Monday, August 6, 2012

Oak aging 2, other questions

Here's another post on the wonders of whisky and what happens when you put it into a barrel.

First I'll repost the 3 questions I hear most often in reference to oak aging of spirits.

1. What does the barrel aging do?
2. If oak is good why not try other woods?
3. Is there another way to age whisky faster?

I tried to cover question one in post 1 on oak aging, I'll do number two here.

First off, many other wood types have been tried for holding liquids and aging wine and spirits. Keep in mind the current wooden barrel is known to date back in basic design and shape to the ones the Celts used as early as the third century B.C. That is plenty of time during which curious people could experiment.

To give a quick and incomplete list of woods that have been tried in the U.S. and Europe: red oak, chestnut oak, sugar maple, yellow birch, douglas fir, beech, black cherry, mulberry, spruce, pine, elm, and many more. This post is going to end up being more about why oak is used, rather than why not other woods, but that's because my experience with other woods has mostly been unpleasant.

The list of things you want in a wood for aging spirits
1. It is not excessively porous
2. It does not contribute unpleasant characteristics
3. It contributes good flavor and aroma
4. It is workable enough to be shaped into a barrel.

When you take these four points into account the only wood to pass the test for long term storage and aging, is white oak of the Quercus family. The rest of the woods above, and the many others not listed, fail at one of these four points in the long term. There are between 9 and 12 species of Quercus white oaks that are commonly used. The two major divisions are American oak and French oak. The two lend very different flavor characteristics but the features that make them liquid tight are similar.

Porosity: Think about what a tree must do to survive. It has to bring all of the water it needs to stay alive up from the ground to the canopy. Trees have to be extremely adept at transporting liquid up and out. That is a major problem in a barrel. Liquid inside the barrel penetrates into the wood during the aging process. In oak, most of that liquid makes its way back into the barrel, but in other woods the liquid is able to move through the natural conduction channels and leak out causing major losses. In a standard 52-59gal oak barrel the loss is ~5% per year (the ~ means roughly) which is bad enough. Oak luckily has a couple of features which are unique and keep this loss to a minimum.

Oak wood has two major anatomical structures that make it liquid tight. They are called tyloses and medullary rays. Basically what they do is prevent liquid from traveling longitudinally to the ends of the staves and leaking (tyloses), and prevent liquid from leaking out through the side of the barrel (medullary rays). Almost all other woods leak from the stave ends because they don't have tyloses blocking longitudinal conduction. The medullary rays on most other woods do not span multiple conduction cells the way white oak rays do. The way the staves are cut during barrel production makes careful use of these structures to retain as much volume as possible in the barrel.

Flavor and aroma characteristics: Oak wood is made of cellulose, hemicellulose and lignin. When the lovely green and supple buds appear in spring they are made of cellulose and hemicellulose. When the material becomes brown and hard it is because the tree is laying down lignin, which is its solidifying encrustant. Happily, in oak it is made out of phenolic components that are pleasing to the pallet. A phenolic compound is a ring of carbon atoms with some other compounds attached. These are known as aromatic rings by chemists because as they were being isolated in the nineteenth century it was noticed that as a group they smelled nice, or so the tale goes. Examples: vanillin, guaiacol (parsely, spice), eugenol (clove). The other great thing about these compounds is that their flavor thresholds are relatively low, so it doesn't take much.

While many other woods have lignin/phenolic content they are often in the company of compounds that have unpleasant sensory characteristics. These other woods may be appropriate for brief exposure as flavor accents but they are not typically used for long term storage or barrel production. The other thing I should say is that over the last 25 years wine and spirits makers have taken to tweeking their products by adding chips of oak wood to their barrels to get some wood extract they're not getting from their barrels (a professor I worked with at Michigan State pioneered the practice). There's probably plenty of room for experimentation with this type of tweeking using alternative woods to add unusual flavors and aromas to spirits, and I we're starting to see some of this in the industry.

Workable: Oak is a hard wood, but is remarkably supple under the right conditions. The process of making a barrel is multi-step and goes like this: drying, seasoning, heating and shaping, toasting or charring. Under the influence of heat, oak wood becomes nice and malleable so it can be shaped into that familiar bilge (the widest part of a barrel) shape. The bilge angle and hoop structure are part of the barrel's liquid tight nature. Many other woods are either too brittle or too soft to hold this shape.

White oak is a big part of the magic of brown aged spirits. Many people don't even know that all spirits are clear when they come off the still. All of the color and much of the flavor of aged spirits comes from the oak. A major contributor to this is the heat treatment of the inside of the barrel, but that could end up being a hefty read so I'll save it for another day.