Whether you’re a lover of Scotch, rye, bourbon or Irish whisky, I’m sure you know that these spirits are aged in oak barrels. These barrels soften whisky’s attack and add additional character and flavour. However, what was in the barrel before it was used for the spirit, the size of the barrel, type of oak and toasting, and how long the spirit spends in it can all change its ultimate flavour.
I don’t believe there is a producer out there who uses raw oak barrels to age their spirit. It’s simply too aggressive and harsh.
Those who use barrels that have not been previously filled with something else, like bourbon producers, will ensure that it is toasted or charred on the inside. This means turning them upside down over an open flame and burning them to a desired char (light, medium, or high). This toasting converts the wood sugars into vanilla and caramel flavours that transfer into the spirit. The more intense the toast, the more vanilla and caramel flavours in the spirit.
Most producers, though, prefer to use barrels that have previously held something else, usually another form of alcohol. Barrels that have held wine seem to be high on the list of those preferred by spirit distilleries. The type of wine the barrel held plays a huge part in the finished spirit’s flavour.
Sherry barrels are widely used. Drier sherries like finos, amontillados, manzanillos and palo cortados seem to impart a nutty, white fruit, spicy component into the barrel and the spirit that ages in it. Sweeter sherries like olorosos and pedro jimenez tend to transfer darker notes like sweet dark fruit, raisin, syrup and richer nuttiness into a spirit.
Occasionally, other wine barrels are used. Those previously holding other sweet wines like Port, Madeira, Sauternes, Marsala and Muscat add more sweetness, tropical fruit and savoury spiciness – perhaps a bit too much infusion! Dry red wine barrels from Burgundy, Bordeaux, Barolo and Amarone have also been used. These add more tannin and dried fruit character … maybe not as desirable.
Barrel size is also important. Whether the barrel has held something else before or has simply been charred or toasted from new, the smaller the barrel, the more interplay between spirit and wood. A smaller barrel will impart much more character and previously ingrained flavours than a larger one.
Oak type makes a big difference too. Most oak for wine or spirits comes from Europe or America. Generally speaking, European oak, especially French, provides more subtle and delicate characteristics such as vanilla and toast while American oak delivers more aggressive flavours and spices, like coconut and fig. Although these major characteristics will be compromised if wine has been in them before being used to age a spirit, they do play a part.
The final aspect of the flavour of spirits aged in previously used barrels is how long they spend in them. Regardless of what the barrel previously held, the more time inside it, the more flavour is extracted.
Keep in mind that a spirit with substantially more alcohol than a wine probably leaches more flavour out of a barrel more quickly, so the number of times it can be used and still deliver character is minimal. Sometimes, these spent barrels are shaved out on the inside and re-toasted, but this can only happen minimally as the barrel loses integrity.