The Effect of Wood - Maturation

The quality and flavour of Scotch whisky is a balancing act of taking the characteristics of new-make distillate and combining it with the maturation process. 

This process has a huge impact on the flavour of all spirits and wines.  Contemporary sensory scientists estimate that maturation can account for as much as 60 - 80% of the flavour in single malt whisky.

Until the mid Nineteenth Century most whisky was sold straight off the still and was available at 70% alcohol.  The process of maturation grew up almost by accident from the use of wooden barrels as transport and storage containers. It became clear that spirit benefitted from storage in casks.  Even after this discovery only a few connoisseurs matured their whiskies before the latter years of the Nineteenth century.  The whisky market during the late Nineteenth century extended dramatically and the industry began to mature both malt and grain whiskies as a basic standard. Finally in 1915 it became law that all whiskies sold in the UK had to be matured for a minimum of 3 years.  This law stands today.

Oak Wood and it's Effect on Flavour

(Also see SMWS article by Dr. Lumsden: The Wood Makes the Whisky)

Whisky is matured solely in oak wood. Experimental maturations in other woods, for example, chestnut have not been successful.

Oak is relatively unique among trees in being free from strong odours.  It contains only small quantities of tannins, all tannins are astringent and bitter but oak tannins are much less so than most and they play a vital role in the oxidation in the cask which is essential for fragrance and delicacy.

The three structural features of all wood are cellulose, hemicellulose and lignin.  The cellulose in oak has little influence upon maturation, however its hemicellulose and lignin are of great importance.  Hemicellulose caramelises the spirit and adds sweetness and colour whilst lignin is a good blending agent which pulls the flavours together, increasing complexity and producing vanilla-like flavours.

It is important to emphasise that oak wood is subjected to degradation by heating during coopering, the partial breakdown of wood structure is vital to flavour, and when applied to casks should be viewed as ‘heat treated oak wood’.  

The changes which must take place in spirit during oak wood maturation are as follows; production of colour, increase in complexity, production of fragrance and estery charcter, removal of off-notes (eg, sulphury, rubbery), production of astringency and development of mellowness (reduction of pungency).

Studies into maturation the of Scotch whisky have shown that the relationship between the composition of the wood and the changes sought during maturation are as follows,

The extent to which oak wood can carry out these changes varies between species and even the growth rate and age of the tree, therefore the selection of oak is extremely important.  Only in recent years has the need to specify oak casks to optimise their role in maturation been fully appreciated. 

Oak species

There are around a dozen oak species commonly used for maturation of wines and spirits around the world.   Scotch whisky is matured most often in American white oak (Quercus alba) and a few related minor species as well as Spanish oak (mainly Quercus robur) which is used to make a high percentage of sherry casks.   The American species gives a sharper and more vanillin type flavour and the European species is more resinous, caramelised and gives more oxidation.  Sometimes a European cask can give astringency to the point of being a fault.

Scotch whisky does not possess a woody or charred wood extract flavour and has therefore relied upon used casks as a means of reducing wood flavour.  New casks are rarely  used as ‘woodiness’ is not a desirable characteristic of Scotch.  The wood effect in Scotch whisky is effectively subliminal and very complex. 

Congeners, What are they and what do they do to flavour? 

In the alcoholic beverages industry, congeners are substances produced during fermentation. These substances include small amounts of chemicals such as acetone, acetaldehyde, and other higher alcohols, esters, and aldehydes (e.g. propanol, glycols, ethyl acetate). Congeners are responsible for most of the taste and aroma of distilled alcoholic beverages, and contribute to the taste of non-distilled drinks.

Types of cask used

The whisky industry relies on two main types of used cask to mature spirit - ex-sherry casks from Spain and ex-bourbon casks from the United States.  Ex-bourbon barrels currently account for around 93% of current cask imports, however, the number of sherry casks currently in use is higher than 7% as they may be used more times and have a longer life span than ex-bourbon casks. 

The Scotch whisky industry re-uses casks as often as they will yield an acceptably matured whisky and every cask is inspected before it is refilled.  Spanish oak has thicker staves than American oak casks (1.25” compared with 1”) and Spanish oak is richer in tannins, it is for this reason it lasts longer than a bourbon barrel. 

An ex-bourbon barrel will typically be used for up to 8 four year cycles. A Spanish oak sherry cask may be used for two 10-12 year single malt cycles followed by up to 6 or 7 four year blended whisky cycles.  If a sherry cask is used for 4-5 year old blends from its initial filling its usage is almost indefinite in practical terms (eg ten times or more or until it falls apart).

More and more experimental maturations are now taking place with oak casks and port pipes and madeira casks have recently been used for whisky maturations. Some distillers employ the practice of ‘finishing’ their mature whisky by decanting it into a second type of cask for the end of its maturation, eg for the last two years.  It is highly likely that we will see more experimental maturations in the near future.

Sherry Casks

Traditionally the major container used for Scotch whisky was the sherry cask, used as a means for shipping sherry to the UK.  As the Port of Leith became a major point of entry of sherry into the United Kingdom, empty sherry casks became readily available for use in the Scotch whisky industry.  Sherry casks soon became established as the finest containers available to the burgeoning whisky industry of the last century.  During all this time, however, no records seem to have been kept as to the identity of the ideal sherry cask for whisky maturation.

The use of sherry casks has, however, dwindled over time as the size of the sherry industry has declined and supplies have become increasingly expensive, particularly since the Spanish government decreed in the 1970s that all sherry had to be bottled in the country of origin.  Today it costs approximately £550 to buy a sherry cask.

The most widely used procedure for sherry cask production today is to use Spanish oak grown in the North West of Spain (Galicia).  Trees are purchased from local farmers and landowners, sawn to stave blanks in the North then transported to the Costa del Sol to season (dry).

Seasoned wood is built into casks using local traditional practices then delivered directly to the sherry producer.  Their procedure is to carry out two fermentations in cask then to use the cask for two to three years for the ageing of dry oloroso wine.  In this ageing process the major change to the sherry is one of oxidation. 

The storage of  sherry casks takes place in bodegas.  Bodegas are warehouses in which the casks are assembled in a pyramid form, typically three or five casks high.  Newly filled casks are placed on the top of the pyramid and when sherry is needed for bottling the product is drawn from the bottom casks (up to one third in a three high pyramid and one fifth in a five high).  These casks are then refilled with wine from the second bottom layer, and so on.  In this way new wine always enters at the top and is fully matured when removed at the bottom.  Casks used for sale as whisky maturation vessels are simply placed on the top layer for two years, or for the specified amount of time agreed with the prospective buyer.

Ex-sherry casks are of two types, butts and puncheons.  Sherry butts have a nominal capacity of 500 litres, whilst puncheons may be a little smaller (around 470 litres).  Butts are the taller of the two at around 54” high whilst puncheons stand at around 44”.  Both may be made from local Spanish oak (from the North West of Spain) or from American oak.  They are typically used for 4-5 years for sherry production before being imported to Scotland for use in the whisky industry.

Effect on flavour

Prior to filling with sherry, butts and puncheons are toasted.  This means that after being left out to dry naturally in the sun they undergo a very light charring. Sherry casks contain wine residues which impart colour and sweet, sherry-like aromas to spirit.

Compared to American oak ex-bourbon barrels the Spanish oak sherry cask produces a more intensely flavoured whisky and is often used as the ‘base’ of a blended whisky.

To a large extent whisky from a fresh sherry cask is dominated by the sherry flavour and for that reason, is not universally preferred, some people believe that the sherry flavour masks the true individuality and complexity  of the whisky. 

The type of sherry cask most often used to mature whisky is that which has been used to mature oloroso sherry, the richest and fruitiest sherry and the one which is darkest in colour.  Casks which have been used to mature fino and amontillado sherries are also used, but less frequently. 

American oak casks which have been used to mature sherry give a different flavour from  Spanish oak wood.  American oak is traditionally used in Spain to mature fino sherry and has a less positive flavour, it tends to make whisky rather light, fragrant and sometimes a little ‘nutty’ in flavour.

Distillers who like to use sherry casks to mature their whisky tend to use almost exclusively the Spanish oak type.  If the mature whisky lives up to their expectations it will be dark in colour, slightly viscous (thick) and will have a clear sherry flavour. 

Bourbon Casks

Ex-bourbon casks fall into two categories; re-made or ‘dump’ hogsheads and ‘after-bourbon barrels’.  Re-made hogsheads are transported from the US as staves and are re-assembled in Scotland with 25% new staves and new heads added to give a capacity of 250 litres.  After-bourbon barrels are transported whole and have a capacity of 200 litres.  Both casks will  have held bourbon for at least 4 years. 

Traditionally American oak was sourced in the Ozark mountains of Missouri, a remote district with poor soils which grew trees too small for most purposes other than making barrels.  In recent times the bourbon industry has been sourcing timber from further east.  This area is more accessible and has better soils where the trees grow faster and promote a greater yield. 

Many American cooperages now kiln-dry their timber rather than leaving it out to season in the open air.  Open air drying takes around 18 months, whereas kiln-drying only takes 23 days.

Prior to being filled with bourbon oak casks are charred. This means that the cask literally has its insides burnt.

Oak is more reactive when a barrel is charred, causing a higher congener level in the product of the first filling than that of a comparable filling in an uncharred barrel. As the barrel is reused, the product congener level approaches that of the uncharred barrel. 

The %ABV  on filling the cask also affects the levels of congeners present in the final product. As the entry proof increases, those congeners that originate from barrel components decrease. Warehouse temperatures are directly relatedto the rate these congeners are formed.

 this releases vanillin, a very sweet substance which imparts sweetness to the bourbon and gives it distinct vanilla tones. Such casks are only used to mature bourbon once so that both the oak and the charring are new. The Scotch whisky industry, however, uses the casks a number of times and can get an even longer life span from an ex-bourbon cask by re-charring or ‘rejuvenating’ it.

Effect on Flavour

Malt whisky which has been matured in a first-fill barrel/hogshead has a typical vanilla-like aroma.  The charring of the ex-bourbon wood assists in removing off-notes.  Ex-bourbon wood produces a mature whisky which is sharper than one from a sherry cask and which ‘pulls’ the blend together.

The recent trend towards kiln-drying in the USA makes no difference to the flavour of bourbon, however, it has a dramatic effect on the second incumbent of the cask.

Swan and Gray discovered in the late ‘80s that to achieve the desired rate of maturation, at least three quarters of the cask should be made from slow-grown and air dried oak.