Tannins are polyphenolic metabolites commonly found in higher plants. Tannins are considered secondary metabolites as they are not directly involved in growth, development, or reproduction (Herderich and Smith 2005). The term tannin refers to a class of compounds that are distinguishable from the broader classification of phenolic compounds in that tannins react with proteins (Herderich and Smith 2005).
In wine, tannin is derived from grapes as well as from oak (e.g. oak barrels) sources utilized during production (Gawel 1998). This class of compounds contributes to the textural properties, color attributes, bitterness, and aging capacity of red wine. The textural sensation imparted by wine tannin is referred to as astringency which is a drying and puckering sensation in the mouth that is considered a desirable characteristic of red wine (Gawel 1998). The tactile perception of astringency is due to both hydrophobic and hydrogen bonding interactions between saliva proteins and wine tannin causing the proteins to precipitate out of solution resulting in a loss of lubrication in the mouth (Hanlin et al. 2010). Tannin also plays an important role in the color of red wine acting as a cofactor in copigmentation and by reacting with anthocyanin forming stable wine pigments (non-bleachable color) (Carew et al. 2013).
Figure 1. Schematic of a condensed tannin molecule. Repeating Flavan-3-ol units with linear 4-8 bonds or branched 4-6 bonds (dotted line) (Source: Wikipedia-public domain)
There are two types of tannin present in wine, hydrolysable tannin and condensed tannin (non-hydrolysable. Hydrolysable tannins are generally extracted from oak barrels used in wine aging and are present in low concentration in comparison to condensed tannin (Smith et al. 2015). Tannin from oak sources will reach a maximum concentration of approximately 250 mg/L (Quinn and Singleton 1985) compared to levels of 2 to 4 g/L condensed tannin derived from grapes (Herderich and Smith 2005). Condensed tannins are present in the grape skins, seeds, and flesh. They consist of repeating flavan-3-ol units (Figure 1) which include catechin, epicatechin, epigallocatechin, and epicatechin gallate (Smith et al. 2015). Skin tannins are comprised of long polymeric chains ranging from 3 to 83 subunits in length linked by interflavan bonds (Smith et al. 2015). The tannins extracted from grape skin contain primarily epigallocatechin-derived subunits as well as trace amounts of gallocatechin and epigallocatechin 3-O-gallate (Smith et al. 2015). Seed tannins consist of much smaller polymers of 2 to 16 subunits of catechin and epicatechin (Herderich and Smith 2005). Grape pulp also contains tannins, but it remains non-extractable during maceration due to a strong association with pulp cell wall constituents (Bindon et al. 2014). The composition of tannin molecules in wine is important because smaller molecular mass tannin (primarily seed tannin) are perceived as bitter and larger tannin molecules (skin tannin) are perceived as more astringent (Fontoin et al. 2008). After condensed tannin is extracted from grapes they undergo many structural changes in wine overtime. The changes are initially induced by yeast, enzymes, and fermentation by-products such as acetaldehyde (Smith et al. 2015). The chemical changes continue as the wine ages which gradually changes the purple hue of a young wine to a brick red color; this is generally accompanied by a reduction in astringency (Smith et al. 2015).
Bindon, K.A., S. Kassara, W.U. Cynkar, E.M.C. Robinson, N. Scrimgeour, and P.A. Smith. 2014. Comparison of Extraction Protocols To Determine Differences in Wine-Extractable Tannin and Anthocyanin in Vitis vinifera L. cv. Shiraz and Cabernet Sauvignon Grapes. Journal of Agricultural and Food Chemistry 62:4558-4570.
Carew, A.L., P. Smith, D.C. Close, C. Curtin, and R.G. Dambergs. 2013. Yeast Effects on Pinot noir Wine Phenolics, Color, and Tannin Composition. Journal of Agricultural and Food Chemistry 61:9892-9898.
Fontoin, H., C. Saucier, P.L. Teissedre, and Y. Glories. 2008. Effect of pH, ethanol and acidity on astringency and bitterness of grape seed tannin oligomers in model wine solution. Food Quality and Preference 19:286-291.
Gawel, R. 1998. Red wine astringency: a review. Australian Journal of Grape and Wine Research 4:74-95.
Hanlin, R.L., M. Hrmova, J.F. Harbertson, and M.O. Downey. 2010. Review: Condensed tannin and grape cell wall interactions and their impact on tannin extractability into wine. Australian Journal of Grape and Wine Research 16:173-188.
Herderich, M.J., and P.A. Smith. 2005. Analysis of grape and wine tannins: Methods, applications and challenges. Australian Journal of Grape and Wine Research 11:205-214.
Quinn, M.K., and V.L. Singleton. 1985. Isolation And Identification Of Ellagitannins From White Oak Wood And An Estimation Of Their Roles In Wine. American Journal of Enology and Viticulture 36:148-155.
Smith, P.A., J.M. McRae, and K.A. Bindon. 2015. Impact of winemaking practices on the concentration and composition of tannins in red wine. Australian Journal of Grape and Wine Research 21:601-614.