Whisky Maturation

Some distilleries will tell you that there are three ingredients used to make whisky - Barley, Yeast and Water but all that will give you is a colourless moonshine.

Oak is the final ingredient which gives colour and flavour to whisky.

It's been compulsory for Scotch whisky to be warehoused in bond for a minimum of two years since 1915 with the age limit raised to three years in 1916. The Scotch Whisky Act of 1988 instructed distilleries to use wooden casks of a capacity not exceeding 700 litres for maturation. The Scotch Whisky Order of 1990 specified that the wooden casks had to be made of oak, but what are they, where do they come from and how are they used?

Let's do some research!

Cask is a generic name for a wooden container used to store foodstuffs, different types of casks have specific name - more of that later. All casks used in Scotch whisky maturation are made from oak, it's both flexible and solid.

Oak

Oak is part of the beech family, there are hundreds of different species of oak but only a few are used for whisky casks:

Species	Quercus alba	Quercus rober	Quercus petraea	Quercus frainetto	Quercus mongolica
Common name	White Oak (aka American Oak)	Pedunculate oak - “Strong oak” (aka European, English, Swedish, Spanish, French (Limousin), Russian Oak)	Sessile oak - “Oak of rocky places“ (aka French,  (Tronçais), Cornish, Irish Oak)	Hungarian Oak (aka Italian Oak)	Mongolian Oak (aka Japanese, Mizunara Oak)
Location	USA & Canada	Europe & Russia	Europe & Near East	Southeastern Europe	Middle East / Asia
Picture
Previous contents	Bourbon	Sherry, Port, Wine, Cognac (Rémy Martin)	Wine, Brandy, Cognac (Courvoisier)	Wine	-
Notes	Q alba tends to be less tannic than the European species it's quite rich in vanillins,  lactones and tyloses. They offer characteristic aromas of coconut and vanilla, bringing softness to distillates.  Straight with coarse, uneven texture and tight grain, usually harvested at 80-120 years of age.	Q robur grows mainly in rich soil, it has a large diameter, offering a large grain, which is more porous, resulting in faster extraction of wood compounds during the maturation process, and yielding complex structure and ample tannins.  Wide straight-grained with a coarse uneven texture, usually harvested at 80-120 years of age.	Q petraea trees grows in light, sandy, loamy and rocky soils. They tend to be lean and tall, with tight grain, so there is a slower extraction during the maturation of a spirit, they have a less porous wood and therefore have less tannins.  Medium-to-large pores, coarse tight grain, usually harvested at 200-250 years of age.	Q frainetto grows in rocky soil which leads to fine-grained wood with light tannins.  It has a tighter grain than other oak containing less tannin.	Q mongolica has a much higher moisture content than other types of oak, has  higher levels of wood vanillins and lactone    and doesn’t grow straight making it much more difficult to work, the wood is more porous making it prone to leaking. They tend to be used for finishing rather than full maturation.  Much softer and knottier that other species.

Whilst all the scientific names sound exactingly precise in fact oak trees are very prone to hybridization; the two European species, Q robur and Q petraea, could therefore share up to 50% of their genetic material within a given forest. In French forests used as coopering sources, this hybridization is common. Limousin oak, for example, is generally considered to be looser-grained because Q robur predominates, whereas Tronçais is primarily Q petraea. In both cases, the two species grow alongside each other, so while an individual tree may be identified as one or the other, their characteristics intermingle.

An oak tree might be 100 years old before it can be felled, even then some may only have a diameter of 30 cm. Some species of oak trees are not normally considered old until they reach around 700 years. Coopers can only use the main, lower trunk of the oak; branches create knots that would eventually mean leaks were they used for barrels, so at the stave mill the rest of the tree is cut away and sold off for other purposes, a single tree may only create a single cask.

         

Fresh cut staves. Photo Ben Leggett

The staves will spend the next two or three years stacked outside the stave mill, seasoning, as this happens large amounts of unwanted sap, moisture and heavy tannic compounds leach out of the wood but core flavour elements such as sugars and vanillins are kept. A lot of oak is now kiln dried these days at 40-60°C for about three weeks.

Casks

Low wood permeability is necessary in order to ensure a tight cask with little leakage. The impermeable nature of oak wood, together with its wide availability, were probably the main reasons for its initial use. The cooper's real skills come to the fore in the creation of a cask. Seasoned staves are heated and constructed into a cask.

A typical whisky barrel consists usually of 31 staves (8 for the heads) and a couple more for a Hogshead. Heating helps to bend the wood and seasons the inside of the cask.

My favourite diagram of this comes from Boutique-y's Ardmore bottlings label.

Where do casks come from?

Different countries have traditionally used different sizes casks to transport or mature  their wares so we now have a wide range of sizes available to the whisky distiller to mature their whisky.

Cask Types
Cask	Capacity (Litres)	Previous Contents	Notes
American Standard Barrel (ASB)	190-200	American Bourbon	By law are single use virgin oak vessels which are usually charred before use. Offers spice, vanilla, sweet oak notes.
Hogshead	225-250	American Bourbon	Build from staves from Barrels, the larger size being better for longer maturation periods
Quarter Cask	45-50	Whisky	Smaller sizes reduce maturation time - usually used to finish a whisky
Barrique	250-300	Cognac or wine	Notes vary according to the wine. Sauternes has notes of fresh bread, tarte au citron, and crème caramel. Bordeaux gives red berry, cherry, and plum.
Butt	475-500	Sherry	Offers dates, walnut, sultana notes.
Puncheon	450-500	Sherry or Rum	After butts, puncheons are the second-most common type of casks used to mature sherry. Dumpier than a butt, these are generally made with Spanish oak staves. Rum puncheons offer Molasses, vanilla, tropical fruits notes.
Port Pipe	550-650	Port	Port pipes are the industry standard cask for maturing port wine. Port pipes are long and similar in proportion to sherry butts, although their width is close to an ASB. Port pipes are generally only used to ‘finish’ Scotch whiskies for a final few years. Offers berry fruits, red currant, sultana notes.
Madeira / Marsala / Malaga Drum	600-650	Wine	Like sherry butts and port pipes, Madeira drums are the industry standard for maturing Madeira wine. However, Madeira drums are significantly squatter than these counterparts, and are built from thick French oak staves. Madeira drums are relatively uncommon for maturing Scotch whisky and are generally used to ‘finish’ aged stocks. Offers sweet fruits, figs, spice notes

Since 1938 bourbon has had to be matured in new oak wood, resulting in great number of reasonably priced used casks available from the USA to Scottish whisky industry. Most casks used to mature Scotch whisky are ex-bourbon. The barrels are usually dismantled for transportation and assembled again in Scotland. Usual practice is to convert the bourbon casks (150-220 litres) into bigger hogsheads (about 250 litres) by adding a extra staves of either new or used wood.

Sherry casks used in Scotch whisky industry are called Spanish oak casks, but in most cases they are Spanish coopered American oak casks, usually toasted but not charred (see below). Although made of the same Q alba oak, these casks produce very different Scotch whiskies from the ex-bourbon casks due to the differences in cooperage practices, seasoning effects the sherry wine and the different sizes of casks. Most sherry casks used to mature Scotch Whisky haven't been used to mature sherry but instead have been seasoned or rinsed with sherry before being sent to Scotland. 

French oak is used extensively in wine cooperages, usually made into 225 litre barriques.

Charring and Toasting

Different oak casks impart different amounts of flavour compounds: the species is the most important factor explaining the differences, but the origin, seasoning and toasting of staves is also significant. 

Speyside Cooperage. Photo @WhiskynStuff

Casks used to mature whisky have usually been used to mature other spirits (like bourbon), sherry, wine or even beer, sometimes charred virgin oak is used. The charring of casks dramatically affects the

volatile composition of the oak wood, and increases the levels of many cask extractives.

Charing involves burning the insides of the casks which converts the wood sugars into vanilla and caramel flavours. The more charring, the stronger the vanilla flavours. The carbon from the burned layer also acts like a filter to remove various impurities, such as sulphur compounds, from the spirit. 

There are four main char levels, simply graded from numbers 1-4, with the lower number representing a shorter exposure to heat and softer char level. 

• No.1 Char: 15 seconds
• No.2 Char: 30 seconds
• No.3 Char: 45 seconds
• No.4 Char: 55 seconds ('Alligator' char)

Toasting is slower at a lower temperature 

• 15 minutes of firing produces light toasting
• 30 minutes medium
• 45 minutes heavy

The picture below shows 7 staves : Untouched, lightly toasted, heavily toasted and char levels 1 to 4.

Photo: BourbonR Blog 

The inside of the cask can reach temperatures of up to 200°C in toasting and over 250°C in charring. Caramelisation of sugars in hemicellulose and lignin and to some extent in cellulose begins at about 140°C. Charred casks are usually rinsed with water and used to fill bourbon. Toasted casks are usually treated with saltwater, coarse wine, burning sulphur or ammonia and steam to kill potentially harmful bacteria (especially Brettanomyces). Filling the cask with liquid also removes loose wooden chips or sawdust and helps spotting the possible leaks in the cask. 

Maturation

Once a cask has been filled it will be laid down in a warehouse to mature, according to the SWA for at least 3 years - most are a little longer.

Two types of warehouse are used: a traditional dunnage warehouse is a windowless earth floor building where casks are usually stacked 2 or 3 high on their sides; a palletised warehouse is a lot taller and casks are stored on their end, on palates 4 or 5 high.

Oak is a porous wood, which means that a cask can be liquid tight, whilst allowing oxygen to enter through its staves. Through evaporation, the angel's share (read more here), the volume of the liquid decreases each year, leaving more room for oxygen inside the cask, essentially further speeding up oxidation. The spirit takes on more colour the longer it is left in an active cask.

Jameson Whiskey Barrels – New Fill, 3yo, 4yo, 12yo  and 18yo

Oak compounds

Flavourful whisky is thought generally to contain high levels of wood-derived compounds, the oak cask affects whisky by extracting the wood compounds that influence the taste directly or together with the spirit compounds or by removing or changing some compounds from the raw spirit.

Oak wood consists of cellulose (38-52%), hemicellulose (25-30%), lignin (22-25%) and extractives of the wood such as Tannin (5-10%). Oak cellulose is a linear chain of up to thousands of D-glucose-molecules and quite inactive in terms of flavour extraction during spirit maturation, extracting only some carbohydrates in high temperatures during toasting/charring. additionally American oak contains Lactone:

Cellulose : is a complex carbohydrate which consists of sugar molecules. It has limited or no effect on the spirit, but is crucial for holding the wood together.

  

Hemicellulose : a sugar based polymer composed of multiple sugars such as glucose, xylose, mannose, rhamnose, arabinose and galactose. When activated by heat they give deep brown colours and subtle almond, walnut, butter and maple notes from a lightly toasted cask. When the heat is turned up even higher to the levels of charring, most of these flavours are pushed aside by butter, rich caramel, caramel toffee and even licorice.

 

Lignin : concentrated in the oak’s cell walls, it's composed of two core structural parts, guaiacyl and syringyl, which are responsible for giving us sweet and spicy aromas, especially vanilla which is pulled from its chemical namesake - vanillin. Lignin congeners can be released very easily with only the smallest amount of activated heat however it’s the effect of heavy toasting and deep charing which breaks down the lignin further into steam volatile phenols responsible for delivering smoky flavour elements.

 

Tannin : there are many different sources of tannin in the natural world (such as grape skins and pips), oak tannins are unique in that most of them can be broken down, not by adding heat, but by simply adding water, they give fragrances like cooked apple. Tannins impart astringent flavours at least in the early phase of maturation and take part in various oxidative reactions removing sulphury off-notes and promoting colour stability. Tannin plays a more important role in long maturations, the wood chemicals reacts slowly with the oxygen molecules in the spirit over time creating a new compound called diethyl acetal which ensures that older whisky is left with rich and complex top-notes. There is a significant difference between oak species is the amount of tannins each contains: Q robur has most tannins, especially the more water-soluble vescalagin, castalagin and roburins. Q petraea has about third of the tannins content of Q robur, the tannin contents in Q alba are very low. These differences are partly due to the growth speed of the species as older trees have less tannins than younger oaks.

 

Lactone : a flavour element influenced by certain oils, soluble fats and waxes simply known as lipids which deliver a unique coconut-woody taste to a spirit. While present in all types of oak, lactones are found in their greatest quantity in Q alba (and much less so in Q mongolica), releasing almost all of its flavour attributes in the first year of a first-fill barrels maturation. Well seasoned and lightly toasted casks release further amounts of lactone while deep alligator charring (#4) can react the other way, reducing its effect.

 

Other important aromatic oak extractives include different eugenols (clove, cinnamon), β-damascenone (fruity, peach, cooked apple), cyclotene (toasty, caramel), hexanal (grass), trans-2-nonenal (saw dust, greasy), 2-octenal (green leaf, untoasted oak). 

When you char a cask the hemicellulose is broken down into its constituent sugars, these caramelize in a tight layer underneath the char in existing unburned wood. Then as the heat radiates and cools lignin breaks down into vanillin and tannic acid begins to turn into the colour red. You can only char a cask for so long before it burns through the staves. With toasting, the wood is not at risk, pre-toasting the wood helps to better control the vanillin and colour. 

The active carbon layer formed in charing has some filtering potential, removing especially sulphury aromas from the spirit. Some oak derived hydrophobic compounds also suppress the release of volatiles from spirit in room temperatures and mask some aromas especially when nosing whisky. 

Casks also allows evaporation and oxidation of the spirit and volatile flavour compounds through the headspace of the cask or by a lesser extent through the surrounding air through the wood or the bunghole.

Refill?

The oak’s effect on the spirit decreases over time and use, so how many times a cask has been refilled also impacts on the flavour. In a smaller cask, the surface of the wood comes into contact with a larger proportion of the liquid than in a large cask. This means maturation often takes place faster in a smaller casks like the Quarter Cask.

Casks can be rejuvenated by scraping the inside of the staves to about 5-10mm deep, recharring the cask and in sometimes seasoning it with sherry or must. This brings new active wood into contact with the spirit and allows the use of cask for refills. The influence of spirit reaches up to 20mm through the wood. Usually casks are rejuvenated after three fillings, although some companies rechar casks after each filling. There are considerable differences between a fresh cask and a rejuvenated cask as some flavour compounds are extracted faster and deeper from the wood than others in previous fills.

Casks can be described as ‘1st fill’ or 'refill'. An American whiskey cask or barrel that is being used to mature whisky for the first time is referred to as '1st fill'. It becomes a ‘refill’ cask when used for a second or subsequent time. ‘1st fill’ casks are more active in the maturation process of whisky, imparting stronger flavours to the whisky from the oak and the previous contents of the barrel. ‘Refill’ barrels, by contrast, are usually less active in maturing whisky, allowing the spirit to dominate the maturation process. Up until the 1980s it was common practice to season the rejuvenated casks as well as some of the new casks with pajarete to create a sherry-cask profile.

Rejuvenating used casks by re-charring increases the amount of colour, solids, fixed acids, tannins and aromatic aldehydes that can be extracted, increasing the viability of the cask. However, the same levels as those found in a new cask will never be reached and viability will once again decline with reuse.

Bottling

Once the Master Blender has decided that a group of casks are ready to bottle they are removed from the warehouse, a number of things could happen:

• they could be bottled as a single cask
• a number of casks could be married together then bottled as a single malt or single grain
• a number of malt and grain casks could be married together and bottled as a blend

Emptying a cask

It could be that the Master Blender decides that the whisky is still not quite ready and that the cask either hasn't had enough influence on the spirit or has had too much - in this case the spirit (or whisky) could be recasked into another oak cask for a period of Finishing or Secondary Maturation.


Finish or Secondary Maturation?

Some whiskies are recasked into other casks for a finish or secondary maturation - but these terms are very loose.

SWA Regs state:

"Some companies choose to “finish” their Scotch Whiskies, particularly their Single Malt Scotch Whiskies, to provide additional complexity to the spirit. This is carried out by further maturation in a different cask. An example of ‘finishing’ is where a Single Malt Scotch Whisky, which has been matured for 12 years in barrels previously used for maturing Bourbon Whiskey, would acquire a different character if “finished” for an additional period of time in a cask which has previously held Sherry or Port wines. The decision as to whether to “finish” a Scotch Whisky or not depends on the character of the spirit the company’s blender wants to achieve."

How long is a finish before it becomes a secondary maturation, what if the whisky spends more time in the second cask than the first?

When a cask has been recasked is it still a single cask whisky? It seems the SWA, through an unpublished technical update given to members, are saying it isn't and shouldn't be called such:

"The SWA’s guidance states that the term “single cask” should only be used when contents of the bottle have spent the entire maturation period in one cask: a whisky that has been finished in another cask shouldn’t be referred to as “single cask”"

Many companies are still using the term single cask for recasked whiskies so I guess it will take time to filter through. I know Loch Lomond have stopped using the term 'single cask' and now call them 'exclusive casks' instead.



Sláinte

"There is still a lot that even the most experienced spirit makers and scientists don’t understand about cask aging spirits. While much can be explained, there’s still an element of unpredictability to the whole science. Yes, maturation is part subtraction, part extraction, and part oxidation, but even two casks filled on the same day and stored next to each other will produce different results in the end. And so, the magic of oak maturation keeps us spirits fans yearning to keep learning—and tasting." Thijs Klaverstijn 2019

In conclusion I think the cask or casks that the whisky was matured in has a huge impact on it's final flavour, both on the nose and palate. Where that cask comes from, what kind or oak it is made from and what it's previous contents were are all really important; but the most important thing is that you enjoy the dram!

You can read the other parts of my 'ingredients' blogs here: Barley (coming soon), Water, Yeast!