wood chemistry. There are five considerations when aging spirits in a wooden barrel: color, wood extraction, absorption, oxidation, and diffusion. Each plays an important role in the overall wood chemistry within a cask.
When distilled spirits are produced, whether it is in a column or a pot still, whether they are heavy or light in flavor, the liquid is as clear as water. Subsequent storage in barrels provides color intensity ranging from pale yellow to straw to gold to amber or even mahogany. Factors that affect a barrel’s influence on a spirit’s color include the number of times the barrel has been used previously and for how long, what the barrel was previously filled with, the size of the barrel, warehouse temperatures, and the duration of time the spirit will spend in the barrel. But wood is an organic product, and aging is complex; it is challenging to maintain a consistent color for spirits, and the same spirit placed in the same warehouse for the same amount of time in two different first-refill bourbon barrels from the same source can come out two different shades. Depending on spirit category, blenders can add caramel coloring to achieve a consistent color. See caramel.
Wood extracts are byproducts of the burning process of the inside of a barrel. The charring or toasting can be controlled either by the intensity or by the length of time the staves are exposed to a flame. By controlling the flame, one can control the degree to which chemical bonds between the cellulose, hemicellulose, and lignin are degraded into smaller units, which will then be extracted into the spirit or wine. See lignin. Depending on the cooper and the requirements of the customer, the inside of the cask is exposed to intense heat for some fifteen to sixty seconds. In general, the extracts provide the vanilla, smoky, caramel, and toffee notes found in the spirit.
The burning of wood’s surface provides a layer of activated carbon (char) that is important for absorptive or subtractive reactions, which remove undesirable sulfur components from the spirit. In whisky, for example, sulfur either comes from the malting process, the raw grain, or is a byproduct of yeast metabolism. Any of these can give garlicy, burnt-match, rotten-egg, or vegetable notes to the spirit. Those can also be removed by a copper interaction in the still, but if there is not enough copper contact time (as with some continuous stills), then the alternative is to use barrels with a thick char layer to absorb any remaining sulfur in the spirit. Charring can be a double-edged sword, however: a thicker layer of char will remove more sulfur, but at the same time it reduces the amount of wood extracts. A 2-mm burn produces more vanilla, caramel, and toffee notes than a 4-mm burn.
Casks are porous and expand and contract due to heating and cooling cycles in a warehouse. This allows movement of oxygen into casks, which causes oxidative reactions. For example, coniferyl and sinapyl alcohols are extracted into the spirit from a barrel and are oxidized into their corresponding aldehydes—coniferaldehyde and sinapaldehyde. The tannins from a barrel also react with oxygen, which releases hydrogen peroxide and converts the tannin to a quinone complex (quinones are brownish in color and also contribute to the overall color of aged spirits). See tannin. In a further reaction, that hydrogen peroxide oxidizes ethanol to form acetaldehyde, which has a green, unripe, and raw aroma. Acetaldehyde further oxidizes into acetic acid and then ultimately ethyl acetate, which gives a green apple or pear drop flavor to the spirit. See acetaldehyde. Oxidative reactions are temperature dependent. In warmer climates, such compounds form more quickly than they do in cooler climates.
Barrel-finishing spirits—resting an already aged spirit for a brief period in a barrel that has previously held another spirit or a wine or beer—can create interesting brand expressions and is a popular trend. Quite simply, wood acts like a sponge and absorbs spirits into its pores. By nature, flavor molecules move from an area of higher concentration to a lower concentration until equilibrium is achieved—this is called diffusion. So flavors in a barrel from a previous fill will diffuse into the next spirit that is to be aged.
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Livermore, D. “Quantification of Oak Wood Extractives Via Gas Chromatography - Mass Spectrometry and Subsequent Calibration of Near Infrared Reflectance to Predict the Canadian Whisky Ageing Process.” PhD diss., Heriot Watt University, 2012.
Pastor-Villegas, J., J. M. Meneses Rodriguez, J. F. Pastor-Valle, and M. Garcia Garcia. “Changes in Commercial Wood Charcoals by Thermal Treatments.” Journal of Analytical and Applied Pyrolysis 80 (2007): 507–514.
Piggott, J. R, K. Y. Lee, and A. Paterson. “Origins of Flavour in Whiskies and a Revised Flavour Wheel: A Review.” Journal of the Institute of Brewing 107 (2001): 287–313.
By: Don Livermore