Grape and Wine Tannins and Phenolics – Their Roles in
Flavor, Quality and Human Health
Leslie A. Weston
Department of Horticulture
Cornell University
Ithaca NY 14853
Leslie Weston, Department of Horticulture at Cornell University. Leslie has worked at Cornell
for 1 1/2 years, with a research and extension appointment. Leslie's research focuses on
bioactive natural products including those with pesticidal and phytonutrient activity. Her
research is concentrated in 3 cropping systems including turf, ornamentals and grapes.
Leslie came to Cornell from the University of Kentucky, where she worked as a
research/teaching faculty in the Department of Horticulture as well for 12 years, and assisting
the fledgling wine industry in this state. In 1997, she spent a year on sabbatic leave in Dijon
France at the Institute Jules Guyot for Wine and Vine Science. She worked on molecular and
biochemical control of production of bioactive stilbene compounds, including resveratrol,
and their roles in plant defense against botrytis. Since 1998, Leslie has worked with Pascal
Durand and Thomas Henick Kling on the development of quality pinot noir wines in the Finger
Lakes region of NY State.
Content in Wine.
From an organic point of view, wine is a hydroalcoholic acid solution containing
various phenolics and other aromatic components (Lubbers and Voilley, 1997). Wine
can contain over 1000 different chemical constituents, making it a very complex
solution, generally consisting of 85-90% water, 10-14% alcohol and 1-5% phenolics. It
also contains much smaller amounts of glycerol and sorbitol, acids primarily in the
form of tartaric, malic, lactic and citric acids (Johnson and Halliday, 1992) as well as
minor
concentrations
of
polypeptides,
amino
acids,
mineral
salts
and
enzymatic/metallic catalysts. Although certain of these components are present in
small quantities, they are presumed to play important roles in the sensory properties
of the resulting wine products (Lubbers and Voilley, 1997). Wine quality is often easier
to sense than to describe or chemically analyze. The balance and harmony
encountered in a wine commonly refer to the taste, mouthfeel and flavor sensations
generated by the presence of phenolics, acids, alcohol and other aromatic
components.
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This paper will focus solely on the role of phenolics including tannins in wine flavor
and quality and their implications for human health as well. A phenolic compound
in wine can be defined as one that is derived naturally from plant or microbial origin,
consisting of a phenyl ring backbone with many substitutive possibilities. Some
phenolics like coumaric, caffeic, ferulic and vanillic acids are relatively simple while
others are more complex polymeric structures such as the tannins, which contribute
strongly to the mouthfeel, quality and palatability of red wines. Phenolic compounds
are commonly found in animal, plant and microbial tissues and contribute to
defensive strategies as many are involved in plant protection as biologically active
growth inhibitors of other living systems. In addition, they contribute to color and
flavor of food items including wine. Since many have strong antioxidant activities,
they also play important beneficial roles in mammalian systems. Red wine
consumption has been clearly linked to reduced incidence of atherosclerosis and
heart disease in humans, most likely due to the presence of numerous bioactive
phenolic compounds in red wine products.
Factors influencing tannin and phenolic content in grape berries.
The content of phenolics in mature grape berries will ultimately influence the flavor
and quality of the resulting wine product. Many factors, environmental and cultural,
will influence the content and quality of phenolics within the grape berry itself. The
highest concentration of complex tannins and other polyphenolics are found in the
skins and seeds of the grape. If the yield of grapes is strongly augmented due to
cultural practices or location of production, the concentration of phenolics in these
grapes is likely to be reduced and less concentrated than desirable (Peyron, 1997).
Simple polyphenols which are present in relatively high proportion in mature grapes
and grape juice include: catechins, epicatechins, gallic acid, flavonols, cinnamic
acids and hydroxycinnamates, and quercetin. Humans ingest over 1g/day of these
types of polyphenolics by consuming food products. Polyphenolics can play
important
health
roles
as
phytonutrients
(Kinsella
et
al.,
1993).
Complex
polyphenolics, in particular the anthocyanins and tannins, are found principally in
solid particulates associated with grape skins and seeds. These compounds possess
many important properties which influence the color and color stability of wine
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besides influencing mouthfeel, depth and astringency (Saint-Cricq de Gaulejac et
al., 1998). These complex structures change over time; with increased storage time,
becoming more complex due to increased polymerization. These compounds can
combine with numerous substances including polysaccharides, proteins, and other
polyphenols to influence wine color, stability and flavor over time.
Numerous factors are important in increasing polyphenols and tannins in grapes as
well as the wine products generated. Increased sunlight and moderate fertility
support vine growth and cluster formation and lead to increased formation of
secondary plant products including phenolics such as quercetin. Quercetin
presence is an excellent indicator of sunlight available to the developing grape
clusters (Price, 1997). In certain cases, too much sunlight can impact quercetin
content and the result is a fine sediment of particulates (mainly quercetin) which
drop out of solution in full-bodied red wines. A Sangiovese produced on a southern
exposure in California with high light intensity developed sedimentation problems as
quercetin accumulated and became insoluble in the wine itself. One solution to this
problem would allow more foliar protection to shield the developing grape clusters.
Ample, but not excessive moisture, as well as adequate but not excessive fertility will
also result in higher concentrations of phenolics and tannins in red wine grapes. A
smaller more concentrated harvest will also result in increased concentrations within
the harvested grapes, in comparison to an excessively large harvest with diluted
flavor and aroma components. Disease pressures can result in reduced berry
quality, leading to reduced phenolic content over time in harvested grapes. Finally,
cultivar choice will also influence phenolic levels in resulting wine products. Full
bodied red wines generally contain higher levels of polyphenolics and tannins, and
are deep red in color due to the presence of anthocyanins. Wines which have
higher tannin content typically include Cabernet Sauvignon and Franc, Sangiovese,
Zinfandel, and Bordeaux style reds.
Impact of enological practices on polyphenolics.
One may ask, is a particular amount of tannins or polyphenolics more desirable in
mature grapes? Just like wine quality, the answer to this question is also subjective.
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One strives to produce a wine with enough body, structure and flavor components
to be interesting and well-balanced, but not overpowering. Too much astringency
provided by complex tannins will not necessarily disappear over time with bottleaging. Peyron and Gerbaux (1997) in Burgundy France have collaborated to study
phenolic, anthocyanin and tannin content in Pinot Noir wines and correlated these
amounts with consumption quality to try and determine optimal time of harvest,
length of maceration, temperature of fermentation and type and length of oak
storage on Pinot Noir quality. Typically, seeds of pinot noir grapes have ten times as
much tannin as the skin. Skins contain variable concentrations of anthocyanin and
tannin which are altered depending on the production year and climatic conditions
encountered and the site of production. Certain general guidelines can be
considered when selecting and pressing grapes for quality red wines such as
Burgundian Pinot Noir. In particular, harvest grapes when mature but not
overmature to maximize tannin and polyphenolic quality. Press grapes in a timely
fashion after harvest and do not allow them to overheat before pressing to maintain
optimal phenolic quality. Be especially careful to control disease problems in the
vineyard before harvest. Fungal organisms such as Botrytis cinerea produce laccase
enzymes which degrade complex polyphenolics and reduce flavor and color
development in a young wine when present (Cavin and Divies, 1997).
Since the greatest concentration of phenolics occurs in the seeds and stems, if one
could destem and even remove seeds, one could minimize the concentration of
complex green tannins and astringency which results from excessive extraction of
these plant parts. Since it is impossible to remove seeds, destemming is generally
practiced when one desires a fruitier wine with less complex astringency. In Pinot
Noirs, whole cluster fermentation requires additional time and is often used in
Burgundian Pinot Noirs, but adds additional complexity to the wine (Johnson and
Halliday, 1992). Full crushing and destemming tends to produce good color, but
often results in a simpler wine with less complexity due to absence of complex
tannins. By macerating before fermentation, one can extract additional color and
tannin from the pomace. The fermentation conditions will also influence phenolic
and tannin content in the resulting wine (Peyron, 1997). In the case of Pinot Noir,
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increased fermentation temperature, especially towards the end of fermentation
period, will result in enhanced color and aroma. This is true for many red wines in
general. The choice of yeast strain will impart different chemical and flavor
characteristics in wine. These differences seem to be intensified in years when less
tannins are extracted naturally because growing conditions were less than optimal
(Watson, 1997). In years with high color, yeast cultures seem to have less effect.
Interestingly, yeast themselves may act as fining agents to remove complexes of
polyphenolics from red wines, by allowing pigment adsorption and promoting
eventual precipitation of polyphenolics.
The greatest impact of yeast on color
enhancement seems to occur with whole cluster fermentations. Pumping over and
stirring also will increase skin contact in red wines and enhance phenolic content.
Post fermentation maceration in reds will result in enhanced extraction of soft
tannins, if greater complexity is desired (Johnson and Halliday, 1992 and Peyron,
1997).
The presence of yeast and bacteria or other fungal organisms in wine and must has
a definite impact on wine quality and flavor and directly influences the metabolism
of phenolics in wine. Free acids in wine and must are susceptible to metabolism by
these microorganisms (Cavin and Divies, 1997). The products produced due to
metabolism of phenolic free acids include aromatic molecules such as vinyl or ethyl
guaicol, and production of aromatic phenolics including vanillins and other
aromatics. In low quantities, these compounds can have positive impacts on flavor
and wine aroma but in higher concentrations, the presence of these compounds
can be detrimental and overpowering.
Influence of oak and aging on polyphenolic content.
Much has been published on the influence of oak on aroma and flavor of wine
through influence on phenolics present in aged wine. Flavor components of oak
include tannins and phenolics, such as vanillin, which are released into the wine
after storage in oak containing these phenolics. Wine character is influenced by the
type of oak barrel selected, species of oak used, age and preparation of the oak,
including firing finish. Oak barrels contribute up to 18 different phenolic compounds
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to the wine, each which has a distinct impact upon flavor and mouthfeel. Older
barrels which have been heavily utilized for wine storage release fewer flavor
components to the wine. Depending upon preference, one may choose to use new
oak barrels for more intense flavoring or older oak barrels for more subtle flavor
extraction. Much has been published regarding the use of French oak versus
American oak on flavor of Chardonnay wines. Distinctive differences in the
chemical components occur in different oak species and locations of productions
and these are correlated with subsequent tannin and phenolic presence in wine
products (Gump, 1999). The recent symposium proceedings on oak – from forest to
glass presented by the ASEV/ES does an excellent job of covering this subject matter
thoroughly. Several factors play important roles in influencing amounts of
extractable tannins and phenolics in oak. These include degree of toast which
influences the spicy and smoky aromatics imparted to the wine. The growth rate of
the oak tree itself influences phenolic content in the staves. A slower rate of growth
is associated with higher quality phenolics present in the barrel stave and resulting
wine. Smaller barrels impart more phenolic and tannin content due to increased
surface area exposure and extraction. Greater seasoning and longer storage of
barrels results in softer tannins and fewer green tannins in the wine product. Tannins
tend to polymerize with increased aging, leading to less astringency and greater
softness.
The aging of red and white wines results in altered and reduced phenolic contents
in the wine product. As tannins and anthocyanins interact with oxygen,
encountered during barrel storage due to permeability of the barrel to oxygen,
these compounds further polymerize and become less astringent. Red wines
become lighter in color and tannins and polyphenolics eventually aggregate into
larger molecules which accumulate as sediment over time at the base of the bottle.
In contrast, white wines often deepen in color, turning darker honey colors as they
oxidize and age.
Measurement of polyphenolics in wine.
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The presence of phenolics and tannins in the wine product has a definite impact on
wine flavor and overall quality. This effect is often hard to characterize chemically
and sensorially, but the impact is great. Phenolics influence taste, odor, color and
clarity. These organic acids, phenolics and mineral salts are less than 2% of the wine
product itself, but have a dramatic influence on sensory quality. We now have
access to formidable tools for measuring phenolic levels and correlating them with
sensory quality. In the past, gas chromatography was used exclusively to measure
volatile constituents in wine. Besides gas chromatography, we can now utilize high
pressure liquid column chromatography to measure
monomeric and polymeric
phenols including all major components in wine (Price, 1997). Monomeric and
polymeric anthocyanins are important for wine coloration and their presence
influences the solubility of other phenolics present in wine. Polymeric phenols and
tannins influence mouthfeel and their presence can be easily measured by HPLC as
well. Steve Price has developed a complete analysis of wine and polyphenolic
constituents including analysis for wine color and flavor components. Spider
diagrams can be utilized to evaluate presence of certain important constituents in a
wine product and these results can be compared to sensory panel results from the
same wine product.
Health benefits associated with phenolics in wine.
As discussed earlier, many of the simpler phenolics present in wine products,
especially red wines, are associated with beneficial effects of wine consumption on
human health. Atheriosclerosis and heart disease are now associated with 50% of all
mortality in the western world today. Great attention has been drawn to the the
fact that red wines have a dramatic impact on incidence of atheriosclerosis and
coronary heart disease in small mammals as well as human studies.
Wine
consumption in the western world is strongly correlated with reduced incidence of
heart disease (Wine Institute, 1997). In fact, Americans consume among the lowest
levels of wine per day and have the greatest incidence of heart disease, with only
Scotland and Finland having higher heart disease incidence. In terms of antioxidant
phenolics present in wines, epicatechin and isomers as well as quercetin are present
in levels of up to 150 mg/ L of wine. Higher levels occur in red wines. Resveratrol is
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also present, especially in red wines such as Zinfandel, Pinot Noir and Cabernet
Sauvignon, but in much lower concentrations than others such as epicatechin. Each
of these phenolics is at least 10 to 20 times more effective than vitamin E in
protecting low density lipoproteins against oxidation, preventing the buildup of
damaging cholesterol in arteries (Frankel, et al., 1993; Folts et al., 1996). Red wines
contain much more flavonoids and phenolics than white wines. Greater reductions
in atheriosclerosis was noted in rabbits and humans consuming red wine rather than
white wine or other liquor products. Moderate daily consumption of wine has been
associated with 40% decreases in human mortality. Other studies of interest include
work done by Jang et al (1997) who showed that resveratrol which is plentiful in red
grape skins and wine has strong cancer chemopreventative activity as well. This
work was done on cancer prone mice. Other compounds such as quercetin and
trans-resveratrol have been linked with a reduction of breast cancer cells as well.
Other research has shown than phenolics in wine are indeed absorbed into the
bloodstream, raising serum antioxidant content in the blood (Wine Institute, 1997).
In conclusion, the presence of phenolics and tannins in grapes and wine products
have dramatic effects on wine flavor, quality and storability. Both viticultural and
enological practices have important influences on resulting concentrations of
tannins and polyphenolics in the subsequent wine products. These compounds can
also play important roles as antioxidants and cancer preventative agents in humans
and other mammalian systems. Further research on the influence of both viticultural
and enological practices on polyphenolic content as it relates to quality and
sensory characteristics in wine is now underway, given the importance of these
structurally diverse chemicals.
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References:
1. ASEV/ES – Oak From Forest to Glass, Practical Management of Oak and Wine. July 1999.
St. Louis, Missouri.
2. Cavin, J.F., L. Barthelmebs and C. Divies. Metabolisme des acides cinnamiques substitues
par les microorganismes et production de phenols volatils. Caracterisation moleculaire
de quelques activites decarboxylases. 1997. 1st International Colloquium BurgundyCalifornia-Oregon, Dijon, France.
3. Folts, D. et al. Red wine, atherosclerosis, and coronary thrombosis. Wine in Context:
Nutrition, Physiology and Policy. Wine and Health Symposium, Reno, Nevada. 1996.
American Society for Viticulture and Enology.
4. Frankel, E. et al. Inhibition of oxidation of human low density lipoprotein by phenolic
substances in wine. The Lancet, 1993; 341:454-457.
5. Gump, B.H. Oak wood constituents: their extraction and contribution to wine flavor.
ASEV/ES – Oak From Forest to Glass. July 1999. St. Louis, Missouri.
6. Jang, W. et al. Cancer chemopreventative activity of resveratrol, a natural product
derived from grapes. Science, 1997; 275:218-220.
7. Johnson, H. and J. Halliday. Vintner’s Art. 1992. Simon and Schuster Press. New York, NY.
8. Kinsella, J. et al. Possible mechanisms for the protective role of antioxidants in wine and
plant foods. April 1993. Food Technology Magazine.
9. Lubbers, S. and A. Voilley. Flavor-matrix interactions in wine. 1997. 1st International
Colloquium Burgundy-California-Oregon, Dijon, France.
10. Peyron, D. Les composes phenoliques du pinot noir. 1997. Objectif Qualite, 5 th
Professional Colloquium of the Wines of Burgundy; 155-160.
11. Peyron, D. Les composes phenoliques du pinot noir de Bourgogne. 1997. 1 st International
Colloquium Burgundy-California-Oregon, Dijon, France.
12. Price, S. Developing analytical tools for winemakers: wine phenolic compounds and
color. 1997. 1st International Colloquium Burgundy-California-Oregon, Dijon, France.
13. St. Cricq de
Gaulejac, N., Y. Glories
and N. Vivas. Recherche des composes
responsables de l’efft antiradicalaire dans les vins. J. Sci. Tech. Tonnellerie, (4), 147-161.
14. Watson, B. Fermentation Processing Effects on Anthocyanins, Phenols, Aroma and Flavor
of Oregon Pinot noir Wines. 1997. 1st International Colloquium Burgundy-CaliforniaOregon, Dijon, France.
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Institute.
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and
Education
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Specific
Benefits.http://www.wineinstitute.org/res ed/wine spec rsch/index.htm. 3/2000.
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