Notes by Jon Stika from presentation at Hoppy Halloween 2014, Fargo, ND by Russ Karasch of Black Swan Cooperage, Park Rapids, MN

White Oak (Quercus alba) is used for barrels because the xylem contains tyloses that are one-way valves that close when the tree dies and prevent fluid from flowing through the xylem pores afterward. White Oak used at Black Swan is sourced from Wisconsin. Rough sawn white oak is shaped into rough staves and allowed to season outdoors for one to three years to reduce tannins in the wood. Staves are then shaped to be narrower at each end than in the middle and beveled. The inside surface of the staves are bored with holes part way into wood to create a honeycomb of end-grain surface area to interact with whiskey. Staves are then immersed in 160F water for 10 to 15 minutes and bent to shape. After the barrel is assembled it is toasted at 365F for 20 minutes to create the flavor profile. The inside of the barrel is then charred as required by law.

Fusel alcohols are larger molecules than ethanol and become trapped in charred wood. The honeycomb pattern bored into wood will allow the ethanol to extract the same amount of flavor (primarily sugars) in six weeks as opposed to 2+ years which is typically practiced by whiskey makers. There are 27 different sugars that can be extracted by ethanol form white oak. 120 proof (60%) ethanol is the optimum concentration for extracting sugars from wood without adversely affecting sugar chemistry. Over time, ethanol links into longer polymer chains. These longer chains are smoother and less “scratchy” going down your throat. Some work is being done with ultrasound to form these longer chains faster so less aging time is needed to achieve a smoother product.

Other woods can be used to flavor whiskey, beer and wine, but they are all too porous for use as barrels and the “angels share” would be very high as the liquid in the barrel would wick through the wood and evaporate. Flavors from these woods can be imparted into alcoholic beverages by inserting wood chips or spirals into the liquid for a period of time instead of aging the liquid in a barrel of that wood.

Scotch was, and still is, aged in used whiskey barrels because the barrels could be had cheaply. Scotch is aged for an extended period of time in used whiskey barrels because it has to permeate the wood to the depth of the original whiskey (~2 years) and then into unsaturated wood to extract flavors and back out again. Hence Scotch whiskey was aged for at least 7 years to give the ethanol time to penetrate deep enough into the used whiskey barrel and back out again to acquire flavors from the wood. This could have been accomplished in half the time if the Scots weren’t so cheap and bought new, instead of used, barrels.

Wooden barrels used to handle beer were made of various species of wood and lined with conifer pitch to seal them from the inside. The beer never made actual contact with the wood, only the pitch that lined the barrel, so the beer did not pick up any wood flavors.

My note: “Beech wood aging” employed by Budweiser is done with Beech shavings in the bottom of their lagering tanks. I asked about this at a tour of a Budweiser facility in CA several years ago. The wood shavings provide additional surface area for the yeast to be in contact with the beer and reduce acetaldehyde and other undesirable flavors during the lagering period of conditioning.