Vol.17, No. 3 May - June, 2002
Bruce W. Zoecklein
Department of Food Science and Technology
VPI & SU - 0418
Blacksburg, VA 24061
Web site: http://www.fst.vt.edu/Zoecklein
Table of Contents
|Environmental and Viticultural Conditions||2|
|Delestage, as Structure/Texture Tool||2|
I. Red Wine Structure/Textural Quality
Phenolic substances influence appearance, color, the sense of acidity, body, astringency, bitterness, oxidative-reductive properties, color stability and aging potential of wines. We now have a greater understanding of the quantitative and qualitative nature of tannins and phenols than ever before, and of the importance of a desirable structural/textural profile. Such a profile has a high volume or body in the fore mouth, contains fine-grained (not dry, dusty or hard) tannins of limited intensity, and has a finish with the absence of bitter tannins. A positive tannin profile is obtained only with a viticultural and enological understanding of tannin management.
Now is the time to review the phenol profile of wines produced last season and to be thinking about the important issues influencing phenols for this upcoming season. What are those issues? What vineyard management tools are available to influence phenols? How is your vineyard management impacting phenols? What are the key processes that affect phenols in the wine and their modification during processing? What are appropriate concentrations and proportions of phenols for specific wine styles?
It is important to fully understand the sensory significance of phenols. All phenols are both bitter and astringent. Bitterness is the result of access to membrane bound receptors and is likely limited by molecular size. Generally, polymerization or binding together increases astringency to a point, due to an increased number of possible hydrogen binding sites. Monomeric flavanoids (relatively low molecular weight single flavanoid phenols) are primarily bitter; however, upon polymerization, their astringency increases more rapidly than their bitterness (Noble, 1998). The relative proportion of bitter to astringent phenolic compounds is important with regard to textural quality. The ratio of bitterness to astringency helps to explain the concept of hard vs. soft tannins. Grapes yielding intense and balanced wines are also characterized as having a high anthocyanin to tannin ratio. If there is insufficient color (anthocyanins), there will not be enough for bonding with non-colored phenols such as tannins. There is never a disadvantage in getting the darkest color possible, but there is a notable disadvantage in having too much tannin.
The interaction of wine structural components, including phenols, is also an important feature governing wine texture as a function of the interaction with other structural elements according to the following relationships: Sweet Acidity + Bitterness and Astringency. This is analogous to the European Suppleness Index: Alcohol-(Acidity + Tannin). See Zoecklein et al., 1999, for additional information. As pH increases, the perception of astringency decreases while bitterness is unchanged. Increasing the alcohol concentration increases the perception of bitterness and reduces astringency. Astringency masks bitterness. This is an extremely important concept as it relates to both aging and fining (see Zoecklein, 1988).
Environmental and Viticultural Conditions:
Important environmental and viticultural conditions influencing grape phenols include: macro, meso and microclimate, seasonal variation, maturity (pulp, skins, seeds, stems), berry size and yield (leaf area per fruit weight ratio). From a practical standpoint, the most important viticultural factors influencing phenols are sun exposure, fruit maturity and berry size. Each of these issues has been elaborated in previous editions. Sun exposure, to a large degree, is controlled by the viticulturist. It is important to note that solar exposure can dramatically increase the phenol concentration of the fruit. This increase is in the form of flavanol glycosides which are particularly hard and harsh. These add significantly to the perception of dry, dusty tannins. Given this fact, is it desirable to use the same degree of fruit-zone selective leaf removal for both sides of the grapevine canopy? The answer depends somewhat on row orientation. North-south running rows have a significantly different solar profile on the east versus west side of the canopy which certainly impacts grape phenols.
Berry size has a important potential impact given that many of the phenols are located just underneath the skin cell surface. Changing the surface to volume ratio can change the phenol concentration of the resultant wine. This has been elaborated in previous issues, and was one of the issues evaluated by one of our students, Danielle LoGiudice, who evaluated the use of the GA growth regulator Apogee.
Perhaps the biggest issue with regard to the wine's phenolic profile is fruit maturity. This includes the maturity of all fruit components including skins, pulp, seeds and cap stems. For more information on this subject see the Enology-Grape Chemistry Group web site under Maturity Analysis for Growers.
Delestage, a structural/texture tool:
We have been evaluating a method of minimizing the impact of immature seed tannins for several years. We have been using a rack and return system (Delestage) which helps deport a certain percentage of seeds during the early part of fermentation. Our Delestage research has been discussed and outlined numerous times in this series over the last few years. The original premise of this work was that the entire berry goes into the red wine fermenter. Given that seed phenols are not always mature in Virginia-produced grapes, given that these immature phenols can add some harshness to red wines and given that approximately 60 percent of the phenols in a conventionally produced wine come from the seeds, it was logical to evaluate methods to abate the problem. Delestage involves at least two features which impact the structural/textural nature of wine, seed removal and oxidative polymerization. Some of our research results have been previously reported and I have encouraged industry members to use this processing method. The following provides some additional research updates.
With each of our trials, the fruit has demonstrated a different pigment profile than the subsequent wine. An example is provided in Figure 1. This shows the relative concentration of three pigment classes, monomeric anthocyanins (bleachable pigments or BP),
Figure 1. Pigments in Merlot Fruit and Wine
small polymeric pigments (SPP) and large polymeric pigments (LPP). As can be seen, the only one of these classes of pigments which is in greater concentration in the wine vs. fruit are the LPP's. Figure 2 compares the concentation of tannin, monomeric or bleachable pigments, LPP and SPP, of Delestage and conventionally produced wines. As can be noted, the concentration of tannins is lower in the Delestage wines. This would be expected due to the removal of seeds. Monomeric pigments are also reduced in the Delestage wines (by an average of 11.5%), likely due to their incorporation into LPP's. Following fermentation, the concentration of large polymeric pigments (LPP) in the wine is consistently higher than the fruit and consistently higher in the Delestage-produced wines (averaged 38% higher than controls).
Despite a reduction in monomeric anthocyanins, Delestage wines are not lighter in color. The incorporation with other phenols adds spectral color which appears to offset reductions in anthocyanins. The incorporation into large polymeric pigments adds to color stability. Additionally, the results illustrated in Figure 2 help to explain the sensory responses panelists provide about these wines. Such wines are perceived to be much more rounded and supple.
Figure 2. 2000 Delestage
Because Delestage also involves oxidative polymerization, we have evaluated the impact of this procedure on wine volatile components. Figure 3 demonstrates the impact of this production practice on selected volatile compounds. Those listed under Delestage are present in significantly higher concentration than in control wines. These listed under Control in Figure 3 are present in significantly higher concentration than in wines produced by Delestage. The most sticking differences are that wines produced by Delestage generally have a higher concentration of esters. These compounds contribute to the fruit notes' in most wines. The control wines have higher concentrations of long chained alcohols, also know as fusel oils. This volatile profile is typical and helps to explain the floral difference between wines produced Delestage vs. conventional long-vatting.
Figure 3. 2000 Delestage Significant Differences
It appears that this processing technique can have a major impact on the sensory features of wine. Because the phenol load is reduced, a wine can hold a much higher concentration of acidity and still be balanced. For this reason, this is an important production tool for grapes such as Norton. Additionally, due to the reduction in hard seed tannins, grape tannin can be added to increase the depth or carpentry of the wine, again while maintaining structural balance. The long-term impact on color stability and aroma is still under investigation.
As has been outlined in previous editions (Vol. 12, No. 2, 4; Vol. 13, No. 2, 6, 7), phenols are also impacted by degree of berry breakage, non-soluble solids, cold soaking, cap management strategies, tank geometry, use of enzymes, sulfur dioxide, fermentation temperature, alcohol at the time of dejuicing, and specific yeast strains. The role of phenolic compounds in wines is complex and diverse. The mouth feel, balance, color and aging potential are all stylistic trademarks. The ability to attain specific stylistic red wine goals is dependent upon the understanding of phenols and their complex interactions.
II. The Virginia Wine Guide
The Virginia Wine Guide is now available on-line at www.virginiawineguide.com. This publication evaluates VA wine, discusses vintages, food and wine pairing, etc. Check it out; it is well done.
III. Student Award
The Italian Food Packaging Association recently selected Bridget M. Archibald to receive an award. Bridget is a masters candidate in the Enology-Grape Chemistry Group at Virginia Tech.
Fourteen U.S. colleges and universities participated in the competition. Students were required to write a paper on Advances in Food Packaging. One winning paper from each school was then sent to the Italian Food Packaging Association for final selection. Six winners were chosen nationwide. The award provides a two-week, all expenses paid trip to Italy to tour packaging facilities. Archibald's paper, Advances in Wine Packaging, was one of the six chosen. The paper focused on the advantages and disadvantages of natural cork, synthetic cork, plastic and rubber stoppers, and screw-caps used to close wine bottles.
IV. Upcoming Meeting
The annual meeting of the American Society for Enology and Viticulture is June 26 - 28, 2002 in Portland, OR. Information is available at www.ASEV.org.
The annual meeting of the Eastern Section of the American Society for Enology and Viticulture is July 10 - 12, 2002 in Baltimore, MD. Additional information is available at www.NYSAES.Cornell.edu/fst/asev.
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Enology Notes and the Vintners Corner Newsjournal are available on-line at: www.fst.vt.edu/zoecklein/index.html.