Enology Notes #106, September 30, 2005
To: Regional Wine Producers
From: Bruce Zoecklein, Head, Wine/Enology-Grape Chemistry Group, Virginia Tech
If fruit such as strawberries or apples are made into wines, the public expects them to have the flavor of fresh fruit. Fortunately, no such expectations apply to grape wines, partly because few consumers have any preconceived notions regarding wine grape flavors.
This allows for a broad degree of stylistic freedom. What the public does expect, however, is a well-balanced wine, one that possesses a whole symphony of integrated flavors and aromas. To produce such a wine requires an understanding of the grape and how each processing variable influences the balance of fruit, wood, bacterial and yeast aroma, and flavor notes.
Lees contact contributes to the complexity of wines by the integration of yeast characteristics with fruit and wood flavors. Lees management is, therefore, an important aroma and flavor consideration. Lees nourish the wine during aging, imparting aromas and structure, by filling out the body and adding depth, length and complexity. Table 1 shows some important considerations regarding lees management.
LEES MANAGEMENT ISSUES
- Non-soluble solids level
- Method of stirring
- Frequency and duration of stirring
- Type and size of vessel
- Duration of lees contact
- Timing and type of racking
- SO2 timing and level of addition
- Frequency of barrel topping
During lees contact, the composition of the wine changes as the yeast commence enzymatic hydrolysis of their intracellular constituents. One important feature is the process of proteolysis, whereby proteins are hydrolyzed to amino acids and peptides. These compounds result in an increase in the available nitrogen content of the wine. It is known that amino acids can act as flavor precursors, possibly enhancing wine complexity and quality.
During sur lie storage, yeast components such as cell wall polysaccharides, and particularly mannoproteins, are released into the wine. These macromolecules can positively influence structural integration, phenols (including tannins), body, aroma, oxygen buffering and wine stability. The interest in lees utilization goes beyond barrel-fermented white wines. As has been discussed in other issues, sur lie storage is becoming much more important in red wine production.
Yeast-derived macromolecules provide a sense of sweetness as a result of binding with wood phenols and organic acids, aiding in the harmony of a wine's structural elements by softening tannins.
As discussed in previous editions, it is important to differentiate between light lees and heavy lees.
Heavy lees can be defined as the lees which precipitate within 24 hours immediately post-fermentation. They are composed of large particles (greater than 100 microns) and consist of grape particulates, agglomerates of tartrate crystals, yeasts, bacteria, and protein-polysaccharide-tannin complexes (Delteil, 2002).
Light lees, on the other hand, can be defined as those that precipitate from the wine more than 24 hours post-fermentation. These are composed mainly of small particles (1-25 microns) of yeasts, bacteria, tartaric acid, protein-tannin complexes and some polysaccharides (Delteil, 2002).
There is no value in storing wine on heavy lees. Indeed, such storage can result in off aroma and flavors, and a depletion of sulfur dioxide.
Light lees storage, however, can have a significant advantage in structural
balance, complexity and stability.
Lees stirring and the frequency of stirring is important, both as a practical and stylistic consideration. Stirring generates an oxidative process which increases the acetaldehyde content, and which may increase the acetic acid concentration. Stirring also changes the sensory balance between fruit, yeast and wood by enhancing the yeast component, and reducing the fruit, and, to a lesser degree, the wood component.
Additionally, stirring may have the effect of enhancing secondary chemical reactions, possibly as the result of oxygen pick-up. Stuckey et al. (1991) demonstrated increases in both the total amino acid content and wine sensory score in wines stored for five months without stirring. The non-stirred wine was perceived to have greater fruit intensity.
MLF reduces the harshness of new oak and aids in the development of complex and mature flavors. Traditionally, stirring is continued until MLF is complete. After that, the lees are said to become more dense, which aids in clarification.
During barrel aging, what we are looking for is slow, well-managed, and controlled oxygenation. Lees allow for this oxygenation and they aid in the prevention of oxidation. Traditionally in Burgundy, wines are racked off the lees in March, usually the time when MLF is completed. Frequently this is an aerobic racking off the heavy lees, then back into wood on light lees, followed by an SO2 addition. Leaving the wine on the light lees helps to nourish the wine. The addition of SO2 helps to protect the wine from oxidation. A subsequent racking often occurs in early July and is in the absence of air.
Timing of SO2 additions, and the quantity of SO2 added, are important stylistic considerations. Early use of SO2 increases the number of components that bind subsequent additions of SO2. The addition of too much SO2 counters the wood flavors and limits oxidation reactions, while too little SO2 may allow the wine to become tired and overaged. Production considerations, such as the method of barrel storage and time of bottling, are factors influencing SO2 levels. Barrel topping is an aerobic process that can result in excessive oxidation. Additionally, wines that spend a second winter in the cellar tend to lose their aroma unless the wine is particularly rich.
Delteil (2002) compared two red wines. One wine was barrel stored on light lees for 9 months; the other, racked several times prior to barreling, was stored for the same period without lees. These two Syrah wines differed significantly in their palate and aroma profiles.
The wine stored sur lie had a much lower perception of astringency and a greater integration of the phenolic elements. The sur lie wine also had a lower perception of oak character, resulting in a higher perception of varietal fruit.
Lees contact is particularly effective at modifying wood tannin astringency by binding with free ellagic tannins, thus lowering the proportion of active tannins. Sur lie storage can reduce the free ellagic acid by as much as 60%, while increasing the percentage of ellagic tannins bound to polysaccharides by 24% (Ribereau-Gayon et al., 2000).
As has been previously suggested, it is important not to overlook the power of macromolecules. Polysaccharides are found in wines at concentrations ranging from 300-1000 mg/L. These can be the result of lees storage or a function of the many commercial products available.
Upcoming Events . The Enology-Grape Chemistry Group will offer a two-day juice and wine analysis short course on January 11 and 12, 2006. This program will be a hands-on, practically oriented laboratory course. It will be conducted in the teaching laboratory of the Food Science and Technology Building at Virginia Tech.
This program will include the following:
- Establishing a winery laboratory
- Good laboratory practices/HACCP planning/ Precision and accuracy
- Fruit processing basics
- Maturity indices
- pH basics
- Titration and titratable acidity
- Fermentable nitrogen
- Protein stability
- Bitartrate stability
- Oxidative stability, aldehydes
- Organic acids
- Volatile acidity
- Sulfur dioxide and sulfur-containing compounds
- Dissolved oxygen
- Carbon dioxide
- Introduction to remission photometry
- Virginia Tech’s analytical lab service
Registrants will participate in hands-on analysis. Analysis will be supplemented with a laboratory manual and discussions concerning the practical winemaking significance of each test.
Enrollment is limited: The short course will be limited to a total of 14 participants. You must register for both days. Registration preference will be given to Virginia bonded winery owners and representatives, and prospective Virginia winery owners who register BEFORE TUESDAY, NOVEMBER 1, 2005. After that date, open enrollment will be offered if space is available. Preference will be given to those already in the commercial wine industry.
You will not be registered until
your check is received!!!
Cost: $450 per person, due by November 1, 2005, checks payable to:
Virginia Tech Foundation (note in memo – Juice & Wine Short Course)
Department of Food Science & Technology
25 A FST Bldg.
Virginia Tech - 0418
Blacksburg, VA 24061
Course fee is non-refundable.
Argentina and Chile Trip. I will help lead a wine and culture visit to Argentina and Chile on March 1-14, 2006. We will spend several nights in Buenos Aires, then fly to Mendoza, Argentina, for three days and nights of vineyard and winery visits. On March 7, we will fly from Mendoza to Santiago, Chile. We will visit Vina del Mar, Valparaiso, the Casablanc, Maipo Valley, Aconcagua Valley, and go to the Colchagua on the wine train. Departure is from Santiago on March 14. If you are interested in joining us, please read the following Adobe Acrobat files. Contact me if you have any questions.
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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