Ex-bourbon, sherry and wine casks and how each one changes whisky

The cask does most of the work

New make spirit comes off the still clear, hot and grainy. Years in oak give it colour, sweetness, spice and most of its aroma. The same new make filled into three different casks can end up as three whiskies that barely seem related.

Two things drive the difference: the species of oak and what was in the cask before. Our guide to whisky maturation and casks covers the basics of charring, toasting and refill. Here we look at the three cask types you will meet most often.

Ex-bourbon casks: vanilla, coconut and honey

American bourbon must be aged in new charred oak containers, so after one use the barrels go out into the world. Most end up with whisky makers in Scotland, Ireland, Japan and India. They are usually American white oak, around 200 litres, with a charred inside.

American oak is rich in compounds called lactones, which smell of coconut. It also gives vanillin from the breakdown of the wood. The char layer adds caramel and toasted notes and filters out some harsher compounds. Expect vanilla, honey, coconut, banana and soft spice. A first fill ex-bourbon cask gives far more than one on its third use, which is why labels sometimes say "first fill".

Sherry casks: dried fruit, nuts and spice

Sherry casks have traditionally been European oak, seasoned with oloroso or Pedro Ximenez sherry. Today most are casks built for the whisky trade and seasoned with sherry for a period before being emptied and shipped, rather than old casks from the solera.

European oak carries more tannin and fewer lactones than American oak. Together with the soaked-in wine it gives raisin, fig, dried orange peel, walnut, clove and a darker amber colour. Oloroso leans nutty and dry. Pedro Ximenez leans sticky and treacle sweet. Sherry casks are large, often a butt of about 500 litres, so they mature spirit more slowly than a barrel.

Wine casks: berries, tannin and a pink tint

Red and white wine casks, plus fortified wine casks such as port and madeira, are now common for finishing whisky. Port pipes bring red fruit, plum and chocolate. Red wine casks give berry notes, extra tannin and sometimes a faint pink or copper hue. Our guide to how fortified wines are made explains why port and sherry casks behave differently from table wine casks.

Wine casks are strong. Too long and the wine can dominate, leaving a whisky that tastes jammy or drying. That is why most wine casks are used for a finish of months rather than a full maturation.

Size, fill and age of the cask

Cask size changes speed. A smaller cask has more wood surface per litre of spirit, so it gives flavour faster. A 200 litre barrel works quicker than a 250 litre hogshead, which works quicker than a 500 litre butt. In Indian heat every one of them runs faster than it would in Scotland.

The number of previous fills matters just as much. A cask gives most of its easy flavour on its first whisky fill, less on the second and much less by the third. Distillers often keep tired casks for long, slow maturation or send them to a cooperage to be shaved and re-charred, which wakes up fresh wood underneath.

Finishing versus full maturation

A "finish" means the whisky spent most of its life in one cask, usually ex-bourbon, then moved to another for a shorter period. It layers the second cask's character over the first. Full maturation means the spirit spent its whole life in that cask.

Neither is better. A finish is a fast way to add a known character. A full sherry maturation tends to be deeper and more integrated. It also costs more, because sherry casks cost far more than ex-bourbon ones.

Reading the cask in your glass

Try a simple exercise: pour an ex-bourbon matured whisky and a sherry matured one side by side. Look at the colour first, then nose each one with a drop of water. Vanilla and coconut against raisin and spice will usually be obvious within seconds.

Colour can mislead, because some whiskies are adjusted with spirit caramel. Aroma is the better guide. Training your nose to separate wood, fruit and spirit is exactly what turns tasting from guesswork into a skill.

Common questions

Why are bourbon casks used for Scotch?

Bourbon must go into new charred oak, so used barrels are plentiful and good value. American oak gives the vanilla and coconut notes many whiskies are built around.

Is sherry cask whisky sweeter?

It often tastes richer and fruitier, especially from Pedro Ximenez casks. Oloroso casks can be quite dry and nutty, so sweetness depends on the sherry style and the age of the cask.

What does first fill mean on a whisky label?

It means the cask was being filled with whisky for the first time after its previous contents. First fill casks give more flavour and colour than refill casks.