Protein-Precipitable Tannins (pTAN)

Figure 1. WINEXRAY's representative phenolic profile of a 21-day macerated Bordeaux varietal fermentation (The Future of Winemaking: Honoring the Vision of Professor Roger Boulton, 2022).

Protein-precipitable tannins (pTAN) react with saliva and are responsible for structure, or the breadth of tannins on the palate. Unlike free anthocyanins, they are very stable and malleable using different controls throughout the grape growing and winemaking process. The Adams-Harbertson Assay was designed to mimic human senses by using Bovine Serum Albumin to precipitate grape-derived condensed tannins. For this reason, it has the highest correlation to astringency out of any analysis in the world. In our experience, pTAN is a better metric for structure, or the breadth of astringency on the palate, as astringency depends on the concentration and activity of the tannins as well as other matrix variables related to the wine (sweetness, ABV, etc.). The assay also does not measure hydrolyzable oak tannins despite their astringent properties. Nevertheless, the vast majority of tannins in wine are from grape skins, seeds, and stems. They vary in concentration, size, composition, and polarity, all of which impact the resulting astringency and texture of the wine. 

The A-H assay’s protein-precipitable tannin measurement has the highest correlation to astringency (r2 = 0.82-0.90) when compared to high-performance liquid chromatography (HPLC) and methyl cellulose precipitation (MCP) (Mercurio & Smith, 2008). This being said, the characteristics of tannin chemistry are too diverse for concentration to represent astringency alone. Instead, we prefer to refer to protein-precipitable tannin concentration as a metric of wine structure because astringency is dependent on several other variables (anthocyanin incorporation in particular). Structure, on the other hand, is the phenolic backbone from which skin-macerated wines are built. Higher pTAN concentrations relate to a wider breadth of astringency on the palate. It is important to note that tannins are much more stable and persistent than free anthocyanins in wine. In practice, we see that protein-precipitable tannins decrease roughly 10-20% within the first year post-fermentation and persist thereafter at very stable concentrations. Another thing to consider is that extraction during fermentation is partial. This informs our fundamental understanding of tannins. They are abundant and they are persistent. The only exception to this rule is in the presence of extreme heat or extreme age. Grapes nearing ripeness exposed to ambient temperatures above 43°C (110°F) show signs of cascading oxidation that deplete skin and seed tannin concentrations (about 30-50%). This type of heat stress also diminishes phenolic reactivity and stability in the finished wine with concentrations continuing to decrease in extended maceration and aging. WINEXRAY’s study with UC Davis in 2022 found an additional 60% reduction in extracted protein-precipitable tannins due to extended maceration of heat-damaged fruit. This is the greatest and most quantifiable consequence we have seen due to climate change. 

Profiling tannin extraction during fermentation is especially useful because changes in sugar, alcohol, and acidity are all interfering with your ability to perceive astringency (Sáenz-Navajas et al., 2010). We also find protein-precipitable tannin in finished wines to be exceptionally useful in evaluating wines by style, vintage, and producer. Below are reference levels for protein-precipitable tannin measured in finished wines.

1. <500 ppm is a low amount of structure found in Pinot Noir and high-yield red wines.
2. 500-1,000 ppm is a classic range for Bordeaux wines.
3. 1,000-1,500 ppm is a classic range for Napa wines.
4. 1,500-2,000 ppm is a range typical for highly structured wines from Napa and Barolo.
5. >2,000 ppm is an extremely high structure found in highly tannic cultivars such as Sagrantino, Aglianico, and Corvina.

Perhaps the most common misunderstanding about wine tannins is the relationship between tannin size and astringency. We often hear speak of soft, polymerized tannins that develop as they age. This is misleading. Not only are larger tannins more astringent, but they also tend to decrease in size with wine age. The average size of tannin polymers in wine is expressed as the mean degree of polymerization (mDP). This includes monomers (mDP = 1), oligomers (2 ≥ mDP <5), and polymers (mDP ≥ 5). One study showed that 21 Bordeaux wines between 3 and 26 years old all had an mDP between 1 and 3 (Drinkine et al., 2007). This is in contrast to grape skin tannins which have mDPs ranging from 34-86 (Morata, 2018). Once extracted into wine fermentation, larger tannin molecules undergo acid catalysis creating small polymers (Smith, 2013). On the other hand, colloids are aggregates of bound anthocyanins, proteins, polysaccharides, and other subunits that snowball into larger and larger macromolecules as wine ages until they eventually precipitate out of solution. The incorporation of other molecules increases the polarity of the tannins thus decreasing their astringency.

Figure 1. Monomeric tannin structures common to V. vinifera

Monomeric tannin composition in grapes and wine consists primarily of the following 4 compounds: catechin, epicatechin, epigallocatechin, and epicatechin-3-O-gallate. This sounds simple, but the total number of unique combinations of these tannins is likely many multiples greater than 65,532 (Adams & Harbertson, 1999). Their combinations are magnificent in their complexity, and their respective concentrations in grapes and wines are largely due to variables outside of a viticulturist's or winemaker’s control. For example, the only significant way to selectively extract skin versus seed tannins is by controlling maceration length. Grape skins have more epigallocatechin which will rapidly extract at the start of fermentation and grape seeds have more catechin which requires more alcohol and time to extract. It’s important to note that even though the wine has completed fermentation, there are still a lot of suspended solids that will mask the underlying astringency. This character is highly specific to site, cultivar, producer, etc. For this reason, we provide phenolic ratios as a reference point for winemakers. If protein-precipitable tannin is a measure of structure, then the ratio of bANT:pTAN is a measure of astringency.

In summary, the diversity of tannin polymers in wine is infinitely complex and surely one of the truest chemical markers of terroir that winemakers have little influence on. Winemakers can focus on managing protein-precipitable tannin concentration to establish structure, form bound anthocyanins to modulate astringency, and incorporate oxygen to decrease tannin activity. Applying this knowledge through and beyond the wine production process lends us powerful insight into its behavior. Rather than seeking to manipulate wine’s complexity, we can observe and work alongside it to reveal its potential. Tannins will always remain one of wine’s greatest mysteries, and the human palate wine’s greatest analytical tool. 

To learn more about the scientific tools used in winemaking and see how they apply to wines from around the world, become a Bound advising client.

References

Colantuoni, G., McLeod, S. WINEXRAY LLC. https://www.winexray.com/

Adams, D. O., & Harbertson, J. F. (1999). Use of Alkaline Phosphatase for the Analysis of Tannins in Grapes and Red Wines. American Journal of Enology and Viticulture, 50(3), 247–252. https://doi.org/10.5344/ajev.1999.50.3.247

Drinkine, J., Lopes, P., Kennedy, J. A., Teissedre, P.-L., & Saucier, C. (2007). Ethylidene-Bridged Flavan-3-ols in Red Wine and Correlation with Wine Age. Journal of Agricultural and Food Chemistry, 55(15), 6292–6299. https://doi.org/10.1021/jf070038w

Mercurio, M. D., & Smith, P. A. (2008). Tannin Quantification in Red Grapes and Wine: Comparison of Polysaccharide- and Protein-Based Tannin Precipitation Techniques and Their Ability to Model Wine Astringency. Journal of Agricultural and Food Chemistry, 56(14), 5528–5537. https://doi.org/10.1021/jf8008266

Morata, A. (2018). Red Wine Technology (1st ed.).

Sáenz-Navajas, M.-P., Campo, E., Fernández-Zurbano, P., Valentin, D., & Ferreira, V. (2010). An assessment of the effects of wine volatiles on the perception of taste and astringency in wine. Food Chemistry, 121(4), 1139–1149. https://doi.org/10.1016/j.foodchem.2010.01.061

Smith, C. (2013). Postmodern Winemaking: Rethinking the Modern Science of an Ancient Craft. University of California Press.