Enology Notes

Enology Notes #103, June 20, 2005

To: Regional Wine Producers

From: Bruce Zoecklein, Head, Wine/Enology-Grape Chemistry Group, Virginia Tech

Subjects: Enological Tannins; Upcoming Meetings: American Society for Enology and Viticulture, Eastern Section; Norton Roundtable; Volatile Sulfure Compounds Technical Meeting.

1. Enological Tannins. Previous editions of Enology Notes have outlined and discussed the importance of oxidation-reduction potential (redox). It could be said that tannins are essentially redox buffers.

There are basically two chemical classes of tannins, hydrolysable and condensed. These two groups differ in nearly every characteristic, other than their ability to bind with proteins. Hydrolysable tannins bind with proteins by hydrophobic interactions. Condensed tannins bind proteins through hydrogen bonding. Grape skins and seeds contain only condensed tannins. Hydrolysable tannins are derived from oak wood or as an additive to wines.

Enological tannins available on the market may differ in a number of respects, including the following:

  • Extraction method
  • Purity
  • Processing method
  • Source, including wood, grape skins, and seeds
  • Toasting variation
  • Degree of oxidation

Tannins for wine addition can be derived from oak, chestnut, seedpods, etc. Most are water or steam extracted, dried, and milled. Different products undergo hydrolysis, pH and color adjustment, and sulfite addition, and may be finished by spray or freeze-drying.

There is a vast array of tannins on the market, and many are tailored to perform different tasks. Tannins are added for the following purposes and problem corrections:

  • Redox buffer
  • Raisined fruit
  • Sun-damaged fruit
  • Unripe grape tannins
  • Structural/textural, mouthfeel modification
  • Increased substrate for microoxidation
  • Limit the activity of laccase
  • Help to precipitate proteins
  • Help to modify aromas, including vegetative aromas
  • Help increase aging potential
  • Possibly to help stabilize red wine color

Many believe that an addition of condensed and hydrolysable tannins is best, depending upon the purpose. Regardless, post-fermentation use should involve careful laboratory fining trials. When conducting tannin fining trials, use a 1% tannin solution prepared in 10% ethanol (See Zoecklein et al., 1999).

The timing of tannin addition may be important, again, depending upon the purpose. Like many agents, their negative impacts are usually less with earlier addition. Adding tannins pre- or during the early stages of fermentation allows for integration with the other structural elements.

It should be noted that during pre-fermentation or fermentation addition, grape proteins might precipitate a portion of the added tannin. The degree of precipitation is dependant upon the grape variety and the season, among other factors. This is one reason why some use multiple additions during fermentation.

2. American Society for Enology and Viticulture, Eastern Section, annual meeting. The 2005 annual meeting of the American Society for Enology and Viticulture, Eastern Section, will be July 13-16 in St. Louis, Missouri, at the Millennium Hotel.

This year’s symposium will involve Norton, Pinot Gris, Traminette, and the cold-hardy Frontenac and La Crescent. Presentations will cover practical information and extensive sensory evaluations. For information and registration, see the website at http://www.nysaes.cornell.edu/fst/asev/.

3. Norton Roundtable. We will conduct a Norton Roundtable July 25, 2005 at Chrysalis Vineyards at 1:30 PM. This will supplement the information provided at the American Society for Enology and Viticulture, Eastern Section meeting in St. Louis, MO, July 13-16, see above.

The following is an outline of some Norton and other red wine production considerations. Many of these topics are covered in more detail in other Enology Notes issues, which are indexed and posted on my website at www.vtwines.info.

Unique Features of the Norton Grape:

  • low tartaric/malic ratio
  • high concentration gallates and diglucoside pigments and mix
  • surface to volume ratio, berry size and seed #, volume/size

Production Challenges:

  • structural/textural balance
  • The seven components of importance in structural/textural balance include: volume (body), acidity, roughness, tannin intensity, astringency, dryness, and bitterness.

Structural/Textural Quality is Impacted by:

  • the quantity and quality of phenolic components in the fruit,
  • the interaction and stabilization of anthocyanins, tannins and certain polysaccharides, and
  • TA and the sense of sweetness.

Structural balance can be viewed as follows:

Sweet/body <--> Acidity + Bitterness and Astringency

In this relationship, the sweet elements of a wine (derived from alcohol, polysaccharides, etc., must be in relative balance with the sum of the acidic elements and phenolic elements (both astringency and bitterness).

Thus, the higher the TA, the lower the desirable phenolic load for balance. A wine with a high level of sweet elements (e.g., alcohol, fermentation and wood polysaccharides) can support a higher level of TA and phenolic elements.

Varietal Aroma Descriptors for Norton: spicy, jammy, earthy, dusty, black pepper, black raspberry, mushrooms, spice, cloves, nutmeg, tobacco, chocolate.

What vineyard management tools are available to influence these aroma/flavor descriptors and phenol elements?

How can we monitor the quantitative and qualitative changes in aroma/flavor and phenols in the fruit and the wine?

What are appropriate concentrations and proportions of aroma/flavor phenols for specific wine styles?

What production methods are available to reach stylistic goals?

Norton Style. Style and textural quality are influenced by quantity and quality of phenols, phenolic maturity, extraction/management, and interaction with other structural elements (acids, polysaccharides, peptides, sugars).

Canopy Management

  • optimum degree of fruit shading
  • optimum degree of leaf shading
  • influence of canopy management on structural balance, e.g. TA, tartaric/malic acid and phenols
  • influence on asynchronous ripening
  • influence of row orientation

Vine Stress

  • soils and soil pH
  • Mg++ nutrition vs. soil concentration
  • monitoring nutritional status
  • correcting Mg++ deficiencies
  • crop load

Defining Optimum Crop Load

  • vine balance
  • avoid delaying maturity

Fruit Ripeness Indicators

  • goal driven
  • indicators should correlate to aroma/flavor and phenol suppleness
  • fruit sampling methods
  • degree of asynchronous ripening: clusters, berries. Sugars are usually fairly uniform (small coefficient of variance), secondary metabolites can be much greater
  • sensory evaluation of skin, stem, pulp, and seed phenolic maturity
  • chew skins, use phenol descriptors (see Mouthfeel Wheel)
  • seed maturity - color, texture and brittleness
  • anthocyanin content and maturity
  • evaluate crushing ability
  • relationship between berry weight, shriveling, and engustment
  • relationship between berry weight and physiological maturity
  • berry softness and diffusion of pulp
  • estimation of berry size (Enology Notes #23)
  • measurement of color
  • sugar per berry
  • relationship between days post-bloom and maturity
  • seed numbers and seed maturity
  • pH, Brix, TA (Enology Notes #6). Brix and aroma/flavor, phenol development are not the same. Changes in pH are not necessarily a function of berry ‘age’, and relate to K+ and vigor. Tartaric/malic ratio low
  • Brix to alcohol conversion rate
  • phenol maturity, TA, and potential structural balance


De-stemming/crushing/whole cluster press

  • planning maceration - know the extent of mechanical resistance from various parts of the fruit
  • importance of gentle fruit handling, low non-soluble solids to wine integration
  • degree of berry breakage

Cap Management Procedures

Cold Soak

  • enhance ‘bright fruit’ character, increase color stability
  • time/temperature
  • w/o enzymes
  • w/o sulfur dioxide
  • w/o oxygen


  • bleed juice has lower arginine/proline ratio which will influence pH evolution


  • This may be the most important varietal in Va. for this processing method. Grape seed tannins differ from skin tannins, in that they contain a higher concentration of monomeric flavan-3-ol, and those which have esterified to gallic acid. As such, they are very harsh and bitter. Délestage can remove as much as 40% of these seed tannins.


  • Brix to alcohol conversion rate/Chaptalization
  • alcohol and structural balance must TA,
  • tartaric/malic ratio, pH
  • TA reduction methods
  • tannin addition, can add mid-palate depth
  • fermentable nitrogen, not a matter of simple fermentation sticking
  • yeast(s), high malate reducer, high polysaccharide producer, high tannin absorber, fermentation rate
  • sulfur dioxide, polyphenol oxidase, phenol polymerization
  • role of oxygen
  • size and shape of fermentation tank
  • open vs. closed tank
  • temperature of liquid and cap
  • MLF strain(s), timing, volume of inoculum
  • w/o wood
  • punch down, pump over, irrigation systems, sweeper tanks, délestage, alcohol at time of dejuicing
  • post-fermentation maceration
  • free run vs. press run

Yeasts. Species and strain have a significant influence on grape and wine phenols, as a result of binding and polysaccharide liberation, and reduction in TA. Desirable strain features include 1) low color absorption, 2) high production of manno-protein polysaccharides available during fermentation and autolysis, 3) malate reducer, and 4) low production of volatile sulfur compounds (some of which can enhance the perception of astringency).

Cellar Management

  • wood(s): sources, seasoning age, fill age, barrel type, etc.
  • lees, very helpful for structural integration
  • sulfur dioxide, keep low (15 mg/L free) for first year
  • oxygen, first year process oxidatively, thereafter anaerobically
  • measure oxygen pick-up

Maceration enzymes may be desirable, particularly if you have a lot of whole berries in the fermenter. Enzymes include pectinase, hemicellulases and cellulases which, like native enzymes, aid in the diffusion and association of anthocyanins, tannins, and polysaccharides.

Cap Management should occur to limit seed tannin extraction and the formation of non-soluble solids. Process to encourage phenolic polymerization and stabilization (oxygen exposure and low sulfur dioxide). Délestage reduces seed extraction, and enhances enzyme activity and oxidative polymerization.

Post-fermentation microoxygenation. This technique is ideally suited to a variety such as Norton, which has a high concentration of hard/harsh tannins.

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Dr. Bruce Zoecklein
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