How To Understand Wine
How To Understand Wine: What Do Oak Barrels Do?
The chemistry and history of oak tannins, malolactic fermentation and more
As any wine-lover knows, wine and oak barrels have a great love affair. Premium wines can sit in oak for over a year, mellowing the juices and bringing out rich new flavours. But why is this? And how did humans discover it? This article explains the differences between American and French oaks, between malo’d and un-malo’d wines…understanding the relationship between oak barrels and wine is a golden ticket to unlocking incredible wines, all over the world.
History
The impact oak had on wines was first observed in wines in oak barrels shipped to Rome from Gaul around 70AD. Barrels were used because of their reusability and convenience of transportation, and made of oak for its relative lightness compared to other woods.
The spell wines spend inside oak barrels is known as maturation, which refers to the period of time post-fermentation and pre-bottling. In this period, the flavour of a wine is still liable to change, and the winemaker has a large amount of control over the finished product. It is at this time that barrels are able to impact flavour using 2 key tools: tannins and malolactic fermentation.
Tannins
Tannins are found in all wines. They are a type of polyphenol which reflect visible light, typically giving them reddish or brownish colours. They are a large part of the colour makeup of the skins of red grapes.
Tannins and Taste
Tannins act as antioxidants, and make wines taste dry by reacting with saliva proteins, causing them to precipitate. Until recently, it was believed that older wines have a softer ‘mouthfeel’ from this precipitation of larger, old tannins in the mouth when drinking. However, there is no scientific evidence for this and it has even been suggested that old wines are softer due to tannins breaking up in older, more acidic wines and becoming smaller. The mechanism for tannins in the mouth is still not widely understood.
There are two different types of tannins commonly found in wines:
- Condensed tannins: Naturally-occurring polyphenols. These are found in the skin, seeds and stems of grapes. They increase in concentration in a wine over time due to a condensation or polymerisation reaction of short-chain polyphenols.
- Hydrolysable tannins: Complex esters of glucose and gallic acid. These are leeched into wine by oak barrels.
The hydrolysable tannins oak leaks into wine have a different, nuttier taste than the condensed tannins naturally present in the wine. The flavours are compounded by the structure of oak itself.
Oak is a porous substance, allowing the flow of a small amount of oxygen between the maturing juice and the cellar air. But because it is relatively tightly grained, it does not allow Brettanomyces (a spoiling bacteria which tastes like mouse urine) or any other undesirables to pass through the barrel walls and allows for only a small, optimal amount of oxidation to occur. This gives the wine a complex, round, nutty taste, which does not occur in un-oaked wines.
Barrels also encourage the diffusion of vanillin, lignins and lactones alongside tannins, but tannins is the key flavour ingredient residing in the wood itself for wines.
Toasting
A nuttier taste can also arise as a result of extensive toasting of a barrel. When oak barrels are made, the wooden staves are held in place at one end by two hoops, and then steamed or heated in order to make the staves flexible enough to bend into the barrel shape. Upon toasting, the tannins are mellowed, some cellulose is degraded into vanillin, the concentration of enolic compounds increases and carbon deposits can form on the inside of the barrel, which also acts as an antibacterial filter.
Barrels can be toasted to various degrees — generally, the longer and more extreme the toasting of a barrel, the greater flavour it can impart to a wine and the darker the colour of the barrel. Burgundian wine barrels are often charred, an more extreme form of toasting which reduces the quantity of coconut-flavoured lactones but increases the amount of vanillin and spicy flavour compounds. The more extreme forms of charring are found in the fortified wine spectrum, as charring results in more carmelized flavours than the spicy vanilla from toasted barrels.
Malolactic Fermentation
Aside from tannins, the other major impact oak can have on wine is malolactic fermentation. This is an optional way of using the barrel which most winemakers seek to avoid. You’ll likely know it as the reason some chardonnays taste extremely buttery and rich, and others fresh and mineral.
Malolactic fermentation is result of the microorganism Oenococcus Oeni. The microorganism converts tart L-malic acid into the buttery L+lactic acid by means of a decarboxylation. These are the acids found in green apples (malic) and sour milk (lactic) respectively, and soften wines into something more buttery and smooth.
The oenococcus oeni occurs naturally in many oak barrels, although nowadays synthetic varietals are mostly used. As an added complication, sulphur dioxide must normally be used in conjunction with the microorganism, to prevent other bacteria which may have heterofermentative pathways and form undesirable end products. Malolactic fermentation in oak barrels can be avoided through very strict barrel cleanliness.
A History of Malolactic Fermentation
Malolactic fermentation has been observed since at least 1837, when Freiherr von Babo described it as a “second fermentation”. Pasteur also observed the transformation of malic acid into lactic acid, but even though his work with wine at the time meant he had just isolated the first wine bacteria ever, he got the culprit wrong. Instead, he blamed the reduction of malic acid on tartrate precipitation, which results in ‘wine diamonds’, or tartaric acid crystals. These crystals resemble shards of glass crashing out of the bottom of wines, and whilst dramatic, they are not linked to malolactic fermentation.
The Swiss oenologist Hermann Müller-Thurgau was the next to attempt to explain the fermentation in 1891, correctly concluding that bacteria were the cause of the reduction of malic acid, and in 1913 remarking that it was caused by bacterium gracile. Eventually, Ribéreau-Gayon then wrote papers lauding the benefits of the bacterial transformation of the malic acid, before Émile Peynaud eventually went on to fully map the mechanism of malolactic fermentation after advances in enzymology. Both Peynaud and Ribéreau-Gayon were extremely influential in the wine world, with Ribéreau-Gayon going on to found the Bordeaux Institute of Oenology and Peynaud becoming one of the most influential wine consultants in the world, as well as promoting the use of sterile, bacteria-free stainless-steel barrels in winemaking. Malolactic fermentation is a powerful and often mis-used tool in the oaking community.
French vs American Oak
Today, most wines are aged in either French or American oak barrels. The two strains of oak have slightly different compositions and impart slightly different flavours. In the 1970s, the great Californian winemaker Robert Mondavi pioneered an experimental approach to barrel suitability. He discovered that American (quercus alba) and French oaks (quercus petraea and quercus robur) have different natural concentrations of hemicellulose, and are able to diffuse oak flavonoid compounds into wines at different rates.
American oak has roughly four times as many lactone compounds as French oak, and therefore is often used for strong red wines and can lead to the over-oaked, buttery taste many drinkers associate with Chardonnays.
French oak has a tighter grain structure, and is dried in open air for 2–3 years, resulting in a mellowing of harsh tannins and flavonoids which does not occur for kiln-dried American oaks. French oak barrels impart a subtler taste due to the slower rate of infusion and overall lower levels of bitter tannins, woody-flavoured lactones, and lignin/vanillin compounds.
French oak barrels also tend to contain lower levels of anthocyanins, reddish-black pigments which can darken wines if left in the barrel for an extended period of time. French oak is also more tight-grained than American oak, so is more effective at keeping oxygen away from the wine and preventing any oxidative degradation.
Old vs New Oak
Newer oak barrels are able to impart more flavour into wines than older barrels, as the stores of naturally-occurring flavour compounds in the wood are leached over time, which has resulted in complex webs of tubes in many French cellars as wine flows from new barrels into old barrels as part of the ageing process.
Oak Barrel Alternatives
Barrels nowadays are very expensive for winemakers, with top-quality French oak barrels costing as much as $1,000 apiece. Barrelmakers known as coopers are represented through specialist agencies to help them cope with demand. As winemakers often require new sets of barrels each year to combat the leaching of flavonoids, this can prove very costly for cellar-masters lacking in financial resources.
The rising cost of oak barrels has led many producers to explore alternatives to barrels whilst still incorporating the effects of oaking into their wines. These alternative methods are highly divisive and there are few artisanal or first-growth houses for whom these methods would be appropriate. However, for industrial winemakers, oak chips and oak staves are nowadays often used. Oak chips are thrown into the vat of wine, the theory being that only the inner oaken surfaces of a barrel make contact with a wine, and inserting them into the wine in chip form maximises the surface area in contact with the wine and therefore enhances flavour in a more concentrated way than a barrel can. This method also avoids paying for the craftsmanship to shape the oak into a barrel, instead relying on a wood chipper and a treetrunk. Similarly, leftover barrel staves are inserted into the stainless-steel vats used to hold the juice and the wine swims around the oak, imparting flavour.
However, winemakers perhaps fail to appreciate that not only does oak impart flavour in the form of flavonoid compounds, but it also allows for a subtle oxidation of the wine to occur through the mildly porous sides of the barrel, which is missing in stainless steel vats, irrespective of the amount of oak placed inside. Fresh staves are often used in this method. These staves do not need to be shaped and therefore the oak is rarely dried out in the sun for 2–3 years as is custom for barrel staves, so many of the tannins remain harsh and the overall effect can be rather crude.
Overall…
After that whistlestop tour, what have we learned? Well, oak barrels are an expensive but effective way to alter the underlying taste of wines. Their use can lead to many different possible flavour combinations, from old French oak with no malo (subtle vanilla); to new charred American oak with malo (strong butter and caramelized sugar notes). They are so ubiquitous that the world’s 50 most expensive wines are all oaked in some way — bear these facts in mind to understand the science behind incredible wines, all over the world.
