To: Regional Wine Producers
From: Bruce Zoecklein, Head, Enology-Grape Chemistry Group, Virginia Tech
Subject: Red Wine Structure.
In a difficult season such as this, it is important to review red wine structure.
The structural features of red wine include those discussed and reviewed during this summers short course with Dominique Delteil: volume, acidity, tannin roughness, astringency, bitterness, and dryness.
This continues the discussion of processing modifications to positively impact wine structure and aroma/flavor (see Enology Notes #81).
The limitations on potential alcohol and the unevenness of fruit development this year can have a negative impact on most red-wine palate components. For the most part, we are averaging at least 2 degrees Brix lower than normal. The alcohol content impacts the above listed structural elements. Therefore, any chaptalization must be done with an understanding of the quantitative and qualitative nature of extractable phenols and acidity.
Depending upon the degree, there can be a difference in body between wines produced by sugar chaptalization vs. grape concentrate.
1. Carbonic Maceration. Carbonic maceration is a French technique in which part of the fermentation is conducted using anaerobic metabolism. Some producers are using this method this season to create blending wine. Carbonic maceration has the advantage of producing a different array of volatile compounds which provide floral intensity. Structurally, these wines can be relatively high in acidity, due to the limited phenol concentration.
This production method should only be conducted using clean-sound fruit, with an actively growing yeast starter and carbon dioxide displacement gas. Volatile acidity levels should be carefully monitored. Additional information is available through my office.
2. Bleeding/Rose Production. As a result of the wet weather this season, many of you will, or have, conducted tank bleeding, or whole-cluster pressed red grapes to produce roses. Some will also bleed, to help to concentrate and increase red wine body and structural depth. Red juices produced by bleeding or whole cluster pressing do not always ferment to dryness, and can have a tendency toward reductive notes and/or are difficult to get to complete MLF.
Amino acids are not equally distributed in the grape berry. For example, with mature Cabernet Sauvignon, about 8.5% of the total are in the seeds, 15% in the skins, and 77% in the pulp. The separation of the pulp juice, as occurs with bleeding and whole cluster pressing, has a significant qualitative influence on fermentable N.
The two amino acids present in the greatest concentration are proline and arginine. Proline cannot be used by the yeast, while arginine can. Indeed, because it has four atoms of N per molecule, arginine is a very good source of fermentable N.
In the case of most red varieties, the ratio of arginine to proline is much greater in the skins than in the pulp. In other words, the pulp juice associated with bleeding and whole cluster pressing has a relatively high concentration of proline (approximately 55%) which cannot be used by the yeast, and a small concentration of the more potent amino acid arginine, and others needed to carry out a healthy fermentation.
Wines so produced should be given a higher concentration of supplemental fermentable N, in the form of amino acids and vitamins, than is required to ferment the juice remaining in contact with the skins. Do not simply dump in DAP. Too much DAP can lower the production of esters.
3. Sulfur Dioxide. The proper use of sulfur dioxide is critical this season. Too much sulfur dioxide (greater than 30 mg/L or so, depending upon the juice pH) will bind phenols and not allow their polymerization, possibly resulting in an increased perception of roughness, astringency and bitterness. Additionally, sulfur dioxide binds thiamine (vitamin B1), which is required by yeast. Elevated incidences of fruit rot should be addressed by fruit culling and a larger yeast inoculation.
4. Thermal Vinification. Methods we have evaluated for modifying the phenol structure of red wines include Microoxygenation and Thermal vinification, or the use of heat.
We have conducted several studies using heat as a winemaking tool for structural modification. In Cabernet Sauvignon must, total glycosides (grape-derived aroma/flavor) rose during fermentation, while skin concentrations dropped. Wines were heated to 42°C post-fermentation, prior to dejuicing to, and held for one day. Thermal vinification resulted in higher total (12%) and colored (18%) glycosides. Large polymeric pigments rose 208%, and small polymeric pigments rose 41%.
The increased degree of polymerization (large polymeric pigments) resulted in a wine which had a much higher volume or body, and lower tannin roughness, astringency, and bitterness. We are exploring the impact of short-term wine heating post-fermentation.
Thermal processing appears to be a viable means for structural modification, that may have a place in Virginia winemaking, certainly in seasons such as 2003.
5. MLF. This season, it may be essential that wines undergo MLF. For many red grape varieties, the acidity coupled with immature tannins will have a tendency to unbalance the palate. I would suggest the addition of a commercial MLF supplement at dryness and before bacterial inoculation.
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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