A Primer on grape tannins:

Everything you always wanted to know

(but were afraid to ask)

By Kelly Fields, Enologist

[Ed. Note: We are indebted to Phelps Preferred member Mike Faber, from Wisconsin, for his thought-provoking letter about tannins. “I have heard that tannin is primarily a texture,” he writes. “but many wine writers…give tannins all kinds of attributes, including dryness/sweetness, ripeness, fruit, viscosity, etc. If tannins are primarily about texture…then how can they be sweet or ripe?…I think tannin is the least understood component about wine,” he concludes. “Why not devote a column explaining it?”

An excellent idea, and herein is our explanation as written by JPV’s resident enologist, Kelly Fields. This is not a simple subject, and we think Kelly did a great job. Let us know what you think.]

Both red and white wines are composed of many molecules: water, ethanol, acids, color, tannin and sugars. Color and tannin are two molecules found in a larger class of molecules called phenols. Phenolics are a group
of compounds naturally found in many substances from aromas to foods to antibiotics.

In relation to wine, phenolics are found naturally within the berry and constitute the color or anthocyanin and tannin. These two groups are found in most plants, flowers and fruits. They serve as natural attractants, such as colorful flowers, or deterrents, such as a bitter taste, in order to discourage animals from eating them. The name tannin comes from its ability to ‘tan’ or soften leather.

There are two main classes of phenolics within grapes and wine, the flavonoids and the non-flavonoids. The two main types that are most commonly referenced in wine are both in the flavonoid class and those are the anthocyanins, which are the color components, and the tannins, which are responsible for astringency, stability and structure of a wine. Grapes contain many types of important phenolic components, but these are the most well known in red wines. Tannin is mistakenly used as a general term to classify all phenolics because it is very prominent in red wines.

Tannin is produced within the seed and the skin of the berry and anthocyanin is produced in the skins while the tannins are maturing. Almost all the tannin that can be made will be produced by veraison – the point at which the fruit stops growing and starts ripening (Figure 1). In the vineyard this can be seen once half the berries on the cluster have changed color from green to red or purple. After veraison the tannins that were produced mature to a less bitter state and the colors within the skins are synthesized (Figure 2).

When grapes are mature and ready to be picked there are many components that are important. As a berry ripens the acid levels decrease, the sugar levels increase, the tannins decrease and become less astringent, and the colors and flavors develop.

All these processes are very important in making the correct picking decisions. Tannin and anthocyanin concentrations vary between red grape varietals with Pinot Noir having smaller amounts than Cabernet Sauvignon, and Merlot concentrations being somewhere in between. White wines have colorless anthocyanin with very low tannin levels because, unlike red wines, they are not fermented on the skins and the seeds.

Anthocyanins are not only important for color. During fermentation anthocyanins are released from the skins which have a finite quantity depending on the grape and its maturity. Tannins are released from both the skins and seeds as the fermentation progresses and the alcohol concentration increases. Tannin extraction is monitored by the winemaker and depends on the style of wine that is being made and the quality and tannin concentration the fruit offers.

Figure 1

Tannin is important in enhancing the longevity of a wine. Tannin concentration is important because it creates a very stable color molecule by linking with anthocyanin molecules. This linking can happen during fermentation and throughout the maturation of the wine during barrel and bottle aging. These molecules will continue to form and will eventually fall out of solution as a red wine ages, leaving colored particles in the bottle.

Tannins are critical to a red wine’s balance and structure. When made from ripe grapes, wine has a balance of tannin, alcohol and acidity which contribute to a wine’s mouth feel. Sometimes wines are described as hard or soft or ripe or green. These descriptors refer to tannin maturity (when the grapes were picked), and its balance with the acid and alcohol. Tannin descriptors are extremely subjective and versatile, so defining them can be difficult, but all of them are describing the mouth feel and texture of the wine.

If a wine is described as big or strong it is simply referring to a wine with a high concentration of tannin. If a wine is described as soft or supple, there is a lower concentration of tannin. When the tannin is in balance with the alcohol and acidity of the wine, it comes across with a nice mouth feel that is not overwhelming. Sometimes ‘sweet’ or ‘ripe’ are also used in conjunction with tannin descriptors. This can be confusing because it is actually describing a wine that is made from healthy, ripe fruit and has a good balance overall, not just with the tannins.

A wine that is described as hard, green, drying or astringent refers to tannins that are
not in balance with the alcohol and acidity and may reflect fruit that has been picked when
it was under-ripe.

From grapes to juice to wine, the vineyard team and winemakers are constantly monitoring the evolution of the phenolic levels. For the general wine taster, it is important to remember that everyone’s palate is different and therefore the interpretation of a wine and its structure may differ from one person to another. A wine that one person thinks has a nice balance of fruit and tannin may seem astringent and out of balance to another.

Phenolics play a huge role in the tasting of any red wine, with genetics and individual knowledge contributing equally to one’s enjoyment of the tasting experience.

Figure 2

Figure 1. The changing color of the fruit signals veraison, when the grape cluster begins to ripen.

Figure 2. Anthocyanin and tannin production during grape berry development and ripening. Berry size is illustrated by the green curve showing the two phases of berry growth between fruit set and veraison, and after veraison when the berries are ripening. Tannin synthesis starts very early in berry development and continues until veraison in skin and for 1-2 weeks after veraison in seeds. Tannin maturation, which decreases extractability of the tannins, occurs during ripening. Anthocyanin synthesis occurs in the skin of red grapes after veraison, and after tannin synthesis is complete. (Figure courtesy of James Kennedy, Dept. of Food Science & Technology, Oregon State University, Corvallis, OR)