February 2007 Issue of Wines & Vines

A Refresher Course on Barrels

The relationship between wood, cellar practices and character

by Chris Fleming

  • The practice of aging wines in small oak barrels is a relatively new development, and is a major factor in the so-called "international style" of winemaking.
  • The contribution of complementary wood tannins and different toasting levels means that many wines are suitable for earlier consumption without the need for decades of bottle age.
  • The aging of wine in new oak barrels has been an evolutionary step forward for the world wine industry.
The effect of oak barrels during elevage--the period when wines undergo aging in wood--is one of the most critical periods of development during a young wine's maturation. In the recent history of winemaking, the use of new 225- and 228-liter French and American oak barrels has become widespread, accompanied by the diminished use of large cooperage several thousand liters in capacity (puncheons, botti or tinas) made of oak or other woods. Many winemakers across the globe have followed a trend away from a "traditional" style marked by prominent acidity, harsh tannins when young, aromatic nuances and complex, multi-layered flavors on the palate, toward a so-called "international" style marked by bold, expressively forward fruit; explosive aromas; deep colors; extracted, concentrated, intense flavors on the palate; sweet, ripe, powerful tannins and high alcohol.

In their youth, traditionally styled wines are recognizable by primary fruit, often accompanied by herbal, woodsy aromas and prominent minerality, which during several years of bottle age become complex, profound secondary and tertiary aromas and flavors of dried roses, mushrooms, leather, truffles, dried berries and woods. "International" style wines display obvious toasted, woody aromas; rich, sweet fruit; ripe tannins and a creamy character on the palate that belies their age. These characters have been largely created utilizing barrel aging and other relatively new winemaking technology.

Ironically, in their pursuit to produce world-class modern wines that can be enjoyed before the years of bottle age necessary for traditional wines, winemakers in the major wine regions that have emerged since the 1970s have adopted traditional vinification and aging methods that originated in Burgundy and Bordeaux.

Wine Barrels
Aging wine in small oak barrels is a relatively new step in the long evolution of winemaking. The interplay between the wood and the wine is complex, and still being explored.
Chemistry's Contributions

Complex chemical changes occur in wines aged in barrel. In addition to a slow micro-oxygenation that softens tannins and imparts a creamy mouthfeel to the developing wine, wines aged in oak are chemically changed by time in wood. The wood components imparted to wine evolve over time, and these become dramatically noticeable characters in the wine's aromas and flavors.

The major odorous compound imparted by wood is lignin, and over some months in barrel, it oxidizes to a few aromatic aldehydes. The most important aldehyde is vanillin, which has the most easily recognizable aroma in barrel-aged wines. With judicious barrel aging, vanillin creates aromas and flavors complementary to most red and white grape varietals, but too much vanillin can overpower primary fruit and complex nuances in delicate grapes like Chardonnay and Pinot Noir. Winemakers say barrel aging imparts "roundness" and greater complexity to wines, but as with a dish prepared with pungent spices, too much oak aging can overpower the wine's fruit and leave it heavy, unbalanced and with a noticeably "hot" alcohol character.

The distinct characters introduced to the wine extend to each individual barrel. Jacques Lardiere, head winemaker at Burgundy negociant Maison Louis Jadot, describes the effect as "a mystery; the wine in each cask is different and unique." Importer Kermit Lynch began to select specific barrels of wines for customers in the 1970s for just this reason, and many U.S. importers have followed his lead.

Although the effect of oxygen in the cask during barrel aging is a slow, controlled process of micro-oxygenation, traditional winemakers have been wary of aging white wine in wood, because whites contain few tannins for protection against contact with oxygen introduced between barrel staves, through wood pores and against the bung. The latter problem has been reduced greatly by newer silicone bung stoppers. The stoppers work so well, contends winemaker Peter Sisseck of the Dominio de Pingus in Spain's Ribera del Duero, that, "They create a vacuum, so it's unnecessary to top up the barrels completely" as was the previous practice with traditional wooden bungs wrapped in cloth.

White wines contain a fraction of the tannins found in reds, so they have less ability to age. However, fermenting white wines in barrel has gained popularity with modern-styled winemakers to produce fat, full-bodied, rich wines with more expressive aromas, greater complexity and a creamy mouthfeel. Yeasts and CO2 from fermentation protect from over-oxidation or excessively extracted tannins, and the process of autolysis, which occurs when spent yeast cells and fermentation sediment absorb tannins and form new odor compounds. This enhances dramatically the aromatic depth and flavor complexity in white wines, particularly in Chardonnay. As a result, traditional Burgundian methods of fermenting and aging on the lees, and of stirring the lees, have gained favor in California, Spain, Italy and other winemaking areas.
F. Mugnier
Burgundian winemaker Frédéric Mugnier believes that the care and treatment of wood staves are vital to the quality of the finished barrel. Even the weather plays a part.

While acidity is critical for allowing a white wine to age, red wines have acidity and grape tannins to help them mature. Tannins combine easily with oxygen, and they work as a natural preservative--the main reason that red wines age longer than whites. Tannins are polymers 2 to 10 molecules long, and young wines contain many short tannins that contribute unpleasantly aggressive, green, "stemmy" characters. On the palate, these tannins seem coarse, bitter and astringent. Over time, these short tannins combine with anthocyanins--color pigments from grape skins--to form longer polymer chains in a process called "polymerization," then fall out of the wine as sediment. In this manner, a mature wine loses its bright purplish garnet color to become a brownish, brick red. The wine loses its youthful color as it ages.

Tannins are important structural polyphenols in wine that contribute volume and a full mouthfeel. In addition to grape tannins from the fruit, barrel-aged wines also contain wood tannins. Tannins accentuate a perception of acidity, so tannin/acidity balance is why red wines seem balanced with less acid than whites. Wood tannins from barrels have a softer, spicier and dustier character, perceptible on the inside of the cheeks and back of the palate.

Because of decreased, young, short tannins, wines aged in wood have a rounder, creamier character on the palate than wines aged in large wood or fiberglass vats or stainless steel tanks. In addition to imparting softer wood tannins, barrel aging helps to evaporate CO2 and volatile ethers, which are often perceptible as a "tanky," gassy, almost effervescent freshness.

Wines aged in wood acquire a number of distinctive characters. There is a general increase in volatile acidity, not solely from ascetic acid, that is sharp on the palate. Among the most important phenol aldehydes or volatile phenols imparted to the wine are cis- and trans-lactones, which exhibit a subtle aroma of coconut. The wine's color becomes more limpid as sediment and tartrates fall out and are separated by racking. Wood molecules from the barrels become part of the wine and increase its viscosity, making it more full-bodied. Descriptors of wood aromas include smoky, nutty, coffee, espresso, cedar, sandalwood and chestnut.

With traditional winemakers, the volume of tannins and color extraction is determined by the length of maceration. Most of the color in red wines is obtained in the first week. Modern-style winemakers in California, Spain, Italy and other areas have adopted the Bordeaux technique of extended maceration, which leaves the skins on contact with the must a month or more after fermentation is complete. Through this process the young wine, characterized by purple color and rough, short tannins, becomes softer. As more tannins are extracted, they polymerize as long tannins, which make the wine gentler on the palate. Some tannins link up with anthocyanins and fall out as sediment, and the color is softened to red. The wine is now softer, rounder and more drinkable at an earlier stage.

Sourcing the Staves

Oak barrels are fashioned from various types of oak staves, which have corresponding different aromatics and flavors obtained from each type of oak and from the tightness of the wood grain. The stave grain also determines the amount and rate of wood tannins extracted into the wine. Today, the barrels in greatest demand are French oak of species Quercus Robur and Quercus Sessiliflora. Fashioned with tight-grained stave wood from the Vosges, Tronçais and Allier forests, and favored by Burgundy and California winemakers for Pinot Noir and Chardonnay, these woods impart the least volume of wood tannins. They are also popular with Spain's top winemakers in Rioja and Ribera del Duero for Tempranillo.

Through the process of barrel aging, tannins become oxidized, aromatic aldehydes and lend complementary and subtle neutral, vanilla and cinnamon characters to the wines. These woods initially impart more of their tannins at a faster rate, and are quite noticeable on the nose and palate. Later this decreases, and finally they lose some of their characters at the end of maturation.

The Domaine Jacques-Frédéric Mugnier is located in the village of Chambolle Musigny in Burgundy's Côte de Nuits. For winemaker Frédéric Mugnier, who has used Tonnellerie Remond barrels since vintage 1993, the handling of the barrel staves is also important. "Drying the staves in the open is an essential part of the process," he says. "The duration is important--some of the woods I use are aged for three years, some for only 24 months--and the weather is important too. It takes a sufficient rainfall to wash out green tannins from the wood. I no longer store (stave wood) myself."

In imparting greater amounts of wood tannins and oxidized aromatic aldehydes, medium- and coarse-grained stave woods lend more overt vanilla, toasty and buttery characters to the wines, and are favored by Bordeaux, Rioja and California winemakers for use with more robust varietals like Cabernet Sauvignon, Tempranillo, Syrah and Cabernet Franc. Coarse-grained staves contribute more so-called "whiskey lactones" and an important aromatic phenol, eugenol, which has an aroma of cloves.

Generally, American White and Gascon oak, Russian, Slovenian, Bulgarian, Romanian and Hungarian oaks produce medium- to coarse-grained stave woods that yield tannins and oxidized aromatic aldehydes quite similar to those yielded by the woods favored in Bordeaux cooperages. These barrels impart a pronounced buttery, vanilla, coconut, dill, cedar and acacia character to the wines.

For wood tannin extraction, these woods initially impart fewer tannins, with the coarse-grained woods slowest of all, but over the course of maturation the rate is steady, and at the end, the total wood tannins imparted which result in aromatic aldehydes are more than double those of fine-grained woods. In the last decade, Eastern European oak has gained favor because of improved quality, lower cost and less aggressive, moderated wood tannins.

Top quality cooperages follow similar production methods to handcraft barrels. Trees are felled in winter when there is no sap, often under a waning moon, and stave wood is split, rather than sawed, to preserve the integrity of longitudinal fibers called "medullary rays," which make the stave watertight.
Australian research has found that staves dried in a hot, dry climate showed increased "whiskey" lactones that can overpower elegant wines. Talley Vineyards consulting executive winemaker Steve Rasmussen referred to a study by Tonnellerie Demptos in which wood staves were aged in Burgundy's cold, wet climate. In staves aged for 18 months under these conditions, glucoses and polysaccharides are released from the wood, which contribute richness to the wine and reduce the astringency of short tannins. Also, lichens grow on the wood, containing enzymes that break down the bitter, harsh, green phenolics.

Many cooperages age barrel staves 7 to 12 months per 10mm of stave thickness, or two to three years for a 30mm thick Burgundy stave, depending upon the wood's forest of origin. Rasmussen noted that the Bordeaux cooperage Tonnellerie Boutes soaks barrel staves in water for a week or two before air-drying to remove any green, harsh characters.

Toasting Choices

The most dramatic impact on a wine's aroma and flavor comes from toasting the staves. This is where there is the most differentiation among cooperages in barrel fabrication. The process heats the staves by oak fires, which softens the wood so it can be shaped into barrels. Aromatic aldehydes increase noticeably when new barrels are toasted, and the main aldehyde is vanillin with its familiar vanilla wood aroma. Ketone and other phenyls, along with the phenol eugenol with its clove aroma, are all accentuated when the staves are toasted, and many of these act cumulatively to enhance the vanilla aromas. The heat caramelizes the sugars, glucoses and polysaccharides released from the wood during stave seasoning.

Seasoning and toasting of the wood are also considered important factors affecting the flavor profile of new French oak barrels. While heavy-toast barrels were in demand a few years ago, today many European and American vintners prefer a more subtle medium or medium-plus toast.

A promising development that allows modern-style winemakers to implement elevage faster is the introduction of micro-oxygenation units. Originating in the Madiran region of southern France, where the Tannat grape has incredibly strong tannins, micro-oxygenation appeared commercially in 1991, and in 1999, micro-oxygenation units were introduced in California. Talley's Rasmussen says he is intrigued: "Some wineries are using micro-oxygenation with and without oak chips, and the results have been very encouraging. It's not only a technique to benefit less expensive, value wines; it has the potential to make better wines, period."

New oak barrels for wine aging have constituted an evolutionary step in winemaking methods and helped create the "international" style of wines. Unlike traditional wines of a generation ago, such as Hubert de Montille's Volnays, which were unapproachable for 12 to 15 years, today's modern winemakers are making wines to enjoy upon release that will also benefit from aging. They are doing it by utilizing the best of old and new vinification methods. It will be fascinating to see what characters tomorrow's wines possess.

(Manhattan-based Chris Fleming has been published in The Robb Report and Wines from Spain News, and has written and consulted for fine wine importer, Kobrand. Currently, he is marketing communications manager at Frederick Wildman & Sons, a NYC importer. Contact him or comment on this story at edit@winesandvines.com.)
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