bANT:pTAN
The bound anthocyanin to protein-precipitable tannin ratio (bANT:pTAN) is a useful metric for understanding astringency in red wines. Free anthocyanins increase the polarity of tannins that they polymerize with, thus decreasing their astringency14,17. Bound anthocyanins are also more prone to folding, making them good precursors to colloids which incorporate structural and flavor compounds together in the finished wine.
bANT:pTAN
- <0.1: The wine is likely very astringent, linear, and drying. These wines typically come from grapes with high tannin and low pigment. They take time to age unless deliberately oxidized to soften astringency. Think Nebbiolo from Barolo.
- 11-0.15: The wine likely has moderate to high astringency. This is a classic ratio that a lot of wines fall under, and a common ratio naturally achieved throughout primary and secondary fermentation.
- 16-0.2: The wine likely has low to moderate astringency, bringing forward the flavor and structure integration. Highly concentrated wines with this ratio are sought after today for their simultaneous drinkability and age-ability. Think Cabernet Sauvignon from Napa.
- 2-0.25: The wine likely has low astringency, imparting a plush and round mouthfeel. This high ratio which has become associated with high-scoring wines, particularly those with high phenolic concentrations.
- >0.25: The wine likely has very low astringency. This ratio is commonly achieved by restraining tannin relative to anthocyanin extraction. We generally want to maximize bound anthocyanins, but over-restraining tannins can depreciate mouthfeel by limiting structure.
Free anthocyanins aren’t the only compounds in wine that modulate tannin astringency, but they are naturally occuring and highly effective. Think of these ratios like archetypes. They are useful for creating generalizations about astringency and wine style, but do not encapsulate the complexity or true nature of a wine. We have found these ratios to be generally true, but certainly not without exception. For instance, wines with very high structure (pTAN) tend to be astringent even with a bANT:pTAN ratio of 0.2. This is because oxygen also plays a critical role in decreasing astringency by decreasing polarity and therefore astringency. The higher the structure, the more Bound Anthocyanin and oxidation are required to modulate astringency into the mouthfeel of the wine. This is why wines from Barolo often need decades to age before consumption, and wines from the Napa Valley have can be consumed sooner despite often having similar ranges of tannins. Nebbiolo is a low color and high tannin varietal, and Cabernet Sauvignon is a high color and high tannin varietal.
Beyond decreasing astringency, bANT also play a role integrating flavors and mouthfeel together. Tannins and flavors are generally hydrophobic, and thus interact and bind to one another. This is why stuctured wines from concentrated fruit often have closed aroma profiles, and why wines lacking structure are often exhibit individual characteristics that dominate the aroma profile, making the wine seem simple. This phenomenon is universal to all winemaking because Iron-Reactive Phenolics from grape pulp also interact with flavors. If a wine is too astringent, then it dominates the mouthfeel and cuts flavor persistency short. If a wine has too little astringency, then the palate can lack structure. Oxidation also plays a key role in modulating astringency by decreasing tannin activity and therefore stickiness. The exact ratio desirable is entirely dependent on the harmony, resonance, and stylistic goal set for each wine. Fortunately, we understand the fundamental variables to modulating astringency so we can craft each wine accordingly.
- Timing: Within the first 100 days post-fermentation, roughly 60-80% of a wine’s free color will degradeX. Anthocyanins need to be abundant enough to complex with tannins, and therefore this 100 day period is critical.
- Concentration: Standard winemaking conditions will polymerize roughly 10% of the peak total anthocyanins extracted from the grapes during fermentation. Higher conversions can be achieved by restricting tannin concentration or through the following factors.
- Warm temperatures above 21°C (70°F): Polymerization is highly dependent on temperature. Although warmer temperatures increase the rate of free anthocyanin decay, they are essential for the formation of bound anthocyanins. At cellar temperature (10-15°C) this reaction slows dramatically, and thus temperatures between 72-74F.
- Delay SO2 addition: SO2 readily binds with anthocyanin and aldehyde, both of which play an essential role in color stabilization. Delay SO2 addition longer to facilitate the reaction.
- Oxidation: Phenols are well known to polymerize as a result of oxidation. Tannins will autoxidize resulting in a cascading reaction called regenerative polymerization, encouraging direct condensation and ethyl-bridging.7