Saturday, December 10, 2016

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.

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. 
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


  1. this is the article i would have wanted to write myself, in a few years, after much research! You just gave me a huge boost in this effort! Do you mind if i refer to your blog ?

    1. You know I won't mind. Go on, enlighten us ;)

    2. Hi. Couldn't find any other way to contact You, so decided to post a comment to the newest post in Your blog. First of all - huge thanks for Your great articles. I'm a whisky making enthusiast and I found a lot of interesting and valuable technological details.

      So, I have 2 questions:
      1. In the "Peat" article You published a table. The third column is named "MIDDLE CUT ABV". Should the figures be understood as a final ABV of newly made whisky right after the second distillation or as a "body" cut point where a specific figure means abv of a distillate in a flow running out of condenser in transition to tails?

      2. There is one more question on which I can't find almost any information. I've read that a whisky mash is always made of a blend of different malts. So, like in beer, there is always a base malt with addition of special malts. Do You have any information on malt blending and use of special malts. What special malts are used - cara, melanoid, munich etc.? What percentages of them are common for whisky mash blends?

      Beforehand grateful for the answer.

      With great respect, Dmitry.
      p.s. my email is jazzminos(dog)google-dot-com.

    3. 1. The middle cut refers to the ABV of the spirit cut. The bigger abv is the "end of heads and start of cut" and the lower abv is the "end of cut and start of feints".

      2. I never heard of a malt mix for whisky (apart from some rare experiments). Scotch malt whisky is made practically always from distilling malt, which is very pale malt (for high alcohol yield). Some of it is peated, but the aim is to keep that pale, too. Compared to brewers, the distillers prefer smaller grain (more enzymes) and low protein.

    4. Huge thanks for Your reply. The information provided is very helpful.

      Considering malt mixing everything is clear. I've made several mashes with pilsner malt. I also make experiments with belgian Castle Malting Whisky malt (30-35 ppm phenols) and Whisky Light (5-10 ppm phenols). These peated malts can be purchased here. Results seem to be almost authentic. Analysis for main congeners can be found here

      Considering the first question. The lower abv ("end of cut and start of feints") is quite clear. But the higher abv ("end of heads and start of cut") is somewhat uncelear. I can hardly understand the start of middle cut at 70 abv and even at 68 abv. In my own distillation practice with a simple direct-flow condenser, heads end at 80-79 abv. I usually take heads about 5% of low-wines total volume. One thing to be mentioned - in home distillations we always aim to get heads at a very low distillation speed (as low as 100 ml per hour) to get heads "sharper". So, my assumptions are:
      1. The construction of traditional pot stills provides extremely low reflux, so the spirit strengthening is very low. So, we get the end of heads at 75-70 abv.
      2. The distillation speed is very high - much higher than I have in my practice. So, again, the reflux with high speed is very low - we get a lot of congeners that lower the spirit content. Again, if the distillation speed at heads stage is high (15-30 minutes total, according to Whisky: Technology, Production and Marketing ed. I. Russell) we "loose" a lot of good spirit with heads and lower the middle cut. In this case the heads appear in a new made spirit in significant quantity. But maybe this is not a problem, because they will evaporate during cask maturation.

      One more thing. I suppose that the middle cut usually is about 25-30% from low-wines volume. If we collect middle cut in the range 68-63 abv - this will be much less than 25% and the good spirit yield will be awfully low.

      My reasoning may be faulty. Can You explain Your vision of this question?

      Thanks in advance.

      With great respect, Dmitry.

    5. The distillers cutting below 70abv probably look for more robust oily peaty style, not floral fruity. As you point out, it might be a way to compensate the relatively fast distilling speed and/or low reflux. The heads fraction is recycled to the next batch anyway so the spirit loss is not important.

  2. Great stuff, glad to see you writing again!

    "According to Valaer in 1940 there was a legal maximum of one pint of liquid caramel per gallon of whisky permitted by the British Customs and Excise Regulations (0.15% or about 1.5 g/l)." I think there might have been a transcription error, as a pint per gallon is 12.5% (16 oz / 128 oz) which seems a bit loony.

    1. True. Certainly not pint of caramel per gallon, although that is what mr Valaer wrote;) Pint per barrel adds up with the 0.15% nicely, so I changed that. Thanks.

  3. Thanks for this.

    The study by Savchuk is really intriguing: Despite the fact that bourbon is aged in a much warmer climate, the older Scotches have a much higher ester count. Why is this? Would the higher temperatures of Kentucky not compensate for their shorter aging time and cause a bit of catch-up chemistry wise?

    1. Probably due to distillation regimes (pot still/column still). Scottish pot still distillation allows (and creates) more esters into the new make. Also more higher alcohols and probably acids, too. And alcohol+acid=>ester. Not that much of a maturation thing in the first place.

    2. To reply very late, I was under the impression that distillation simply concentrated alcohol and acids which then gradually reacted into esters with time, as opposed to the bulk of esterification being during distillation.

    3. The heat (and probably copper) accelerates the ester formation. How much, I don't know, would be great to see some numbers (not too hard to do, looking at you Heriot-Watt). You are probably right about the bulk happening in the cask.

    4. I would not presume i know enough so i can enlighten you already! Much learning to be Done before i can do that ;) im planning a little experiment myself for the relationship between ABV and wood interaction speeds :)

    5. This is an interesting discussion to have, Teemu.

      Another confusing wrench to throw in is that most Bourbon is made using sour-mash method. This would make it very high in volatile acidity (specifically lactic + acetic). Again this would mean the conditions for esters forming are more ideal, both because of the low pH in both the boiler and columns, and also because of the amount of ester precursors is very high for the same reason.

  4. Love this site! Makes learning about wine fun. Thanks for sharing!

  5. Wery good article!!!

  6. Thank you for article. Assumptions about the nature of Jerez and Montilla-Moriles casks abound. To think the lauded 19th century and early to mid 20th century single malts were often aged in pajarete barrels. Pajarete and PX are kin to drinking liquid raisins. Both seem to pass on a certain residual barrel sweetness to single malts. I know cellulose contributes, but I still taste an inherent sweetness diff from cellulose. Maybe I pick up on the nuances bcs I am experienced with the full gamut of sherry. Either way, seems hypocritical for the whisky industry to not admit how revered malts of yesteryear were aged in "sweetened barrels" (PX or pajarete) not "dry wood" barrels. Would like your input.