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. |