Oak Aging and Wine

Let’s talk chemistry. When wine is discussed, the focus is almost exclusively on the grape vine, Vitisvinifera, in its many varieties. But there are two other organisms crucial to wine production that are often forgotten about. The first is the yeast, Saccharomyces, without which our favourite tipple would just be grape juice. The second – and the subject of this article – is the oak tree, Quercus.

An odd choice, you might think. However, oak’s accidental association with wine has been a critical one. The majority of fine red wines are dependent on oak barrels for a vital component of their flavour, as are a good number of whites.

The reason oak barrels were initially chosen for storing wine had nothing to do with the flavouring effects they have: it’s simply that in the past barrels were used as all-purpose containers, and oak is a tight-grained wood capable of making leak-proof barrels ideal for storing liquids in. The shape of the barrel makes it extremely strong and once on its side it can be moved by rolling, even when full. Fully-closed barrels were first developed during the Iron Age (800-900 BC), and by the first century BC were widely in use for holding wine, beer, milk, olive oil, and water but wood had been used for open vessels in ancient Egypt since 2500 BC.

The barrel manufacturing process involves heating the staves over a brazier so that they can be bent into shape. Somewhat fortuitously, this slight charring – referred to as ‘toasting’ – coupled with the chemical properties of the wood, means that the interaction of the wine with the inside of a new barrel imparts pronounced flavour characteristics to the wine. When used appropriately, new barrels can have a significant beneficial impact on the wine that is aged in them.

Another equally important, but less talked-about, effect of ageing wine in barrels is that this allows a very slight and controlled exposure to oxygen. Normally, winemakers do all they can to avoid exposing their wines to air, but in this case the low-level oxidation that barrels permit is beneficial to the structure and character of the wine. “Angels' share" is a term for the portion (share) of a wine’s volume that is lost to evaporation during aging in oak barrels. This aids in the polymerization of tannin into larger molecules, which are perceived on the palate as softer. This very gradual oxidation results in decreased astringency and increased colour and stability. It also evolves the fruit aromas to more complex ones. Through a program of topping the wine (filling up the barrel) while it is in the barrel and racking the wine from barrel to barrel to clarify it, just enough oxygen is introduced to the wine to have these beneficial effects over a period of many months.

So how does oak actually affect the flavour of the finished wine?

Oak wood is composed of several classes of complex chemical compounds, each of which contributes its own flavor or textural note to both red and white wines. For red wines, such as Shiraz or Cabernet, barrels often add a little spice, enhance the structure, and may add some sweet vanillin characters. A white wine, such as Chardonnay, that has been fermented and aged in barrels will often have a noticeable nutty, buttery character along with the spice and vanilla characteristics that reds often pick up. Barrel aged wines are generally a little more complex and have a more interesting texture than those aged in tank, although some white grape varieties, such as Riesling and Sauvignon Blanc rarely benefit from being oaked.

A key difference in the effect of oak barrels is whether they are made of French or American oak. If you ever get a chance to try side-by-side cask samples of the same wine aged in French and American oak, take it: the differences are marked. The relatively wide-grained American oak imparts a much stronger flavour, with more obvious sweet vanilla flavours and spicy notes. French oak has a more subtle, slightly more savoury effect. Producers have to decide which suits their wine better, although cost can be a factor here: French barrels are much more expensive than American ones. The degree of ‘toast’ and even the manufacturer of the barrel are also important factors in the effect barrels have on the wine.

A crucial variable here is the age of the barrel. New barrels impart the most flavour, and this effect is subsequently diminished with each re-use of the barrel, such that third-use barrels don’t add much flavour at all. Many producers juggle their barrels carefully, ageing their wine in a mix of new and used barrels to avoid over-oaking it. Great care must be taken with the use of older barrels, since they can harbour bacteria and yeasts that might contaminate the wine.

Because new barrels are expensive, their use is usually reserved for premium wines. But winemakers are only human: they want the beneficial effects of oak for their cheaper wines as well, without the high cost. As a result, barrel substitutes have become increasingly popular. These can range from small oak chips in teabag-like nets to barrel staves bolted into the inside of the tank. Results can be variable, and are generally not as good as those achieved by barrels. If you see the words ‘oaked’ on the label of an inexpensive wine without mention of barrels, the chances are one of these alternative techniques has been used.

A hi-tech twist on this theme is a technique called micro-oxygenation. This process aims to simulate the gradual, low-level exposure to oxygen that occurs in barrel. A specialized device is set up inside the tank that releases a slow stream of tiny oxygen bubbles in a controlled manner. It’s a technique that’s becoming widely adopted. Converts claim that it enhances the structure, stabilizes the colour, and removes unwanted vegetal notes from red wines treated in this fashion. Increasingly, micro-oxygenation is being used in tandem with oak chips in a sophisticated emulation of the process of barrel ageing, but at a much-reduced cost.

However whatever happens in the future, it seems likely that the historical association between wine and oak is likely to be an enduring one. So we may say that it is a happy, historical coincidence that wine and wood marry together to form a richer, more complex flavor and texture than wine would have were it stored in a totally non-reactive container.