Peat Terroir

Arbroath maltings

In 1800's the peat used in maltings was commonly local and the kilning was done in almost every distillery. During the latter part of 1900's and the invention of industrial malting processes the maltings have been concentrated to bigger facilities, such as Diageo's Roseisle, Glen Ord and Port Ellen, Simpson's Tweed Valley, Baird's Abroath and Inverness, Greencore's Glen Esk and Buckie and Edrington's Tamdhu. Several distillery maltings are still operational, but only a few distilleries malt all of their barley on-site (Springbank, Glen Ord, Tamdhu). The use of peat has diminished as more economical fuels have been introduced, but some peat is still used to create smoky flavoured malts. The peat used in Scotch whisky maltings is sourced mostly from the north-east of Scotland, Islay and Orkney.

Peat is formed in waterlogged lands by partial degradation of organic matter. There are some differences between peat compositions based on different climate, vegetation, bog type and cutting depth.

Peatlands can be divided into bogs, fens, marshes ja swamps.  Bogs are formed by heavy rainfall (therefore called ombotrophic) and usually contain more sphagnum moss and less woody vegetation than the other peatland types, which are formed mostly by waterlogging from the ground water (minerotrophic). Fens (aka basin bogs or valley bogs) contain more sedges and grass. Marshes are treeless intermittently waterlogged areas and usually accumulate peat very slowly. Swamps are very minerotrophic and contain large amounts of wood and nutritients. Average peat contains 90% water and 10% dry matter, of which 92% is organic and 8% inorganic (practically ash). Organic matter consists mainly of residues of lignin and different carbohydrates, but there are considerable variations depending on the vegetation and the bog type. Ombotrophic peats are richer in phenols and aromatics, but due to poorer vegetation they lack carbohydrates, lignin and nitrogen. Western Scotland and the islands are especially abundant with blanket bogs. About 10% of Scotland in covered by blanket bogs and 1% by basin bogs.

Peat bog	Maltings	Location	Bog type
Glenmachrie	Laphroaig	Islay	basin
Gartbreck	Bowmore	Islay	basin
Castlehill	Port Ellen	Islay	blanket
Hobbister Hill	Highland Park	Orkney	blanket
St Fergus	various	Aberdeenshire	basin
Tomintoul	various	Speyside	basin
Machrihanish	Springbank	Campbeltown	blanket

The extraction depth is also important as the surface layers are usually rich in carbohydrates and poor in phenols and the deeper layers might have too much harmful nitrogen- and sulphur compounds due to anaerobic fungal and bacterial metabolism. On the other hand some fungi seem to produce vanillins such as acetovanilline from lignin, for some unknown reason especially so in island peats.

Location of peat bogs used in whisky production

Islay peat bogs (from left: Gartbreck, Glenmachrie, Castlehill)

It seems that the most significant factors in peaty aroma are the bog location and the cutting depth. The bog type and the vegetation play some role, but they are not that important for peaty flavour. The peats from Islay, for example, are very similar with each other as the peat from the basin bogs Glenmachrie (Laphroaig) and Gartbreck (Bowmore) cannot be identified by infrared spectroscopy. 

The Castlehill peat for the Port Ellen Maltings somewhat different from the basin bog peats from Islay, probably due to different microbiology of the blanket bog and a greater amount of woody material are therefore more lignin-derivatives and carbohydrates. 

Surprisingly, Hobbister Hill peat from a blanket bog is chemically more similar to the Islay basin bog peats (Gartbreck/Glenmachrie) than the blanket bog Castlehill peat. The local microbiology of peat bog might therefore have a greater impact on the peat composition, influencing both the nitrogenlevels and phenol-concentrations in the peat. Another explanation is that the bog types are overlapping, for example Hobbister Hill bog might have some basin bog properties (standing water bowls) especially in the deeper layers; this might explain the similarity of peats cut from the deeper layers of Hobbister Hill bog with the Islay basin bog peats. 

Tomintoul peat is different from the Island peats; a basin bog, as expected, contains great amounts of carbohydrates, but there is also lots of sphagnum moss, which for some reason does not result in the same amounts of phenols as in the islands. It could be speculated that this is due to different climate, microbiology or variety of Sphagnum in the western islands and the mainland.

St Fergus peat is rich in woody material and therefore rich in lignin derivatives, especially syringyl. There is however a great amount of phenol-compounds in the St Fergus peat, although it is a basin bog and the sphagnum-content is lower than in for example Tomintoul peat. The location of St Fergus bog is closer to the sea than of Tomintoul bog. Maybe the proximity of sea results in greater amounts of phenols in peat? Explanation might be for example lesser temperature fluctuations or differences in microbiology or drainage.

There are no scientific analyses available on Machrihanish (Springbank) peat, but since it is cut from a blanket bog and located near sea in the western Scotland, it probably is closer to the Islay and Orkney peats than the mainland peat.

Some breweries use artificial smoke flavourings in their malting processes, apparently common raw materials are extracts from burnt deciduous trees, such as beech or birch. These are likely to produce more aromas from syringyl, guaiacol and carbohydrates resulting in softer but not as phenolic aroma as from peats. I don't know whether Scotch whisky industry uses smoke flavourings in their maltings, but it is not prohibited in the Scotch whisky regulations. A black alder smoked malt could be an interesting experiment (a free hint for any progressive distillers reading this).

Anyway, there clearly is a peat terroir and even some local variations in the compositions. Also the cutting depth is important and it will be interesting to see wheter the peat composition will change as the limited amounts of peat for example from Islay are utilised and either the cutting depth or the cutting location changes.

REFERENCES AND FURTHER READING:

Bozkurt  S et al. Peat as a potential analogue for the long-term evolution in landfills. Earth Sci Rev 2001; 53; 95-147
Bryce JH et al (ed). Distilled spirits, production, technology and innovation. Nottingham Univ Press, 2008
Da Porto C et al. A study on the composition of distillates obtained from smoked marc. Anal Chim Acta 2006; 563; 396-400

Guillén M et al. Carbohydrate and nitrogenated compounds in liquid smoke flavorings. J Agric Food Chem 2001; 49; 2395-2403 

Guillén M, Manzanos M. Study of the components of a solid smoke flavouring preparation. Food Chem 1996; 55; 3; 251-257

Guillén M, Manzanos M. Characterization of the components of a salty smoke falvouring preparation. Food Chem 1996; 58; 1-2; 97-102

Harrison B, Priest F. Composition of peats in the preparation of malt for scotch whisky production. J Agric Food Chem 2009; 57; 2385-2391

Harrison B et al. Differentation of peats used in the preparation of malt for scotch whisky production. J Inst Brew 2006; 112; 4 ; 333-339

Jefford A. Peat, smoke and spirit. Headline 2004

Jounela-Eriksson P. The aroma composition of distilled beverages and preceived aroma of whisky. Academic Press 1978

Kostyra E, Barylko-Pikielna N. Volatiles composition and flavour profile identity of smoke flavourings. Food Qual Pref 2006; 17; 85-95
Lehtonen M. Phenols in whisky.  Chromatographia 1982; 16; 201-203

Russell I (ed). Whisky, technology, production and marketing. Academic Press 2003

Swan JS, Howie D. Sensory and analytical studies of regional influence on the composition of Scotch malt whisky. Institute of brewing, 1983.

Valaer P. Scotch whisky. Industr Engineer Chem 1940; 32; 7; 935-943

Walker GM, Hughes PS. Distilled spirits, new horizons: energy, environment and enlightenment. Nottingham Univ Press, 2010

Caramel E150

E150a-d

Spirit caramel (E150) is allowed as a colouring agent in Scotch whisky production. There are four different types of E150, labeled from a to d, or from class I to IV, according to the manufacturing process used. E150a (plain caramel) is "prepared by the controlled heat treatment of carbohydrates (commercially available food grade nutritive sweeteners which are the monomers glucose and fructose and/or polymers thereof, e.g., glucose syrups, sucrose, and/or invert syrups, and dextrose). To promote caramelization, acids, alkalis and salts may be employed" (not ammonium compounds or sulphites). Sulphites are allowed in the production of E150b (caustic suphite caramel), ammonium compounds are allowed in the process for E150c (ammonia caramel) and both suphites and ammonium compounds are allowed in the process for E150d (sulphite ammonia caramel). Common raw materials for caramel colourings are corn syrups, wheat, glucose syrup or sucrose. Additives may include a variety of acids, alkalis and salts. Different raw materials produce different caramels and the use of additives influences the resulting color, viscosity, ionic charge and pH and there are hundreds of different spirit caramels available from several producers.

Caramels from DD Williamson. 570 is their most used spirit caramel.

Caramel samples from Sethness. Typical spirit caramel is 0.075-0.110

Most caramel colourings are very dark in color and are usually used in tiny quantities. Fructose produces the darkest color, probably because it starts to caramellisate in 110ºC, as sucrose, glucose and galactose caramellisate in 160ºC and maltose in 180ºC. Viscosity varies, but in general low viscosity caramels are used in beverages. The ionic charge of caramel is important because of possible flocculation. For example, if negatively charged caramel is added to positively charged beverage, there will be some flocculation or even percipitation. Correctly selected caramel colouring has some emulsifying properties, in other words it helps the oils to mix with the water. In fact, the Coca Cola Company first patented the caramel colouring as an emulsifying agent, not as a colourant. Whisky and most soft drinks are negatively charged, but beer, baked goods and herbal liqueurs have usually a positive charge. E150c is positively charged and used in breweries and never in whisky. E150a has the best stability in high proof alcohols, especially when the raw material has been sucrose. Wheat and corn based syrups are widely used, but they are usually less stable in alcohol. E150a can tolerate up to 75% abv as most E150d is guaranteed to work up to 50% or 60% abv. E150b is used in the presence of tannins, especially in sherries, wines and some brandies and the residue sulphites of E150b probably also help to preserve the wine from excess oxidation. The amount of caramel varies, in spirits it is usually about 0,1-0,5% or about 1-5g/litre as in comparison some 0,4% of E150d is used in cola soft drinks, 0,01-0,3% of E150c in beers and up to 10% of E150d in (cheap) cocoa powders. EU (EFSA) recently lowered the acceptable daily intakes (ADI) for caramels; 300mg/kg/day for E150 and 100mg/kg/day for E150c, so a man weighing 80kg is allowed to consume 24g of caramel color a day. Hopefully not many of us get that from blended whisky, but probably some will get those amounts from cola and bakery stuffs.

Most producers give caramel colors a two year shelf stability guarantee, if stored in room temperature and protected from sunlight. Sunlight fades the caramel colors rapidly, in matter of weeks to months. E150a is the most resistant to fading and it fades evenly in all wavelenghts, as E150d usually fades more of the higher wavelenghts (red fades first).

Loch Dhu, mit farbstoff

The majority of single malts and virtually all the blends are coloured with E150a. According to The Scotch Whisky Regulation in 2009 only "plain caramel" (E150a) is allowed, although the EU laws permit the use of "spirit caramel", which is not exactly defined in law and can be any E150 and so at least some E150b and E150d could have been used in Scotch whiskies.

Toasting a barrel (vinography.com)

Due to the variety of raw materials and additives, the chemical structures of caramels are complex and there are lots of variations even between caramels of the same subgroup. In the production the main reaction is dehydration as water (hydrogen and oxygen) is extracted (boiled) out of the sugars. This results first to sugar monomers as polysaccharides (for example sucrose) are divided into glucose, fructose, galactose, xylose and maltose. As the heating continues, the monosaccharides lose water and react with each other producing big polymers, mainly caramelans (C₂₄H₃₆O₁₈), caramelens (C₃₆H₅₀O₂₅₎ and caramelins (C₁₂₅H₁₈₈O₈₀₎, which give the caramel most of its color. Additionally some residue sugar may be left in the product and several flavour components are produced, for example different furans, diacetyl, maltol, esters and lactones. Furans are probably the most influential flavour components in most caramels, they are formed especially from fructose or sucrose (containing fructose) in acidic encironment. They also form during the toasting of oak barrels, but the relative amounts of different furans are different as the raw material are either lignin and (hemi)cellulose in oak or simple sugars in caramel. Caramellisation of simple sugars produce less furfural (almond, walnut, grainy) and more 5-hydroxymethylfurfural (butter, musty, waxy, caramel) than caramellisation of oak (there are also significant differences between different oak species, see previous blogs). 5-HMF is not produced if sulphites or ammonium are used in the process (E150b or E150d), but is formed in large quantities in the plain caramel (E150a). 5-HMF is also used as a flavour enhancer in milkpowders, honey, juices and even cigars. Furfural acts as a reactant with various compounds in the spirit; it has some antioxidative properties that slow the oxidation reactions and help to stabilize the color from antocyanins (in especially wine), it also reacts easily with H₂S (rotten eggs, nasty sulphur) producing furfurylthiol (strong coffee), decreases the volatile sulphur compund concentrations and potentiates the odor of oaklactone (vanilla,coconut). Some other furans such as hydroxyacetylfuran (sweet), hydroxydimethylfuranone (also known as furaneol, additive in baked bread, coffee and chocolate) and dihydroxydimethylfuranone are also produced. Diacetyl imparts a buttery (butterscotch) flavour and maltol (aka E636) gives freshly baked bread aromas. Esters and lactones (here from sugar, not oak) are usually fruity. Also increased levels of E2-nonenal (cardboard, stale beer) and less hop flavour are also found in caramel coloured beers, but this possibly results from changes in fermentation process and is not studied properly in spirits.

So, caramel does affect the flavour and it is not inert in whisky, but are the quantities used in Scotch whisky industry enough to affect the overall flavour significantly? No reliable scientific fact exists, but my guess is that they probably are significant. Does caramel impair the flavour? It could, but then again in some cases caramel might even improve the taste.

References and further reading:

Abalos D et al. The use of furfural as a metabolic inhibitor for reducing the alcohol content of model wines. Eur Food Res Tech 2011;232;663-669

Blanchard L et al. Formation of furfurylthiol exhibiting a strong coffee aroma during oak barrel fermentation from furfural released by toasted staves. J Agric Food Chem 2001;49;4833-4835

Boscolo M et al. Spectrophotometric determination of caramel content in spirits aged in oak casks. J AOAC Int 2002;85;3;744-750

European Union Directive 95/45

Furukawa Suarez A et al. Impact of colour adjustment on flavour stability of pale lager beers with a range of distinct colouring agents. Food Chem 2011;125;850-859

Laws DRJ, Peppard TL. The stability of flavour constituents in alcoholic beverages. Food Chem 1982;9;131-146

Quesada Granados J et al. Influence of aging factors on the furanic aldehyde contents of matured brandies: aging markers. J Agric Food Chem 1996;44;1378-1381

Ratsimba V et al. Qualitative and quantitative evaluation of mono- and disaccharides in D-fructose, D-glucose and sucrose caramels by gas-liquid chromatography-mass spectrometry di-D-fructose dianhydrides as traces of caramel authenticity. J Chrom A 1999;844;283-293

Rodriguez Dodero MC et  al. Phenolic compounds and furanic derivatives in the characterization and quality control of brandy de Jerez. J Agric Food Chem 2010;58;990;997

Scotch Whisky Regulations. Scotch Whisky Association 2009.

Sousa A et al. Isolation and structural characterization of antocyanin-furfuryl pigments. J Agric Food Chem 2010;58;5664-5669

Tsai PJ et al. Interactive role of color and antioxidant capacity in caramels. Food Res Int 2009;42;380-386

www.caramel.com (DD Williamson)

www.sethness-roquette.com

Cask sizes

Just a quick table about cask sizes.

Cask	Litres	Imperial gallons	US gallons
Bourbon barrel	159 (official barrel)		42	size varies 150-220l, commonly 180-200l
US beer barrel	117		31
UK barrel	164	36
Hogshead	250	54
UK ale barrel	146	32
Tun	955	210		"a ton"
Sherry shipping butt	491	108		30 arrobas,56 cases of 0.75l bottles or ½tun
Bota gorda, sherry cask	600			32arrobas
Sherry puncheon	667			40arrobas
Cognac	350			varies 270-450l
Cognac shipping cask	500-600
Bocoy	600-800			Spanish wine cask
Wine barrique	225			Bordeaux barrel
UK wine shipping barrel	119	26.25
Port pipe	528	116		varies 400-800l
Madeira cask	419-437	92-96
Marsala pipe	396	87
Marsala hogshead	200	44
Rum puncheon	410-455	90-100
Kilderkin	82	18		½ UK barrel
Firkin	41	9		Quarter of UK barrel
Bloodtub	34	7,5
Whisky quarter cask	125	27		½ hogshead
Whisky octave	46	10
Whisky puncheon	387-500	85-110		staves from 2 barrels

References:

Calabrese S. Cognac, liquid history. Cassell&Co 2001

González Gordon M. Sherry. Cassell Ltd 1972 
Kilby, K. The cooper and his trade. John Baker Publishers Ltd 1971
Rainbird G. Sherry and the wines of Spain. 1966

Pot still distillation

Wash still (bladnoch.co.uk)

Scotch malt whisky is distilled typically twice in copper pot stills. The first distillation is carried out in a wash still, usually slightly bigger than the spirit still used in the second run. Copper is used because of its good malleability and heat conduction but also for its catalytic properties and the ability to neutralize some of the sulphur compounds and off-notes. The stills can be heated either directly by burning gas, peat or coal or indirectly by steam coils running inside the still. If direct firing is used, there must be a rummager on the bottom of the still to prevent charring, at least in the wash still. The stills usually have a broad pot and a narrowing swan neck that turns to a lyne arm leading to a condenser, which can be a traditional worm, a shell&tube condenser or a plate heat exchanger. Various shapes of stills are used and it is believed that the bulbs and boiling balls etc increase the copper contact at the right spots to produce better spirit. Some distillers use a purifier in the spirit still, which is a pipe reversing some distillate from the lyne arm back to the swan neck producing more reflux.

Worm-tubs	Purifier	Direct firing
Balmenach	Ardbeg	Glenfarclas
Benrinnes	Glen Grant	Glenfiddich
Cragganmore	Glenlossie	Macallan
Dalwhinnie	Glen Spey	Springbank (wash) GlenGarioch (wash) Tobermory(wash)
Edradour	Strathmill
Glen Elgin	Talisker
Glenkinchie	Tormore
Mortlach
Oban
Royal Lochnagar
Talisker
Springbank (wash)

 
Basically the first distillation is a simple distillation of volatile compounds (such as alcohols), producing low wines of 20-25% abv from 6-9% abv wort. Wash is usually preheated to prevent excessive temperature differences and charring inside the still. The first distillation is usually deemed complete when the distillate is under 1% abv and about one third of the wort has boiled over to the wash safe. Wash distillation usually takes 5-8 hours to complete, depending on the size, shape, temperature and charge of the still. The residual is called pot ale.

The wash distillation is mixed with the foreshots and the feints from previous spirit distillations and distilled in the spirit still. The second distillation is a fractionated distillation; the most volatile compounds boil first and are called foreshots or heads, the second part is called the middle cut (spirit cut, heart), the third part is feints (tails) and the remaining liquid in the still is called spent lees.

Panek&Boucher 1989

Panek&Boucher 1989

The foreshots consist of the most volatile compounds, such as methanol, acetaldehyde (ethanal), some of the ethyl esters such as ethyl acetate and volatile sulphur compounds (see table of boiling points below). Along with them come some of the residues of the previous distillation, which are not high volatiles but soluble only in higher strenghts of alcohol, for example fatty acids and their esters. These compounds are therefore condensed into the still at the end of previous distillation and as the next distillation starts, they are made soluble again by the high alcohol strenght and swept up to the spirit safe along with the foreshots. The foreshots are either unpleasant in flavour or dangerous to health (or both) and therefore not included in the final spirit but returned to the next distillation. The long-chain fatty acids and other residues from the previous distillation tend to form a cloudy mixture when diluted to 80 sikes proof (45.7% abv).

	Boiling point C	Odor
acetone	56,5	nail polish remover
glycerol	290	sweet
acetic acid	118	vinegar
aldehydes
acetaldehyde	20,2	pungent fruity, green apple, metallic
furfural	161,7	almonds
alcohols
methanol	65	sweetened ethanol
ethanol	78	ethanol
1-propanol	97	fruity
2-propanol	82,5	fruity
butanol	118	banana, solvent
amyl alcohols	102-138,5	sharp, burning
2-phenyl ethanol	219	floral, rose
esters
ethyl acetate	77,1	pear,sweet
ethyl butyrate	121	pineapple
ethyl formate	54	rum, raspberry
hexyl acetate	171,5	fruit
sulphur compounds
hydrogen suphide	-60,3	rotten eggs
sulphur dioxide	-10	burnt sulphur
dimethyl sulphide	37	cabbage, vegetables
fatty acids
lauric acid	299	bay oil, soap
palmitic acid	351	waxy, creamy, soapy

The traditional way of making the first cut point is by mixing the foreshots with water in the spirit safe; as the spirit turns clear, the foreshots have mostly passed and the middle cut is collected into the spirit receiver. Some producers use timed foreshot runs and do not bother with the demisting test and therefore are are likely to cut the spirit a bit lower as a precaution and probably getting less of the highly volatile compounds into the spirit. The second cut can be determined by taste, abv or time according to the distillery practice. The feints are then run usually down to 1% abv and added to the next wash.

The feints are the last volatile fraction of spirit distillation. Feinty aromas increase slowly towards the end of distillation, developing from quite pleasant mushroom, cereal and popcorn aromas to leathery tobacco notes and further to ashy, fishy and even cheesy aromas not usually approved in whisky. The feints are usually rich in phenols and smoky aromas important for peaty whiskies. Therefore the second cut point must be determined with care to produce peated but not feinty spirit with off-notes. The feints are usually run quite fast to save time, but this makes some of the fat-solubles to adhere to the still. These compounds (mostly fatty acids) must be purged by an adequately long and slow run of foreshots in the next distillation, otherwise the whisky might become feinty no matter how early the second cut is made.

The spirit cut is usually about 72-65%, but there are considerable differences between distillers. To produce a very light aetheral whisky (richer in high volatiles) the distiller would be likely to use a tall still with a low charge run slowly in low temperatures for maximum reflux, a purifier, an ascending lyne arm, a purifier and a shell&tube condenser for maximum copper contact and cut short foreshots and long feints (earlier second cut) for maximum amount of high volatiles and less of the heavier aromas. To produce a peaty whisky the distiller must try to catch all the phenols from the latter part of the middle cut but also avoid excess feints.

The distillation is not only a simple process of separation, but the heat and the copper contact alter some of the flavour compounds, too.  Heat promotes Maillard reactions producing furfural and sulphur compounds, especially thiophenes and polysulphides, which at low concentrations add a pleasant roasted and complex flavour, but produce pungent and unpleasant cereal and sulphury notes at high concentrations. Heat also promotes aldehyde reduction to alcohols and acids and their conversion further to esters. Lignin-derived components such as coumaric and ferulic acids can evolve to more spicy guiaicols. Acrolein (peppery) can form from bacterial fermentation products in presence heat. Fusel oils, 2-phenethanol (rose, flower) and furfurals (caramel, burnt sugar) are formed in pot-stills, but not significantly in column still distillations and therefore are likely to be generated by heat differences during distillation.

Copper removes most of the sulphury, cereal, feinty and meaty aromas during distillation. Especially the copper contact of the first wash still distillation is important. Total removal of copper contact in the spirit still has surprisingly minimal effect on the mentioned off-notes.

References and further reading:

Adams AB. The distillation of alcohol. J Ind Eng Chem 1912; 8-14

Herstein KM. Chemistry and technology of wines and liquors. Van Nostrand Co 1935

Jounela-Eriksson P. The aroma composition of distilled beverages and preceived aroma of whisky. Academic Press 1978

Lea GH, Piggott JR. Fermented beverage production 2nd ed. Kluwer Acad 2003. 

Monica Lee KY et al. Origins of flavour in whiskies and a revised flavour wheel. J Inst Brew 2001;107;5;287-313

Piggott JR, Paterson A (ed). Understanding natural flavors. Blackie academic&professional 1994

Russell I (ed). Whisky, technology, production and marketing. Academic Press 2003

Udo M: The Scottish Whisky Distilleries. Black & White 2006

Walker GM, Hughes PS (ed). Distilled spirits, new horizons: energy, environment and enlightenment. Nottingham Univ Press, 2010

Webb AD (ed). Chemistry of winemaking. Am Chem Soc 1974

A medical view on maturation of whisky

A fresh medical view on the maturation issue and a shameless rip-off from The Lancet.  Just in case someone does not know The Lancet, it is a highly respected English medical journal, established in 1823 and still one of the big three among the medical journals. Below are two articles first published on 11.1.1902 and 19.3.1904 discussing the definition and consumption of whisky.

Some interesting points are made concerning for example malt and grain whiskies
"There is a marked difference between the physiological action of spirits derived from different sources...The potentialities of these substances for good or evil depend upon the materials from which the spirit is produced."
and the quality of blended whiskies
"...age is essential in order to reduce spirits to state fit for consumption...Even genuine malt whisky must be matured, whereas grain spirit...featureless as regards flavour...age can have no advantegeous change [on grain spirit]."
and wine finishes
"Grain whisky... either has to be flavoured by adding a small proportion of full-flavoured malt whisky, or...storing it in sherry casks in which malt whisky has been ripened"
and the disappearing age-statements
"It seems to us that it avails nothing to the consumer to be sure of getting all-malt spirit when he asks for it if he cannot at the same time receive some guarantee also as to its age."
and the rise of NAS-whiskies
"The whole object of the Bill would be destroyed if...it were possible to supply true malt whisky so immature as to be injurious to health."
and the decline of properly aged quality single malts
"The detractors of all-malt whisky might be expected for their own purposes to supply...pure malt whisky...which though genuine could lay no claim to...age."
All this a bit over hundred years ago. Tradition?


References:
Lancet 1902;159;4089;109
Lancet 1904;163;4203;816-817

Rejuvenation

Scotch whiskies are predominantly matured in ex-bourbon casks. The previous filling with bourbon extracts much of the flavours, but there is still a lot left for the refills, too. The water-soluble extracts seem to exhaust faster than the lignin-derivatives or the lipid-solubles, although the filling strenght does influence extraction rates. For example oaklactones and soluble oaktannins, especially ellagitannins are largely depleted during the bourbon fill and the first Scotch fill. By extending the maturation period it is possible to create whiskies with considarable oak flavours, although the proportions of flavour compounds tend to differ from the first fill maturations (see table 1 below).

 

Table1. Extracts from ex-bourbon casks used for Scotch malt whisky. From left: 1st, 2nd and 3rd/more refill (modified from Piggott&Conner 2003)

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Lignin does break down in presence of alcohols, but most of the linkages are resistant to ethanol. Therefore lignin does not degrade completely even in the presence of high filling strenghts during long maturation, but the release of monoligninderivatives tends to slow down considerably leading to an exhausted cask. The relative absence of guaiacol ja syringyl compounds is thought to be a good marker for cask exhaustion. The guaiacol concentrations are probably quite similar to those of ferulic acid concentrations shown in table 1 and the amount of syringic acid is likely to represent the amount of total syringyl compounds (for information on lignin construction see previous blog 15.2.2011).

Exhausted casks can be rejuvenated by scraping the inside of the cask, removing the used wood and bringing new wood in contact with the spirit. Untoasted wood however gives green and raw aromas and scraped casks are usually toasted or charred again. Recharring might impart too much colour and change the aromas towards bourbon-style whiskies and therefore retoasting in apparently preferred in Scottish cooperages. Regenerated recharred casks differ considerably from ex-bourbon casks used for the first fillings with Scotch. In table 2 are presented the sensory profiles of 3 year old grain whiskies matured in refill bourbon casks compared to decharred recharred exhausted ex-bourbon casks. The recharred casks seem to produce more sweet and woody notes as refill casks tend to bring out the drier woody notes. In terms of flavour compounds the recharring promotes fast lignin breakdown and caramellisation of hemicellulose thus extracting considerably more guaiacols, isoeugenol and vanillin commonly associated with new wood barrels. Surpisingly the concentrations of oaklactones are only slightly greater in recharred casks compared to refills. Perhaps oaklactones are already extracted from the deeper layers of the wood exceeding the scraping depth in rejuvenation and the recharring affects primarily wood layers that are already poor in oaklactones. Another possible explanation is the lack of seasoning of rejuvenated casks after the scraping which might lead more green wood notes and less oaklactones. A recharred cask with more guaiacol and vanilla extractives is more likely to promote smoky and sweet flavours of the spirit than a refill cask, although the sweet notes differ from those of the first fills as they are probably caused more by caramellisation products and vanilla than oaklactones. The charred layer is likely to remove some of the off-flavours, especially some sulphury aromas, but as seen in table 2 toasting also considerably reduces suphury aromas, so the filtration by active carbon layer might not be the only explanation for this.

A common practice during the 20th century was to season the rejuvenated casks as well as some of the new casks with pajarete to create a sherry-cask profile. The first intentional wine-treatments were made in 1880s and the last larger scale treatments in the whisky industry were terminated prbably during the 1980s.

A recent develepment has been a treatment of exhausted casks with salt catalyst before recharring or retoasting; this "curing" increases the extraction from rejuvenated casks (tables 2&3 below). The salt solution probably increases the heat influence in deeper layers of wood especially during medium heating (toasting). It is also likely that salt increases the solubility of extractives into spirit, although the precise mechanism is not yet described.

Tables 2 and 3. Sensory profiles of 3 year old Scotch grain whiskies matured in different ex-bourbon casks (brutal modification from Reid KJG et al 2008, sorry...)

Rejuvenated casks are not particulary appreciated by malt whisky aficionados, but they probably are of good use in maturing young grain whiskies as they remove effectively some of the common off-notes associated with new make spirits. Also lighter style malt whiskies might benefit from a subtler cask influence lacking especially the overly sweet oaky notes.

REFERENCES

Boudet AM et al. Biochemistry and molecular biology of lignification. New Phytol 1995; 129; 203-236 

Conner JM et al. Changes in wood extractives from oak cask staves through maturation of scotch malt whisky. J Sci Food Agric 1993;62;169-174

Clyne J et al. The effect of cask charring on scotch whisky maturation. Int J Food Sci Tech 1993;28;69-81

Lea GH, Piggott JR. Fermented beverage production 2nd ed. Kluwer Acad 2003.

Mosedale, JR. Effects of oak wood on the maturation of alcoholic beverages with particular reference to whisky. Forestry 1995; 68; 3; 203-230

Piggott JR, Conner M. Whiskies. In Fermented beverage production. Kluwer Acad 2003.

Piggott JR et al. Effects on scotch whisky composition and flavour of maturation in oak casks with varying histories. Int J Food Sci Tech 1993;28;303-318

Reid KJG et al. Understanding and enhancing cask performance. The Scotch Whisky Research Institute 2008.

Russell I (ed). Whisky, technology, production and marketing. Academic Press 2003.

Viriot C et al. Ellagitannins and lignins in aging spirits in oak barrels. J Agric Food Chem 1993; 41; 1872-1879

Cask variations

A range of different casks are used in Scotch whisky maturation. Before 1990 the casks were allowed to be of any wood, although only an odd chestnut cask has probably been used as most casks were made of oak. Now all casks have to be made of oak wood. Most casks are refill casks from bourbon industry made of Q.alba and to a lesser extent casks previously used to mature sherry (mostly Q.alba, sometimes Q.robur), port (Q.alba, Q.robur, Q.pyrenaica), wine (mostly Q.petraea), rum (mostly Q.alba) or even cognac (Q.robur). The first filling extracts much of the flavour compounds from the oak as most of free solubles are extracted during the first year of maturation. Spirits with higher filling strenghts reach deeper into the wood and extract more alcohol-soluble congeners, such as lactones, lipids, fatty acids, lignin-derivatives, phenolic acids and aromatic aldehydes. Lower strenght beverages and even fortificated wines extract more water-soluble components, such as tannins, glycerol and sugars. Esters are extracted practically evenly despite different ethanol concentrations. The breakdown of lignin continues as maturation is prolonged and more extractives are available at a considerably slower rate, higher alcohol-strenghts increasing the ligninolysis.

The high lactone-content explains the heavy sweet coconut aroma of bourbons, but the aroma is toned down in the refill casks as the lactones are largely depleted in the first fill with spirit. Wine extracts lactones too; a six month wine maturation decreases the extraction of oak lactones to about 30-40% in the second fill. The average oaklactone concentration in a new Q.petraea cask is about the same as in a wine-refill Q.alba and a bourbon-refill Q.alba is very likely to be much poorer in oaklactones. Trans-oaklactones extract themselves more easily during the first fill, so the cis-trans ratio is likely to be greater in refill casks, producing more spicy than sweet oak flavours. Q.robur is usually very poor in oaklactones even in a new wood cask, but Q.crispula (Japanese oak) is very rich in oaklactones, especially the cis-isomer, giving rise to spicy incence oak aromas, which are often further fortified with a sherry-treatment before filling in the whisky. Toasting increases the amount of lactones and although heavy charring diminishes the lactone-concentrations, the increased permeability through charred wood usually increases the total extraction into the spirit.

Spicy tannins and vanillins are typical for Q.robur and Q.crispula, but their concentrations in Q.petraea seem to vary considerably, although some of the "odd" single results might be explained by hybridization with Q.robur or other oak species. Seasoning of the staves is preferred in Europe instead of kilning, probably due to more astringent and tanninic taste profiles of the European oaks compared to Q.alba. Tannins soften during seasoning by the effects of rain, bacteria, yeasts and oxidation. Vanillin, eugenol, furfural and cis-oaklactone are reported to decrease in colder climate and increase in hot climate seasoning, but this may be also partly explained by different microbes present in Australia compared to those in France and the USA.

Smoky and spicy guaiacols are created from lignin in toasting/charring and through ethanol-induced lignification during maturation. Higher temperatures increase the guaiacol-concentrations which are likely to enchance the smoky flavours in whisky. Especially 4-vinylguaiacol is very soluble in spirits and is usually depleted fast in the first fill. Guaiacols can be generated through ligninolysis and extracted into refills too at a slightly slower rate. Furfurals (caramel, burnt sugar) are also created by heating and quite rapidly extracted in the first fill.

The colours extracted from the oaks are different, as Q.alba and Q.crispula are usually red, Q.petraea is usually lighter, almost pink in colour and Q.robur extracts a strong yellow colour. Again, charring decreases the colour extractives in the surface layer but because of the increased permeability it usually adds to the typical red/orange colour typical of bourbon.

The East European Q.robur is usually a bit closer to Q.alba in terms of the flavour compounds as it contains usually more oaklactones, eugenols and vanillin but less tannins than Q.robur from France or Spain.

The cask absorbs some of the spirit or wine used in the first fill. A large bodega butt can gain up to 25kg in weight during several decades of solera maturation and it is very likely that such cask would impart quite a lot of sherry aromas to a refill spirit regardless of the oak species used. A barrel sized cask (usually 180l) soaks about 9 litres of 50% abv whisky during a bourbon maturation. A typical wine cask is used for 5 years and several refills and is likely to absorb several litres of wine. Some of the liquid used in the first fill are bound to enter the refill whisky, but the magnitude is not known. Recharring the cask before a refill burns most of the alcohols and probably flavours previously absorbed by the wood, but re- or decharring is apparently not an usual practice for first (scotch) fill bourbon casks. Rinsing of a cask is not likely to remove significant amounts of the previous liquid soaked into the wood. If a cask has been used in fermentation of wine, it is likely to lose some weight, probably because of the wood degradation by the yeasts. It is believed that the fermentation yeasts remove some of the bitter tannins and other undesired aromas from a wine cask, although not much wine is fermented in casks anymore.

The size of the cask affects the result too, larger casks having less wood surface and headspace per spirit volume. Spirit in smaller casks mature faster but suffer from greater oxidation per volume. Smaller casks impart probably more sugars, glycerol, eugenol and galloyl esters but less lignin-derivatives, vanillin and ellagitannins and this may result in relatively sweet and spicy aroma profile different from the profiles of the larger casks.

In conclusion, the new wood casks impart the most sweet oak, coconut, caramel and smoky aromas as the refills tend to be drier, subtler but still quite tanninic. The cask maturation is very rapid in first fill casks during the first months as the maturation in refills is usually more linear and predictable, although significantly slower. Different oaks have very different tasteprofiles and the strenght and the aromas of the first filling does affect the refill maturation of whisky. Seasoning softens the astringent aromas as kilning increases the sweet aromas but does not affect so much the tannins. Below is a table showing the main differences between the species considering whisky maturation.

	Q.alba	Q.robur	Q.petraea	Q.crispula
ring width	+++	+++	+	?
oaklactones	+++	+	+(++)	+++
-cis	(+)	(+)	++	+++
-trans	+++	+	+	++
eugenol	+(+)	++	+	?
vanillin	+(+)	++	+(++)	+++
furfural	+	+++	++	+++
tannins	(+)	+++	+	++(+)
roburins	(+)	+++	+	?
phenolics	+	+++	++	?
colour	red	yellow	pink	red
tyloses	+++	+	++	(+)

	seasoning	kilning	toasting	charring
oaklactones	+	(+)	++	+++
eugenol	-	+	+++	++(+)
vanillin	++	+	+++	++(+)
furfural	++	+	-(+)	+
tannins	---	-	---	-
roburins	---	-	--	-
guaiacols	(-)	(+)	++	++
colour	-	(+)	+	++

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