Enology Notes #116, August 23, 2006
To: Regional Wine Producers
From: Bruce Zoecklein, Head, Enology-Grape Chemistry Group, Virginia Tech
1. Tannins, Color and the 2006 Season. The phenolic compounds in wines are the result of a host of factors, including those depicted in Figure 1. This begins a multipart series adapted, in part, from a presentation titled “Grape and Wine Phenols,” that I conducted in Chile in late 2005.
Approximately one-third of the carbon produced by grapevines is used to produce phenolic substances. Therefore, phenols are an important constituent of both vines and wines. Most winemakers think of phenolic compounds in terms of mouthfeel, balance, color, and aging potential. Balance refers to the integration of the structural and textural elements, and has been discussed in previous editions (See Enology Notes #52, 68, 69, 76, 84, 107, 108, and 113).
An important scientist and leader of our industry, Emile Peynaud, added the following to his 1984 text, Knowing and Making Wine.
“The faster the scientific advances, the greater the risk of widening the gap between what we know and what we do.”
This quote may be appropriate to our industry regarding certain beliefs that guide our practices. Grape and wine color receives attention due to the impact on sensory perception, because color absorption spectra are easy to measure, and because of certain assumptions. As Boulton (2005) suggested these beliefs drive our practices, but are they correct?
- Grape and wine color strongly correlate to aroma/flavor? These are not always strongly correlated.
- Anthocyanin concentration strongly correlates to aroma/flavor? The biochemical pathways for the production of anthocyanins and aroma/flavor compounds are different and operate independently. Why should they relate? Indeed, the biochemical pathways for anthocyanins and tannins are different.
- Color is a good indicator of yield? The correlation between color and yield depends on what component(s) of yield are investigated.
- Desirable grape color will assure desirable red wine color. While correlated, these two are different, as outlined below under Factors Impacting Red Wine Color.
Tannins. The term tannin defines a very heterogeneous group of phenolic compounds that are identified, based on certain properties:
- reaction with ferric chloride
- the ability to bind with proteins, e.g., tannin leather - hence the term tannins
It was their characteristic interaction with proteins that traditionally differentiated tannins from other phenols. However, not all phenols that bind with proteins elicit an astringent response, and tannins are not the only compounds in wines that cause astringency.
Phenols, including tannins, have the ability to polymerize, or associate, with themselves and other compounds, including anthocyanin pigments. As polymerization occurs, the molecule becomes larger. The number of subunits bound together is referred to as the DP number, or degree of polymerization. The DP number increases with wine age, a reason, in part, why wines become more supple or less astringent, and go from bright red to tawny in color.
So-called tannin “quality” refers to:
- degree of polymerization
- the association of tannins with other molecules
- stereospecific nature of the molecule, which can make it harsh and hard, or supple
Grape tannins derived from the skin, seeds, and stems differ in their length, overall subunit concentration, and sensory properties. They differ in the following:
- stereospecific nature
- number of hydroxyl groups
In general, seed and stem tannins have a relatively short DP length, the average being about 15. Skin tannins, on the other hand, have a DP number that averages about 40.
Pigment Polymerization. In grapes and wines, anthocyanin pigments can be either free monomers, that is, unbound, or associated with other phenols to form polymers. In the lab, we can easily characterize two sizes of polymers: small polymeric pigments (SPP), which are 5 subunits or smaller, and large polymeric pigments (LPP), which are more than 5 subunits.
Polymerization can impact the sensory quality of the pigments by increasing color stability, and by changing the spectral color from bright red to red-brick. Additionally, the LPPs are loosely associated with softer, less harsh astringency.
A limited degree of polymerization occurs in the fruit during maturation. In wines, tannin polymerization continues until an anthocyanin molecule binds the terminal end, then polymerization is believed to stop. Therefore, the ratio of anthocyanins to tannins is important in impacting polymerization. The ratio of free anthocyanins to tannin is important, due to the following:
- tannin and anthocyanin concentrations affect concentrations of LPP
- low concentrations of tannin cannot form polymers
- whichever is limiting will determine amount of polymeric pigment
- impacts color, color stability, and mouthfeel
Grapes producing the most intense and balanced wines usually have a high anthocyanin to tannin ratio. Indeed, wine quality may be dependent, in part, on this ratio, a reason why some add enological tannins (see Enology Notes #8, 103).
The anthocyanin-to-tannin ratio of wines for optimum color stability is usually about 1:4. An example of Bordeaux red wine ratios is as follows (source: Scott Labs):
Red wines: Anthocyanin 0.5 to 1 g/L, Tannin 1.5 to 5 g/L
Rosé wines: Anthocyanin 0.2 to 0.5 g/L, Tannin 0.25 to 1 g/L
Factors Impacting Red Wine Color. Color is an important wine attribute, because humans are visually oriented. As such, wine color can certainly bias evaluations. A classic example of color bias is to change the color of a white wine, such as Chardonnay, with red food coloring. In blind evaluations, the color-adjusted wine frequently receives a different sensory rating for attributes such as fullness, body, and complexity.
As such, richly-colored wines are assumed to have high volume or body, and softer tannins. Conversely, a wine with less color is automatically assumed to have ‘green’ or ‘harsh’ tannins.
Spectral color in wine is a function of these three elements:
- anthocyanin concentration
- concentration of cofactors, or certain non-colored compounds, which bind with anthocyanins
- polymeric pigments
Hyperchromicity, also known as copigmentation, is an interesting phenomenon that allows more visible red color, than would be expected due to the anthocyanin concentration alone. Cofactors are non-colored compounds that have the ability to bind with anthocyanins, creating more color than the unbound pigment, hence the term hyperchromicity.
The concentration and type of cofactors vary greatly from variety to variety, season to season, but include some non-flavonoid phenols, flavonols, and the amino acid arginine. It is not likely that enological tannins contain compounds that act as cofactors.
We need to learn more about the impact of seasonal and processing variations on cofactors. Some are easily oxidized, which can affect red wine color intensity dramatically. Previously, we assumed that fining agents bound with anthocyanin molecules directly, thus reducing red color. It appears that their action may also be removing cofactors and other macromolecules, or impacting cofactor binding with anthocyanins.
Because red color is a function of three elements (anthocyanin concentration, cofactor concentration, and polymeric pigments), it is possible to have the following (Boulton, 2005).
- Change in grape anthocyanin concentration = Change in wine color
- Change in grape anthocyanin ≠ Change in wine color
- No change in grape anthocyanin = Change in wine color
The above highlights several points:
- Variation in cofactors and polymeric pigment concentration may be more important to spectral color, than simply anthocyanin concentration.
- Grape pricing based on anthocyanin concentration alone may not be desirable.
- Harvesting based on anthocyanin concentration will not necessarily assure desirable red wine color.
2. VT Service Lab. The Enology-Grape Chemistry Group at Virginia Tech has established a fee-based, full-service enology laboratory. This lab provides chemical, physical, microbiological, and sensory analysis, including standardized reagents.
The laboratory now conducts fining trials.
Details regarding this service are available at www.vtwines.info.
3. Brettanomyces Slide Show on-line. My slides from our recently concluded workshop titled “Practical Monitoring and Management of Brettanomyces” and adapted from my presentation at the Cool Climate Symposium, Christ Church, NZ, is posted at www.vtwines.info. Click on-line publications.
4. Maturity Evaluation for Growers on-line. An extension publication titled “Maturity Evaluation for Growers” is available at www.vtwines.info. Click on-line publications.
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Professor and Enology Specialist Head Enology-Grape Chemistry Group
Department of Food Science and Technology, Virginia Tech
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: http://www.vtwines.info/
Phone: (540) 231-5325
Fax: (540) 231-9293
Cell phone: 540-998-9025