The role of oak in wine

By Jamie Goode | 21st September 2022

If it wasn’t for oak barrels, many wineries would look just like factories. Lots of shiny stainless steel, occasional bits of machinery lying around, and perhaps some pipes running across the floor. I should add an aside here: this all depends on when you visit. For nine months of the year there’s usually not a lot of activity: making wine, and specifically the busy period when grapes are being harvested and brought into the winery, is very seasonal. Wood is what stops most wineries looking like factories, and it’s remarkable that the traditional way of moving wine around has become an integral part of winemaking in most cellars that persists to this day.

This is because oak barrels are capable of imparting flavour to wine, in two ways: the subtle way and the unsubtle way, and we’ll discuss these shortly. Initially, oak was merely used to make the containers that took wine to market, and which also proved handy for finishing some wines off in. Then, people realised that these barrels could change the way that wine tasted, especially on their first and second use. This oak flavour has become an important signature of the taste of many wines, and this is the unsubtle way oak flavours wine. Then, when stainless steel vessels began to fill wineries, people realised that the flavour was a bit different, and that the way the barrel allowed small amounts of air to get to the wine (and with it, the all important oxygen) had an effect on the way the wine aged. It’s for these two reasons that you’ll still see stacks of oak barrels in modern wineries, even though they are expensive, their use requires a lot of work and space (wine is lost from them gradually to the air around, and this means they need topping up, and they take up lots of room compared with tanks for the same volume of wine), and they are hard to clean properly.

Oak is the wood that is mostly used to make barrels, and it comes from old forests in France, central Europe and the USA. Over time, it proved to have the right combination of workability, air exclusion (it lets just enough oxygen in to be useful) and flavour impact that it is the dominant wood for wine cooperage. Generally speaking, oak adds some spiciness, notes of vanilla and coconut from compounds called oak lactones, and some tannin (which adds to the structure from any tannins in the grape skins). Also, because in the manufacturing process heat is needed to bend the staves into the right shape for the barrel to be assembled, and this heat is usually from a flame, the inside of the barrel chars a little – this is called the degree of toast. This can cause the barrel to impart smoky, spicy and toasty flavours to the wine. The origin of the wood affects the flavour it imparts: American oak releases more of the buttery, vanilla-scented oak lactones, and also more spiciness. As a result, wines matured in American oak tend to have lots more oaky flavour than those that have dwelt in French oak barrels. Eastern European oak tends to give a more savoury, spicy feel to the wine, and is less common than the other two.

Barrels usually hold 225 or 228 litres of wine (the classic Bordeaux and Burgundy sizes), but some winemakers like to use larger barrels of 400 or 500 litres because these have less of a flavour impact, and allow the wine to develop a little more slowly. This is because the ratio of surface area of wine in contact with wood to volume is lower in larger barrels. These larger barrels are especially favoured by white winemakers. While we are talking about barrels, it’s important to recognise that in traditional wineries it’s common to find much larger barrels, known as foudres or botti or some other local term. These are made of thicker staves of oak, are usually assembled in the cellar, and can hold thousands of litres of wine. The wine world went through a phase of getting rid of these large oak barrels and replacing them with tanks, although now it has become quite fashionable to have large oak in the cellar again, because it allows a little bit of oxygen (but much less than small oak), and wines develop really nicely in them over quite a long time.

This oxygen is important for some wine styles. Aside from the flavour impact of oak, barrels are appreciated because of the way they allow a bit of oxygen in, which helps with the development of the wine. Quite a bit of oxygen comes in from the bung hole, which is usually sealed with a silicone bung, but some also comes through the wood. The barrels are usually topped up to prevent a large air space developing as the level drops a bit. How much wine evaporates from the barrel depends on the humidity of the cellar. A dry cellar sees quite a bit being lost, while a very humid one is better in this respect. But if it’s too humid, then the winery walls will grow mould and it won’t look very pretty.

Barrel use differs for red and white wines. Red wines are usually fermented in tanks or vats, and then after pressing the wine, which has usually finished its alcoholic fermentation, matures in barrel. It would be impractical to ferment red wines in barrel because they are fermented on their skins, and this mass of skins and sometimes stems would be impossible to get in and out through the bung hole. For fermenting reds in barrels, you have to stand them up and then remove one of the ends, which has to be replaced once fermentation is over and the skins have been removed. For white wines, barrel fermentation and then maturation is the norm. The grapes are pressed and the juice goes to barrel. It ferments, then the dead yeast cells fall to the bottom and form what’s called lees. These can then impart flavour to the maturing wine. Also, because of wine chemistry, whites that are fermented in barrel pick up less oak flavour than wines that are fermented first, and then just matured in the barrel.

When you look on the back label of a wine you’ll often see information about the time spent in oak, and also the proportion of new oak. Very ambitious wines are sometimes matured entirely in new oak (100% new oak), which can make them taste quite oaky. But the very top red wines can sometime absorb this without showing too much oak character, and it helps set them up for a long life. It’s more common to have, say, 15-20% new oak. This allows wineries to replenish their barrel stock each year as some older barrels are taken out of production. Some winemakers only work with older oak, because they want the benefits of small amounts of oxygen without any flavour impact.

Some varieties seem to have an affinity with oak. Chardonnay and oak are almost inseparable, and most ambitious examples of this grape will be barrel fermented. Riesling is rarely fermented in oak, with the exception of classic wines that are fermented in large format oak barrels that don’t contribute any oak character. Most Sauvignon Blanc is unoaked, but some are made in a different style with some barrel fermentation. And whether or not oak is involved in the production of Chenin Blanc is one of the key determinants of style of this versatile grape variety.

Over recent years, in some winemaking circles there has been a move away from oak. Concrete tanks and eggs, and also terracotta amphorae allow some oxygen exposure without any flavour impact, and both concrete and clay have characteristics that affect how the wine develops. At the high-end, there’s been a lot of interest in these materials for wine ageing, and there’s been quite a bit of experimenting. But it looks like the good old barrel is here to stay, a nice connection with wine’s past, but also serving a useful role.