Why Oak?

Let’s talk biology. When we discuss wine, we focus almost exclusively on the grape vine, Vitis vinifera, 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 piece – 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 premium 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 in 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.

The barrel-manufacturing process involves heating the staves (the oak slats that make up a barrel) 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.

So how does oak affect the flavour of the finished wine? For red wines, barrels often add a little spice, enhance the structure, and may add some sweet vanillin characters. A white wine 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, often sacrificing the vitality and freshness that is intrinsic to these aromatic grapes.

A key difference in the effect of oak barrels is whether they are composed 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 significant. The relatively wide-grained American oak imparts a much stronger flavour, with more obvious sweet coconut flavours and spicy notes. French oak has a more subtle, slightly more savoury vanillin effect. Imagine the two oak influences to be like differing vanilla ice creams - American oak is a big, buttery creamy yellow ice cream, whereas the French oak is whiter, softer, finer and milkier

Both have their fans, with a concerted push in recent years towards French oak in an attempt to reign in overt new oak influences. Ultimately, producers have to decide which oak characters 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, so 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. The exact scientific basis isn’t clear, but 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, this is a technique still in its infancy, and it’s hard to gauge its likely take-up with winemakers currently using the real thing. Whatever happens, it looks likely that the historical association between wine and oak is likely to be an enduring one. (www.wineanorak.com)