A stand of Sessile oak in France.

A Wood Primer

A huge credit for much of the information in this presentation goes to Dr. Jim Swan who has done a tremendous job of consulting with us at Compass Box to help us better understand the vital subject of wood maturation for the production of the best quality whisky.

By John Glaser

Wood Primer

• Why Oak?

• But It’s Not Just the Oak…

• Three Effects of Oak on Spirit

• Constituents of Oak That Influence Spirit

• Air Seasoning, Toasting & Charring

• Oak Species

• Quality Measures

Introduction.

Why Are the Winemakers Getting All the Good Wood?

Let’s Look at the Production Process for Top Quality

Cooperage Oak

Why Oak?

• Because of its unique physical and chemical nature– Strength: Physically, its wide radial rays give

strength when shaped into a cask– A Pure Wood: unlike pine or rubber trees with

resin canals that give strong flavours– Pleasing Flavours: during coopering,

pleasant-tasting oak lactones are produced

But It’s Not Just the Oak Itself…

It’s the transformation of the oak by seasoning and heat

treatments in the coopering process.

Three Effects of Oak on SpiritAdditive Adds organoleptically desirable

elements from the caskEg, vanillin, oak lactone (coconut, bourbon character), toastiness, wood sugars, colour

Subtractive Removes undesirable elements from new make spirit

Eg, sulphur compounds, immaturity

Interactive Adds extractive wood elements from the cask and converts them to organoleptically desirable elements

Eg, tannins to acetals;

acetic acid to fruity esthers

Constituents of Oak and How They Influence Spirit

• Cellulose

• Hemicellulose

• Lignin

• Oak Tannins

• Lipids and Oak Lactones

Cellulose

• Virtually no effect other than to hold the wood together

Hemicellulose

• Consists of simple sugars that breakdown when heated and provide:– Body: by addition of wood sugars– Toasty & Carmelised Aromas & Flavours– Colour

Lignin

• Consist of the building blocks which yield (when heated):– Vanillan– Sweet smoky and spice aromas

Oak Tannins

• Play an essential role in maturation by enabling oxidation and the creation of delicate fragrance in spirits– Tannins combine with oxygen and other

compounds in spirit to form (over time): acetals

Oak Tannins Help Form Acetals

• Acetals:– “Have a strongly ethereal influence on the

product giving it delicacy and topnote…without it, spirits are dull and flat.”

– Dr. Jim Swan

Oak Lactones

• Resulting from lipids in the oak, they increase dramatically during toasting.– Strong woody, coconut character– Give Bourbon its distinctive character– Higher concentrations in American oak than

European varieties

The Quality Measures forCooperage Oak

• Species and Source

• Grain tightness– function of slow growth versus fast growth– which is a function of forest “terroir”

• Seasoning: length and type

• Heat Treatment: type and quality of toasting, charring

Oak Species

• Of hundreds of oak species, just three are the main species used for wine and whisky cooperage:– Quercus alba, “White Oak” (America)– Quercus petraea, “Sessile Oak” (Europe)– Quercus robur, “Pedunculate Oak” (Europe)

Quercus alba, “White Oak”

• Known also as “American oak”

• The most commonly used variety in whisky cooperage

• More vanillan than European varieties

• High in lactones when toasted which provide woody and coconut flavours

Quercus petraea, “Sessile Oak”

• Found across Europe, notably in France

• Most commonly used for wine cooperage

• Slow growth, fine tannins and more vanilla (compared to Pedunculate)

• Most common species in Tronçais forest

Quercus robur, “Pedunculate Oak”

• Found across Europe, notably in France

• Most commonly used for Cognac cooperage

• Commonly used for sherry cooperage

• Fast growth, more tannins, thus more oxidative characteristics in the matured products (compared to Pedunculate)

• Most common species in Limousin forest

Impact of Oak Growth Rate: Slower is Better

• Winemakers are convinced of the relationship between oak growth rates and the flavour and quality of their wines

• In whisky, this is not widely considered!

• Slow growth oak has more “good stuff”: especially vanillans and oak lactones

Air Seasoning: Vital to Make Quality Cooperage Oak

• Converts chemical compounds in wood to more desirable types

• Dries out the wood• 24 months ideal for wine• Whisky usually uses 6-12 months, sometimes less!• Air Seasoning better than kiln drying

– Reduces tannic astringency

– Releases more vanillan

Toasting: Transforming the Flavours the Wood Can Give

• The level of toasting affects the spectrum of compounds and flavours the oak will provide:– Vanillans

– Lactones

– Toastiness

– Spice characters

– Tannins

• Different toasting techniques provide different profiles

Charring: Further Transformation Including the All-Important Char• Takes place after toasting• Required by law for Bourbon:• Char (carbon) removes sulphur compounds

and immaturity from new whisky spirit• Typical: 40 seconds to 1 minute, but some

experiment with up to 3-4 minutes• Creates dramatic changes at surfaces;

effectively toasts layers beneath

A stand of Sessile oak in France.