Fact File


 

No. 1:
Oak and
Oak Barrels

Oak and its sources

Oak is a hard, supple and watertight common hardwood of the temperate northern hemisphere that has a natural affinity with wine and is physically easy to work. There are three main types: Quercus Alba (found mainly in the U.S.); Q sessiliflora (sessile oak); Q robur (pedunculate, mainly European). The top 5 producers (in million m³) are: US (700), France (420), Yuogoslavia (180), Hungary (100), Bulgaria (80). UK is 12th (33).

American oak is much less porous and can be cut in curved stave shapes, whereas European requires cutting (or, preferably,splitting) parallel to the grain and bending into shape. The best American oak is reckoned to come from Minnesota and Wisconsin, and like all US oak has a more obvious vanillin flavour and astringengy compared to European oaks which are smoother and more subtle. It is traditionally used in powerful wines from Spain (eg Rioja), N and S America and Austarlia.

European oak is mainly robur which tends to have a wider grain than sessile, although the two are rarely distinguished in coopers’ workshops. France: produces the standard by which all other oaks are judged. It has 40% forest of which a third is oak. Barrel oak comes mainly from northern forests: W Liore/Sarthe (tight grain, highly prized); Limousin (wide grains, tannic, popular for brandy); Nievre/Allier (includes individually-named forests, such as Troncais as well as universal ‘bois de centre’; tight-grained); Vosges (clear, white, tight-grained); Jura/Bourgogne (mainly for Burgundy); Argonne (little used for barrels). Surprisingly for France’s love of delimitation there is no AOC, leading to some confusions of nomenclature. Some oak comes from Eastern Europe, but is declining, as is oak from the former Yugoslavia, although still popular for large vats/casks used in Italy. Oak from the Baltic/USSR regions were once prized, but through mis-management future use is uncertain.

Oak is used in wine-making in various forms: as the wood of barrels used for storage, transportation and in the wine-making process itself; and in the form of oak chips, shavings or other flavour additives. These and their affects on wine are described in the following sections of this fact file.

Barrels and barrel-making

Barrels (often referred to in the UK as casks) are cylindrical wooden containers made of staves with hoops, circular heads at either end and at least one bung hole. The term refers mainly to such containers for the storage and, especially, transportation of wine and as such of a size capable of being moved. Larger wooden vessels are known as vats, though all regions have their own terminology.

Most scholars acknowledge the celts as being the inventors, as 7th C BC Armenian barrels made of palm-wood are known to archeology and silver fir-wood barrels were encountered by the Romans in the 50s BC in a fully-formed state. Whilst these were certainly used for wine they were also used for other liquids and for salt.

Barrels are made from long sections of oak trunks from trees ideally 100 to 150 years old. Logs are cut and split into four lengthwise, the bark and sap wood removed. American oak is cut curved by saw whereas European oak is cut or cleaved (split) along the grain into staves which are then bent. Staves are then dried in air for 18 to 36 months or kiln for a max of 12 months. Air drying, either in stacks in the forest or in the cooper’s yard (which can allow for periodic water spraying to keep the moisture to about 17/18%), is preferred as it dries the wood more evenly, reduces tannic effects and retains aromatic qualities, through the conversion of bitter components to neutral ones and release of glucose and polysaccharides by enzymatic moulds on and in the stored wood.

Staves are trimmed into oblong lengths with a double taper, traditionally called ‘dressing’ and are joined on a jointer known as a colombe and given their final shape. They are then fitted onto a frame and arranged around an iron ‘raising up’ hoop. Shaping requires heat which also modifies the wood’s physical and chemical composition. Heat can be provided by natural gas, steam or boiling water or the flames from burning wood chips - or a combination of any of these. With fire the barrel is assembled over a metal fire-pot called a chaufferette. The cooper hammers home temporary iron hoops whilst dowsing the wood with a damp cloth. Non-fire methods can be controlled in such a way as to prevent the wood blistering and are often therefore preferred.

However, many coopers - and winemakers - use fire to give different degrees of toast to the inside of the barrel which affect the taste of the wine. No or light toasting exposes wine alcohol (an effective extractor of flavours) to the maximum amount of wood tannins which can make the wine more tannic and atsringent and so is used for very fruity and strong wine. Medium toast (about 15 minutes burning) increases coffee and vanilla flavours in wine but with less tannins. Heavy toasting creates a barrier between the wine alcohol and the wood tannins, so is much more suitable for lighter, more subtle wines, wherein toasted bread, roasted coffee, ginger and smoked meat flavours are imparted.

The heads of the barrels, comprising five or six straight staves pinned together, are shaped (usually circular or oval) to fit into a groove (the croze) cut in the inside ends of the side staves. To finish, the outside is planed smooth and the barrel is filled with steam or water: if it doesn’t leak the bunghole is drilled and the temporary iron hoops are replaced with metal or chestnut ones. Shaving and re-toasting of used barrels is possible, but rarely pleases.

Barrel maintenance

Before use new barrels are often rinsed with cold or hot (never luke warm) water and/or steam, or amonia and sometimes vigorously agitated to remove harsh wood tannins. Soda ash used to be used but was found to remove toast and therefore accentuate tannin transfer. The exterior of the barrel is occasionally treated with linseed oil or a commercial mould retardant and sometimes marked with a cellar management identifier. Storage conditions can affect the wine: too cold and it will not develop; too warm and off-flavours and bacteria will form and ageing takes place too rapidly. If the atmosphere is too dry excessive loss through evaporation may result. The ideal is 13ºC/55ºF and over 70% humidity. Too high a humidity can result in mould problems, and if empty formation of bacterial blight and volatile acids.

Because of the expense of new barrels and also due to choices made by wine-makers to use older or pre-used barrels (where time or previous use tends to reduce the harsher or more obvious influence of the wood, eg in Burgundy), it has become relatively popular to re-use barrels. Such barrels, if not used directly without further treatment, can be ‘enhanced’ by being disassembled, the staves shaved of all pigmented wood and then re-toasted. Only if the wood is less than three-years old is this effective. A marginally better alternative has been to insert new oak boards into the head.

Bungs are made of glass, plastic, rubber, earthenware, silicon or wood and act as the barrel’s stopper. Barrels can be stored so that the bung is at the top - ‘bung up’ (which allows gas escape) or at 2 or 10 o’clock - ‘bung over’ (which keeps the bung moist and sealed). Traditionally, in Bordeaux and Burgundy barrels are filled through a top bung and racked through a racking bung in the head of the barrel.

Most wineries ‘top up’ barrels, between twice a week to once every six weeks, to compensate for evaporation which varies in direct relation to temperature. Unused barrels can become susceptable to acetobacter growth, so nowadays are usually treated with sulpur dioxide and inverted unbunged to allow moderate air circulation, although close management of barrel stocks to avoid residual excess of barrels is much more widely practised.

Barrel types

These vary considerably from region to region and before the 1960s growth of stainless steel vats included types which strictly speaking were vessels larger than barrels, many of which still remain in the lexicon such as tonneau (Bordeaux 900-litre, equivalent to four barriques and still the measure used in Bordeaux trade and equals 100 cases of wine) and fuder (Mosel 1000-litre, similar to the 1200-litre Stück from the Rhine).

The barrique bordelaise of (since 1866) 225 litre capacity and about 95 cms high is probably the most famous and used widely all over the world. The ‘chateau’ model comes equipped with head crossbars and both top and racking bungs. The Burgundian equivalent is the slightly squatter 228 litre pièce standing at 88 cms high and having a more pronounced bilge to collect lees since racking happens less in Burgundy. Chablis used to have the diminutive feuillette of 132 litre capacity but this is now rare although prices are often quoted in this unit. Cognac barrels have recently settled down to a standard 350 litre type. Other types and capacities are found all over France ranging from the demi-muid of Chateauneuf (600-litre) to varying sizes of foudres common in Alsace.

Spain uses a butt of various sizes for sherry and normal barriques for other wines. Portugal’s measure is the pipe but cooperage varies considerably in size and shape. Italy has large but variable-size botti barrels as well as increasing use of the barrique (called a carato) although since the 1970s rise of super-Tuscan wines which use barriques the French name is incorrectly used for any small barrel: vin santo uses 50-225-litre caratelli. Hungary traditionally uses 136-litre Gönci for Tokay.

In the new world the USA uses either old 190-litre American oak barrels from the whiskey business or most commonly imports French barriques. Australia and New Zealand have quite widespread use of the mostly 300-litre hogshead as well as French barriques and pièces and also employs 450-500-litre puncheons but these are often too large to move about and for many wine-makers don’t impart sufficient wood flavour.

How barrels are used in wine-making

Barrel fermentation

Many top quality whites undergo barrel fermentation before barrel maturation, helping to integrate wine and wood. In some areas, notably Burgundy, California and Australia some winemakers put partially fremented red wine into barrel to complete the process. For whites it provides protection against oxygenation, allows extraction of additional flavours and facilitates lees stirring - the latter contributing to softer and better oak integration with yeasts biochemically enhancing aromatic molecules and complex sugars leading to an apparent richness and length of flavour on the palate. Stirring of lees also acts as a barrier against over-extraction.

Barrel fermentation also leads to paler white wines since oak phenolics extracted from the wood cause colour compounds to be precipitated out. The general higher-than-vat temperatures in barrel during fermentation cause loss of floral and tropical fruit flavours and reduction of perfume/soapy esters. Because barrels are small and each fermentation unique this often leads to a greater range and complexity of flavours which go to make the final wine.

Barrel maturation

Maturation in barrel encourages stabilisation and clarification, deepens the colour, softens the tannins and increases the complexity of a wine. Along with wood flavours a secondary flavour effect is that of slow oxygenation which reduces grapey primary aromas, agglomerates tannins so that white wines become golden and red wine anthocyans change from colourless to emphasise the wine’s redness.

Bordeaux provides the paradigm for Cabernet Sauvignon/Merlot maturation: light/medium toasted barrels are used immediately after malolactic fermentation for two years, racking every quarter with fining at the beginning of the second year. Pinot Noir on the other hand was found to respond better to different treatment due to its relative lack of tannic structure: in Burgundy and the New World heavier toasted barrels are used to act as a buffer between the oak and the wine and used immediately after alcoholic fermentation, allowing the malaolactic to take place in the barrel producing better wood-wine integration and increased complexity of flavours. This has now spread to New World Cabernet, Syrah, Zinfandel and Merlot production.

An alternative to barrel maturation is cask ageing where large wooden containers are used which impart no wood flavour (often due to great thicknesses of tartrate-crystal build-up on the vessel walls through repeated re-use - sometimes cellar workers are lowered in to scrape these off between fillings!) but do help mature the wine (the tartrate lining helps to precipitate this out of the newly-filled wine).

How oak influences wine

The substances which are directly extracted from wood are:

  • Lactones: give a coconut-like oaky aroma, increased by toasting, decreased by air seasoning
  • Phenolic aldehydes: such as vanillin the vanilla-essence flavour compound which is increased by toasting and air-seasoning, but reduced by barrel fermentation
  • Terpenes: essential oils, found in fruit and tea, and more dominant in American oak - thought to influence flavours but in an unknown way
  • Carbohydrate degradation products: such as furfurals produced from toasted wood sugars producing a bitter almond flavour.
  • Maltol and cyclotene, also products of the toasting process, have caramel-like flavours but also act as flavour potentiators, increasing the perception of other flavours
  • Tannins and other phenolics: give colour and astringency but also act as buffers or sponges against excessive oxidative/reductive aromas

In addition wine is biologically active: yeasts transform bitter furfurals into a range of flavours from smoked meat to leather; bacteria, particularly in white wine, transform a number of flavourless extracted compounds from the wood into highly aromatic ones imparting smoke, cloves and coffee notes.

In summary the following factors affect how wood influences wine:

  • type of wood (or type of oak)
  • manufacturing process - split or sawn; seasoning method (air- or kiln-dried, and prevailing temperature and moisture); toasting degree and intensity
  • size of container - ratio of wood surface area to volume of wine
  • age of container - can range from new/unused to successively older/more re-used barrels
  • time - or length of exposure of wine to the wood
  • vintage - its character provides the basic wine structure and degree to which it is susceptible to wood influence
  • wine-making techniques - fermentation or maturation in barrel
  • storage conditions - temperature, humidity, draughts, movement.

Magic or slight of hand?

The market for oaked wines has been so influential that some winemakers have sought to obtain the influence of oak in their wines without the expense of using barrels. Whilst it is legal in most areas to add oak shavings or chips (from pencil-shaving to cashew-nut size), oak essences or powders are not permitted. In both cases they can add oak flavour but can raise acid levels unacceptably and are unlikely to contribute to the ageing potential of the wine. Chips are, however, considerably cheaper than barrels and can reduce costs by more than 20 times. Lables with phrases such as ‘oak maturation’ or ‘oak flavours’ only may indicate extraction of such flavours without cooperage. Some feel that this is a slight of hand and not really in keeping with wine as a quality product

But there is no doubt that the use of oak can have a magical affect on wine and the wine-maker is placed in the role of magician. For them they have to make judicious choices, but the real magic is in the harnessing all the factors set out above to produce a wine which is more than the sum of its parts, true to its character and terroir, and is an evocative expression of a living, organic product of nature. And that's no mean feat!

Acknowledgements/Further reading

There are many wine books on the market, but few go into the intricacies of the use of oak in great detail. The best sources of information and those used here are:

The Oxford Companion to Wine edited by Jancis Robinson (ISBN 0-19-866159-2), which is the almost biblical source for facts on
nearly every aspect of wine

The Art and Science of Wine by James Halliday and Hugh Johnson (ISBN 1-85732-422-6), a most delightful exposition of the artistry and the technical expertise required to make good wine, expressed in everyday language - an excellent book for the keen wine enthusiast who wants to know a little bit more about wine-making.


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© Cooden Cellars 2005

Last updated: 18 April 2005

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