When you contemplate any barrel-aged spirit there are many flavors and aromas that will confuse and astound your palate. Look at any review of these products and you will get my point. A lot of the characteristics you experience come from the internal chemical and biological attributes of the wood itself. Many distillers attribute around 40% to 80% of the overall characteristics experienced in whiskies, provided it hasn’t been influenced by other means, are produced by the interaction of the spirit with the wood. To understand how the wood does this, you have to delve into the inner workings of the tree.
The main structure of a barrel is composed of multiple staves that are cut from the heartwood of a tree. There are two internal structures inside the plant’s cell wall known as hemicellulose and lignin that influence the character of the spirit. Hemicellulose is made up of organic compounds and numerous sugars that are soluble in alcohol, and with the application of heat will produce color and caramel notes. Lignin is a source of Methoxy phenols, such as vanillin and syringol. These are naturally occurring organic compounds and they change with the help of heat and acidity, creating the smoky vanilla flavors and aromas in the spirit.
Throughout the wood, there are four broad components that further influence the spirit; tannins, lactones/trans-lactones, phenolics, and acids. Tannins produce an astringent, mouth drying characteristic that creates structure. Lactones/trans-lactones create coconut, clove, and butterscotch flavors and aromas. Phenolics and acids create over 400 different flavor and aromatic compounds including Ethyl syringate (tobacco and fig), Ethyl ferulate (spice and cinnamon), Ethyl vanillate (burnt, smoky and vanilla), and Methyl salicylate, which gives off minty wintergreen notes. These components also help to create the aromatic differences found in various wood varietals. Some examples include American white oak (Quercus alba), which has aromas of vanilla, coconut, pine, cherry, and spice. European oak (Quercus robur) has aromas of dried fruit, clove, raisin, and orange peel, and Japanese oak (Quercus mongolica) has intense, perfumed notes of spice and sandalwood. All of these properties are enriched and balanced during a curing process, where the cut oak is seasoned in the elements for 1 to 3 years before use. This helps with reducing astringency in the tannins, and allows for airborne bacteria and fungi to collect and grow. These organisms help breakdown complex carbohydrates in the wood, making it easier for further chemical reactions to take place inside the barrel.
After the curing process, the wood is shaped into staves and the main body of the barrel is built, from there it is toasted and charred. This application of heat effectively changes the outer structure of the staves, and chemically changes the sugars in the wood. Toasting affects the wood in two ways; oak tannins are degraded, giving color to the spirit, and the lignin degrades, producing vanilla flavor and aroma. Charring changes things a bit further, the hemicellulose is broken down into ten simple sugars, which then caramelize into what’s known as the “red layer,” creating flavors and aromas of caramel and chocolate.
Once the spirit is in the barrel, chemical and environmental reactions begin to shape the final product. Fluctuations in temperature expand and contract the barrel, forcing the alcohol in and out of the wood, extracting flavor congeners and sweetness. During this time the alcohol passes through a thin carbon filter on the inner surface of the barrel created during the charring process, this smooths out the spirit by absorbing aldehydes and sulfur compounds. As the temperature reaches higher levels it causes the evaporation of around 3% to 10% of the liquid yearly — the famous “angel’s share.” Humidity levels during this process influence the loss of liquid and effect the alcohol percentage. Higher humidity causes alcohol to evaporate more readily than water, and decreases the level of alcohol over time. In lower humidity the water is first to evaporate, causing an increase in alcohol percentage.
After evaporation has occurred the headspace created in the barrel is replaced by oxygen. This enters the barrel through pores in the wood and dissolves into solution with the spirit, this forms esters, aldehydes and acids that create fruity, nutty and vanilla flavors. At this point in maturation the flavors and aromas continue to concentrate in the reduced volume of spirit. During this period the spirit’s natural characteristics start to diminish, and the complex flavors and aromas of the wood start to take over; this is controlled by the length of maturation, climate and the size of the barrel. Typically, you will see barrels that range from the standard american 53 gallon barrel, all the way up to the 132 gallon sherry butt. The spirits matured in these larger barrels tend to take longer to appreciate in complexity because of the higher volume of liquid and the larger vessel. Aging in smaller barrels allows for more play between spirit and wood, due to an increased surface to volume ratio. In these small barrels, characteristics of the wood such as a darker color and oaky vanilla flavor and aroma are more readily infused in a shorter period, but the uptake of other components such as tannins which can quickly overpower the spirit is also accelerated.
The use of smaller barrels has been popularized within craft distillation because of their ability to produce a richly flavored and colorful product in a shorter period. A lot of these products can be quite vibrant and complex, yet some experts argue against their ability to create a well aged spirit. They propose that the longer periods of maturation are integral to the formation of flavor and aromatic compounds. While there is some validity in that statement, many aged craft spirits on the market today have shown great promise and continue to gain in popularity.
The act of aging spirits in wooden barrels has been a tradition for a long time, and has inspired some of the most sought after bottles in the history of alcohol production. The complexities brought forth from the interaction between spirit and wood will continue to astound and perplex the senses, creating a want for more experimentation. We are now seeing a multitude of new techniques being applied in wood aging: Different types of wood, re-use of barrels, and experimental maturation processes continue to create varied and expressive end products. Because of this, it is essential to educate yourself on the inner workings of wood, allowing for a greater understanding of what you are experiencing. Although this only scratches the surface of how wood influences alcohol, I hope it makes things a little easier the next time you pour a dram.