Sugars in whisky

There are small amounts of sugar in all whiskies. Scottish whiskies have some sugars dissolved from the oak cask and often some from optional caramel colouring (E150a). Total amount of sugars is quite low, usually well below 1 g/l, but in certain cases it is quite possible to reach a few grams per liter. The sweet aromas of whisky matured in refill bourbon or new oak casks mostly come from sweet aromatic vanillin and fruity esters, not from the sugars. However, sugars can have a significant role in the case of casks previously used for sweet wine or sweetened spirit.

Typical composition of a liquid E150a colouring

Sugar gets to whisky from the oak cask used in maturation and its possible previous contents, especially sweet wine, and often from optional caramel colouring. New make spirit does not contain significant amounts of any sugars and it is not legal to add sweeteners to Scottish whisky, Irish whisky or bourbon, unlike to most rums, other American whiskies, Canadian whiskies, cognac, armagnac, vodka and so on. However, caramel colouring is an allowed additive in Scotch whisky. The E150a colouring used in whiskies consists of mostly of water (30-50%), carbohydrates (50-70%) and ash (0-4%), depending on the manufacturer. The colour comes mostly from furans and furfurals, mainly 5-hydroxymethylfurfural (5-HMF). The carbohydrate composition varies a lot, but usually the amount of simple sugars (mono- and disaccharides) is about 10-20% of the total volume and about 20-40% of the total carbohydrates. Caramel colouring as such is not sweet but rather bitter, although it probably enchances the perception of vanillin and some other sweet notes while supressing some sulphury notes. The amount of caramel added to whisky is usually well below 1 g/l, but there is no legal upper limit, although 2g/l of E150a (0,2-0,4 g/l simple sugars) would render the whisky very dark. In one independent study the highest caramel solids amount found in a Scotch whisky was 0.97-1.10 g/l (simple sugars ~0.2-0.4 g/l) depending on the analysis method (I suspect L*******g, but no names) and the average amount was 0.278 g/l caramel (from 0.01 to 1.10 g/l). According to Valaer in 1940 there was a legal maximum of one pint of liquid caramel per barrel of whisky permitted by the British Customs and Excise Regulations (0.15% or about 1.5 g/l).

Caramel content of various whiskies (Boscolo et al 2002)

Another source of sugars in whisky is wood. Oak is composed of three main macromolecules; cellulose is a long-chain D-glucose-fiber. Hemicellulose is a mesh-like branched chain of 5-carbon sugars (xylose, arabinose, galactose, ribose, rhamnose) and glucose. Lignin is a complex matrix of polypropane and polyphenols. Cellulose breaks down very slowly in alcohol-water solution extracting very small amounts of glucose to whisky. Most sugars extracted from the cask come from hemicellulose, hence most of them are 5-carbon sugars. Arabinose, xylose and rhamnose taste about half as sweet as glucose, where as fructose is almost twice as sweet compared to glucose.

About 100 mg/l of solids (including sugars) is extracted from new charred oak casks during the first year of bourbon maturation and after 1-3 years typically significantly less and only as little as 4 mg/l/year extraction has been reported from old refill casks used for example for cognac (or Scotch) maturation. A toasted cask extracts considerably more sugars compared to a charred cask, but typically about 150-250 mg/l/year is reached for the first year(s) of maturation. For comparison, a 40 years old brandy matured in a toasted oak cask had 2 g/l sugars (averaging 50 mg/l/year), while a 30 years old cognac (probably mostly matured in a refill casks) had only 0.5 g/l (17 mg/l/year).

Sugars dissolve in water better than in ethanol, so for example 55% ethanol-water solution can extract almost two times more sugars from the wood compared to 70% abv solution. The difference between 40% abv and 15% abv seems to be just about significant, so the alcohol strength is probably important only for high proof spirits. High entry proof is likely to significantly diminish the sugar extraction. Toasting and especially charring of the oak lowers the amount of extractable sugars, converting them to for example furfurals ("caramel"). On the other hand, charring contributes to sweetness by increasing the formation, extraction and perception of other sweet-tasting compounds, especially vanillin.

Analysis of different spirits by GC-MS (Savchuk 2001) Note the high levels of vanillin from new charred casks.

Probably the most important factor considering the amount of sugars in Scotch is the previous content of the cask. The most usual type of cask, the ex-bourbon cask would impart minimal amounts of sugars, often well under 50 mg/l/year and overall well below 1 g/l even during a long maturation, usually in the range of few hundreds of milligrams per liter. The charred American oak cask used for bourbon is quite low on sugars to start with and the bourbon will extract a major part of the available sugars. A toasted cask previously used for cognac, armagnac, brandy of other non-sugared spirit could extract slightly more, but most likely still not over amounts 1 g/l during refill maturation.

Venezuelan rum, 40 g/l sugars

Rum can, and often does, contain added sugar or other sweeteners. Most sweetened rums are sugared at the bottling phase, but sometimes sugar (or molasses, cane juice, fruit etc) has been added already into the cask and some of that sugar is bound to get into a ex-rum refill Scotch. Up to 40 g/l sugar contents are commonly reported in rum (100 g/l in spiced rums), but the amount of sugar added directly to cask is likely to be in the range of 5-20 g/l.

Pedro Ximenez sherry,
 470 g/l sugars

Significant amounts of sugar can be extracted from a sweet wine cask. Sherry is by far the most used ex-wine cask in the Scottish whisky industry. Most sherries, such as fino, manzanilla, amontillado, palo cortado and most olorosos, are dry wines (0-5 g/l sugars) and add only little sugars to the cask, some might even extract more than impart. Most sweet blended sherries (cream, dulce, sweet olorosos) are often blended with sweet must prior to bottling, not in maturation casks. The sweetness perceived in the olorosos often comes from high amounts of glycerol, not as much from sugars. A notable exception is Pedro Ximenez wine, which nowadays is vinified almost exclusively in Montilla-Moriles. It is a sweet wine produced from late harvested dried PX grapes and it typically contains by law over 212 g/l sugars. It is quite sweet even compared to other sweet wines, such as port (usually 100-150 g/l), Sauternes (100-200 g/l), Tokaji (60-500 g/l) or icewine (180-320 g/l).

The Scotch whisky industry has preferred sweet sherries for seasoning of their whisky casks from the 19th century and while most modern sherries are quite dry, most ex-sherry whisky casks were used for (or seasoned with) sweet sherries. In the 1800s and the early 1900s sweet(ened) sherries were imported to Britain in shipping casks. Britain was a major importer of sherry and emptied shipping casks were often used for maturation of Scotch whisky. By the late 19th century sherry casks were used extensively in Scotland and the first attempts to rejuvenate or season used casks were made in the 1890s. After the sherry casks were used for Scotch whisky maturation, they were seasoned with sweet sherry or paxarette, sometimes using pressure to soak the wood with the wine. By the early 20th century the use of sherry casks was quite wide spread, for example in the late 1920s a big whisky blender Johnnie Walker matured all of their whiskies in sherry casks or sherry-treated casks.

Aberlour 18yo
sugars <2 g/l

Shipping casks were mostly Spanish coopered lightly toasted American oak (Quercus alba) butts or puncheons (often called bocoyes in Spain). In 1972 Manuel González Gordon stated that "in recent years some Spanish oak has been used, due principally to the difficulties of importing American timber, but its greater density and hardness and lower porosity make it less suitable than the American wood". In the 19th century and the early 20th century they were often used for fermentation prior to shipping. Oak influence was not that detrimental for sherry during fermentation period and the fermentation process was believed to extract some of the excess tannins and bitterness from the wood prior the use in a solera or shipping.

The fermentation phase added plenty of sugars to the oak. For example, a typical 1980s (arguably Spanish oak, but at least Spanish coopered oak) shipping cask previously used for fermentation extracted 2.25 g/l sugars to whisky during only 6 years of maturation (375 mg/l/year). On the other hand an American ammonia-treated cask without a previous fermentation use would extract 770 mg/l (154 mg/l/year) sugars from a fino-seasoned cask and even less 655 mg/l (133g/l/year) from a similar oloroso cask. According to the table below, the previous use as a fermentation cask is very significant in the terms of sugar extraction.

JM Philp 1989

Ardbeg Dark Cove
4 g/l sugars

The most important factor considering the sugar extraction is probably the sweetest liquid to occupy the cask previously. According to Manuel González Gordon "large solera butts which have been used for years in a solera sometimes gain up to 25 kilos" in weight. In case of fermenting must or sweet wine, the refill cask can extract at least up to about 400 mg/l/year sugars. A typical sherry treatment used for Scotch whisky casks used in the 1890-1970s was made with about 35 litres of paxarette or sweet sherry per hogshead, but even with a relatively high pressure only a small part of the wine was absorbed into the cask. If we presume that as much as 5 litres of paxarette (375 g/l sugars) was absorbed into a hogshead and half of its sugars (just a figure from top of my hat) was extracted to the whisky, it would have resulted into 1875 g/ 220 litres = about 4.3 g/l of sugars, with a hypothetical gram or two from the cask itself could theoretically bring the sugar content up to 6 g/l during a very long maturation, say an average of 300 mg/l/year for 20 years. If were take a purely fictional solera butt used for an extremely sweet PX-wine (400 g/l) with 25 kg of PX wine absorbed into the cask and then used for whisky maturation, it could mean 10 kg of sugars in the cask and with a purely fictional extraction of about half of the sugars would result roughly to the amount of 9 g/l sugars into the whisky in an extreme(ly long) maturation from an extreme cask. It is probably safe to say that most spirits with over 10 g/l sugars contain additives and most spirits with over 2 g/l were matured in ex-sweet wine casks. Nowadays the sherry casks used in the whisky maturation are no longer used for fermentation or exposed to high-pressure paxarette/PX-treatment. The seasoning sherries are probably mostly medium oloroso/raya sherries with a sugar content in the range of 0-100 g/l, so a typical sugar extraction from that kind of cask could be in the range of 100-300 mg/l/year and diminishing through maturation, resulting to a maximum of a few g/l sugars into a modern whisky even during decades in a cask.

REFERENCES AND FURTHER READING:
Alañón ME et al. Monosaccharide anhydrides, new markers of toasted oak wood used for ageing wines and distillates. Food Chem 2010;119;505-512
Blanco Gomis D et al. Evolution of sugars in cider brandy aged in oak barrels, a contribution to its characterization. J Agric Food Chem 2003;51;923-926
Boscolo M et al. Spectrophotometric determination of caramel content in spirits aged in oak casks. J AOAC Int 2002;85(3);744-750
Boudet AM et al. Biochemistry and molecular biology of lignification. New Phytol 199;129;203-236
Clyne J et al. The effect of cask charring on Scotch whisky maturation. Int J Food Sci Tech 1993;28;69-81
González Gordon M, Sherry. Cassell Ltd 1972
Haldane FF. Casks; their manufacture and treatment. J Inst Brew 1906;12(7);688-711
Hills, P. Appreciating whisky. Collins 2000
Kansallisarkisto, Helsinki. http://www.arkisto.fi/en/the-national-archives-service/arkistolaitoksen-vaiheet-2 
Liebmann AJ & Rosenblatt M. Changes in whisky while maturing. Ind Eng Chem 1943;35(9);994-1002
Martínez Montero C. Estudio de parámetros alternativos como indicatores del envejecimiento y de la calidad del brandy de Jerez. Thesis, Universidad de Cádiz, 2006
Martínez Montero C et al. Sugar contents of brandy de Jerez during aging. J Agric Food Chem 2005;53;1058-1064

Mosedale, JR. Effects of oak wood on the maturation of alcoholic beverages with particular reference to whisky. Forestry 1995; 68; 3; 203-230
Mosedale JR & Puech JL. Wood maturation of distilled beverages. Trends Food Sci Tech 1998;9;95-101

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
Piggott, JR et al(ed). The Science and technology of whiskies. Longman 1989

Read J. Sherry and the sherry bodegas. Sotheby's 1988
Savchuk SA et al. Application of Chromatography and Spectometry to the Authentication of Alcoholic Beverages. J Anal Chem 2001;56(3);246-264
Tolman LM & Trescot TC. A study of the methods for the determination of esters, aldehydes and furfural in whisky. J Am Chem Soc 1906;28(11);1619-1630
Tsai PJ et al. Interactive role of color and antioxidant capacity in caramels. Food Res Int 2009;42;380-386
Valaer P & Frazier WH. Changes in whisky stored for four years. Ind Eng Chem 1936;Jan;92-105
Valaer P. Scotch whisky. Ind Eng Chem 1940;32(7);935-943
Valaer P. Foreign and domestic Rum. Ind Eng Chem 1937;Sep;988-1001
Williams JF. Rapid determination of alcohol in distilled spirits and of color in whisky. Ind Eng Chem 1926;Aug;841-843

Bengt Thorbjörnson in Scotland, 1929

Bengt Thorbjörnson (1891-1975) was a Swedish chemical engineer. After graduating from Kungliga Tekniska Högskolan of Stockholm he worked in sulphite factory in Bergvik (1916-1917), which manufactured sulphite spirits as a side product from 1911 at least until 1917. In 1917 he moved to Kramfors to work in a cellulose factory (1917-24) also producing industrial spirits as a side product. After a short spell at a margarin factory in Kalmar (1924-26) and a visit to Nashwalk Pulp & Paper Co in USA (1924-1925) he was appointed as the chief chemical engineer for Vin & Sprit AB (Wine & Spirit). He continued to work for the biggest Swedish alcohol producer for 30 years until 1957.

Bengt Thorbjörnson on the left

 Thorbjörnson toured Scottish whisky retailers and distillers in 1929-1930. The main object was to investigate whether it was profitable to start whisky production in Sweden. After the tour the Finnish alcohol monopoly consulted Thorbjörnson on the same subject and the record of the Scottish tour is still available in Swedish from the Kansallisarkisto (The National Archive) of Helsinki. Thorbjörnson visited the warehouses of W.H. Chaplin, John Walker & Son and The Distillers Company, as  well as the distilleries in Caledonian, Mortlach, Cardow (Cardhu), Glen-Mhor, Glen Albyn and Adelphi.

He preferred the Highland malts and considered the Campbeltown malts were "lacking midtaste". The factors most affecting the flavour of whisky according to his studies were: 1) Water, which should be low in calcium. The water of River Spey was good as it ran through soil rich in granite and sand. 2) Climate, which should be quite cold but even to allow long stable distilling times. 3) Amount of peat used in drying the malt. 4) Bacteriae flora, as the local bacteriae influenced the quality of the brew. 5) Experience of staff. 

The first place to visit was the wine and spirit retailer W.H.Chaplin & Co in London, who were at the time the sole representatives of the popular Long John brand. Their warehouse at Tower Hill consisted of 10 floors, of which 3 were underground. Huge glazed concrete cisterns (136 000 l) were used to vat and cold-chill-filter port wine. The whisky was vatted on demand in smaller concrete vats of size about 22 000 litres from different casks and different distilleries. According to other sources, Ben Nevis was the leading malt for Long John.

At John Walker & Son Thorbjörnson was hosted by the manager Sir Alexander Walker. At the time their Johnnie Walker blend was the most sold whisky brand in the world. Only sherry casks were used at the time. About 20% of them were new casks seasoned with sherry and the rest were refill casks rejuvenated with a small amount (about 35 litres) of sweet dark sherry for six weeks during which they were turned regularly. After the sherry-seasoning they are treated with pressure to impregnate more sherry into the wood. The sherry in the cask was then poured off and used several times for other casks. The pressure treatment had been developed by WP Lowrie in late 19th century and at the time of Thorbjörnson's visit it was used by many other blenders and distilleries as well. The Walkers had recently shifted to mainly hogshead size casks to ensure even quality. At the present time the Johnnie Walker recipe consisted of 10 parts Highland malt (Mortlach, Benrinnes, Ord, Cardow, Glenlossie, Dailuaine, Aultmore, Coleburn or Clynelish), 2 parts of Islay malt (Talisker (classified as Islay!), Caol Ila or Lagavulin) and 2 parts of Lowland malt (Rosebank or Glenkinchie). The blended malt was then again blended with grain whisky (mostly Caledonian) and the malt content varied between 40-60%. Long John blend consisted of 65% grain, 15% Lowland malt, 15% Highland malt and 5% Islay malts.

Caledonian distillery in 1966 (scotlandsplaces.gov.uk)

Caledonian grain distillery produced 40 000 gallons per week, which equalled 9 000 000 litres 50% abv- spirit per year. Maize was the most common cereal, but wheat and barley were sometimes used, too. About 30% of the mash came from barley malt dried over coal fire to gain diastase power. The maize flour was pressure cooked to prevent bacterial contamination. Brewers' yeast from Edinburgh was used to ferment in covered washbacks of 225 000 litres capacity (Cardow used pressed yeast). Carbon dioxide was collected and sold to mineral water producers. Column distillation with 20 plates in the analyser and 40 plates in the rectifier was used to produce new make spirit of 67 degrees over proof (95,3% ABV). The spirit was cut with water to 11 over proof (63,4% abv) for maturation. Draff was given for free to farmers. At the Adelphi grain distillery weekly production was 36 000 gallons/week and the only significant differences were the cooking of maize (not pressurised in Adelphi) and the fermentation time (72h in Adelphi, 96h in Caledonian). About 125 men were employed in each.

Mortlach produced high quality Highland malt with "quite old-fashioned means". A total of 24 men were employed to produce 8 000 gallons per week. Production was bigger than at the Invergordon distilleries, as Glen-Mhor and Glen Albyn managed only 2 500-3 000 gallons per week each. Distillation was carried out from September to end of May and the spring production was considered to be of the best quality. Mostly foreign barley was used and floor malted on site. Two kilns were used to dry malt for 50 hours in up to 77C after 9 days of germination. About 18 kg peat for every 120 kg of coal was used in kilning. Fermentation time in seven 60 000 gallon washbacks varied between 46-56 hours. After each fermentation the washbacks were washed with lime and peat was burned on the bottom of the washback to avoid bacterial contamination. The spirit was double distilled (no mention of the Wee Witchie or even partial triple distillation) to a very high proof of 45 over proof (82,8% abv) and reduced to standard 63,4% before maturation. Rummagers for wash still and direct firing with coal for both stills were used. 

Thorbjörnson calculated that the blending and maturation was cheaper in big English warehouses compared to the Swedish Reymersholm or Slottet warehouses. He also thought that the flavours came mostly from the malt whiskies and therefore the malt content of the Swedish blends (Crown Blend and Black Label) should be increased. On the other hand Scotch grain whisky could be replaced with cheaper domestic neutral potato spirit to cut costs. He also made a a costs-analysis for building a Swedish malt distillery with a 500 000 litres capacity per 6 months, which apparently never came to be.

References and further reading:
Kansallisarkisto, Helsinki. http://www.arkisto.fi/en/the-national-archives-service/arkistolaitoksen-vaiheet-2

Koch B. Från idé till produkt. Svenska Uppfinnarföreningen, 1963.

Morrice, P. Schweppes guide to Scotch. Alphabooks 1983
Spiller, B. Cardhu. John Walker & Sons, 1985.

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	-	(+)	+	++

References:

Cutzach I et al. Identification of volatile compounds with a toasty aroma in heated oak used in barrelmaking. J agric food chem 1997;45;2217-2224

Doussot F, De Jeso B, Quideau S, Pardon P. Extractives content in cooperage oak wood during natural seasoning and toasting; influence of tree species, geographic location and single-tree effects. J Agric Food Chem 2002; 50; 5955-5961
Gallagher et al. Whisky losses during aging. Ind Eng Chem 1942; 8; 992-995

Garde-Cerdàn T et al. Effects of composition, storage time, geographic origin and oak type on the accumulation of some volatile oak compounds and ethylphenols in wines. Food Chem 2010; 122; 1076-1082

González Gordon M, Sherry. Cassell Ltd 1972
Gomez Plaza E et al. The effect of successive uses of oak barrels on the extraction of oak related volatile compounds from wine. Int J Food Sci Tech 2004;39;1069-1078

Gougeon RD et al. Expressing Forest Origins in the Chemical Composition of Cooperage Oak Woods and Corresponding Wines by Using FTICR-MS. Chem. Eur. J. 2009, 15, 600 – 611

Gougeon RD et al. The chemodiversity of wines can reveal a metabologeography expression of cooperage oak wood. PNAS 2009; 106; 23; 9174-9179 Mangas J et al. Volatiles in distillates of cider aged in american oak wood. J Agric Chem 1996; 44; 268-273

Martinez J et al. Effect of the seasoning method on the chemical composition of oak heartwood to cooperage. J agric food chem 2008;56;3089-3096

Monica Lee KY, Paterson A, Piggott JR. Origins of flavour in whiskies and a revised flavour wheel. Review. J instit brew 2001; 107; 5; 287-313

Mosedale JR, Puech JL. Wood maturation of distilled beverages. Trends in Food Sci Technol 1998; 9; 95-101

Mosedale, JR. Effects of oak wood on the maturation of alcoholic beverages with particular reference to whisky. Forestry 1995; 68; 3; 203-230
Mosedale, JR. Variation of the flavour and extractives of european oak wood from two french forests. J Sci Food Agric 1996; 70; 273-287

Mosedale JR, Charrier B, Crouch N, Janin G, Savill PS. Variation in the composition and content of ellagitannins in the heartwood of european oaks. Ann Sci For 1996; 53; 1005-1018

Perez Coello MS et al. Analysis of volatile components of oak wood. J Chromatograph 1997;778;427-434

Prida A, Puech JL. Influence of geographical origin and botanical species on the content of extractives in american, french and east european oak woods. J Agric Food Chem 2006;54;8115-8126

Prida A, Ducousso A, Petit RJ, Nepveu G, Puech JL. Variation in wood volatile compounds in a mixed oak stand: strong species and spatial differentiation in whisky-lactone content. Ann For Sci 2007; 64; 313-320

Prida A et al. Relation between chemical composition of oak wood used in cooperage and sensory perception of model extracts. J Sci Food Agric 2009;89;765-773

Spillman PJ, Sefton MA, Gawel R. The effect of oak wood source, location of seasoning and coopering on the composition of volatile compounds in oak-matured wines. Aust J Grape Wine Res 2004; 10; 216-226

Viriot C et al. Ellagitannins and lignins in aging of spirits in oak barrels. J Agric Food Chem 1993;41;1872-2879
Withers SJ et al. Comparison of Scotch malt whisky maturation in oak miniature casks and american standard barrels. J Inst Brew 1995;101;359-364

Coopering, part2

Most whisky casks are made in the USA, where over a million bourbon casks are produced per annum compared to circa 500 000 (mostly wine) casks in France. Estimated 800 000 used American bourbon casks are sold for Scotch whisky industry per year. It is estimated that about 18 million casks at a time are being used in maturation of whisky in Scotland, suggesting that an average lifespan of one cask is typically 20 years, including the possible refills and rejuvenations.

Cooperage oak is usually harvested at an approximate height of 20-30m, diameter of 35-60cm and age of 60-250 years, according to the species, soil and climate. Fast growing Q.alba and Q.robur are harvested usually at the age of 80-120 years, but Q.petraea grown in dense forests (as in central France) might need up to 250 years. Harvesting time and the age of wood influence the composition of wood. It is believed that wood harvested during winter develops less "green wood"-notes and is easier to cooper due to greater amount of tyloses forming in the spring wood and the astringent tannins diminsh as the tree ages.

The oak used in coopering is cut in quarters to prevent the staves from warping, cracking and leaking. Quartered staves are approximately 25% more waterproof than tangentially sawed staves.

American oaks, especially Q.alba, tend to grow straight and with only a few branches making the coopering easier. High concentration of tyloses and the relative absence of knots allows machinery to be used in cooperages more than in Europe and the staves are often quartered by sawing instead of cleaving in American cooperages. Cleaving is more labour intensive and timeconsuming, but it produces tighter casks and might also affect the permeability of some flavor components. Some Portuguese coopers also use sawing, especially when coopering coarse-grained Q.robur or Q.alba, but usually Q.petraea is cleaved.

After quartering the staves must be dried to ease the coopering and to prevent cracking, shrinking and leaking. The traditional method is to season the staves in columns in oper air for 1-3years, but since the 1800s kiln-drying in 40-60C for about three weeks has been used too, especially in the USA. The French prefer to season their staves much longer, up to 20 years, especially the staves used in maturation of quality cognac or armagnac. The method used to dry the staves does affect the properties of the cask, reducing tannins and adding some flavours from the local microbes.

Seasoned staves are heated and constructed into a cask. A typical whisky barrel consists usually of 31 staves (8 for the heads) and a hogshead of a couple of more. Heating helps to bend the wood and seasons the inside of the cask. American coopers usually use hot steam in bending and a gas burner to char the inside of the cask. Charring is fast; light char is achieved in 15 seconds, medium char in 30s and heavy char in 45s. Further charring results in alligator/grade4 char with blistering burned wood. Europeans commonly use burning wood chips in bending and toasting. Toasting is slower, generally 15 minutes of firing produces light, 30 minutes medium and 45 minutes heavy toasting, although there are variations between cooperages and coopers. Usually the inside of the cask reaches 100-200C in toasting and over 250C in charring. Caramellisation of sugars in hemicellulose and lignin and to some extent in cellulose begins at about 140C.

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. In the Jerez region it was customary to ferment the must in new barrels to remove excess resin and tannins from the cask and to produce a furry deposit on the inside of cask, probably consisting of fermenting yeast and bacteria, which was believed to improve the cask. After 1960's most big sherry producers have used stainless steel tanks for fermentations.

Rejuvenation of casks is used in Scotch whisky industry. It usually consists of scraping the inside of the staves about 5-10mm deep, recharring the cask and in some companies seasoning it with sherry or must. This brings new active wood into contact with the spirit and allows the use of cask for refills. Scraping the staves too much increases the risk of leaking or cracking, but on he other hand if the scraping is too thin, the cask may not be active enough. The influence of spirit reaches up to 20mm through the wood. Usually casks are rejuvenated after three fillings, although some companies are rumoured to 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. Recent innovation in rejuvenation has been a treatment of cask with salt water before recharring or retoasting, which improves the extraction of color and flavour from refill casks.

Various cooperage practices such as wood selection, sawing/cleaving, seasoning/kilning and charring/toasting affect the maturation properties of casks.

REFERENCES:

Chatonnet P, Dubourdieu D. Using Electronic Odor Sensors To Discriminate among Oak Barrel Toasting Levels. J Agric Food Chem 1999; 47; 4319-4322

Clyne J, Conner JM, Paterson A, Piggott JR. The effect of cask charring on Scotch whisky maturation. Int J Food Sci Technol 1993; 28; 69-81

Conner JM, Paterson A, Piggott JR. Changes in wood extractives from oak cask staves through maturation of Scotch malt whisky. J Sci Kilby, K. The cooper and his trade. John Baker Publishers Ltd 1971.

Deguilloux MF, Pemonge MH, Petit RJ. DNA-based control of oak wood geographic origin in the context of the cooperage industry. Ann For Sci 2004; 61; 97-104

Food Agric 1993; 62; 169-174

González Gordon M, Sherry. Cassell Ltd 1972

Jackson, RS. Wine Science. Academic Press 2000

Mosedale JR, Puech JL. Wood maturation of distilled beverages. Trends in Food Sci Technol 1998; 9; 95-101

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

Puech JL. Characteristics of oak wood and biochemical aspects of armagnac aging. Am J Enol Vitic 1999; 50; 4; 503-512

Ronde I (ed. Malt whisky yearbook 2011. MagDig Media 2010

Spillman PJ, Sefton MA, Gawel R. The effect of oak wood source, location of seasoning and coopering on the composition of volatile compounds in oak-matured wines. Aust J Grape Wine Res. 2004; 10; 216-226

Wanikawa A, Hosoi K, Kato T, Nakagawa K. Identification of green note compounds in malt whisky using multidimensional gas chromatography. Flavour Fragr J 2002; 17; 207-211

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

Coopering



Cooper in the Guinness brewery, late 19th century

Wooden barrels have been used to store wine since ca 500BC. Whisky has probably been stored in barrels from the invention of distilled malt spirits, altough the significance of maturation has been commonly appreciated from the  latter part of 19th century. In 1915 it was made compulsory for Scotch whisky to be warehoused in bond for a minimum of two years and the age limitation was extented to three years one year later. In 1988 the Scotch Whisky Act declared that the maturation had to take place in wooden casks of a capacity not exceeding 700 litres. The wooden casks were specified to be made of oak in Scotch Whisky Order 1990.

Until the 16th century the oak used in cooperages of United Kingdom was mostly English. After that shipbuilding and construction increased the demand for quality oak and imports of oak staves increased. There was even a legistlation from 1543 prohibiting export of casks larger than barrels and making exporters import a corresponding amount of timber for casks. The British imported oak came mostly from the Hansa region (Denmark, Baltic), Russia and America (Virginia, New Orleans). Used wine barrels especially from Spain and Portugal were also common. Spanish and Portuguese coopers preferred American oak, whereas British coopers preferred Memel oak over American, European and English oak.

Memel (now Klaipèda) was an important port in Baltia (Lithuania) providing timber from the Baltic region and Russia. Memel was the golden standard of quality oak for the British until 1930s, but during and after the second world war it became unobtainable in UK due to rise of Soviet Union in Baltia. In addition the Spanish civil war (1936-1939) decreased the amount of imported oak casks from Spain. In the 1930s and 1940s Polish, American and English oak was used. In the 1950s Memel oak was already available, but because of the loss of woods in the wars and difficulties in trade the price was high.

Speyside cooperage

From the war years until 1956 as much as 90% of oak used in UK breweries was Persian oak, probably of species Q.brantii (Persian oak, a white oak), Q.macranthera (Persian oak, caucasian oak, not exactly white oak but similar Q.mesobalanus) or Q.mirbeckii (Q.canariensis, a white oak). The quality of Persian oaks varied greatly, probably due to different species harvested and the long transportations in too hot climates. The amount of Persian oak casks used in whisky industry is not recorded. Wine and spirit shipping casks were used widely and apparently preferred for maturation of whisky, especially sherry casks from Spain and port wine casks from Portugal.

Oak maturation of quality American whiskey became a norm during the late 1800s, whisky from Kentucky being sold with age statements from 1840's. Most American bourbon is supposedly matured in charred new Q.alba, but also other American white oaks and their hybrids are used either on purpose or by mistake, since the species can be quite hard to distinguish even with DNA-testing and practically impossible to differentiate only by inspecting the staves.  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. In the 1960's the American cooperages produced up to 2.3 million casks a year, about twice as much as today. Because of the surplus bourbon casks the prices dropped and resulted in diminishing use of sherry casks during the last decades, especially as the amount of sherry shipped in casks dimished. Today over 90% of Scotch whisky casks are ex-bourbon. The barrels are usually dismantled into shooks during transportation and assembled again in Scotland. Usual practice is to convert the barrel sized (150-220l) bourbon casks into bigger hogsheads (about 250l) by adding a couple of extra staves of either new or used wood. Some bourbon producers are now using hogshead-sized casks, too. The casks in Scotch whisky industry are filled usually three times before discarded or rejuvenated by seasoning and/or toasting. Other types of American whiskey (not bourbon) have been allowed to mature in refill oak casks since 1970. Whether the Scotch whisky industry has utilized these American refill whiskey casks is not known. Some American red oak has been used in maturing spirits and beer, most notably Guinness beer from late 1800s to early 1900s, but generally red oak casks are not used anymore.

Manuel González Gordon

Sherry producers use mostly new American oak, usually toasted and not charred, but at least some Spanish Q.robur from Galicia is used. In the 1960's and the 1970's Costa Rican red oak varieties Q.costaricensis, Q.eugeniifolia and white oak Q.copeyensis were used (and esteemed) in the Sherry region, but I have no information of the scale of this nor whether they are still used. The sherry producers have been using at least some American oak from the 1600s and nowadays use it almost exclusively. A note from 1807 by Spanish botanist Esteban Boutelou (1776-1813): "The great wine shippers cooper themselves the casks they require for their trade...They use oak almost exclusively; and they esteem more than any that which comes from the United States of America, next the Northern oak, then the Italian and lastly the Spanish... wines ferment better in butts and wooden vats than in earthenware, which belief is shared by the Sanlucar growers.". Hardman (1878) reports that American oak is used exclusively for sherry and marsala production. The great sherry authority Manuel González Gordon writes in 1948 (and in later editions 1972 and 1990) that "In Jerez no other wood has been used for many years, as American oak has given the best results for the fermentation, maturing, ageing and shipping of Sherry." and "In recent years some Spanish oak has been used [for shipping sherry], due principally to the difficulties of importing American timber". Of the big sherry importers Gonzalez-Byazz, Sandeman, Williams-Humbert and Lustau declare they use mostly American oak and I have not yet found a firm using only European oak. It is safe to assume that the whole Sherry region has used mostly American oak for at least the last two centuries and the majority of the Spanish oak casks used in Scotch whisky industry are in fact Spanish-coopered American oak casks. Some European oak (Q.robur, Q.pyrenaica) sherry casks have sold to Scotch whisky producers either by chance or by special order from for example Edrington Group (owner of Macallan, Glenrothes, Highland Park,Tamdhu, Glenturret), Glengoyne and Gordon&MacPhail. Sherry shipments by the cask have practically ended since Spain became member of EEC in 1986 and bulk shipments of wine fortified over 15.5% ABV became prohibited inside EEC. Usually only some fino sherries are below 15.5% ABV and the olorosos preferred in whisky maturation are often over 18% ABV. In addition, if the lower strenght sherry wines with the status of Denominación de Origen (DO) are to be shipped in bulk, the containers must be approved and sealed by the Consejo Regulador de Jerez and the wine must be accompanied with detailed certificates of analysis. Only the biggest sherry houses (Bodegas de Crianza y Expedicion) are allowed to ship sherry in bulk (64 different houses at the moment). Understandably practically all sherry casks are shipped empty or dismantled to shooks.

Apparently there is no official definition for "sherry cask" in Scotch whisky industry, so some of the so called sherry casks might be ex-bourbon casks treated with sherry or similar products such as paxarete wine (a heavy sweet wine from the Sherry region). The size of sherry cask is by law under 1000 litres, usually a butt contains 30 arrobas (about 500 litres), although a variety of different size casks has been used (octaves, hogsheads, gordas, bocoys etc).

In Portugal "imported oak is preferred". American, Polish, French and even Brazilian (no idea of the species) oak has been used, although considerable amounts of Q.pyrenaica and even Castanea sativa (chestnut) has been used as well, at least in red wine maturation and port wine shipping casks. Since 1986 the same EEC shipping rules apply to fortified port as to sherry (see above). The size of port cask (pipes) is usually slightly bigger than the cask size for sherry or other wine casks.

French oak has been used extensively in wine cooperages and lot of research has been made concerning diffrent oak species and local variance of wood as well as its influence on wine maturation. Traditionally local oak has been used to mature local wines, but recently the knowledge on oak qualities has lead to various experiments with different oaks and combinations of oak species. There are lots of variation between the species and the forests, but generally Q.petraea is considered to be of better quality for wines, especially for the whites. Wine casks are usually barrique sized 225l.

Q.robur (Limousin, Gascony, Cîteaux, Galicia) is used to mature spirits, most notably cognac and armagnac. Armagnac producers do use different species of oaks, but the most appreciated is "black oak" of Monlezun forest in Gascony. Black oak is probably a variation of Q.robur. Cognac producers prefer Q.robur from Limousin; new casks are used to mature inferior cognacs or for just a short period (months) in the beginning of maturation before reracking the spirit into old (usually bigger) casks for longer maturation. Cognac casks are usually sized 270-450l, the most popular size being 350l.

Q.crispula, Q.dentata, Q.serrata and probably Q.mongolica are used in Japan. The Japanese mizunara casks are commonly 500l casks and often seasoned with sherry.

General rule of the thumb seems to be that casks made of European Q.robur or American Q.alba are preferred in spirit maturation over Q.petraea in various spirit producing areas and for longer maturation periods refill casks are preferred, except for bourbon.

http://whiskyscience.blogspot.fi/2011/02/coopering-part2.html

References:

Calabrese S. Cognac, liquid history. Cassell&Co 2001
Crowgey HG. Kentucky bourbon, the early years of whiskeymaking. Univ Press Kentucky 2008.

Deguilloux MF, Pemonge MH, Petit RJ. DNA-based control of oak wood geographic origin in the context of the cooperage industry. Ann For Sci 2004; 61; 97-104

Doussot F, De Jeso B, Quideau S, Pardon P. Extractives content in cooperage oak wood during natural seasoning and toasting; influence of tree species, geographic location and single-tree effects. J Agric Food Chem 2002; 50; 5955-5961

González Gordon M, Sherry. Cassell Ltd 1972

Gougeon RD et al. Expressing Forest Origins in the Chemical Composition of Cooperage Oak Woods and Corresponding Wines by Using FTICR-MS. Chem. Eur. J. 2009, 15, 600 – 611

Gougeon RD et al. The chemodiversity of wines can reveal a metabologeography expression of cooperage oak wood. PNAS 2009; 106; 23; 9174-9179
Halley, N. Sandeman: two hundred years of port and sherry. Granta Editions 1990.
Hardman, W. The wine-growers and wine-coopers manual. Wm Tegg London 1878

http://eur-lex.europa.eu 

Jackson, RS. Wine Science. Academic Press 2000.

Kilby, K. The cooper and his trade. John Baker Publishers Ltd 1971.

Lepais O et al. Species relative abundance and direction of introgression in oaks. Molecular Ecology 2009; 18; 2228-2242

Monica Lee KY, Paterson A, Piggott JR. Origins of flavour in whiskies and a revised flavour wheel. Review. J instit brew 2001; 107; 5; 287-313

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

Page CE. Armagnac, the spirit of Gascony. Bloomsbury Publ 1989

Prida A, Ducousso A, Petit RJ, Nepveu G, Puech JL. Variation in wood volatile compounds in a mixed oak stand: strong species and spatial differentiation in whisky-lactone content. Ann For Sci 2007; 64; 313-320

Prida A, Puech JL. Influence of geographical origin and botanical species on the content of extractives in american, french and east european oak woods. J Agric Chem 2006; 54; 8115-8126

Read J. Sherry and the sherry bodegas. Sotheby's 1988

Reid KJG, Swan JS. Assessment of Scotch whisky quality by pyrolysis - massspectrometry and the subsequent correlation of quality with the oak wood cask. J Anal Appl Pyrol, 1993; 25; 49-62

Singleton VL. Oxygen with phenols and related reactions in musts, wines and model systems. Am J Enol Vitic 1974; 38; 69-77

Swan JS. What's best for barrels: air or kiln-drying? Wines & Vines. July 1993

Twede D. The cask age: the technology and history of wooden barrels. Packag techol Sci 2005; 18; 253-264
www.legislation.gov.uk/ukpga/1988/22