Vanillin and oak lactones in whisky

Sweetness without sugar

Most whisky contains no added sugar, yet many drinkers describe it as sweet. A large part of that impression comes from aroma: vanilla, coconut, toffee and cream. The nose reads these as sweet even when the tongue finds nothing to back it up.

Vanillin and oak lactones are two of the most important sources. Both are extracted from the oak cask during maturation. Our guide to how whisky is made shows where maturation fits.

Vanillin comes from lignin

Oak cell walls contain lignin, a tough polymer that gives wood its strength. When a cooper heats the inside of a cask to bend the staves, then toasts or chars it, some of that lignin breaks down into smaller aromatic molecules. Vanillin is the best known. It is the same molecule that gives vanilla pods their main aroma. Related compounds such as syringaldehyde form alongside it and add a gentler sweet, spicy note.

Spirit in the cask dissolves these compounds over years. Toasting is the step that creates most of the vanillin, so the depth of toast matters as much as the oak itself.

Oak lactones bring coconut

Oak lactones, often called whisky lactones, are present in the raw wood. They smell of coconut, fresh sawn wood and sometimes celery or hay at high levels. There are two forms, known as cis and trans. The cis form is the more aromatic of the two.

American white oak usually contains more oak lactone than European oak. That is a big part of why bourbon, aged in new charred American oak, often smells of coconut and why Scotch from ex-bourbon casks picks up the same note. European oak, used for many sherry casks, gives fewer lactones and more tannin.

What the cooper changes

• Seasoning. Staves left outdoors for a year or two lose some harsh compounds. Long seasoning tends to soften the wood's flavour.
• Toasting. Moderate toasting builds vanillin. Very heavy heat can start to break down lactones and vanillin again.
• Charring. Charring creates a charcoal layer that cleans the spirit. The toasted layer just behind the char is where much of the vanillin sits.
• Previous fills. A first-fill ex-bourbon cask gives far more vanillin and lactone than a cask on its third fill. The wood gets used up.

Our guide to whisky styles of the world shows how these cask choices shape regional character.

How to find them when you taste

Pour a small measure of a whisky matured in ex-bourbon casks. Nose it gently, glass slightly below the nose. Look for:

• Vanilla custard, ice cream or vanilla pod: vanillin.
• Desiccated coconut, sun cream or fresh pale wood: oak lactones.
• Toffee, butterscotch or toasted bread: caramelised wood sugars.

Then try a sherry cask whisky. You will usually find less coconut and more dried fruit, nuts and drying tannin. The contrast is the easiest way to train your nose on these compounds. Add a few drops of water to each and nose again.

Indian heat and the sweet notes

In a hot warehouse, spirit moves in and out of the wood more often, so extraction is faster. Whiskies matured in India can show strong vanilla and coconut at a young age. That is pleasant, but too much can flatten everything else. Blenders working in heat often watch for the point where the cask's sweetness starts to cover the spirit's own character. You can try this with malt and blended whiskies from hot and cool climates side by side.

Vanilla in other drinks

The same compounds turn up wherever oak meets alcohol. Barrel-aged wines, especially those in new American oak, can show vanilla and coconut. So can barrel-aged beers, rums and brandies. Once you can pick out vanillin and oak lactone in whisky, you will start noticing them in a barrel-aged stout or an oaked red wine as well.

It also helps to know what is not from oak. Fruity notes like pear and banana usually come from esters made during fermentation. Smoky notes in many Scotches come from peat used to dry the malt. Separating wood from grain and fermentation is one of the most useful skills a taster can build.

Train with a vanilla pod and a coconut

Before your next tasting, smell a real vanilla pod and a piece of desiccated coconut, one after the other. Then nose an ex-bourbon whisky. With the reference smells fresh in your mind, you will pick out vanillin and oak lactone far more easily. Sip slowly, keep water to hand and never drink and drive.

Common questions

Where does the vanilla flavour in whisky come from?

Mostly from vanillin, formed when lignin in the oak cask breaks down during toasting and charring, then dissolves into the spirit.

Why does some whisky smell of coconut?

Oak lactones, found especially in American white oak, give a coconut and fresh wood aroma. Ex-bourbon casks pass this on to Scotch.

Does sherry cask whisky have vanillin?

Some, but European oak usually gives fewer lactones and more tannin, so dried fruit and spice tend to show more than coconut.