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.

Pajarete and the wine treatment

The ruins of Torre Pajarete
Castillo de Matrera, Villamartin, Andalucia, Spain
Foto www.laspain.com

Originally pajarete (paxarete, paxaret, paxarette, pacharetti) meant the Paxarete vineyards of Cartusian Convent of San Hieronomo near Jerez de la Frontera, probably east towards Villamartin and Prado del Rey. The vineyards were planted with sweet grape varieties Millar and Pedro Ximénez grapes along with some Moscatel and Palomino. After phylloxera there is no evidence of the dark and sweet Millar variety. Sherry wines called abocado, vino seco and paxarete were made in the monastery. The first grapes to ripen were picked first and set on esparto grass mats to dry. Palomino (Listán) was picked last and pressed usually on the same or the following day. The harvest lasted for 1-3 weeks allowing the Millar, PX and Moscatel grapes to dry longer, usually about 10 days. As a result the early ripening grapes became raisined and produced natural sweet wine of 8-15% abv and about 300-400 g/l of residual sugar, which was often fortified with brandy de Jerez. Abocado was a dry oloroso style oxidized sherry (sometimes sweetened with Palomino Dulce) and Vino Seco (or Fino Seco) a dry fino/amontillado wine, both of which could be still referred as (dry) Paxarete wines in the 19th century. Similar sweet wines were made in Rota from dried Tinta grapes producing bitter-sweet dark Tintilla de Rota. Color de Macetilla or Mistela was an oloroso, which had been fortified before it fermented completely, leaving it sweet, in the style of port wines. Another similar product was mosto apagado, which was done by adding sweet mosto into the wine producing partially fermented sweet wine and usually used only in blending of sweeter sherry styles. Dulce was made by fermenting must from (mostly palomino) grapes in a cask that contained brandy; the higher alcohol stopped the fermentation before all the sugar was used, producing sweet high-alcohol white wine.

Pedro Ximénez grapes dried on grass mats

The sweet paxarete was valued highly in Spain, but in the early 19th century Britain sweet sherries were considered more of a ladies drink, men preferring drier fortified wines, beer and spirits. During the 1830s the sherry trade boomed the first time and in 1838 there were 43 registered shippers of sherry. Paxarete was one of the most expensive types, costing up to 7 times more than the cheapest real Jerez sherry (probably raya) and usually a bit more than a good aged amontillado or fino. Sherry trade reached its peak of the 19th century in the 1870s as the French strugled with vine diseases. At the same time the branding and marketing of wines by the shippers really began, which made it easier to blend different wines to be sold as sherry, including those from different regions. Blending wines with mosto (grape juice), brandy, sugar and colourings was very common and not just in the sherry business. As dried grapes were easier to transport, some paxarete was often made from grapes imported from other areas. Málaga and Montilla became major producers of pajarete sherry in the late 19th century.

Malaga Pajarete from 1968,
Imported by the Finnish church
for sacramental wine

During the latter part of 19th century the English taste shifted towards lighter drier wines, partly because of the taxation and the flood of French wine into the market after 1860 as the customs embargo was dropped. Paxarete was still imported regularly and it was one of the most esteemed "malmsey" wines in Victorian Britain. Sherry barrels were used for whisky maturation probably from the early 19th century and as the blending and branding became common towards the end of the century. Sherry casks, and most likely also sherry as an additive, were used to give body to especially the lighter and younger grain spirits. Oxidized and sweet sherries such as pajaretes, olorosos, rayas and cream sherries were preferred because they gave the darkest colour and an agreeable flavour.

Pajarete was not a legally defined until 1999, so some producers from for example Málaga and California were selling pajarete/paxarete wines of their own, sometimes consisting of Pedro Ximénez wine, but quite commonly of caramel coloured, sweetened, oxidized and fortified wines. A notable imitation sherry was the very affordable Hamburg sherry, which consisted of potato spirit, caramel and spices. It was was banned in 1870 due to "health reasons". Most big sherry shippers had their own paxarete, PX or dulce brands; The Brown Bang Sherry (Sandeman), Gran Orden PX (Garvey's), Pio IX (Pedro Domecq), Delage Dulce, Osborne Paxarete and MacKenzie's Pajarete Solera. Some producers turned their raya wines (lower quality olorosos) into paxarete by maturing them outside in the sunshine, producing more cooked and concentrated wine. Vino de color, arrope or sanchoco were commonly used in blending of paxarete and cream sherries. Vino de color is any wine cooked and sometimes additionally sweetened with mosto. Sancocho is very sweet and concentrated wine boiled to one third of the original volume and arrope is thick syrup boiled to one fifth of the original volume. The tradition of boiling wines into syrups probably comes from the Moors, as they did not drink wine for religious reasons.

Arrope syrup

In the USA the boiled sweet fortified wine was classified as "Spanish-type blending sherry" by law in 1949 and manufactured mostly in California spesifically and exclusively for the whisky industry. The blending sherry was often sweetened with sugar or caramel, cooked to a reduction and infused with oak chips. It was not taxed nor was it possible to sell it as wine for consumers. Málaga and Montilla became the biggest producers of PX Pajarete wines in the 20th century Europe and Chile has its own DO Valle de Huasco producing pajarete wine. Cyprus made numerous bulk imitation sherries; the popular Cyprus Cream sherry was even sweeter than the natural paxaretes, probably due to added sugar. Imitation sherries were often colored with vino de color or caramel and fortified with industrial alcohol. The imitation sherries really stormed into the British market in the 1950s mainly in the lower price segment by agressive pricing.


Pajarete from Tarragona,
made of Moscatel and Garnacha

In 1999 Pajarete became a protected wine variety of DO Málaga and defined as "wine liquor, or natural sweet wine with a total sugar content of 45-140g/l produced without the addition of syrup [arrope], being aged and amber to dark amber in colour.". The Málaga pajarete is usually be made of Moscatel, but some Pedro Ximénez is also used. The pajarete style wines from Jerez are nowadays called PX, Pedro Ximénez, Moscatel or Dulce sherries and most of them are made according to the original soleo method.

Pajarete probably evolved from high quality sweet natural dessert wine to the blended, sweet(ened) and fortified bulk wine in the early 20th century. An affordable sweet brown sherry wine was just what the Scotch distillers were looking for, as it made possible the fast rejuvenation of whisky casks and enabled faster maturation.

WP Lowrie?

William Phaup Lowrie (1831-1916) pioneered the whisky cask management in the late 19h century. Lowrie was a merchant for both Port Ellen whiskies and González Gordon sherries, a cooper and later a co-founder of Glentauchers distillery. He was supposedly the first blender to use (sherry) wine finishes for whisky and the first cooper to import American pre-cut staves for UK cooperages. The brewers had used steam washers for new casks for decades. The process was automated already in the 1870s and it was common to use salt, hydrochloride acid, bisulphate or bicarbonate soda as additives when rinsing beer casks prior to refills. WP Lowrie introduced the steam pressure treatment to whisky industry primarily to test the integrity of casks in 1888, but soon after the steam was used also to remove harsh tannins from the fresh wood and to impregnate the wood with wine.

The wine-treatment of new casks became common in early 20th century and in 1929 it was reported that all of the new casks to be used for Johnnie Walker whiskies were seasoned with approximately 35 litres of sweet dark sherry per hogshead for 6 weeks. The casks were stored in an individual warehouse and turned regularly for even distribution of sherry into the wood. After the seasoning the casks were lightly pressurised ("with 0,2kg pressure", probably per one square inch?) and after that the remaining sherry was poured out. About 20% of the casks were said to be new, all of which apparently were UK-coopered wine-treated hogsheads made from import oak staves from the US or the Baltic. Older UK-coopered puncheons and Spanish-coopered butts were still being used as refill casks at the time.

The use of wine-treatment was introduced to save coopering costs and to ensure the availability of fresh casks as the sherry shippers had already started their own bottling plants in Spain in the early 20th century and the availability of home-emptied sherry casks was insufficient for whisky industry. Pedro Domecq started their bottling operations in Jerez in 1920 and González Byass were to follow gradually during the interwar period. Despite that, only 20 percent of sherry imports were in bottles during 1926-1940. Sandeman bottled some of their sherries and ports in location as early as 1880, but the bottling of sherry in UK by Sandeman ceased in 1969. Harvey's were the last big shipper to bottle all their sherries in England, as they bought a winery in Jerez from MacKenzie in 1970 and since then have been bottling all their sherries in Spain. In 1983 most (54.4%) of the sherries exported were bottled in Spain. The markets of EEC were easier to reach from Spain than from UK, especially after the preferential trade agreement with Spain and the EEC in 1970 and Spain's accession into EEC in 1986. Bottling of sherry is nowadays done almost exclusively (96.5% in 2000) in Spain and full sherry casks are no longer imported.

Harvey's bottling hall in the 1960s

As imported sherry casks became scarce due to the second World War in the 1940s and especially due to the increased bottling of sherry in Spain during the 1960s and early 1970s, the distillers rejuvenated the exhausted casks with pajarete. According to Philp in 1989 "a typical cooperage procedure was to add 500ml Paxarete per hogshead or 1 litre per butt, pressurise at 48 kPa / 7 psig for 10 minutes and then disgorge any absorbed paxarete.". Until 1972 the DCL also coopered their own sherry casks, which were made of American oak to a size of a puncheon (~558 l), wine-treated with paxarete and then used primarily for grain whiskies before used for malt whiskies. The American ex-bourbon barrels often received the same treatment; first fill with grain whisky, then after wine-treatment refilled with malt whisky.

Whether the Scotch distillers used real natural pajarete wine or sweetened imitations or even arrope or sancocho for seasoning is not clear, most likely they were not too fussy about the right provenance or traditional production methods concerning their "blending sherry". The typical seasoning paxarete of the 1980s was rich in tannins, esters and especially acids. A chemical analysis of different sherries by Philp in 1989 indicates that paxarette used at that point was very likely boiled and/or sweetened with arrope or sancocho as it contained 3,2 times more sugars (375 g/l), 5,7 times more tannins (4,7 g/l), 3,6 times more esters (278 g/l) and 6,9 times more acids (22 g/l) than an ordinary sweetened oloroso. In comparison the current version of Harvey's Bristol Cream has only 3,1 g/l acids and 130 g/l sugars and the naturally very sweet Don Guido 20yo PX sherry from Williams & Humbert has only 4,5 g/l acids despite of whopping 400 g/l sugars.

Photo: Albert Watson

As the Spanish sherry producers shifted from the traditional cask fermentations to use of modern steel tanks, the properties of casks changed. Fermentation was thought to remove many unwanted flavours from the fresh wood, especially bitter and sulphury off-notes. A special Gomez treatment was invented in the 1960s to simulate the effects of fermentation in the cask. It consisted of steaming the cask with ammonium hydroxide steam at 55 kPa (8 psig) for 60 minutes, which stripped much of the tannins and colour from the inner surface of the cask. This produced very neutral casks suitable for especially fino sherry maturation, but when subsequently used in Scotland they tended to produce light-coloured and slow maturing whiskies. The Gomez treatment was used from about 1965, but during the 1970s most of the Scotch distillers stopped buying these casks and began ordering their own spesified casks for whisky maturation. The casks were used for fermentations and maturation of sherries in Spain before they were shipped usually as empty standing casks to Scotland. According to the distiller's order, both American (Quercus alba) and Spanish (Q.robur) oak were used, as well as fino and oloroso sherries.

According to a rumour the Scotch Whisky Association banned the use of paxarete in the late 1980s or early 1990s, but there is no official evidence of that. The Scotch Whisky Order from 1990 bans the use of additives other than water and caramel E150a, so the blending of whisky with sherry wine including pajarete was indeed banned at that point. PX sherry casks are still quite often used especially for single malt production, but the use of sherry concentrates and high pressures in rejuvenation and seasoning has apparently ceased or at least diminished greatly. Philip Hills wrote in 2000: "wine-treatment of worn-out casks has been widely used to simulate the effects of new sherry wood... a sweet dark sherry such as pedro ximenes [sic] is introduced and the cask is pressurised in order to force the potion into the wood. For many years, a potion called paxarette was used, which combined sweet dark sherry with other flavourings and colourants... the results are not impressive: whisky from a treated cask of this sort smells slightly sulphurous and sweet, but that is all. Of the lovely aromatic flavours of a true sherry-cask maturation, there is no trace.". It is likely that the wine-treatment was dropped because of the poor results rather than an SWA order. Below is a comparison of different cask types and maturation temperatures.

Philp 1989

Oloroso and raya sherries are probably the most common seasoning sherries used nowadays, seasoning times varying from months to couple of years. Cask fermentations are rarely done in the Scotch-bound casks. The sherry bodega casks used for maturation of commercial sherries in solera systems are discarded usually after 80-100 years and sometimes  used for Scotch maturation after recoopering.

REFERENCES AND FURTHER READING:

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
Crawfowd, A. Bristol wine trade. Historical association 1984
Glasgow Herald 27.5.1969

González Gordon M, Sherry. Cassell Ltd 1972
Halley, N. Sandeman: two hundred years of port and sherry. Granta 1990
Harrison, G. Bristol Cream. Batsford 1955
Hills, P. Appreciating whisky. Collins 2000
Hotelli- ja Ravintolamuseon arkisto, Helsinki.
Jeffs, Sherry. 1970
Lea GH, Piggott JR. Fermented beverage production 2nd ed. Kluwer Acad 2003
Morewood, S. A philosophical and statistical history of inventions and customs of ancient and modern nations in the manufacture and use of inebriating liquors. 

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

Reid KJG et al. Understanding and enhancing cask performance. The Scotch Whisky Research Institute 2008.
Sykes WJ, Ling AR. The principles and practice of brewing. Griffin 1907
Wright HE. A handy book for brewers being a practical guide to the art of brewing and malting, embracing the conclusions of modern research which bear upon the practice of brewing. Lockwood & Son 1897

Alcohol strength

Scotch whisky is usually diluted before filled into casks. The usual filling strength for malt whiskies is 63,4-63,5% ABV (111 UK proof) and 68% ABV for grain whiskies. These are industry standards based probably on the history, minimizing evaporation losses and warehousing costs, but also creating an acceptable flavour profiles and uniform products for the blending industry. Bourbon and corn whiskey producers can by law fill at a maximum of 125 US proof (62,5%) and the Irish have commonly filled casks at 71%. Before bottling whisky is usually diluted to 40% or 43%.

Known unusual cask filling strengths are Aberlour 69,1% (121 UK proof), Bruichladdich (undiluted 70-72%), Port Charlotte (undiluted, probably over 70%), Glenrothes (63,5% and "some casks" undiluted about 70%) and Glen Scotia 62,5%. Bladnoch has experimented with higher and lower strengths, but apparently is now filling all the casks at 63,5%. Grain spirits are usually filled at 68%, but North British fills at 62-68% and Girvan at 74% (at least the ones going to Grant's). Most malt distilleries use 63,4-63,5% fills.Since 1848 the strength of whisky warehoused in a distillery was legally from 22 under proof (44,6%) to 25 over proof (71,4%) and the maximum strength of whisky sold out of a distillery was 111 proof, so at least the grain distillers were diluting their new make. Irish whisky was said to be warehoused at 14-16 over proof (65,1-66,3% abv) in 1808 and at the same time Scottish whisky for home consumption was taxed at 107 proof (61,1%). Illicit distillers most likely did not dilute their spirits, but the small stills and wide cuts probably did not produce much higher abv.

Ross states in 1970 that Scottish malt whisky is filled at 11-12 over proof (63,4-64% abv) and Irish pot still at 25 over proof (71,4% abv). The dilution to an uniform strength probably eased the common practice of exchanging cask between the different blenders. During the years of Scottish whisky overproduction in the late 70's and early 80's at least DCL "uncommonly" filled casks at full proof to cut cask- and warehousing costs. In the early 60's there were problems acquiring enough casks and that would probably have had the same effect on filling strengths. It is likely that dilution of malt whiskies down to 63,5% has been used at least for the most part of the 20th century. During the WW I the maximum bottling strength was temporarily (1915-16) adjusted to 35 under proof (37,2% abv). The alcoholic strength was indicated usually in Sykes proof until 1980, after that it has been replaced with alcohol per volume (abv).

Higher filling proof means savings on the casks and warehousing, but the faster alcohol evaporation and slower maturation curves favor dilution before casking. Very high proof spirits mature slower as they extract less color per absolute alcohol and especially less solids from the cask. Below 120-125 US proof (or not-so-coincidentally just below 63,5% abv?) the extraction is more even and efficient. Entry proof has an impact on the flavour profile, too. More alcohol solubles are extracted at high proofs, for example coumarins (often bitter, spicy), aromatic aldehydes (almonds, vanilla...) and terpenols (lapsang souchong, pine, resin...). 
 

Entry proof of whisky and congener extraction (Reazin 1981)

The water solubles such as sugars are extracted better at lower proofs. On the other hand, higher proof alcohol breaks the lignin and hemicellulose a bit faster to increase the amounts of sugars in the spirit.


Extraction of sugars (Reazin 1981)

Tannins, esters and fats are extracted quite evenly despite changes in the ethanol concentration, but there are some important equilibriums and reversible reactions happening in the spirit. Simplified: esters+water <=> acids+alcohol and acetals+water <=> aldehydes+alcohol. The addition of water or evaporation of alcohol results in proportionally less esters (fruit) and acetals (flowery, aetheral) and more acids, aldehydes and higher alcohols.

Ethanol micelles (from blog.khymos.org)

On the other hand, ethanol forms micelles trapping hydrophobic compounds at over 23% abv. Addition of water opens up these ethanol micelles and releases some of these hydrophobic compounds but also increases the surface tension of the solution. The short chain esters (fruity, solventy, glue) are usually more volatile and evaporate in proportionally larger amounts than the long chain esters (fruity, soapy). The oak solubles in turn increase the surface tension especially at high ethanol concentrations, decreasing the overall evaporation.


Effect of ethanol concentration on the maturation of cognac (Cantagrel & Galy 2003)

Cutty Sark ad from 1977

It probably makes a difference whether the dilution is made prior the cask filling, just before bottling or in the glass. Theoretically, the dilution before casking might produce a sweeter, cleaner and less fruity and bitter-oaky spirit. The cognac producers usually add water or lower abv spirits just a small amount at the time, probably to avoid sudden changes in ethanol-water equilibrium. Dilution before bottling might have some dimishing effect on the sugars and some volatile off-notes such as glue and acetone, while probably increasing the proportional amount of linalool-oxides and bitter woody off-notes. Dilution in the glass, especially below 23% abv would release more of the hydrophobic aromas, such as esters with fruity, but also solventy and soapy off-notes.

References and further reading

Conner JM, et al. Release of distillate flavour compounds in Scotch malt whisky. J. Sci. Food Agric., 1999; 79; 1015–1020
Conner JM, et al. Agglomeration of ethyl esters in model spirit solutions and malt whiskies. J. Sci. Food Agric., 1994; 66; 45–53
Conner JM, et al. Interactions between ethyl esters and aroma compounds in model spirit solutions. J Agric Food Chem 1994;42;2231-4
Conner JM, et al. Contributions of distillate components to disperse phase structures in model spirit solutions. J Agric Food Chem 1998;46;1292-6

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

Piggott JR, et al. The influence of non-volatile constituents on the extraction of ethyl esters from brandies. J. Sci. Food Agric., 1992; 59: 477–482

Reazin GH. Chemical mechanisms of whiskey maturation. Am J Enol Vitic 1981;32;4;283-9

Ross, J. Whisky. Routledge & Kegan Paul Books 1970

Udo M. The Scottish whisky distilleries. Black&White Publishing 2006.Taylor AJ, Mottram DS. Flavour Science: Recent Developments. Woodhead Publishing 1996.

Oaky flavours

Hundreds of different flavour compounds have been identified in whisky. The synthesis and degradation and synergistic properties of these compounds is still poorly understood as there are so many aspects contributing to the result of cask maturation.

Oak cask affects whisky by extracting wood compounds that influence the taste directly or together with the spirit compounds or by removing or changing some compounds from the raw spirit. Cask also allows evaporation and oxidization of spirit and the volatile flavour compunds through the headspace of the cask or by a lesser extent through the surrounding air through the wood or the bunghole.

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

Toffee

Hemicellulose contains several defferent sugars (xylose, mannose, galactose, rhamnose, arabinose, glucose) and forms branched chains of hundreds of molecules. Hemicellulose breaks easily when heated, producing a range of extractable aroma compounds, such as furfural (almond, walnut, grainy), hydroxymethylfurfural (butter, musty, waxy, caramel), maltol (malt, sweet) and cyclotene (maple, caramel, licorice).

Lignins are very complex macromolecules consisting of three monolignol units p-hydroxyphenol (not present in oak), guaiacyl (32% in oak lignin) and syringyl (68% in oak lignin) derived from dehydration and polymerization of cinnamyl alcohols. Usually natural lignin includes various other molecules joined in to the structure, including different sugars, acids and aldehydes. Heating breaks parts of lignin to soluble p-coumaryl-, coniferyl- and sinapyl-alcohols. They can transform into their respective aldehydes, acids and phenols including very aromatic compounds such as guaiacol (smoky), 4-vinylguaiacol (clove), phenyl ethanol (floral, rose), vanillin and vanillic acid. At higher temperatures a range of other volatile phenols are formed. Lignin breakdown continues at a slower rate during maturation by the effect of ethanol. Most of the lignin derivatives and extractibles decribed above are present also in the malted grains, peat and new make spirit.

Lignin synthesis, monolignols. From Nature Reviews

Oak wood extractives include two different natural isomers of oak lactones. The cis-oak lactone gives sweet coconut-vanillin aroma. The trans-oak lactone is spicier (coconut,cloves, celery, incence), but 2,5-20 times weaker if all the synergetic influences of other oak extractives are not taken into account. The trans-lactone is believed to promote the taste of cis-lactone and various other flavour compounds in whisky, producing heavy coconut amd incence aromas at high concentrations. Various other lactones are described as fruity, peach-like and vanillic.

Tannins can be divided into hydrolysable (gallo- and ellagitannins) and the non-hydrolysable condensed tannins, for example proanthocyanidins common in red wines. Oak tannins are more hydrolysable than the more stable wine tannins from grape skins and pips and thus more volatile and active during maturation. Ellagitannins consist of vescalagin, castalagin, their oligomers or their variations such as roburins or grandinin. Tannins impart astringent flavour at least in the early phase of maturation and take part in various oxidative reactions removing sulphury off-notes and promoting color stability, lignin breakdown and alcohol oxidation into acetals producing etheral top-notes.

Dried cloves (Eugenia aromatica)

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

Several extractives from oak wood show significant synergetic effects between each others and lower the odour treshold levels of aromas, for example vanillin and vanillic acid lowers the treshold for lignin-derived aldehydes.

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

In conclusion, oak and especially the toasted/charred layer of the cask adds flavour compounds to the spirit and removes some undesired compounds by carbon filtration and oxidation reactions.

Vanilla planifoli

Different oak casks impart different amounts of flavour compounds. The species is the most important factor explaining the differences, but also the origin, seasoning and toasting of staves are significant. Three most common oaks used in whisky cooperage are Quercus alba, Q.robur and Q.petraea. Q.alba grows in northeast America and the latter two in Europe (see previous blogs).

Main differences between species are most likely in the concentrations of oak lactones, eugenol, tannins and other polyphenols. The variation inside the species is most distinct in Q.petraea, as Q.alba and especially Q.robur tend to be more predictable in terms of whisky maturation.

Coconut (Cocos nucifera)

Oak lactones are important flavour extractives in oak wood. The cis-isomer is usually dominant and imparts sweet vanillin and coconut aroma. The trans-oaklactone is more spicy and herbal in low concentrations, but in high concentrations produces heavy coconut and incence, part of this phenomenon is probably due to synergistic nature of trans-isomer with other lactones and polyphenols. The ratio of cis/trans-isomers differs between species: It is highest in Q.alba and almost non-existent in Q.robur. The ratio in Q.petraea varies, but it is usually less than in Q.alba. The only oak with greater proportion of trans-isomer is Japanese oak (reported as Q.mongolica, but it is more likely Q.crispula). Japanese Q.dentata is similar to American Q.alba and Japanese Q.serrata is similar to European Q.petraea in both oaklactone-concentrations and cis/trans-ratios. Total amount of oak lactones is highest in Q.alba and Q.crispula, usually less in Q.petraea (although some very high concentrations have been measured) and very low in Q.robur.

Grapes (Vitis vinifera)

Another significant difference between oak species is the amount of tannins and other polyphenols. Q.robur has most tannins, especially the more water-soluble vescalagin, castalagin and roburins. Q.petraea has about third of the tannins content of Q.robur, but the tannin contents in Q.alba and presumably in Q.crispula are very low. These differences are partly due to the growth speed of the species as older trees have less tannins than younger oaks, but the growth speed alone does not explain the differences between the concentrations.

The amount of lignin is quite similar in all the oaks, although it seems that Q.robur might be richer in lignin than Q.alba. Again there probably is more variation between different Q.petraea trees. It seems that Q.alba has little less soluble monolignols than the European species. Lignin- and hemicellulose- derived vanillin and furfural contents are usually highest in Q.robur. The differences in coopering and especially in toasting/charring practices influence the monolignol and vanillin contents apparently more than the variation between the species.

It was earlier believed in wine industry, that tight-grained (slow growth) oaks produced less tannins and more sweet notes, but the grain width is not significant if the species are taken into account. The observation is true in the sense that Q.petraea is usually tight-grained oak (about 1mm) and has less tannins than coarser Q.robur. Q.robur and Q.alba have usually coarser grains of about 3mm.

The origin of oak has some effects, too. East European oaks (not depended on species) have usually more lactones, eugenols and vanillin but less tannins than French or Spanish oaks.

It should be noted that although the sherry casks used in Scotch whisky industry are called Spanish oak casks, they most likely often are Spanish coopered American oak casks. Although made of the same Q.alba oak, these casks produce very different Scotch whiskies from the ex-bourbon casks due to the differences in cooperage practices, seasoning effects the sherry wine and the different sizes of casks.

References and further reading:

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

Bryce JH et al (ed). Distilled spirits, production, technology and innovation. Nottingham Univ Press, 2008

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 et al. Release of distillate flavour compounds in scotch malt whisky. J Sci Food Agric 1999; 79; 1015-1020

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

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

Günter Berger R. Flavours and Fragrances: Chemistry, Bioprocessing and Sustainability. Springer 2010
Jounela-Eriksson P. The aroma composition of distilled beverages and preceived aroma of whisky. Academic Press 1978

Mangas J et al. Volatiles in distillates of cider aged in american oak wood. J Agric Chem 1996; 44; 268-273

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

Nykänen P, Suomalainen H (ed). Aroma of beer, wine and distilled alcoholic beverages. Akademie-Verlag, Berlin 1983.

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

Poisson L, Schieberle P. Characterization of key aroma compounds in an american bourbon whisky. J Agric Chem 2008; 56;5820-5826

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

R. Rowell (ed.) The Chemistry of Solid Wood’ . Am. Chem. Soc.,Washington D.C. (1984)

Reale S et al. Mass spectrometry in the biosynthetic and structural investigation of lignins. Mass Spect Rev 2004; 23; 87-126

Ronde I (ed. Malt whisky yearbook 2011. MagDig Media 2010
Russell I (ed). Whisky, technology, production and marketing. Academic Press 2003.

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

Tanaka H et al. Determination of total charge content of whiskey by polyelectrolyte titration: alteration of polyphenols. J Food Sci 2002; 67; 8; 2881-2884

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

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

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