Oak & The Cask Ageing Of Spirits

The absence of amphorae beyond the Iron Age means it is fair to conclude that wooden containers had by this time replaced clay ones. The existence of straight-sided, open wooden containers, employing the craft of the cooper, is visible in Egypt as early as 2500 bc. Enclosed wooden barrels were first made during the Iron Age and by the 1st century bc were widely used for holding liquids such as wine, beer, milk, olive oil, and water.

With the development of trade and transport routes, carters and shippers discovered that sealed wooden containers were stronger than clay vessels, and the skills of barrel-making grew in importance. Thus, the craft of cooperage and the subsequent development of the cooper’s guild was established, probably originally in Rome. By the 2nd century wooden chests, casks and barrels had largely replaced many other containers.

The most significant advantages of wooden barrels were, firstly, their strength and ability to take the odd knock; secondly, the barrels could be easily rolled on and off carts and vessels ; and thirdly, certain goods—such as wine, beer and spirits—actually benefited when stored in wood.

By the Middle Ages coopering was a structured industry in Europe and by the end of the 17th century it had also established itself in America. The Spanish were the first to take American oak logs back home, thus beginning a tradition of ageing Jerez and Rioja in American oak barrels. When metal and plastic containers became available during the 19th and 20th centuries they became the receptacles of choice for most other industries, so today the wine and spirits industry is virtually the only remaining market for wooden barrels.

Casks are still made of many types of woods, including chestnut, cherry acacia, and of course oak. Oak wood is preferred to most of the others for spirits because it is harder and has sweeter and more aromatic phenolic substances—tannins and lignins—that aid the development of complex flavours in the spirit. One of the most important aspects of the wooden cask is its re-use, often necessitated by scarcity or price, this second use has enabled distillers to produce variations in the spirit character from a single distillate.

Ageing spirits in oak barrels is all about the complex interaction between the wood and the spirit coupled with the oxidation and evaporation levels. High-strength spirit will dissolve some of the oak’s elements and with time these develop and become absorbed into the spirit forming complex flavours and aromas. It should be noted that the process also includes the subtraction of undesirable components, particularly sulphides, by absorption into the char and through evaporation.

Maturing in Oak

Maturation has often been described as controlled oxidation, which helps to soften and mellow the natural harshness of a raw spirit. The density of the oak plays an important part here as it only allows slow oxygenation and slow evaporation of the spirit. This evaporation—referred to poetically by the French as la part des anges or the angel’s share—results in some loss, which over many years makes the spirit creamier, richer, smoother, more unctuous and more concentrated. Depending on the climate the rate of evaporation per year varies for example it could be as little as 2% in Scotland but as much as 8 % in the Caribbean.

Aside from the type of oak, the most important factors are the age of the wood (affecting how much the wood allows the spirit to extract) and the size of the cask (determining how much wood is in contact with the spirit). Careful management of new and old wood will allow the spirit to develop the desired flavours.

The Composition & Structure Of Oak

The trunks of an oak tree are constructed of tubes, which are plugged by structures known as tyloses. The presence of tyloses determines whether an oak species is porous or non-porous. There are two types of oak tree, red and white, of which the former is porous and therefore of no interest here.

The various species of white oak are, depending on their relative tylose content, to a greater or lesser extent non-porous. Barrels made from them do not leak yet their density allows slow oxygenation and evaporation of maturing spirits. The heartwood of the white oak consists of four main elements:

Insoluble in both ethanol and water, this is the fibre that holds the wood together.

Soluble in both ethanol and water, A family of aromatic compounds, notably vanillin, coniferaldehyde, syringaldehyde and synapaldehyde produced from the lignin during oak breakdown and toasting.

The wood’s natural sugars, especially glucose, arabinose and xylose which yield soluble caramels after the charring or toasting of the cask. The heavier the toast, the more caramelised the sugars, and therefore, the greater the potential for various sweet aromas, flavours and colour.

Tannin itself can be between 1-2 % of American oak and 7-8 % of French oak. The main components of tannins are polyphenols which can oxidise during maturation to give further more complex aromatic flavour compounds. Lactones are of two major types, cis-lactones which give woody flavours and trans-lactones which give coconut type flavours. American oak can contain as much as twenty times the lactone content of European oak so one should expect ex-Bourbon casks to be high in these components. The level of tannin and lactone released will depend greatly on the type of oak and the char level. Heavy charring, is also known to increase conversion of what has become known as the ‘whisky lactone’ cis-3-Methyl-4-octanolide which is detectable to the human palate at one part per million!

Relevant White Oak Species

Prevalent in the east-coast forests of Maine and Pennsylvania through to Wisconsin and Minnesota and then down to Missouri and Arkansas ; this species is particularly non-porous. A very strong oak which offers structural differences in its hemicellulose and lignin resulting in more intense vanilla, wood sugars and toastiness. Its density, high tylose content and straight grain means higher yields, machine cutting and subsequently lower costs. Canadian oaks have similar properties but produce less vanillin.

Found throughout Europe this hardy oak is lower in tannin than other European species. These trees grow slower and are smaller, creating fine grain and extremely subtle extraction. Research shows that its hemicellulose breaks down more easily than other similar species, thereby forming a different spectrum of toasty aromas. More commonly known as sessile oak, this tight-grained wood is much desired by winemakers.

Grown in the Alliers, Nevers, Tronçais and Vosges Forests, this species of sessile oak produces spice notes that stem from naturally occurring extracts. With lower tyloses content than Quercus alba this fragile oak needs to be split along the grain in order to produce non-porous barrels.

Grown in the Limousin Forest this is a more solitary tree. Its preference for soil rich in water results in its more rapid growth, thicker trunks and coarser grain, which subsequently allows above-average evaporation from casks made with it. A wide-grained wood with excellent tannin levels but offering less in the way of lignins than other species. As with Quercus petraea it must be split along the grain rather than sawn in order to ensure that the resultant barrels do not leak. It is also found in Bulgaria, Hungary, Russia and Slovenia where it has a finer grain, moderate tannin content and a lower quantity of aromatic components.

How Are Barrels Made?

The preferred oak type is selected, and block staves are cut. In America the wood is sawed into staves and there is little waste. Sawing, however, exposes more of the grain to the spirit, and this tends to raise the tannin levels and the astringency of the oak. European oak, however, is more porous than American oak and must be split along the grain of the wood, rather than sawn, in order to avoid subsequent barrel leakage. Not only is this much more time-consuming, but there is considerable waste; accordingly European oak barrels can be twice as expensive as those from America.

The wood then needs to be dried; there are two primary options:

A quick and relatively straightforward method. However, its detractors argue that it does not always dry the wood properly which results in leakage, and that it retains too much of the feistier elements of wood character which may result in imbalanced spirits.

This method can take up to three years and requires unpolluted air. It has the effect of leaching out tannins and lignins, thus imparting a more subtle character to the spirit.

Much American oak is kiln-dried, most French oak is air-dried and many European countries use a combination of the two. The ideal result is a humidity level of 12-15% within the wood.

The cut staves next need to be trimmed and shaped to fit the cask. Using heat, the wood must be bent so that the rigidity of shape and the metal hoops are all that are required to hold the barrel together. The method chosen to heat the staves—options include open or gas fires, steam and immersion in boiling water—determines in many ways the character of the spirit ultimately aged in the barrels. The barrel ends are fitted, sometimes in a different oak to allow additional vanillin or tannin extraction and the barrel is filled.

When fire is used the inside of the barrel becomes toasted and the end user may specify a light, medium or heavy toast when purchasing barrels. The heaviest toast or char, where the inside of the barrels are actually set alight, is very popular in America. This breaks open the grain enabling huge amounts of the oak’s elements to be absorbed as well as imparting charred notes to the spirit. In general, toasting the staves breaks down the woods lignins making it easier for a spirit to absorb them, but it has an inverse effect on the tannins, which become progressively less accessible to the spirit as the level of toast increases. Almost all spirits casks are toasted to some extent, during or following production.

The capacity of a barrel comes from both practical experience and historical use. From large scale storage in vats that can also be used for blending, to smaller moveable barrels and casks.

In one case the volume of the wine barrel was equal to the amount of wine from grapes harvested from a certain piece of land. This was most common in vineyard areas as a vine worker’s wage could be measured in barrels, which in turn lead to measuring the harvest of an entire vineyard in numbers of barrels ; this was generally around 225 litres from 800–1000 vines planted on 500 acres, or half a hectare.

American oak casks produced for the bourbon trade are between 180–200 litres in size, the hogshead often used for whisky is approximately 250 litres, the most common cognac casks are 300–350 litres and in the Armagnac region casks are often 400–440 litres or even up to 800 litres.

Wine barrels are different again with the traditional Bordeaux cask holding 225 litres and made up of 26–30mm x 22mm staves while the more robust Burgundy is 228 litres in capacity and slightly shorter and wider than the Bordeaux with 27mm thick staves. Sherry and port butts are generally 470–500 litres while the port pipe is smaller in girth but greater in length.

Having decided that our spirit needs to be aged in oak, we now need to decide on the type, size, and toast/char of the cask; the alcohol content of the spirit on entry to the barrel and the length of ageing our flavour profile demands. The latter is complicated further by the age of the cask and the conditions of the ageing cellar itself.

Alcohol Strength

The proportion of lignin derivatives (lignin undergoes what’s been termed ethanolysis -ethanol reacts with the lignin to break it down into some of the important flavour compounds, such as the aromatic aldehydes note above), tannins, sugars and colour that can be extracted from the oak and therefore the weight of congeners that can be formed from these are dependent on the alcohol concentration.

Alcohol Content and Extract from American Oak Casks of 200l

Abv% Total Extract
20 6.31%
40 7.44%
60 7.68%
80 6.53%
95+ 3.70%

At this point it should also be noted that the higher the original alcohol content, the more reduction will likely take place when the final liquid is deemed ready for bottling and post-ageing reduction also reduce the cask’s contribution, the flavours we have been trying so hard to capture and marry with our original spirit.

So, for different spirits in standard 200-250 litre casks it has been determined that there are optimum barrelling strengths; for malt whisky 55-65% and for rum perhaps 65-75%. These offer the potential to maximise the balance of extraction and colour, without taking in too much tannin. At 55-65% abv, it was also found that the oak had a higher propensity for porosity of water rather than alcohol, therefore allowing the alcoholic strength to remain higher and also allowing for an increased retention of the fusel alcohols, acids, esters and aldehydes. This however appears to be reversed in casks that have been used more than once so grain whisky and rums can age for longer in older casks as the porosity is reduced and many of the harsher tannins have been diminished by the previous contents.

Warehouse Conditions

Humidity affects the porosity of the wood and alters the balance of water to alcohol evaporation. Lower humidity has been found to aide extraction of lignin and vanillin and increases the higher alcohol evaporation levels.

Temperature also works in two ways; the higher it is, the greater the evaporation of both water and alcohol and the faster the oxidative esterification reactions take place. A potential advantage in building flavour but one which costs in terms of lost alcohol. Tropical temperatures, especially when there are variations between night and day, are ideal for faster aging, but altitude because of air pressure difference may hinder this. It is known that constant movement of the casks during the aging period also encourages the process but this is obviously not a practical approach.

The Length of Ageing

If we consider the distillate we produce and our warehouse conditions do not alter greatly during the ageing period, it is the final flavour profile we want that will determine the length of time in cask. In whiskies and brandies, there are reactions that only take place over lengthy periods or compounds that require building to certain levels that produce our intended flavour. This brings us thankfully back to human choice!

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