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Winemakers face many choices when using oak in to produce their wines, and the interaction between oak and wine is complex and only partly understood.
As a vessel for transportation, the oak wine barrel has been used from Roman times (or earlier) until well into the last century, before being replaced by bottles, food grade tanks and advanced bladders filling entire shipping containers. The effects of oak on flavor, aroma and texture now appreciated by drinkers are by-products of the need to find suitable watertight containers. But today their use is more integral to the winemaking process, and the modern winemaker – budget notwithstanding – has an array of options with which to shape the character of the wines.
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At one extreme, a wine can be aged in new oak, which can bestow – among other things – vanilla or toasty aromas, spiciness, a literal wood character or an extra backbone of tannin. The effect of the oak is reduced greatly after one use; however, a used barrel may be desirable if a winemaker wants subtler wood effects, perhaps in a lighter wine like a Pinot Noir, rather than a heavier Cabernet Sauvignon or Syrah. Wine barrels of differing ages can also be combined to achieve a desired style, to suit a grape variety or to fit a certain price point, and some of the grapes might be vinified or aged in tanks made stainless steel or other inert materials.
At the other extreme, the impact of old oak, used for multiple previous vintages, comes via the degree to which it allows oxidation of the wine. This can provide nutty, Sherry-like flavors, floral notes and a softer texture. Wines may also be fully or partly fermented in oak – a practice more common with white wines – to give texture and weight, and oak aromas that are more integrated than those achieved through extended barrel aging.
Oak’s main component is cellulose (40-45 percent of dry weight), which has little effect on flavors or aromas in wine production. It can however provide sugars to feed on for spoilage microorganisms when toasted.
Hemicellulose (25-35 percent of dry weight) is a mix of polysaccharides that releases simple sugars and compounds when toasted, providing butterscotch, toast and burnt sugar flavors and aromas, and increased body and color in the wine.
Lignin (20-25 percent of dry weight) is a binding polymer in the wood, and provides phenolic aldehydes such as vanillin, which provides creamy, vanilla flavors, guiacol and eugenol, which both offer smoky notes. Seasoning and toasting have a strong effect on the lignin-derived flavors and aromas in a wine.
The remainder is taken up by phenolic compounds (mostly hydrolyzable tannin in the form of gallotannins and ellagitannins), which influence the structure of wines and can help regulate oxidation. These can be broken down by seasoning and/or toasting into softer acids and sugar.
Oak is seen as the best wood option for its combination of subtle flavors, effective, watertight storage and durability. Chestnut, beech, acacia and other genera are also used, but mainly for larger storage vessels intended to have a less-complex interaction with the wine. The combination of grapes and oak has proven to be a lasting one.
Compared to other tree families, oak has a high proportion of medullary rays, which radiate like wheel spokes, conducting water and nutrients between wood and bark. These give strength and resilience to oak products. Also, in oak species used in wine, drying the wood plugs the xylem vessels that transfer water and nutrients up from the roots, forming xyloses, which create the necessary level of watertightness.
Only three species within the Quercus family are commonly used in vinification and maturation of wine – not including cork, of course. But a winemaker is still presented with a huge array of choices and variables when considering oak, including where the oak comes from, the size and shape of barrel and how it is prepared, and whether the wood is new or used.
Oak is usually divided into geographical zones of origin, giving French oak, English oak (Quercus robur) , European oak and American (Quercus alba or white oak), although there are three main species.
Though French oak is dominant, oak trees are grown in various European countries including Croatia (Slavonia), Hungary, Slovenia and Romania. Russian oak was widely used in France in the 19th Century when local forests were dwindling, and, though rarely seen today, is still regarded as being of comparable quality to French oak.
© Pernod Ricard | Two species of European oak are most commonly used. Quercus sessilis (sessile oak) is the predominant species in the forests of Tronçais, Neviers, Alliers and Vosges, and prefers drier shallow hillside soils. It has a tight grain and releases its components slowly, is relatively soft, and is rich in aromatic compounds. It is a popular choice in the top Pinot Noir regions.
Quercus robur (pendunculate oak) is found in the Limousin and other southern French forests. It has a courser grain and has a different balance of phenols and polyphenols compared to oak from northern French forests, with less aromatic compounds but bestowing more astringency and bitterness. It is used most heavily in Cognac, and also for Chardonnay, and when winemakers wish to raise the tannin levels in their wine.
Compared to American oak, both these species used for French oak wine barrels have a higher concentration of water soluble extractable compounds and a greater content of extractable tannin (especially ellagitannins). Both pedunculate and sessile oak have around 2.5 times higher total tannin phenolics, so give more a more spicy character to wines.
One disadvantage with European oak is that it must be split rather than sawn, to preserve watertightness. Only two 225-liter barriques can be manufactured from the heartwood of a tree of 80-120 years of age. American white oak typically has much thicker xyloses, meaning that staves can be sawn (not split) without losing watertightness. Production per tree is more efficient, even if growth is not managed for straightness.
Less permeable American oak barrels have a lower evaporation rate, meaning there is less need to top up the contents, saving labor but, more importantly, creating more stable conditions for wines to mature. However they are heavier to move around.
American oak has a higher amount of methyl oak lactones (before toasting), which have a higher potential for coconut or vanilla characters and can often be found in Chardonnay or Chardonnay blends, as well as some of the bigger red wines. Therefore it is usually avoided for Pinot Noir wines. It tends to impart less spicy and nutty characters – «old-school» white Rioja gets its nuttiness from oxidation in old white oak, not from extraction – and levels of extractable phenolics are only around 40 percent of those in European oak.
Once the wood for the barrel is cut, there are still many options at the cooperage. A freshly cut log is 50 percent water; barrel wood (or firewood for that matter) needs to be dried to an ambient moisture level.
American oak is usually dried in kilns rather than slowly outside, which speeds up production. Along with saw-cutting and less-managed forests, this is a key factor in explaining why American 225-liter (59-gallon) oak casks cost around US$750-800 against $1100 or more for French examples.
The size and shape of the barrel can have an influence on the ratio of oxygen ingress and flavor compound transfer. A 500-liter (132-gallon) puncheon has 33 percent less surface contact by volume than a 225-liter barrique. Larger barrels have thicker staves, giving better insulation and less permeability. Shape may also have subtle effects on the finished wine. The Burgundy shape (a standard Burgundy pièce holds 228 liters) is more rounded, while Bordeaux barriques and American wine barrels are longer and less curved.
Availability and cost are key considerations; in countries outside France, less usual sizes, such as 400 and 450 liters, and 600-liter demi-muids may need special ordering. The very top French brands are often strictly allocated in export markets. Traditionally barriques are sized to be more easily handled by one person, which may compensate for their purchase cost. Winery space and stacking options may also influence choices here. Some countries have a deeper background with other materials; for example, New Zealand’s wine industry grew up with stainless steel partly because an industry already existed making dairy equipment.
In the last few decades, primary fermentation in barrel has become more popular, particularly for white wines. It is generally regarded that a better integration of oak flavors is achieved if the wine is fermented and aged in oak, rather than just matured in barrel. This can be done for red wines, like Cabernet Sauvignon, but requires larger-volume containers to manage the grapes’ skin contact, before the juice is transferred to smaller barrels to finish primary fermentation and any subsequent malolactic fermentation that may be part of the winemaking process.
Heat management during barrel fermentation is challenging. Stainless steel is a conductor of heat, and a heating or cooling jacket can effect near instantaneous change in temperature on the win. Oak is an insulator, and the equipment is less precise, more costly by wine volume, and more labor intensive.
It is often suggested that ferment heat pulls more «oakiness» out of a barrel. But the precise role of heat in the relationship between wine and oak is not yet agreed. The interactions in barrel fermentation are much more complex than during maturation, and results are more subtle.
© Brown Forman | Barrels provide ample oxygen in the early part of fermentation, allowing yeast to reproduce more rapidly, usually producing higher alcohol levels. As the alcohol is produced, it begins to inhibit oxidation of polyphenols (antioxidants) and extraction of flavor compounds from the wood.
Chemical reactions during barrel fermentation are complex. The presence of yeast makes a big difference to the interaction of wine and wood. Yeast cells coat the inside of the barrel reducing contact with wine, and can also absorb some compounds from the wood, changing them into less aromatic ones. The wine’s colloid structure is enhanced, giving more texture and weight, and the overall effect is to create a more harmonious interaction of wood and wine. Then, as yeasts die, they release polysaccharides, which reduce bitterness from wood tannins, add to flavor complexity, wine clarity, prevent oxidation, and increase color. Vanillin levels are much higher when wines are aged in new oak – but not if fermented in new oak.
Aside from flavor influences, oak aging is all about offering control of oxidation. When wine oxidizes, acetaldehyde is produced by the oxidation of hydrogen peroxide and ethanol, and, aided by wood tannins, can prevent the precipitation of color compounds (antho- and pro-cyanins) and development of brick-red colors. Polymerization and condensation of quinines can soften tannins. Oak tannins scavenge oxygen and in the presence of alcohol can produce diethyl acetal giving fragrant top notes. Elligatannins from the barrels also help to slow down oxidation as the wine ages.
New oak is at its most air- and watertight but oxygen can still get through pores in the wood, as well as around the bung, and during the racking process when wines are moved from their barrel to separate the lees. Barrels need to be kept topped up as evaporation otherwise creates an airspace which would lead to rapid oxidation.
Using old oak can increase the rate of oxidation, as the barrels become less air and water tight. On the other hand many old barrels are allowed to become lined with tartrates from successive uses, which renders them largely inert in terms of flavor enhancement.
Usually used barrels are refilled with the same grape variety or wine type. Occasionally winemakers will use barrels previously used for other wines – perhaps a Chardonnay or Viognier barrel to give lift to a Syrah. Sometimes a barrel which has just been used to ferment white wine, such as Chardonnay on its lees, is then filled with red wine. However, here the influence of the lees is the key element.
The high cost of new barrels means that many winemakers’ oak policy is based on what they can afford. Of course there are several other materials commonly used for fermentation and maturation of wine, notably stainless steel tanks, concrete, epoxy and plastics. Cost and stylistic considerations mean that many of the wines we drink combine partial use of oak barrels combined with other options; many whites ferment in oak, but mature in tanks. A winemaker might recoup some of the cost by onselling an old oak barrel, which might well find a new lease of life as a whiskey barrel.
Other formats may be considered for adding oak characters to wine. Individual oak staves and oak chips are all cheaper options used most commonly in fermentation; placing staves in stainless steel tanks costs around 10 percent of the price of equivalent barrels for the volume, while chips might cost five percent. Neither staves nor chips fulfil the oxidative function of barrels, though the need to top up barrels is removed. Care is needed with dosage with many winemakers choosing to «overdose» a few tanks then blend with unoaked lots. Similar choices in level of toast and seasoning still remain.
Oak essence may also be used, though this is unlikely to be advertised and was more prevalent in the 1980s and 1990s. One further alternative is to shave used barrels to expose surface area retaining some of the physical and chemical attributes of new oak.
A more direct challenge to the primacy of oak barrels may come from modern plastic vats, which can be supplied with a range of air permeability levels. Combined with oak staves, these can deliver oxidative processes along with oak flavoring, are inherently more hygienic and easier to maintain than barrels, and can be used again and again, without degradation in performance.