How does oak aging work?

[PaulDistiller]

Hello Distillers!

It's been a long time since I posted here as I had to give up distilling when I moved. I have moved again and have retaken up the art.

I am very interested in oak wood and other wood aging but I do not understand how it works. Google has produced many varying explanations, most of them youtube vids and advertisements.

I'm here to get the info straight from the people I trust, this community.

So: How does oak aging work? How does it reduce off tastes and add flavors?

I've heard that the alcohol moves through the capillaries of the wood, picking up flavor and depositing chemicals. This implies, bigger capillaries and more capillaries = faster results. I will compile the answers and edit this post for any future users.

Thank you all!

I apologize if this has been asked an answered a thousand times, the search feature does not accept words shorter than 4 characters and "oak" just happens to be an important word.

[T-Pee]

The following is IMO.

Oak doesn't age. Time and molecular changes are responsible for aging.
Oaking is for flavor. Depending on the temperature and length of time in toasting, the oak imparts those flavors as the liquid moves through and removes the flavors in the wood.
The charring is responsible for the filtering of impurities as the liquid moves through it.

Maybe oversimplfied but that's how I see it.

Someone else may get into the molecular science of the whole thing but that won't be me. I just know what works.

tp

[MyUncleMo]

I just read through this paper...
here is cut and paste... or you can go to the paper itself - page 33.

2.2.2.5.1 Oak Wood for Maturation
Although many woods have been used for spirit maturation, only oaks are
important as maturation barrels (Singleton, 1974). Some of the qualities that make oaks
suitable for liquid containers are the presence of multiseriate rays, tyloses, durable, tough,
bendable wood with high extract to inhibit rotting organisms. The oaks used for
cooperage are white oaks from the Leucobalanus or Lepidobalanus subgroups. European
species are, Quercus rubor, and Q. petraea, in North America there are many species but
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the most important one is Quercus alba. Red oaks are not suitable for cooperage due to
the poor tylose formation. Tyloses are spheres of 28% lignin that expand from the cells to
plug the sprinwood pores of the heartwood, this prevents the liquid from leaking out of
the barrel. Q. alba is more fully plugged than the European species. Some other
differences from American and European oaks are that European oaks have more
extractable phenols, while American oaks have more aromatic compounds. In general the
components of different oak species are similar, varying in concentration and relative
amounts. Some conditions that may influence the composition of the oaks, and the
maturation that can be achieved with the barrels are tree age, growing rate, and such.
Older trees have larger medullary rays which are barriers to the migration of
liquids. This may be a reason why old, larger trees are better for staves. Wood chemical
and physical composition varies between and within trees. The youngest heartwood,
nearest to the sapwood has the highest and most diverse content of extractable
components. As the tree becomes older, the heartwood near the pith diminishes in
extractable and phenol compounds. Wood of slow grown trees is easier to work and bend,
and has greater extracts, this is why coopers prefer it to faster growing wood. European
staves have higher extractable solids (161%) than American oak, and have more phenol
per unit (154%) (Singleton, 1974). The general composition of dry heartwood is 50%
cellulose, 20% hemicellulose, and 30% lignin. Cellulose is the framework of the wood,
hemicellulose the matrix, and lignin the encrustant. Cellulose is a polymer of glucose,
hemicellulose is an heterogeneous polymer that includes xylose and other sugars,
especially pentoses. Lignin in oak is a three-dimensional polymer of phenylpropane
derivatives of guaical and syringyl units substituted in the four th position with the
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aliphatic side chain and cross-linked by oxidation. Some other compounds such as
tannins and carbohydrates are attached to lignin (Rowell, 1984). In sufficient quantity,
the extractives of the wood compounds affect the flavor of the final beverage. Water and
ethanol extracts of oak wood contain color, carbohydrate derivatives, gallo- and
ellagitannins, lignin fragments, together with their precursors and degradation products
(Singleton, 1995). These compounds are flavorful and modify the aroma of the beverage
during maturation. The soluble extractives of oak are depleted readily from the layer
extracted, while the major structural polymers are broken down, if at all, during the
process of maturation (Singleton, 1995).
The ideal ethanol concentration that yields the maximum solid extraction is
approximately 55% (v/v) (Singleton and Draper, 1961). For the production of lighter
products, lower proofs can be used yielding a product with less extractives and having a
different composition. A barrel for 200 L has as much as 90 cm2/L of wood surface.
Evaporation, extraction, oxidation and component reactions are maturation effects related
to the conditions of the barrels, and they will increase as more wood surface is in contact
with a unit of beverage (Singleton, 1995). Each mm penetration of the beverage to the
wood will contribute some 9 cm3 of extracted oak or 5.4 grams of soluble oak solids,
assuming a density of 0.6 g/cm3, and some 10X flavor threshold (Singleton, 1995).
Once maturation results suffice, a common practice is to reuse or refill the barrels.
After refilling, barrels never seem to lose their physical effects, and never seem to be
depleted, even when they are not shaved off. This may be due to the faking and cracking
that exposed some new wood (Singleton, 1995).
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Some consideration has to be given to cooperage as the rays have to be tangential
to the barrels circumference. The staves should not be dried too rapidly, in order to
prevent checking. The ideal moisture content of the staves for cooperage is 8% to 10 %
(Rassmussen and McMillen 1956). According to Francis et al (1992), woods that are
seasoned in warmer, drier climates possess enhanced vanilla, buttery, caramel, and cedar
notes. These notes plus nutty, and a decreased raisiny note are present in wood that is
thermally treated at 105°C for 24 h. Other considerations are that due to tree-to-tree
differences, the staves used for a single barrel should be randomly selected from several
trees. This will decrease the barrel-to-barrel variation (Singleton, 1995).
Charring and toasting of the barrel wood produce various effects in the extractives
which are transferred to the beverage during maturation. Charring is a more radical
treatment and produces mainly carbon and some pyrolysis products as furfural and HMF.
It may transfer some undesirable smoky flavor to the beverage. Toasting is a less drastic
treatment and can be accomplished to different degrees and depths (Singleton, 1995).
High temperature treatments cause a reduction in hemicellulose and increased furfural
and HMF derivatives unless the heat treatment is so severe that the compounds
polymerize (Hillis, 1984).
During maturation water and ethanol losses occur in the barreled spirits. About
2% to 7% loss in volume can be expected per year during maturation in standard barrels
in the tropics. Many factors such as container size, temperature, relative humidity, and air
circulation influence evaporation, other factors include water’s liquid state, heat of
vaporization, self association, adsorption to barrel carbohydrates and vapor pressure
(Singleton, 1995). Given a certain temperature higher humidity lowers the water loss rate.
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Ethanol is also lost during maturation. For a constant ethanol concentration in the
beverage, the relative humidity should be between 65% and 70%. While water and
ethanol are lost, the content of non-volatile compounds increases during maturation.
2.2.2.5.2 Contribution of Oak to the Aroma of Rum
Some of the components found in oak wood that contribute to the aroma of spirits
are 4-methyl-g-octalactone, g-nonalactone, and eugenol. Table 2.8 lists compounds
identified in oak wood. Acetic acid increases during maturation in oak casks, it is a main
component of pyrolegneous acid, and it derives from the acetyl group of hemicellulose by
degradation (Nishimura et al., 1983).
The products of lignin degradation obtained from charred oak are much larger
than those obtained from uncharred oak. These components include extracts, with
aromatic aldehydes such as vanillin, syringaldehyde, and sinapaldehyde. The higher the
toasting temperature, the higher the amounts found. Aromatic aldehydes are directly
extracted, while vanillin and syringaldehyde are products of the oxidation of
coniferaldehyde and sinapaldehyde (Nishimura, 1983). Nishimura et al (1983), found that
the ethanolysis of lignin yields more vanillic acid and syrinaldehyde, and less
sinapaldehyde and coniferaldehyde than oak chips. Native lignin of low molecular weight
yields about the same amounts than with oak chips. However, due to the relative low
amounts of native lignin in wood, they suggested that this is not the main source of
lignin-derived compounds. Later they studied oxidation and esterification of these
aromatics, and suggested that the lignin-derived compounds are formed by the following
pathways (Figure 2.4, and 2.5).
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Table 2.8 Compounds Identified in Oak Wood.
Aliphatic hydrocarbons
Tetradecane
Pentadecane
Hexadecane
Heptadecane
Octadecane
Nonadecane
Eicosane
Aliphatic acids
Acetic acid
n-butyric acid
i-butyric acid
n-valeric acid
i- valeric acid
caproic acid
heptanoic acid
caprylic acid
nonenoic acid
nonanoic acid
caprilic acid
decanoic acid
undecanoic acid
lauric acid
myristic acid
tetradecanoic acid
pentadecanoic acid
pentadecenoic acid
pentadecadienoic acid
palmitic acid
Other aliphatic
compounds
Cis- 4-methyl-g-octalactone
Trans-4-methyl-g-
octalactone
g-nonalactone
g-decalactone
1,1-dimethoxynonane
1,1-dimethoxydecane
b-Ionone
Aromatic hydrocarbons
Naphthalene
a-methyl naphthalene
b-methyl naphthalene
a-ethyl naphthalene
b-ethyl naphthalene
dimethyl naphthalenes
trimethyl naphthalenes
tetramethyl naphthalenes
biphenyl
acenaphthene
acenaphthylene
1,1-6-trimethyl-1-1,2-
dihydronaphthlene
fluorine
1,2-dmethyl-4-allyl
benzene
Phenols
Phenol
guaiacol
o-cresol
p-cresol
p-ethyl phenol
4-methyl guaiacol
eugenol
i-eugenol
chavicol
syringol
4-methyl syringol
4-ethyl syringol
4-allyl syringol
vainillin
propiovainillone
Other aromatic
compounds
Benzyl alcohol
phenethyl alcohol
phenethyl acetate
acetopehenone
Other aromatic
compounds
1-indanone
benzothiazole
methyl salicylate
benzoic acid
phenyl acetic acid
cinnamic acid
Furan compounds
Dibenzofuran
2-furoic acid
3-furoic acid
Terpene compounds
a-Muurolene
g-muurolene
b-bisabolene
a-cadiene
g-cadiene
d-cadiene
d2-cadiene
a-curcumene
calamene
a-calacorene
cadalene
terpineol
borneol
myrtenol
elmol
epi-cubenol
b-eudesmol
a-eudesmol
g-eudesmol
a-cadinol
T-cadinol
Verbonene
Geranyl acetate
Source: Nishimura et al., (1983).
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Other effects of maturation on the composition of beverages are an increase in
acetaldehyde, derived from ethanol + acetic acid. The already minor quantities of sulfur
compounds, such as dimethyl sulfide, dimethyl disulfide, methionyl acetate, ethyl
methionate, and dihydro-2-methyl-3(h)-thiophane decreased with aging. Toasting of oak
wood produced other non- lignin volatile compounds such as furfural, 2-methylfurfural,
guaiacol, and 4-methyl guaiacol (Nishimura et al., 1983).
Acetovainillone Propiovainillone
Vainillin Vainillic acid Ethyl vainillate
Coniferaldehyde Ferrulic acid
Sinapaldehyde Syringaldehyde Syringic acid Ethyl syringate
Figure 2.4 Reactions of Lignin Components During Storage in 60% Ethanol Solutions.
(Nishimura et al, 1983).
Extraction Oxidation
Esterification
B D
O2 Charring A Ethanolysis C
Figure 2.5 Pathways for Lignin Derived Compounds Formation (Nishimura et al., 1983).

[thumper123]

A most thorough treatise, UncleMo. As you point out, it's a really complicated process.

I think this can be paraphrased into three actions caused by white oak aging.

A. Subtractive: The removal, or more correctly, the absorbtion of unwanted alcohols and acetates by the charcoal
B. Additive: The addition of vanillans, smoke and other flavors.
C. Reactive: Changes in the liquor due to chemical reaction.

Hope this isn't overly simplistic.

[BoomTown]

This is some good stuff....

Boom

[PaulDistiller]

Wow,

Great information! I read through it and I have some more questions

White oak is less porous according to the paper. I am still assuming capillary action is important here. So I should use red oak instead of white as red is more porous.

The article did not mention the age of the wood after it was cut. But I can glean from the forums aged oak with no sap is important.

I also found this paper that discusses reactions at length. Here is the paper
http://www.tuthilltown.com/wp-content/u ... ation1.pdf" onclick="window.open(this.href);return false;" rel="nofollow

I have not yet read it but from the paper already available, it seems the flavors come from the wood and not from converting the alcohol. So it seems perhaps the aging process could be produced by charcoal filtering and adding the final flavors immediately (vanillin, etc)
Thoughts?

Edit: Forgot the questions.
1. Is capillary action important?
2. Will carbon filter impurities from alcohol? I tried using activated carbon on crappy vodka years ago and the results were pretty crappy even after weeks of soaking the activated carbon in the vodka.

[shadylane]

Hello Distillers!
So: How does oak aging work? How does it reduce off tastes and add flavors?

Depending on the final product, the standard for flavor was set in stone, after century's of storing alcohol aged in oak barrels.

[MyUncleMo]

Here is the link to my other thread with the link to the paper lol...
http://homedistiller.org/forum/viewtopi ... =6&t=51549

or direct to the paper...
http://etd.lsu.edu/docs/available/etd-1 ... thesis.pdf" onclick="window.open(this.href);return false;" rel="nofollow

Frankly there are several Scientific technical breakdowns regarding Oak.
The above mentioned is related to Rum production.

HD has many other descriptions of how Oak works as well... I think Oak is too short of a work to search for. Hmmmmm

[T-Pee]

White oak is less porous according to the paper. I am still assuming capillary action is important here. So I should use red oak instead of white as red is more porous.

Nope. Red oak is high in yucky tannins.

The article did not mention the age of the wood after it was cut. But I can glean from the forums aged oak with no sap is important.

The wood should be seasoned for at least a year.

I have not yet read it but from the paper already available, it seems the flavors come from the wood and not from converting the alcohol. So it seems perhaps the aging process could be produced by charcoal filtering and adding the final flavors immediately (vanillin, etc)
Thoughts?

You can't rush aging. You know? AGEing? Some use a microwave but I don't.
Oak flavors come from patience, NOT additives.

Is capillary action important?

Up to a point.

Will carbon filter impurities from alcohol? I tried using activated carbon on crappy vodka years ago and the results were pretty crappy even after weeks of soaking the activated carbon in the vodka.

You just answered your own question, didn't you?

tp