The process of aging alcoholic beverages is just as old as fermentation and distillation for the simple reason that the products needed to be stored and transported. Storing, be it in clay pots, barrels, or simply bottles, exposes the alcohol to both the air and the storage materials themselves for long periods of time, altering the chemical structure of the alcoholic products. This can change the colour, aroma and flavour in various and mostly beneficial ways. (While beer and wine have their own science and research and aging can take place in several kinds of materials, this article will focus solely on spirits aged in wood barrels).
To understand these chemical reactions, one first must understand the basic components:
- Wood barrels were designed for transportation, enabling one man to move large weights around easily by just rolling the container. This is the purpose behind the wheel-like shape of the barrel, which was achieved by bending the wood. By heating the wood, traditionally by lighting a fire inside the barrel, it could be bent into shape. The charred wood on the inside of the barrel adds flavour and colour to its contents. Oak was and is the most common wood used for barrels because it is durable, easy to shape, fairly inexpensive, and more inert than other woods (meaning it is less reactive to its contents).
- Spirits are created through distillation of a fermented product, be it wine, grain mash, fruit, plants, and so on. Distillation yields primarily ethanol, but also aldehydes, esters, and fatty acids, all of which have very specific flavours and aromas. It is the unique combination of these chemicals that make spirits different from one another. Multiple distillations and filtering can remove many of these compounds to create a "clean" or aroma-less and flavourless spirit (i.e. vodka). The choice of raw materials, the fermentation process, and the distillation technique and equipment will all contribute to the overall chemical composition and therefore smell and taste of the final product.
By combining these components and exposing them to the surrounding environment, numerous chemical reactions occur. The following are the most important factors:
Time
Distilling alcohol creates beneficial byproducts, as mentioned above, but also creates bad-tasting and poisonous byproducts, including butane, methanol, hydrazines, acetates, and acetaldehydes. Both good and bad byproducts are grouped together under the label "congeners" (a category of some debate, none of which will be discussed here). Fortunately, the good congeners happen to be quite stable, whereas the bad ones break down over time. Aging a distilled product allows this time, thereby decreasing the amount of bad tastes and poisons and increasing the amount of good flavour and aroma components.
Interestingly, time seems to be the most unpredictable of barrel-aging factors. Products can go from tasting great to awful or vice-versa in a matter of days or years. There is a consensus, however, that there is a decay rate of change. In other words, at some point the changes to the spirit stop and further aging becomes fairly moot. Some researchers have approximated the effects of barrel aging by using sine wave mathematics (a very common composite representation of natural processes). This research shows that basically all the effects of aging occur by fifteen years (99.5% when assuming that no signs of change in the product are noticeable after a period of thirty years, a consensus among many distillers). In fact, almost a third of the full effect of barrel-aging occurs within the first year, and almost half after the second. Many bourbon producers claim that eight years is the magic aging number, and by this mathematical approximation 93% of aging effects will have taken place by this point. Interestingly, most Scotch ages twelve to fifteen years. This spirit is very often aged in used barrels, which would slow down the reaction rate between spirit and wood. As for very long-term aging, this mathematical explanation shows only a 0.5% change in chemical composition from fifteen to thirty years in the barrel. Again, this is an approximation used to understand the concept and aging is by no means this simple. Information from distillers and tasters alike do seem congruent with its generalizations and for the purpose of discussion it is both useful and interesting.
Air
Oxygen is extremely reactive, and makes up for about one fifth of our atmosphere. Almost any material undergoes a chemical reaction when exposed to oxygen, a common example being metal rusting. Exposing a distilled alcohol to oxygen will cause chemical reactions with the unwanted congeners mentioned above, facilitating their removal from the product. Wood is porous, allowing for only small amounts of air to pass in and out of the barrel, controlling the rate at which the congeners are oxidized. During this process, some ethanol will also oxidize or simply evaporate, and the proof (alcohol-by-volume) will drop slightly. This is called the "angel's share," as early distillers saw the volume drop by itself and so believed that angels watched over their products and took some as payment. The type of climate aside from temperature does play a small role here, though not a very important one. For example, barrels aging near the ocean will absorb small amounts of salt water, but this is very low on the list of reactions affecting the spirit.
Temperature
The climate temperature where the barrels are aging will affect the rate of oxidation, and can cause evaporation as well. Warm climates will cause faster reactions, and the aging will cause changes more quickly but more harshly, occasionally removing desirable components as well. Barrels even "breathe," expelling gas during the day when temperatures are warmer, and drawing gas in at night when it's cool. To better control these conditions, barrels are usually placed in underground cellars where temperatures can be regulated and climate changes minimized.
Barrel-Type
Some barrel regulations exist for whiskies. For example, bourbon can only be aged in new charred oak barrels, so will absorb and react only with the components in the oak itself. Irish and Scottish whiskies, however, can be aged in used barrels, such as those previously used for sherry or bourbon aging. This will impart flavours and aromas from the previous occupants giving the spirit new and unique characteristics. As for the type of wood, spirits are basically aged in one of two kinds of wood: American or French oak. Due to specifics in harvesting and production as well as climate there are some differences between the two that affect spirit aging. French barrels are lower in tannins due to production procedure and if toasted contribute smoky and spicy flavours. American oak is naturally lower in tannins, the production of the barrels is a harsher process that releases more flavouring compounds, and the barrels are charred more heavily which gives stronger vanilla flavour and often that of smoke.
Barrel Char
The amount of char, or how long the barrel was burned, has great effect on the flavour of an aged spirit. Heavily charred barrels impart much stronger flavours of smoke, whereas lightly charred ones tend to be sweeter and offer subtle smoke or "toast." The reason for this goes beyond the obvious (more fire means more smoky taste). Plants contain sugars called hemi-cellulose (a combination of eight sugars altogether) that caramelize when burned. Oak also contains other compounds like lignin, tannins, and vannillins that all break down when the wood is charred. The resulting chemical products give colour to the spirit, as well as sweetness and a collection of subtle flavours. Charring the barrel too much will break down these compounds too far and negate their flavour contributions, as will a harsher handling of the wood during barrel construction.
Humidity
If humidity is very high, more ethanol evaporates during aging, while if humidity is low, more water is lost. Aside from the changes in proof this will cause, water and ethanol act as solvents in the barrel and as such contain many other compounds responsible for flavour and aroma. Water will contain more sugars and colours, whereas ethanol will contain the lignins, vanillins, and tannins that contribute more to depth of flavour. For this reason, distillers in more humid climates will typically work with higher cask strengths (fill the barrels with spirit at a higher proof) to avoid losing too many of these components in the ethanol solution.
The above are the most obvious and measurable of contributing factors to the chemical process that spirits undergo while sitting inside a barrel, but there are countless others. The quality of an aged spirit is dependent on so many circumstances and controlling these as best as possible is just part of what makes distillers such dedicated masters of their craft. Understanding the science and subtleties of the process gives an even deeper appreciation for the final product. As the drinking populace becomes more knowledgeable, hopefully aged spirits lose their social stigmas and are enjoyed by all - particularly those of higher quality and those of small distilleries.
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