Box Whisky - Advanced Master class

Box Advanced Master Class No. 1

Number 1 Toasting levels s the first in the Box Advanced Master Class series. The intention is for the series to be educational and aimed primarily at people wishing to broaden their interest in whisky.

The contents of this box were matured for 30 months in 100 litre casks made from new Swedish oak with five different toasting levels. It is therefore not old enough to be called whisky, but this is of no consequence here as the intention is only to understand how different degrees of toasting influence aroma and palate.

Oak is, and has always been, the predominant type of wood used for storing whisky, although casks have been made from e.g. chestnut in the past. In Great Britain whisky must be matured for three years in oak casks with a maximum volume of 700 litres, even though other types of wood may be used in the remainder of the EU. Oak has proven to provide superior maturation casks as it is relatively dense and imparts fantastic tastes to all kinds of alcoholic drinks. The oak genus (Quercus) includes several hundred species and probably just as many hybrids, but only a handful of species are used for maturing whisky.


Oak is usually 80 to 225 years old when it is felled and cut into suitably long pieces. The logs are split or sawn into staves which are allowed to dry and season in stacks. The fast-growing American oak is harvested earlier than the European variety. Seasoning usually takes place outdoors in the elements for between 24 and 36 months until the moisture content of the oak is reduced from around 55 per cent to 15 per cent, although in the USA up to six months’ seasoning and one weeks’ final drying in large kilns is normal. Because more than nine out of ten casks in the whisky industry originally contained bourbon, most whisky casks are kiln dried. During drying an important breakdown of substances in the oak takes place and the major proportion of the tannins are leached out. American white oak only contains one eighth of the amount of tannin of European oak and does not require as much drying time. The lead time from log to completed bourbon cask can be as little as 60 days. Fastidious distillers and winemakers still usually prefer American oak to be seasoned outdoors for around three years like its European counterpart.


In order to bend the staves for a cask they must be heated to make them pliable. It is the lignin in the oak that must be softened. This not only applies to cooperage, but to all forms of processing involving shaped wood. The process is called toasting and is crucial for the flavours in a whisky. Different flavours are created depending on the temperature and time taken for the toasting. Substances are transformed, caramelized and charred to varying degrees and provide whisky with widely different flavours during maturation. Toasting the oak not only provides new aromas but also removes a number of undesirable aromas from the untreated oak. The treatment is categorized into different levels of toasting, usually divided into light, medium, medium plus and heavy. There is currently no standardized scale, so a medium plus toasting from one cooper may very easily be equivalent to heavy toasting from another.

The upright staves, which normally have a thickness of 22 to 27 mm, are placed over a heat source which traditionally consists of a cresset that burns waste oak from production. After around 20 minutes heating the cask is turned and the staves are bowed to their final shape. The cask is placed one more time over the fire and toasting continues for a further 20 to 40 minutes.

In addition to cressets burning oak waste there are other heat sources such as gas burners, hot water, microwave, infrared and convection heaters. The latter can be connected to a control system to regulate heat and any moisture over time to achieve special toasting profiles where aromas emerge at different temperatures. By controlling the temperature over time it is possible to create casks with dominant aromas such as vanilla and mocha.

Traditional toasting

Controlled toasting


In addition to toasting, which is a long, deep-seated heat treatment, the oak is sometimes allowed to catch fire for around 10 to 60 seconds so that a layer of charcoal is formed on the inside of the cask. This is most common with American oak as there is a demand for charred casks in the bourbon industry. In addition to the heavy heat treatment on the surface with e.g. the caramelization of wood sugars, the charcoal layer also acts as a filter that absorbs sulphur compounds and other undesirable substances in the spirits. A more heavily toasted or charred cask can impart certain smoky tones to spirits made from unsmoked malt. Some coopers have up to five different degrees of charring. Char nos. 1 to 4 are the norm, where no. 4 is the heaviest, but occasionally one comes across a special fifth level of charring. Char no. 3 and Char no. 4 are the most common in the bourbon industry where they are also known as alligator skin. A cask that is only heated enough to allow shaping before it is charred will impart an entirely different character to the whisky than a cask that was toasted more heavily before charring. In other words, it is possible to specify how a cask is toasted before charring, but unfortunately this is rarely requested by distillers.

Oak species for maturation casks

Around 95 per cent of all casks for the whisky industry are made from American oak. The preeminent species is Quercus Alba, but there is also a smaller proportion of other oak species from the American flora. Even though an unbelievable number of casks are made, the cooper’s share is barely 1 per cent of the total harvest. So if the oak, despite new growth, runs out at some time in the future, it will not be the fault of whisky or bourbon.

Quercus Alba
American White Oak
Bourbon, Sherry, Wine

Quercus Robur (Quercus Pendunculata)
English Oak, Pedunculate Oak, French Oak
Cognac, Sherry

Quercus Petraea (Quercus Sessiliflora)
Sessile Oak, Cornish Oak, Durmast Oak

Quercus Mongolica
Mizunara Oak
Japanese whisky

The Constituents of Oak

Cellulose (38–45 per cent): Cellulose is a polysaccharide; it constitutes the greater part of plant cell walls, and it is the most common organic substance in nature. It has no direct influence on the taste of whisky, but it is one of the oak’s building blocks.

Hemicellulose (22–28 per cent): Hemicellulose is a branched polysaccharide but with much shorter chains than cellulose. During toasting it breaks down at around 150°C to form furfural and simple sugars to provide sweet aromas, caramelized sugar, body and colour. At 215°C, other, heavier, toasted tones emerge.

Lignin (24–30 per cent): If cellulose constitutes the building blocks of oak then lignin is the mortar that binds them together. The lignin is broken down by heat, UV light, hydrolysis and oxidation during drying and toasting to form several extremely important flavouring substances such as vanillin, eugenol and guaiacol.

Tannins (0.8–10 per cent): Often associated with tannic acid. They impart body and complexity and constitute an important component for oxidation during maturation. Too high a content gives rise to harsh, bitter tones and therefore drying and toasting are crucial. European oak contains around 6 to 10 per cent while American oak has 0.8 to 2 per cent. The amount of tannin varies depending on where in the oak log a stave originated. The tannin content is highest lower down the trunk and closer to the bark. Tannin content also increases with the age of the oak.

Lactones: Cis-lactones impart coconut flavours while trans-lactones provide the character of fresh oak and coconut. Cis-lactones have more powerful aromas than trans-lactones and a sensory threshold that is up to 20 times lower.
Despite a lower ratio between cis-lactones and trans-lactones in Japanese oak compared to American oak, whisky from Japanese oak casks has a more robust coconut aroma. This effect is thought to arise from the synergistic effects of the presence of trans-lactones’ boosting cis-lactones, among others. A mixture of cis and trans-lactones also acts as a good insecticide. Perhaps it’s not just the sweet-and-sour aspects of whisky matured in Sherry casks that are the reason why only whisky tumblers are where flies go to die.

Furfural: An aromatic aldehyde with aromas of almonds, toffee, crème brûlée, and butter toffee.

Vanillin: Vanilla.

Eugenol: Cloves, spicy. Eugenol and isoeugenol have similar aromas and tend to be more distinct in seasoned oak.

Guaiacol: Smoke, medicinal, Creosote. The breakdown of lignin at higher temperatures provides a broad spectrum of volatile phenols including guaiacol and 4-methylguaiacol. The smokiness of some phenols is perceived more as an aroma (e.g. syringol) in some and as flavour in others (e.g. guaiacol). (Guaiacol gives smokiness, 4-methylguaiacol a bacony smokiness.)

Sensory experience

It is not always easy to describe the aromas of different substances. The way we perceive tastes and smells may vary depending on the amount of the substance present. The table below shows how we perceive cis-lactones in whisky depending on their concentration (ppm = parts per million).

Cis-lactones Effect
0,1 ppm No effect
0,5 ppm Traces of oak
1 ppm Oak and traces of coconut
2 ppm Oak and coconut
5 ppm Heavy coconut and traces of oak

Compounds that are chemically similar may in combination lead to discernible sensory effects even where they do not individually achieve the threshold normally required for their perception. This can also take place between unrelated aromatic compounds. For example, it is up to 50 times easier to detect lactones if vanillin is also present. And vanillin, which provides sweet vanilla tones, is present to a greater or lesser extent in all whisky even though it is more predominant in American oak.

Analysis of the box contents

Vanillin µg/ml


Furfural µg/ml

Almonds, caramel, crème brûlée, and butter scotch.

Guaiacol µg/l

Smoke, medicinal, creosote (primarily on the palate).

Syringol µg/l

Syringol, smoke, medicinal (primarily on the nose)

Eugenol µg/l

Cloves, spicy

Influencing factors

There are a great many more factors in addition to oak types and degrees of toasting that influence whisky maturation in casks.

Alcohol content

A liquid with a higher alcohol content penetrates deeper into the oak and tends to draw out more of certain alcohol-soluble aromas from the oak such as lactones, phenols, fatty acids and lipids. On the other hand, high alcohol content may require greater dilution at bottling with the consequent watering down of all the aromas. A lower alcohol content often generates a higher concentration of water-soluble substances such as tannins and sugars. The amount of sugars drawn from the cask is around twice as high in 55% spirits as in 70%. Casks that are stored with higher alcohol contents lose greater amounts of alcohol through the unavoidable evaporation known as the Angel’s share. In Scotland it is normal to water spirits down to 63.5% when filling casks even though some choose to fill at full strength of around 68 to 72%. The aim of using full strength is to take up less space in the warehouse and to avoid the risk of a really old whisky falling below an alcohol content of 40% thus disqualifying it from classification as whisky. At Box, casks are normally filled at 63%, but full strength also occurs.

Spirit quality

A cleaner distillate will clear more quickly than a distillate containing sulphur compounds and fusel oils. It is often said that grain whisky matures more quickly than malt whisky. However, this has nothing to do with the raw products and a lot to do with the distillation equipment. A spirit distilled to almost 94.7% will be much purer than a distillate of 70%. A major part of the sulphur in the spirits is removed by the charcoal layer if it is permitted to remain sufficiently long in fresh casks. These days it is eminently possible to produce high-class malt spirits free from sulphur and troublesome fusel oils. It’s just a matter of allowing the distillate sufficient contact with copper during careful distillation with well adjusted cut-off points and finishing off with cold cooling water.

Tannin from oak shavings after five months

The table shows the extraction of gallic acid from oak shavings at different alcohol levels.

Cask size

The size of the cask also plays a major part. Small casks have a larger internal surface area in relation to their volume and provide faster maturation than bigger casks. The most common casks associated with whisky hold 200 and 250 litres. While smaller casks are susceptible to greater evaporation, they need not remain as long in the warehouse. The maturation process in a 40 litre-blood tub is three times faster than in a 200-litre barrel.


The climate in the warehouse is of great importance. Generally speaking, the higher the temperature the faster the maturation process. A whisky takes appreciably longer to mature in Scotland’s temperate climate than one in e.g. India. But perhaps even more important are the temperature changes that create pressure differences in the casks causing the whisky to rise and fall in volume so that the spirit migrates in and out of the oak. This effect enhances what is known as interactive maturation wherein substances in the spirits react with each other or with other substances in the oak to form new ones such as fruity esters. Attempts have been made to create small, rapid pressure changes to accelerate interactive maturation by means of e.g. music and ultrasound. However, these methods appear to be unbelievably complicated when a well-situated warehouse provides the same effect with temperature differences during the day and the seasons. Humidity is another parameter. The alcohol content of whisky maturing in a humid warehouse falls but rises in a dry environment. Studies have shown that vanilla emerges more distinctly in whiskys in dry climates.

Facts about the casks concerned

In November 2008, 299 m³ of oak aged between 75 and 150 years were felled. Lower trunks from the old oaks with a volume of 15.47 m³ were approved for the manufacture of wooden casks. This represents a yield of 5.2 per cent of the felling which is excellent compared with the normal average of 2 per cent of the total volume of felled oak used for casks. The timber was split and sawn into stave billets in December 2008. It provided a total of 5.07 m³ of billets, which represents a yield of 32 per cent. Normally no less than 75 per cent of fine oak is waste. The billets were seasoned outdoors in Ölmbrotorp for 26 months.

The casks were made in March 2011.

Type of oak: Quercus Robur
Cask size: 100 litres
Forest: Mixed forest / Dense pasture
Landowner: Niklas Lind, Hornsö Lamnehult 402
Town: Blomstermåla
Country: Sweden
Age: 150
Stave thickness: 25
Drying method: Seasoning
Seasoning time: 26 months
Toasting method: Open flame (cresset with oak waste)

Oak forest: Hornsö, Blomstermåla

Toasting process at Thorslundkagge

Toasting begins by heating the cask body and shaping it over an open fire for 35 to 45 minutes. Heat treatment then continues until the desired toasting level is achieved.

Lightly toasted casks are toasted for 5–10 minutes over a low fire until the temperature a few millimetres into the wood on the inside reaches around 80°C. The surface temperature on the inside will be around 150°C, and the colour almost bare wood.

Medium toasted casks are toasted for 20 to 30 minutes over a medium fire until the temperature a few millimetres into the wood on the inside reaches around 100°C. The surface temperature on the inside will be around 200°C, and the colour reminiscent of nougat.

Medium plus is slightly more than medium toasting, but not quite as much as heavy toasting.

Heavily toasted casks are toasted for 35 to 45 minutes over a medium fire that is intensified somewhat at the end. The temperature a few millimetres into the wood on the inside reaches around 110°C. The surface temperature on the inside will be around 225°C, and the colour black, but the cask has not caught fire.

Charred casks are heated over an intense fire for 5 to 10 minutes until they catch fire. The cask is then allowed to burn with 2-metre high flames for 10 seconds before being doused with water. The temperature a few millimetres into the wood on the inside reaches around 100°C. The inside is black and crackled.

The bottoms of the casks are toasted or charred using gas burners. Toasting takes place under low heat for up to two minutes and charring under intense heat for 30 seconds.


TypType: Dunnage warehouse. A traditional warehouse with stone walls and an earthen/sand floor. Relatively humid environment with temperature changes spanning both days and seasons.

Location: Box Warehouse 1, April 2011–October 2012. Box Warehouse 3, October 2012–November 2013.


A total of 24,940 kg malt made from the grain types Tipple, Quench and Publican were loaded at Vikingmalt in Halmstad on 14 March 2011 at 14:05 for delivery to Box Destilleri.

Batch nos. 40–53 were mashed with 1,200 kg groats and 3 water; 6.300 litres of wort per batch was cooled to 20°C for subsequent fermentation for an average of 81 h 17 min.

Low wine distillation took place for an average of 6 h 3 min. from 18/03/2011 to 19/04/2011.
Spirit distillation with the first cut to the heart after 10 minutes and the second at 67% ABV.
Distilate temperature downstream of the condenser: around 4°C. The spirit was diluted with water from 72% to 64% prior to filling into casks on 26/04/2011.

Water source: Bålsjön

Yeast: Fermentis Safwhisky-M1

Cask log

Each barrel was filled on 26th April 2011. The cask type is 100 liter drums of new Swedish oak. The distillate was bottled November 1, 2013.

Distillation batch number is 40B-53A, distillate phenol content of 0 ppm and filling strength was 64 percent ABV.

Cask number Weight empty Weight full Weight spirit Volume Cask info LPA (litres pure alcohol)
2011-114 30.0 kg 124.65 kg 94.65 kg 105.17 l 11050 Swedish oak, Light toasting 67.3 l
2011-115 30.2 kg 124.95 kg 94.75 kg 105.28 l 11051 Swedish oak, Light toasting 67.4 l
2011-116 29.2 kg 123.7 kg 94.5 kg 105.00 l 11052 Swedish oak, Medium toasting 67.2 l
2011-117 28.55 kg 122.8 kg 94.25 kg 104.72 l 11053 Swedish oak, Medium toasting 67.0 l
2011-118 29.2 kg 123.0 kg 93.8 kg 104.22 l 11054 Swedish oak, Medium plus toasting 66.7 l
2011-119 28.8 kg 123.0 kg 94.2 kg 104.67 l 11055 Swedish oak, Medium plus toasting 67.0 l
2011-120 28.7 kg 121.25 kg 92.55 kg 102.83 l 11056 Swedish oak, Heavy toasting 65.8 l
2011-121 29.25 kg 123.05 kg 93.8 kg 104.22 l 11057 Swedish oak, Heavy toasting 66.7 l
2011-122 29.45 kg 123.3 kg 93.85 kg 104.28 l 11058 Swedish oak, charred 66.7 l
2011-123 28.7 kg 123.35 kg 94.65 kg 105.17 l 11059 Swedish oak, charred 67.3 l

Tasting notes

RK = Robert Karlsson, Malt Maniac member

LK = Lars Karlsson, Highly ranked whisky connoisseur

MN = Micke Nilsson, Whisky Manager at Akkurat whisky bar Stockholm

Tasting notes by toasting level:


RK: Fresh on dry pears. White peppery, lively and refreshing. Hints of blackcurrants. Clean classic ex-bourbon vanilla style.
Taste is reminiscent of the nose. Really refreshing, dry green pears – perhaps some apples also. Very clean, white peppery aftertaste. Remains true to style with water. A favourite.

LK: Lots of coconut, fresh oak, anise, hay, leather, metallic, some gun powder and bitter almond

MN: Light and sweet with peardrops, candifloss and a light maltiness. Slightly herbal. Sweet, ”green” taste. Malty with herbal overtones – sage.


RK: Buttery vanilla of the fatter kind. Some mandarins thrown in. The young distillate breaks through a bit. Calms down somewhat with water.
Taste is a bit on rough wood and lots of vanilla. A bit edgy and a tad too youngish for my tastes at first. Improves with time, on peppery liveliness and a more subdued nervousness.

LK: Coconut, stovewood, earthiness, some black powder. Light body and sligthly peppery finish

MN: Soft, sweet, slightly fruity nose with fresh cucumber and gorse. Subtly floral. Sweet and bitter and oaky with hints of orange. Spicy aftertaste.

Medium plus

RK: Quite evident young style. High alcohol. A bit darker breadiness, perhaps hints of dried fruits. Ripe pears below that. Grows considerably on vanilla with a dash of water. With more water it turns oilier – younger in style.
Taste is intensely (a bit surprisingly) lively, on pepperiness. High ABV here most likely. Clean maltiness, some darker styles mainly in the aftertaste. Some rough wood influence appear also.

LK: Sweet nose with vanilla and candy, some leather and tobacco, slightly medical and spicyness, mainly clove and some coconut

MN: Malty with bourbon-y notes. Green kiwi-like notes. A flowery edge and grape marmalade(!) Toasted and spicy woodnotes. A little chocolate, dough and nutmeg perhaps?


RK: A quite mellow nose, on darker fruits and a well-fused maltiness. Oranges, raisin, nutty. Rather attractive, although some some younger notes show through initially.
Round taste, sweet, some dark-styled spices. Rounds off even more with a dash of water. Sweet aftertaste.

LK: Lots of spicyness, white pepper, nutmeg and clove. Vanilla, slighty peaty, wet leaves, some butterscotch, red cabbage, sweet and sour in the taste, red wine vinegar with a peppery finish.

MN: Ginger bread, burnt wood and spicy toast. Warm sand and wood chips. Spicy with clove and roasted wood. Tannic and full.


RK: A rather calm round nose. Compact, on darker spices and dried fruits. Chocolate, nuts, candied oranges perhaps. An attractive vanilla lurks in the background. Well-balanced. Grows on alcohol with a dash of water, surprisingly. The favourite.
Complex darkly sweetish taste. On dark sugars, suryp even?, dried fruits and candied orangey fruits. Well-composed throughout, and feels soo much older than it is.

LK: Very sweet, butterscotch, caramelized suger, overriped banana, clove, hints of chocolate. Fulled bodied with lots of sweetness.

MN: Soft spiciness – cola and hints of strawberry. Sweet caramel, creamy fudge, shaved pencil and oak. Hot with rounded spice. Cola and a touch of strawberry jam.


Sweet, Caramel

Smoke, Medicinal


Coconut, Fresh oak

Before the tasting

Can you pair up the five bottles with the right toasting levels with the aid of their aromas and tastes?

Ideally, carry out the tasting using black taster’s glasses.

Questions for discussion

  • What can be said about the colour of the respective toasting levels?
  • Is there a difference between the level of maturity and the different toasting levels?
  • Which toasting level do you prefer?