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IR Spectroscopy Tests Charcoal for Tennessee Whiskey

Photonics Spectra
Jul 2006
Daniel S. Burgess

To enable distillers of Tennessee whiskey to produce a better and more uniform product, scientists at the University of Tennessee in Knoxville and at the US Department of Agriculture Forest Service’s Southern Research Station in Pineville, La., are analyzing wood charcoal using mid-infrared spectroscopy. With the chemical information they gather, they hope to apply near-IR spectroscopy and other techniques for online process monitoring in the spirits industry.

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The manufacture of Tennessee whiskey is distinguished by the use of the Lincoln County process, a days-long mellowing step in which the newly distilled spirit is filtered through a 10-ft-thick layer of charcoal made from sugar maple. Infrared spectroscopy offers distillers a means of verifying that the charcoal they produce for the process is of the proper species of maple. Courtesy of Nicole Labbé.

The range of flavors, aromas and colors of the whiskeys is the product of an interplay of factors: the type of grains in the mash, the number of distillations, the duration of the aging process and the action of impurities known as congeners during maturation. According to whiskey lore, a Tennessee distiller named Alfred Eaton developed an additional manufacturing step in the early 1800s that imparts a characteristic sweetness and bouquet to the result, and thereby invented Tennessee whiskey. Called the Lincoln County process after the site of the discovery, it involves the filtering of the newly distilled spirit for more than a week through a 10-ft-thick layer of wood charcoal from sugar maple before securing it in charred white oak casks for maturation.

Given the importance of tradition in the branding of spirits and the anecdotal evidence that suggests that the quality of Tennessee whiskey depends on the species of maple used to make the charcoal, there is a strong incentive for distillers to confirm that sugar maple is the source material, explained Nicole Labbé, an assistant professor at the university’s Tennessee Forest Products Center.

“The whiskey maker wants to make sure that what he is selling is really what he says it is,” she said.

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Tennessee whiskey matures in casks of white oak that have been heat-treated and charred to impart color and flavor to the spirit. Infrared spectroscopy offers chemical information that can be used to confirm that the composition and structure of the wood are sutiable for the aging process.

Moreover, she noted, the chemical composition and structural characteristics of the charred oak barrels in which the whiskey matures are particularly important to its flavor and color. During coopering, the white oak staves are heated so they can be bent into the proper shape, and this treatment caramelizes the sugars and tannins in the wood. Before casking, the interiors of the barrels are exposed to flame, which results in a thin layer of charcoal over the caramelized wood with which the spirit interacts over its years of aging.

Wet chemistry and thermogravimetric techniques may be used to confirm that the charcoal for mellowing is of the desired species of maple and that the barrels have been prepared properly, but these approaches are relatively slow and are unsuited for online process monitoring. Near-infrared spectroscopy, Labbé said, is an ideal solution, but because it does not offer enough information about the chemistry involved, the researchers first are employing mid-infrared spectroscopy to acquire the data necessary.

In experiments that they reported in the May 17 issue of Journal of Agricultural and Food Chemistry, the scientists analyzed ground charcoal from sugar maple, red maple, silver maple and white oak using a Fourier transform IR spectrometer from PerkinElmer Inc. of Wellesley, Mass., fitted with a ZnSe attenuated total reflectance accessory. To identify thermally dependent changes in chemical composition of the materials, the samples of the various types of wood were prepared by heating them to 250, 275, 300, 325 and 350 °C before analysis over the 4000- to 650-cm–1 spectral range at a resolution of 4 cm–1.

Multivariate analysis of the resulting data revealed a number of spectral peaks, mainly associated with lignin, that can be used to distinguish charcoal made from the various maple species. The researchers further found several indicators of the thermal decomposition of white oak that can verify that casks have been properly charred.

Because wood is a variable and complex material, Labbé said, the investigators expect to continue collecting spectral data from samples and performing multivariate analysis for the time being. As the work progresses, they plan to meet with a distiller that is funding the project to introduce statistical process control to improve its manufacturing processes.

Contact: Nicole Labbé, University of Tennessee, Knoxville, Tenn.; e-mail: nlabbe@utk.edu.



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