape seeds, tannins are rich in epicatechin, catechin and epicatechin gallate, while in skins and stems, tannins are rich in epicatechin, catechin and epigallocatechin / gallocatechin.
Grape tannins are biosynthesized during the berry development and their concentration tend to decrease after véraison until harvest maturity. The environmental factors such as sun exposure, temperature and the grape variety have an impact on the concentration and structure of these tannins in seeds and skins. Even though the polymerization pathway is unknown, a decrease of the mean degree of polymerization and concentration of tannins in seeds and skins has been observed during grape ripening in 2009 vintage with hydric stress (Lorrain et al., 2011). The size of grape tannins can be as high as about 50 constitutive units that is about 14,500 g/mol or Daltons and be present at a concentration of about 1.5 mg/g berry in Vitis vinifera grapes (Springer et al., 2016). In interspecific hybrid grapes, tannin content is much lower at about 0.5 mg/g berry with a size of about 4.2 in Marquette grape must (Manns et al., 2013).
What happens during winemaking?
During winemaking, multiple steps lead to a decrease of tannin concentration in wine compared to grapes. In V. vinifera wines, the tannin content varied between <100 to 1600 mg/L in Pinot noir and Cabernet sauvignon, respectively while in interspecific hybrid grapes, the tannin content is often lower than 100 mg/L (Harbertson et al., 2008 and Springer et al., 2016).
The first winemaking step is grape crushing, which leads to grape cells disruption and potential interactions happen between cell wall material and tannins, enzymes, etc. After crushing, the maceration step is one of the most important for the extraction of tannins from skins to red wines helped by the alcohol content. This step is the main difference between red and white wines that does not contain that much tannins. Long maceration time are generally required to extract tannins from seeds, which is much slower due to the seed structure. Many factors influence the extraction of tannins in red wines:
- Extended maceration time up to several weeks favors the extraction of grape tannins.
- Cap management has an impact on tannin extraction as submerged-cap tend to extract more tannin than punch-down.
- High temperature of fermentation tend to extract more tannin from grapes as observed by the application of flash-détente or thermo-vinification process.
- The use of enzymes during maceration tend to increase the extraction of tannins and reduce the adsorption of tannin by the cell walls in V. vinifera grapes as those enzymes can degrade the cell wall material. However, in interspecific hybrid grapes, the application of enzymes does not show any significant improvement of tannin extraction in red wines (Manns et al., 2013).
During red wine aging, changes are observed especially on the formation of polymeric pigments that are due to the reaction of anthocyanins with condensed tannins. It leads to more stable compounds during V. vinifera wine aging due to the structure of anthocyanins (mono-glucosides). In interspecific hybrid wines, anthocyanins are mono-glucosides and di-glucosides, therefore the formation of stable polymeric pigments is not elucidated, yet. Acetaldehyde is a chemical compound formed by the oxidation of ethanol by yeasts and some bacteria, which accelerate the formation of new wine compounds with tannins. Also, as a result of oxidation reactions during wine aging, the mDP of tannins tend to decrease due to a cleavage of interflavan bonds.
Why Grape Tannins are important in wine?
As explained above, grape tannins are oligomers or polymers of flavan-3-ols that are able to interact with proteins. Those compounds are responsible for the red wine astringency perception or mouthfeel. Monomers and dimers of flavan-3-ols are associated to the wine bitterness (taste) while tannins are responsible for wine mouthfeel, the astringency. This mouthfeel is known as a 3-steps mechanism from the interactions between red wine tannins with proteins from saliva when tasting wine, followed by the formation of tannin-protein complexes that aggregate and precipitate (Figure 2). Those precipitates lead to a loss of in-mouth lubrication providing the drying / astringent sensation. This astringency perception is a characteristic of wine quality and is appreciated by consumers. However, a too low or too high astringent wine is not appreciated as it can lead to unbalanced wine, which reduces its quality.
Also tannins are important for the red wine color as explained above in the copigmentation and reaction with anthocyanins. Moreover, tannins have antioxidant properties that can be used in winemaking as a barrier against oxidation as it will be explained further in the “Tannins from wood” article.
If you have any suggestions or questions about the research/winemaking topic, feel free to send me an email at watrelot@iastate.edu.
References to learn more:
Harbertson, J. F., Hodgins, R. E., Thurston, L. N., Schaffer, L. J., Reid, M. S., Landon, J. L., Ross, C. F., & Adams, D. O. (2008). Variability of tannin concentration in red wines. American Journal of Enology and Viticulture, 59(2), 210–214.
Lorrain, B., Chira, K., & Teissedre, P.-L. (2011). Phenolic composition of Merlot and Cabernet-Sauvignon grapes from Bordeaux vineyard for the 2009-vintage: Comparison to 2006, 2007 and 2008 vintages. Food Chemistry, 126(4), 1991–1999. https://doi.org/10.1016/j.foodchem.2010.12.062
Manns, D. C., Lenerz, C. T. M. C., & Mansfield, A. K. (2013). Impact of Processing Parameters on the Phenolic Profile of Wines Produced from Hybrid Red Grapes Maréchal Foch, Corot noir, and Marquette. Journal of Food Science, 78(5), C696–C702. https://doi.org/10.1111/1750-3841.12108
Morata, A. (Ed.). (2018). Red Wine Technology (1 edition). Academic Press.
Springer, L. F., Chen, L.-A., Stahlecker, A. C., Cousins, P., & Sacks, G. L. (2016). Relationship of Soluble Grape-Derived Proteins to Condensed Tannin Extractability during Red Wine Fermentation. Journal of Agricultural and Food Chemistry, 64(43), 8191–8199. https://doi.org/10.1021/acs.jafc.6b02891