The Oxford Companion to Spirits and Cocktails Definition of maturation

maturation is the aging of spirits in order to improve their flavor. In most parts of the world, this is done by putting them in oak or other wooden containers after distillation and leaving them there for a period that can be as short as a few days and as long as forty years or more. See barrel. Some spirits, however, are aged in semi-porous earthenware jugs, or occasionally even large, loosely sealed glass bottles. Moreover, even those spirits that are not destined for further maturation are generally rested after distillation for at least a week or two in stainless steel tanks or other such nonreactive containers, to let flavors integrate and some of the most volatile compounds break down, evaporate, or get reabsorbed.

Chemically, maturation proper is a highly complex process that is not fully understood, or at least not to the point that spirits can be synthesized that are indistinguishable from traditionally aged ones, a thing that distillers and rectifiers have been trying to do since the eighteenth century. See rectification and speed-aging. In fact, maturation involves not one process but three. There is extraction, where the spirit becomes infused with compounds from the wood it is being aged in (obviously, this does not apply when nonreactive containers are used). There is oxidation, where the congeners in the spirit (basically, anything that survives distillation beyond water and ethanol) and any wood extracts react with the oxygen entering the container and with each other, thus breaking down and forming new compounds. See congeners. And there is concentration, where the evaporation of ethanol and water through the pores of the container concentrates the congeners and wood extracts and their breakdown products in the liquid left behind.

Before we discuss these in detail, we must emphasize that how they play out is highly dependent on the spirit that goes into the barrel: on its raw material, on its fermentation, on its distillation proof, on the type of still that was used, and on the proof the spirit was reduced to before barreling. These all govern the type and amount of congeners that will be present, and congeners are the key to maturation: without them, aged spirits would just be wood-flavored vodka. See starka. The aging process is also quite sensitive to temperature, like any chemical reaction.

Extraction

Extraction is the most visible part of the maturation processes and has the fastest results, particularly when new oak barrels are used. Some woods, such as the îpe-roxo used in Brazil, are relatively nonreactive; oak is not one of them. See cachaça and oak alternatives. With ethanol and water both being powerful solvents, in a matter of days the spirit is on its way to becoming, as the British chemist Peter Shaw (1694–1793) described French brandy in 1731, “a dilute tincture of oak.” Among the compounds that leach out of the wood are various phenols, such as ellagic acid, eugenol, and gallic acid, and various lactones. See ellagic acid; eugenol; gallic acid; lactones; and phenols. They are joined by things such as lignin and furfural, activated by the toasting or charring of the barrel during its construction (although the latter can also be a product of fermentation and distillation), vanillin, and small amounts of wood sugars. See lignin; vanillin; and wood chemistry. These all give the spirit a certain impression of sweetness and roundness. There are also, however, tannins, particularly if the barrel is new. These extract fairly quickly and give the spirit an astringency and underlying bitterness. This can be pleasing, but it needs watching, particularly since tannins and related substances such as gallic acid continue to leach into the spirit for as long as it remains in the barrel. See tannin.

Oxidation

Oxidation is the result of storing a spirit in a porous container. The most important reactions involve the esterification of fatty acids with ethanol, producing the flavor-bearing compounds known as esters. See esters. Some fermentation and distillation esters also decay, taking with them their bright, floral new-spirit notes. Other higher congener acids reacting with ethanol also form aldehydes, and acetaldehyde in particular. See acetaldehyde and aldehydes. The eye-watering, peppery acrolein, a bacterial compound sometimes found in grain spirits, breaks down. Sulfur compounds transform into less-pungent ones. Many of these reactions are quite gradual, taking years to play out, but they end up softening the spirit and adding depth of flavor to it. These reactions will also occur in earthenware containers, such as the large jars baijiu is often stored in. Indeed, some baijius are thus aged for as much as fifty years, producing a spirit that is greenish in color, soft, and grassy. See oxidation.

Concentration

That fifty-year-old baijiu also has a silky, thick texture on the tongue, the result of the third maturation process, concentration, which is the end result of evaporation. In that case, the clay jar has acted like a wooden barrel, which—as Charles Crampton and Lucius Tolman of the United States Internal Revenue Service described it in their landmark 1908 study of whisky maturation—“acts in many ways like the porous membrane of an osmotic cell and has a very decided selective action on the materials passing through it.” Ethanol is highly volatile and water moderately so, making them among the substances that will pass through that barrier and evaporate, leaving the heavier, less-volatile flavor compounds behind (ultimately, the viscosity of a well-aged spirit can end up at three times that of water).

This evaporation means that a barrel of, say, rum or whisky will lose anywhere between 1 or 2 percent and 12 percent or more of its content each year to evaporation, depending on the climate in which it is stored. Producers will lament (and lament) this “angel’s share,” which can be a greedy one indeed: over the eight years of the Crampton and Tolman study, the ryes and bourbons lost an average of 42 percent of their volume. See angel’s share. But without evaporation there is no oxidation and no concentration, and without concentration there is just woody new spirit. (This was proved anew in the early 2000s by the many small-barrel-aged two-year-old whiskies released by new distilleries, where the barrel size had allowed disproportionate contact with the wood, making for a spirit that was as woody as an older one but not as rich or smooth.) Furthermore, there is a certain disingenuousness to bemoaning the angel’s share: while a two-year-old bourbon, for example, might fetch some $15 a liter at retail, an eight-year-old one will get at least $45, more than making up for the lost volume.

Another effect of the climate on evaporation is seen with the proof of the spirit. A barrel stored in a hot and dry climate will lose water faster than it loses ethanol, resulting in a spirit that increases in proof the older it gets. In a moist, cold climate, however, it is the alcohol that leads, and the proof decreases. A good cellarmaster has ways of managing this. See cellarmaster. Another thing that requires management is tannin levels: tannins are not volatile, and as ethanol and water evaporate, their concentration increases. At a certain point, they will take over the spirit, making it mouth-puckeringly astringent, bitter, and woody. There are ways of avoiding this—in particular, the French art of managed maturation known as élevage—but it is a constant risk, particularly for spirits stored in new or nearly new oak. See élevage.

There is no one rule for how long a spirit should stay in the barrel. Spirits made from pungent materials that have undergone a long fermentation and are then pot-distilled to relatively low proof will take much longer to mature than spirits from neutral materials that are fermented quickly and distilled in a column still to over 90 percent ABV. Spirits can stay longer in well-used barrels than new ones. Spirits that take twelve years to mature in Scotland or Scandinavia might take four in Jalisco or Queensland. But when maturation is done well, when the three processes are made to work together, it is as Smith wrote in 1731: the spirits’ “lying so long, as it were in a state of slow digestion, wonderfully takes off from that hot, acrid, and foul taste, peculiar to all spirits or brandies newly distilled, and gives them a coolness and a softness not easily to be introduced by art.”

maturation

Extraction

Oxidation

Concentration

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