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.
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.
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.
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
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.