The chemistry of maturation is a pretty tough subject which, if you don’t already know a lot of chemistry, will require a good many years of study. However, there are certain basic things which happen to alcohol which you ought to know about and can be outlined fairly briefly.Aldehydes
When an alcohol is exposed to air, oxygen combines with the hydrogen of the alcohol leaving it short on a couple of hydrogen atoms per alcohol molecule. In that state it may be described as having been dehydrogenated. Hence the name given to the resulting class of compounds – aldehydes. The compound produced by the dehydrogenation of ethanol is called acetaldehyde and its molecular formula is C2H4O. Acetaldehyde occurs naturally in beers and wines through exposure to oxygen and by the action of yeasts.Acetaldehyde is one of the main causes of hangovers. An enzyme called alcohol dehydrogenase, which is found in the liver, converts ethanol in the blood to acetaldehyde and makes us feel bad the morning after. There is a common belief that whisky does not go off in bottle and nothing need be done to keep bottled stocks in good condition. This is not the case: whisky left in the presence of a relatively large volume of air for a long time will form acetaldehyde.Ethanol is not the only alcohol to undergo dehydrogenation. Formaldehyde, which comes from the oxidation of methanol, is perfectly ghastly. It’s used to preserve dead flesh and gives a dissecting room its stench. Fortunately, the aldehydes of the longer-chain alcohols are far more agreeable to the nose and palate than those which contain only a few carbon atoms. Aldehydes formed by some of the higher alcohols are vital flavour congeners in whisky. Vanillin, C8H803, is an aldehyde and also a flavour component of good malt whisky. While its main source is the oak wood used in maturation, it also results from the dehydrogenation of a higher alcohol, eugenol, C10H12O2, which is the active component in oil of cloves and closely related to nutmeg. Cinnamaldyde, C9H8O, is what we taste as cinnamon.The higher aldehydes tend to appear during distillation in the pot-still only: in patent-still distilling the higher aldehydes are discarded along with the fusel oils which is one reason why patent-still whisky is less flavoursome than pot-still.Acids and Esters
There is a further stage in the interaction of alcohols with oxygen, of which you should be aware. This is oxidation proper – the gaining of an oxygen atom rather than the losing of hydrogen. When the aldehyde molecule gains one oxygen atom, it becomes acetic acid, C2H4O2. This is the acid which is formed when wine and beer go sour. It’s also the acid in vinegar.Acetic acid is not normally a problem as regards the taste of whisky: oxidation does not usually proceed much beyond the aldehyde stage in distilled liquors. It, and the acids (such as butanoic acid and propanoic acid) formed by oxygenation of the higher alcohols, are important in the development of whisky flavours. The best known is the reaction of ethanol with acetic acid to form ethyl acetate which gives many fruits and berries their characteristic scent. Since ethanol is the main alcohol present in whisky and acetic acid the principal organic acid, ethyl acetate is the ester most likely to be found. It and amyl acetate are common organic solvents: they are likely to be encountered as the solvent in such things as glue and nail polish. Amyl acetate, which is the product of amyl alcohol and acetic acid, has the smell of peardrops. Its presence is one of the indicators of a well made, well-matured whisky.Esters are the stars in the worlds of whisky and perfume: they are essential for an attractive nose. Over a hundred esters have been identified in malt whiskies. Some of the more common ester odours are shown in the table printed here. Research into whisky flavour has revealed more than 700 compounds, each of which may contribute to the flavour of the mature spirit. There are 15 basic flavours detectable by the amateur sensory analyst. If you can identify all of them in a whisky, believe me, you will indeed be one in a million.Esters are formed during fermentation and maturation. During maturation, acids from the cask, which are a different lot from the acids formed in the production process, react with alcohols in the whisky to produce even more and different esters.Ketones
Molecules which contain the carbonyl group C=O are called ketones. They are the source of many natural odours. Diacetyl, one of the simplest, CH46O2, is the ketone based on butane. Also known as butanediol, it is the molecule which is mainly responsible for the smell of cheese.
A rather complicated molecule called 3-(para-hydroxyphenyl)-2-butanone, though it is in reality one of the smaller simpler ketones , has the molecular formula C10H12O2. It is the molecule responsible for the scent of ripe raspberries. Newly picked Raspberries have a wonderfully fresh smell due to a compound of the same three atoms in a slightly different combination. This is ionone, C13H20O, and is the essential oil extracted from violets: it also causes newly dried hay to smell so pleasant – which of course is one of the aromas to be found in good whisky.Smokiness
Before we begin,it would be as well to dispel a widely-held belief: the presence of peat in the water which is used to make a whisky causes it to taste peaty. It doesn’t. You can make whisky with water which has so much dissolved peat that it is almost black and you will get no flavour of peat in the finished spirit if you use unpeated malt. The more peat used, the smokier the whisky. The degree of peating is measured in parts per million (ppm) of phenols (the main smokiness-producing family of compounds) and the higher that rating, the wilder the smell of smoke. Phenols are aromatic compounds whose odour most people will recognise as tar. The basis of all aromatic compounds is benzene. It is a hydrocarbon, but with the difference that the carbons are arranged in a hexagon. When we think of alcohol, smoke is not a natural association. However, there is a similarity in the molecular structure and component elements. Both alcohols and phenols are hydrocarbons with an added hydroxyl group. The difference lies in the fact that in alcohols, the carbons run in a chain, while in the phenols they are arranged in a ring (for which read hexagon). Phenols are very persistent – they stick to the barley grain, dissolve in water during mashing, survive brewing and distilling, and then after ten or twenty years in the cask they still haven’t gone away. The intensity of their taste does diminish with time. You can’t always assume that the level of peating is a constant. Practice varies from time to time and distilleries will for various reasons, mostly to do with their perception of future demand, vary the level of peating of the malt they use. Ardbeg uses malt peated to 30 parts per million, where before it was 50 ppm. Ditto Laphroaig.Fruitiness
Fruity aromas are mainly caused by esters which are formed from reactions between alcohols and acids. It is well-established that esters are formed during fermentation – whether they are also formed during maturation is less clear.Experiments with bourbon casks have shown that the rate of ester formation is constant throughout the maturation period. This suggests that the distillate is the source of the esters: the rate of formation of flavour congeners derived mainly from the wood tends to decline over time. However, it has been shown that storage of spirit in a sealed container with no access to alcohol, does not produce esters in appreciable quantity. The acetic acid required for ester formation is formed by an oxidation reaction. The requirement for oxygen appears to be the reason why whisky stored in a sealed container will not mature: the oxygen which permeates through the walls of the cask is part of the maturation process. Floweriness
Esters are commonly represented among floral odours by benzyl acetate, C9H10O2, which is a large part of the odour of jasmine. It is formed by the replacement of one of the atoms in the methanol molecule with a benzene ring. Another ester, geraniol, C10H18O, is found in the leaves of geraniums and the flowers of the rose.Vanilla
The vanilla-like aromas in whisky are caused by two main compounds: vanillin and diacetyl. Diacetyl gives the buttery flavour to toffee. In whisky, it is formed during fermentation. The vanilla flavour we get from another single-compound organic odour molecule, C8H8O3, vanillin. The flavouring can be extracted from the fermented seed pods of the vanilla orchid or it can be synthesised by the oxidation of eugenol. It’s an important element in the character of whiskies. It is perceptible in extremely low concentrations and once it is present the flavour of vanilla does not increase appreciably, irrespective of how much of the compound is present.Vanilla is extracted from the barrel wood during maturation. Lignin in the oak undergoes oxidation when in contact with spirit and oxygen to form aromatic aldehydes, of which vanillin is one. The amounts of vanillin present vary with the type of oak wood used, American white oak generally yielding lower concentrations of vanillin than do European types. How then do we explain the very strong scents of vanilla to be found in bourbons which are all matured in American oak? The answer is to be found in the practice of charring. The oak staves which are used to form a cask are bent into the required shape by steaming or toasting. The heating process degrades the lignin in the oak and produces aromatic aldehydes, of which vanillin is the most important.In addition to heat-treating the oak wood for construction purposes, the bourbon distillers subject the finished cask to a further charring, which is purely for flavour. This produces the sweet flavour which is characteristic of bourbon. The Scotch whisky industry has experimented with recharring as a way of rejuvenating casks. Casks which, though virtually inactive through long use, are still watertight are heavily charred and then refilled with whisky. It is successful up to a point but, as with other such expedients, the spirit matured in a treated cask cannot compare with that which has lain in a new sherry or bourbon wood.Cereals
Descriptors of this flavour usually associate feints with grass, hay, barley, oats and other such grassy and cereal vegetables. Some of them are ketones, such as ionone, mentioned earlier. Soapy and Sulphury flavours
Typically described as being waxy, soapy flavours derive from long-chain esters formed during fermentation. Organo-sulphur compounds on the whole don’t taste pleasant and some of them are vile. Sulphur compounds form during fermentation and, to a lesser extent, in distillation where they tend to result from a late spirit cut. Caramel
The flavour of caramel is well known and identifying it isn’t as easy as you might think. Caramel is added at the bottling stage to give colour to most blends (and some malts, though no malt distiller will admit to the practice). It is the only additive permitted by law – though some companies today are steering pretty close to that law by using treated casks in which substantial residues of the treating compounds remain.Nuttiness
Nutty flavours are almost universally associated with good whisky. They derive from benzaldehyde produced during fermentation and from compounds extracted from the oak wood during maturation. Benzaldehyde, C6H5CHO, is the compound which gives us the flavour of almonds. It is related to cinnemaldehyde and vanillin. New wood flavours
There are certain new wood flavours which are occasionally to be found in whisky. These are the odours of pine resin and new wood and are not to be confused with the tannic flavours referred to when whisky is usually described as ‘woody’. They almost certainly are extracts from the cask wood but their chemistry is not understood. Over two hundred volatile components of cask wood have been identified and there are probably a lot more, so it isn’t as surprising as it may seem.Sour flavours
Some whiskies have a sour, vinegary, cheesy odour. While this odour may be tolerable in low concentrations it is not desirable. Such flavours arise from the formation during fermentation of excessive quantities of acids such as acetic acid and butyric acid.Mustiness
Occasionally one comes across a disagreeable, musty odour in whisky. This can have a number of causes, most of which occur during maturation or bottling. Compounds of naphtha, a polycyclic hydrocarbon, have been implicated in this. Sweetness
A variety of sweet flavours are found in the taste of whisky. Obviously tastes differ in this as in all other matters but there is general agreement that, though minimal levels of sweetness are tolerable, any more makes for a bad whisky.
The formation of sugars in the malting barley is an essential part of the production process and it has been suggested that some sugar finds its way through the distillation process. Exactly how this could happen is not clear and the source of sugars in mature whisky can more profitably be sought in the cask.Tannins
Tannins in whisky are oak wood extracts, unlike in wine, where they come from the grape, from the wood and in some cases, from additives. Low concentrations of tannins are acceptable in whisky. Generally, the longer the whisky lies in cask the more tannins it will acquire. This leads some misguided souls to think that the presence of tannins in whisky is an indicator of quality – it isn’t. It usually only indicates age and old whisky isn’t necessarily good whisky.Tannins are a group of phenolic compounds which are synthesised in the growing oak. There are large numbers of different tannins and their chemistry is complex. Opinions vary as to the mechanisms by which tannins contribute to maturation.