To: Regional Vintners
From: Bruce Zoecklein
Subject: Wood Fermentations and Sur lie Storage of Red Wines
Now is the time to consider last years wine quality in relationship to your HACCP-like plan. What changes, if any, need to occur for you to attain your quality and stylistic goals?
Wood fermentation and sur lie storage are methods of modifing structural balance, color and complexity. There is increased interest in using these in conjunction with several other processing variations, such as Microoxgenation and Delestage (for more information on our reseach in these areas, see the Enology-Grape Chemistry Group web site at www.fst.vt.edu/zoecklein/index.html.
Fermentation of Red Wines with Wood. There are two types of wood fermentation used in red wine production, the Australian red method and fermentation with cubes. The Australian Red approach, so named due to its origin, involves several steps. Crushed and destemmed fruit is cold soaked with maceration enzymes for 24 to 48 hours. Cold soaking is effective in aiding the extraction of anthocyanins and stabilizing color. Additionally, co-pigmentation promoters are extracted. Wine is dejuiced at 18-16 °Brix, depending upon the initial maturity, and barrel fermented. The alternative is to add cubes (not chips) to the fermentation.
During wood fermentation, two factors help to influence the structural integration of the wood. Mannoproteins are released by the yeast which help to bind phenols. Additionally, yeast adsorb about 1/3 of the ellagic tannins, lowing the perception of astringency. The process of wood fermentation is used by some to create wines for blending.
Sur lie and structural component integration.
As has been previously suggested in several Enology Notes, do not overlook the power of macromolecules. Polysaccharides are found in wines at concentrations ranging from 300-1000 mg/L. They are of two general types, those found in the fruit, such as pectins, and those produced by yeast and bacteria during fermentation and released during autolysis. A number of compounds classified as polysacchardes also have significant protein components and vice versa.
The yeast cell wall is composed largely of ß-glucans and mannoproteins. Mannoproteins are released during sur lie storage. This release post-fermentation is the result of enzyme hydrolysis of the lees caused by ß-glucanases, present in the cell wall. This enzyme retains activity for several months after cell death, releasing mannoproteins into the wine. This increase can be as much as 30% in four months of aging sur lie. Heavy lees contact results in a high concentration of polysaccharides compared to light lees; the difference can be as much 200 mg/L (Ribereau-Gayon et al., 2000).
Wine balance can be depicted as an equilibrium relationship between (Sweet) and (Acid + Phenolic Elements). This relationship suggests that as the perception of sweetness increases, the perceptions of those elements on the right-hand side of the equation will decrease. The converse is also true, as the sweetness diminishes, the perception of the acid and phenolic elements (bitterness and astringency) increases. Lees contact can have a major impact on structural balance. Polysaccharides have the ability to bind with phenols, thereby lowering the perception of both the tannin elements and, therefore, acidity.
The impact of polysaccharides on astringency is evidenced by a reduction in the gelatin index (see Zoecklein et al., 1995). This reduction can cause an increase in the wines volume or body. Lees contact is particularly effective at modifing 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).
Lees and color. High lees concentration can reduce color, as a function of adsorption onto the yeast cell surface.
Lees and oak bouquet. Lees modify oaky aromas, due to the ability to bind with wood-derived compounds such as vanillin, furfural and methyl-octalactones.
Lees and oxidative buffering capacity. Both lees and tannins act as reducing agents. During aging, lees release certain highly reductive substances which limit wood-induced oxygenation. Wines have a higher oxidation-reduction potential in barrels than in tanks. Inside the barrel, this potential diminishes from the wine surface to the lees. Stirring helps to raise this potential. This is a primary reason why wines stored in high-volume tanks should not be stored on their lees. Such storage can cause the release of reductive or sulfur-containing compounds. If there is a desire to store dry wines in tanks sur lie, it is recommended that the lees be stored in barrels for several months, then added back to the tank (Ribereau-Gayon et al., 2000).
Lees and white wine protein stability. The greater the lees contact, the lower the need for bentonite or other fining agents for protein stability. It is not believed that lees hydrolyze grape proteins or that proteins are adsorbed by yeast. Rather, lees aging produces an additional mannoprotein, which somehow adds stability. The production of this compound is increased with temperature, time, and frequency of stirring.
Lees and bitartrate stability. Mannoproteins produced by yeast can act as crystalline inhibitors. The longer the lees contact time, the greater is the likelihood of potassium bitratrate stability.
Winery Planning and Design Workshop Proceedings Available. Proceedings of the Winery Planning and Design Workshop conducted in July are available. The 104 page proceedings covers establishing a business plan, and winery design considerations, including gravity flow, winery tank selection, sanitation, etc. Send $45 payable to:
Dr. Bruce Zoecklein,
Department of Food Science and Technology,
Virginia Tech (0418),
Blacksburg, VA 24061.
Proceedings are used to support our enology graduate education efforts.
Subscription to Enology Notes. All past Enology Notes and Vintner's Corner newsjournals are posted on the Enology-Grape Chemistry Group's web site at: www.fst.vt.edu/zoecklein/index.html. Enology Notes are different in content from this subscription-based Vintner's Corner newsjoural. To be added to the Enology Notes list serve send an email message to firstname.lastname@example.org with the word "ADD" or "REMOVE" in the subject line.
Dr. Bruce Zoecklein
Associate 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: www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293