Tannins – types and amounts in grapes and wines
Tom Collins
Senior Research Enologist
Beringer Blass Wine Estates
Formerly the Manager of East Coast Grower Relations and Vineyard Operations for Canandaigua Wine
Company, Tom Collins is now a Senior Research Enologist with Beringer Blass Wine Estates in St.
Helena, CA. His viticultural areas of interest while with Canandaigua included evaluation of new
sprayer technology, vineyard mechanization and vineyard redevelopment. In his position with Beringer
Blass, Tom oversees the development and implementation of a broad range of winemaking research
projects, including yeast strain trials, yeast nutrition trials, color enhancement and stability trials for red
wines and the winemaking evaluation of various vineyard research projects. He is also involved in the
evaluation of new technologies for the winemaking laboratory, as well as lab proficiency training and
testing in all of Beringer Blass’ wineries in California.
Tannins are complex polymeric compounds that are responsible at least in part for many of the sensory
attributes of wines, particularly red wines. Tannins are involved in the tactile sense of astringency in red
wines, they play an important role in the intensity and stability of color in red and tannins are responsible
in many cases for the brown color that develops with age in both red and white wines. The flavonoid
compounds that polymerize to form some types of tannins are also thought to provide many of the health
benefits associated with the moderate consumption of wines.
The grape berries themselves contain some tannin, particularly in the seeds and skins. Certain types of
tannins arise from polymerization of monomeric phenols during wine processing and aging. Wines aged
in wood cooperage or wines that have had exposure to oak dust, chips or staves will also extract some
types of tannins from the wood. Finally, pure or relative pure mixtures of tannins may be added to the
wine during processing or aging.
Tannins are polymers of simpler phenols, which can generally be classed as either flavonoid or nonflavonoid. The simplest of phenols is phenol itself, which consists of an aromatic ring and a single
hydroxyl group (Figure 1).
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Figure 1. Phenol
Catechin, (Figure 2), is an example of a flavonoid, in this case a flavan-3-ol. Flavonoids have a common
structure that consists of an aromatic “A” ring, an oxygen containing heterocyclic “C” ring and a second
Figure 2. Catechin, a flavan-3-ol
aromatic “B” ring. There are several classes of flavonoids that differ primarily in the structure of the
heterocyclic ring. From these and similar building blocks, tannins are polymerized. There are two
primary classes of tannins found naturally in grapes and wine. The first class, the hydrolysable tannins, is
based on non-flavonoid phenolics. The basic unit in a hydrolysable tannin is a polyhydroxy molecule,
Figure 3. Gallic acid
usually a sugar, to which are bound multiple non-flavonoid phenols, often gallic acid (Figure 3) or ellagic
acid, which is a dimer of gallic acid (Figure 4). Gallotannin (Figure 5) is an example of a hydrolysable
tannin. These tannins are readily hydrolyzed under acidic or basic conditions, hence their name. Nonflavonoid phenols and hydrolysable tannins are also found primarily in the skins and in the pulp.
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Figure 4. Ellagic acid
Figure 5. Gallotannin (“G” is a gallic acid unit)
The second primary class of tannins in grapes and wines are the condensed tannins. These tannins are
polymers of primarily flavan-3-ols (catechin and epicatechin), along with the anthocyanin pigments. If an
anthocyanin is incorporated into the tannin, the resulting polymer is often colored. As a covalent bond is
formed between the individual units within a condensed tannin, these tannins are not readily hydrolyzed.
Typically the linkage occurs between the 4 position (on the heterocyclic ring) and the 8 position on the
aromatic A ring, although it is possible for the linkage to occur at the 6 position as well.
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Figure 6. An epicatechin tetramer
In Figure 6, the tetramer consists of linkages just between the 4 position and the 8 position of the adjacent
epicatechin units. These polymers can include both catechin and epicatechin units.
The epicatechin and catechin monomers are found primarily in the seed coat in grapes, and to a lesser
extent in the skins. Polymers of these two compounds (condensed tannins) are found in the seeds, skins
and to a lesser extent, in the pulp. Monomeric anthocyanins and polymeric pigments are found primarily
in the skins of red grapes.
Polymerization of the condensed tannins continues as the juice is processed into wine and as the wine
ages. As the number of units in the polymers increases, their color changes from colorless to yellow to
brown. As the number of units increases, the solubility of the polymer decreases, and eventually the
polymer will precipitate from solution.
If the tannin incorporates an anthocyanin moiety, this
precipitation will result in slowly declining color as the wine ages.
As tannins are very complex compounds, common methods for determining their concentrations in grape
tissues or wines are often indirect or empirical in nature. Comparison of results from different studies can
be problematic.
Most analyses of tannins begin with an attempt to differentiate them from their
monomeric building blocks. This may be done either by a separation based on the larger size of the
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tannins or by selective precipitation or extraction. Once separated from the monomeric phenols, the
tannins may be analyzed by a number of available methods for the analysis of phenolic compounds. The
most widely used is probably the Folin-Ciocalteu colorimetric method. Gallic acid is used as the standard
for this method and results are reported as gallic acid equivalents (GAE). It is not possible, given the
complex mixture of tannins present in grape tissue or wine samples to convert GAE into an exact amount
of tannin, but GAE are never-the-less useful as a tool for comparisons amongst samples or treatments.
Using a low pressure chromatographic method for separation of non-polymeric and polymeric phenols,
Kantz and Singleton (1991) reported that grape seeds contained the highest amount of polymeric phenols
of the grape tissues they looked at, with polymeric phenols ranging from 21 to 27 mg GAE/g fresh
weight. Polymeric phenols accounted for 60 to 70% of the total extractable phenolic content of the seeds.
Grape skins contained much lower total and polymeric phenols and were much more variable, ranging
from 0.1 to 5 mg GAE/g fresh weight of skins; polymeric phenols accounted for from 6% to 43% of the
total extractable phenols in the skins. Thorngate and Singleton (1994) subsequently showed that most of
the phenolic material in the seeds was actually in the outer layers of the seed, with very little found in the
seed endosperm.
Of the total pool of phenolic material available in the grape berry, only a part is subsequently found in the
resulting wines. Incomplete extraction, adsorption with yeast lees and other solids, precipitation with
grape or yeast proteins and a host of other reactions all tend to limit the total phenol and polymeric phenol
levels in wine. Singleton and Draper (1964) studied the extraction of polymeric phenols from grape seeds
into wine; complete extraction of the tannin in the seeds should result in a tannin content of 200 to 400
mg/L. Under typical red wine fermentation conditions about half that amount is common. That level is
then further reduced by other reactions and by precipitation. They also showed that in the same length of
time, more complete extraction is favored by increased temperature and increased ethanol content.
References:
Kantz, K. and V.L.Singleton. Isolation and determination of polymeric polyphenols using Sephadex LH20 and analysis of grape tissue extracts. Am. J. Enol. Vitic. 42:309-16 (1991).
Singleton, V.L. and D.E. Draper. The transfer of polyphenolic compounds from grape seeds into wines.
Am. J. Enol. Vitic. 15:34-40 (1964).
Thorngate, J.H. and V.L. Singleton. Localization of procyanidins in grape seeds. Am. J. Enol. Vitic. 45:
259-262 (1994).
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