Liberation of recalcitrant cell wall sugars from oak barrels into bourbon whiskey during aging

Abstract

Oak barrels have been used by humans for thousands of years to store and transport valuable materials. Early settlers of the United States in Kentucky began charring the interior of new white oak barrels prior to aging distillate to create the distinctively flavored spirit we know as bourbon whiskey. Despite the unique flavor and cultural significance of “America’s Spirit”, little is known about the wood-distillate interaction that shapes bourbon whiskey. Here, we employed an inverse method to measure the loss of specific wood polysaccharides in the oak cask during aging for up to ten years. We found that the structural cell wall wood biopolymer, cellulose, was partially decrystallized by the charring process. This pyrolytic fracturing and subsequent exposure to the distillate was accompanied by a steady loss of sugars from the cellulose and hemicellulose fractions of the oak cask. Distinct layers of structural degradation and product release from within the barrel stave are formed over time as the distillate expands into and contracts from the barrel staves. This complex, wood-sugar release process is likely associated with the time-dependent generation of the unique palate of bourbon whiskey.

Sources of variation in bourbon whiskey barrels: a review

“…Barrel staves, like any other wood product, comprise a matrix of biopolymers (cellulose ( 15 ) and hemicellulose) and phenolic compounds (lignin) ( 16–18 ) . Within this matrix, components reside such as hydrolysable tannins ( 5, 6, 10, 19–21 ) , tyloses, lipids ( 22 ) , whiskey lactone ( 23 ) , and other trace compounds ( 24 ) .…”

“…These two processes influence how cellulose breaks down during charring. Cellulose makes up about 35% of bourbon barrel wood ( 15 ) . Chemically, cellulose is a linear polymer of Β‐ (1‐4)‐D glucopyranose units in a 4 C 1 conformation.…”

“…Hemicellulose is a complex carbohydrate polymer that consists of several different monosaccharide subunits that vary in their composition but overall make up about 33% of the wood used in the construction of bourbon barrels ( 15 ) . Hemicellulose is composed of glucose, mannose, galactose, xylose, arabinose, 4‐O‐methyl glucuronic acid and galacturonic acid residues that form a polymer of roughly 150 units ‐ considerably fewer than cellulose ( 55, 89, 91, 105–108 ) .…”

“…As described above, identifying where the variation in these compounds arises can be difficult. But if variation leads to shifts in the composition of cell wall biopolymers that are available to be degraded during maturation ( 15, 140 ) , then in turn, it is expected that measurable changes in the volatiles that are formed will occur ( 65, 142 ) . This is evidenced by changes in the volatile profile of whiskey that undergoes sequential maturation in different barrels ( 64 ) .…”

“…Whiskey maturation within barrels is complex as extractable compounds are formed from biological factors and modified through thermal treatments. Cell wall compounds are hydrolysed and then undergo further reactions that generate volatile compounds ( 15–17, 65, 82, 140 ) . There are two major chemical reactions that occur during maturation – transesterification ( 64, 65 ) and Fenton chemistry ( 143, 144 ) .…”