Curious about Oak: What Does It Do to Wine?

Oak barrels in a winery

Oak plays a major part in maturing wine—from the cheapest dribbles to the classiest sips. In fact, according to Wine Folly, the 50 most expensive wines in the world are oak-aged in one way or another.

Yet have you ever asked yourself, “Why oak?” Because you never hear of wine being aged in any other kind of wood. In fact, even within the oak family, the focus is very narrow. It’s mostly French or American.

By now, you’re probably thinking it’s all a conspiracy against other kinds of wood. Read on to find out. (It isn’t.)

Oak soak

Oak, in the shape of barrels, wasn’t originally intended to be used for maturing wine. It was merely a way of getting wine from a winemaker to be drunk elsewhere*. Along the way, people discovered that while it was in the barrels, the oak did something to the wine. And by a wide margin, people decided it was improving the flavour. So much so, winemakers started fermenting wine and maturing it in oak barrels to add those flavours on purpose.

So what does oak do? And how?

Oak is porous. Not wine-gushing-out porous, rather wine-gradually-evaporating porous. Nevertheless, the amount of volume lost can be considerable. Over the course of a year, as much as 10% of the wine can vanish—mostly in the form of water and alcohol. As a result, flavour and aroma compounds in the wine are more concentrated. The void left by evaporation is topped up with additional wine. A little oxygen helps wine during maturation. A big, persistent bubble of it doesn’t.

Oak barrels allow oxygen to seep gently into the wine. This subtle addition of oxygen helps soften the tannins in oak-matured wines compared with wines aged in stainless steel.

Oxygen also changes the colour of the wine. Reds go from deep crimsons and purples to a softer brick reddish colours. White wines aged in oak are typically paler than their steel-barrel-matured counterparts.

Finally, oak adds its own flavours and notes to the wine. The most common flavours associated with oak-aged wines are vanilla and caramel. You will also hear people talk about ‘spice’, ‘smokiness’ and ‘cream’. In Chardonnay, the oak can add coconut, cinnamon or clove notes. In red wines, oak is responsible for mocha and coffee notes.

How strong the impact is of putting wine in an oak barrel depends on a dizzying combination of factors:

  • the kind of oak used.
  • how it was dried.
  • the size of the barrel.
  • how much the inside of the barrel is charred (toasted).
  • how long the wine spends in contact with oak.
  • at which point the wine was introduced to the oak.
  • how often the barrel has already been used for making wine.

Choices, choices, choices

Barrel made from French oak

French oak has a tighter grain than American oak. This means the wood needs to be split carefully rather than sawn, otherwise the resulting barrels won’t be watertight. Because this is a less efficient use of the wood, barrels made from French oak are more expensive than those made from American oak. Pic: Jeanine Smal from Pixabay

Before we get into oak itself, let’s address why it has come to dominate the wooden wine barrel business in the first place. Long story short, oak is the only wood that is easy to work with, has the right porousness and enhances the flavour of the wine. Other woods have been tried, but have proved either too temperamental to work with (palm, redwood), too leaky (chestnut) or make the wine taste absolutely horrible (apple, cherrywood).

So oak it is.

But not all oak is created equal. Some oaks are easier to work with than others. Others impact the flavour more subtly. So, while several types of oak are used in winemaking, two dominate: French and American oak. French oak splits into a further two species, and where they are grown in France is important too, but for the sake of everyone’s sanity, we’ll treat all French oak as the same.

If you want a loud, in-your-face wine, you want something aged using American oak. American oak has up to four times as much of the chemical that adds the vanilla flavour to wine compared with French oak. That’s great for Chardonnay, but you’d avoid American oak for something like Pinot Noir. Or, at least, you’d avoid using a brand new American oak barrel for Pinot Noir, when its effects are most potent.

American oak is also easier and more economical to work with due to its structure. Unlike American oak, French oak needs to be split rather than sawn, which makes the cooperage (barrel making) less efficient.

French oak is typically seasoned outdoors for as long as three years before being used in winemaking. This allows the natural tannins to leach out of the wood and makes for wines with smoother tannins. American oak is either kiln dried or allowed to season outdoors. Kiln-dried American oak still has much of its tannin content, so makes for bigger tannins in the wine.

French oak is less porous than American oak, so changes to the wine take place more gradually, resulting in a more elegant and refined flavour profile.

A toast to barrels

The insides of oak barrels are charred to help with the development of flavours in wine. The lighter the charring (‘toast’), the more of the natural oak and vanilla flavours are passed on to the wine. Heavier toasts add coffee and caramel notes.

Also important is how long the wine and oak are together. The oak does most of its work in the first few months. Longer contact allows the effects of the oak to smooth out and become more nuanced.

Toast and the length of time the wine is exposed to oak aren’t the only ways of increasing or decreasing the impact of the oak on the flavour of the wine. The size of the barrel plays a role too. The smaller the barrel, the higher the ratio of oak to wine, and the more pronounced the impact of the oak is.

The effect of the oak also depends on how often the barrel has been used before. The impact of maturing in an oak barrel diminishes significantly after the barrel’s first use—the vanilla enhancing chemical especially. However, oak barrels are expensive so they are reused, either as they are, or by shaving down the insides and adding a new lining of toasted oak staves. Eventually, many oak barrels pass through the wine industry into the whiskey-making business, where they are used to mature whiskeys over many years.

Wine meets oak, sooner or later, one way or another

Oak isn’t only used in the maturing of wine. Oak barrels can also be used earlier in the process, during fermentation. This is done for white wines in particular, which may or may not then also be aged in oak.

When wine is fermented in oak, the vanilla impact is less pronounced because some of the chemical reaction is with the yeast used to encourage fermentation, which also coats the inside of the barrel, reducing the interaction between oak and wine further.

And not all oak-associated flavours are added to wine by using oak barrels for fermentation or maturation. You can get oaky delicious by adding in oak chips. The chips work much more quickly than oak barrels and give winemakers lots of options to control the flavours and notes in their wines by combining different oaks.

Winemakers also use oak powder or staves of oak dipped in barrels to add oaky goodness to their wines.

You might feel this goes against the tradition of winemaking. And there’s certainly a lot to be said for that. On the other hand, it takes an oak tree 80 to 120 years to grow big enough to be used in barrel making. In other words, some of the barrels being produced at the moment are being made from trees dating back to the first half of the last century. And the wine industry isn’t blind to the issues around using a precious natural resource that takes so long to develop.

As a result of both environmental and cost considerations of using oak, winemakers are starting to look at plastic alternatives. We know: plastic as a green alternative to wood. Who’d have thought? Plastic barrels can be made with varying degrees of porousness and can be re-used nearly infinitely in combination with fresh staves of oak.

Regardless of how it gets there, though, there is no doubting that oak and wine is one of those perfect matches. And a great many wine drinkers have come to adore its influence on their wine.

* Yes, we are clever, thank you for noticing: “from a) winemaker to b) drunk elsewhere”.

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