No. 5 September - October, 2003
Bruce W. Zoecklein
Department of Food Science and Technology
VPI & SU - 0418
Blacksburg, VA 24061
Web site: http://www.vtwines.info/
Table of Contents
|The Wine Lab of the Future||2|
I. Bleeding/Rosé 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 rosés. 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.
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, 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.
II. Malate-Reducing Yeast
This season we are evaluating a yeast which can significantly reduce the malic acid content. Reduction of malic acid can be a benefit, with immature fruit, in cool seasons, and with high-malic varieties, such as Norton. The advantage of using Schizosaccharomyces pombe for biological de-acidification is that it can reduce malate to ethanol, vs. MLF, which reduces malate to lactic acid and carbon dioxide. A high concentration of lactic acid can detrimentally impact a wine’s palate and flavor structure.
This recently-developed product contains encapsulated yeast in a double layer of calcium alginate. This allows for the Schizosaccharomyces pombe to stay within the capsules. After the appropriate malic acid reduction, the capsules are removed, and the fermentation is completed by inoculation with a standard strain of Saccharomyces cerevisiae.
We hope to have wines available for industry evaluation, at Winemaker Roundtable meetings after the harvest.
III. The Wine Lab of the Future
Advances in Infrared (IR) technology are changing wine analysis. Currently on the market are Fourier Transform Infrared (FTIR) systems which can measure metabolites such as sugars, ethanol, pH, TA, volatile acidity, and tartaric, malic, and lactic acids.
IR frequencies (wavelengths) result in the simultaneous absorption of radiation by the different components in a sample. Changes in the chemical composition of the sample will result in a change in the absorption characteristics.
We are now testing a system which uses NIR (Near Infrared) to test Brix, pH, TA, and other metabolites of the fruit, in a non-destructive way. The system can scan fruit on the vine!
We hope to relate some of our findings this winter and spring, at Winemaker Roundtable meetings.
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.
Nutrition is as important an issue for malic acid bacteria, as it is for yeast. After yeast fermentation, wine is a relatively-harsh medium for the growth of Oenococcus oeni, which has rather complex nutrient requirements.
Oenococcus oeni requires organic nitrogen, in the form of amino acids and peptides, and will not be supported by DAP. Additionally, alcohol, sulfur dioxide, high tannin levels, pesticides, and fatty acids can inhibit growth.
Yeast cells release B-complex vitamins, a reason why MLF is frequently easier immediately following yeast fermentation. Some yeast are more compatible with MLF for a variety of reasons, including the production of mannoproteins. Mannoproteins released from the yeast cell wall can help soften tannins. This may be an important yeast selection factor. Additionally, the binding of some tannins may reduce the tannin inhibition on the bacteria.
There are several nutritional supplements for bacteria on the market, which can help assure an even and complete MLF.
V. Enology Notes
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