Malt whisky is usually aged in casks previously used to mature bourbon or sherry, with bourbon barrels having been charred on the inside, while sherry casks are toasted. This is done by applying a flame, with the heat ‘releasing’ various flavour compounds bound up in the oak. A certain level is extracted when the casks are used to age bourbon and sherry, but a significant level remains to be extracted by malt whisky. Consequently, if casks weren’t charred or toasted they would contribute significantly less flavour (and excessive oakiness) to malt whisky.
Exactly what instigated the tradition of charring and toasting is uncertain. The essential difference between them is that bourbon barrels are exposed to more intense heat.
“You’re looking to flame the interior of bourbon barrels, but not to ignite the interior of a Sherry cask. All cooperages have their own individual regimes to achieve this, that’s why bourbon distillers tend to stick with one cooperage to promote consistency,” says Dr James Swan, a consultant renowned for his research into the influence of oak.
Bourbon barrels are charred to varying degrees. A number one char resembles burnt toast, with the heaviest being a number four, known as an ‘alligator char’ as it looks like alligator hide. Most barrels receive a number three, with fewer number fours, and far fewer number one or two chars.
“At the Brown-Forman cooperage barrels (without the ends) are charred on a ‘production line’ fitted with separate stations. At the first station a flame from a gas burner comes into contact with the base of the staves. It takes about a second for the interior of the barrel to ignite, and become engulfed in flames. The barrel is then moved to the next station and allowed to continue flaming, for approximately 25 seconds to achieve a number three char, and 30 seconds for a number four char. At the next station flames are extinguished by a water mister, spraying water onto the staves,” says Chris Morris, master distiller, Brown-Forman Woodford Reserve.
The layer of char is typically two mm deep with a number one char, and four mms with a number four. As flavour compounds within the char have been combusted by flames, the char doesn’t contribute aroma, flavour or colour to whisky.
The intense heat also results in a layer of oak beneath the char becoming toasted, which instigates various reactions within a 12 mm layer (staves are about 25 mms thick). Compounds within the oak include cellulose (comprising glucose units), hemicellulose (comprising glucose and other sugars), and lignin (a complex, not fully understood compound that helps bind cells together and so strengthens the oak).
“Cellulose is the main structural element in oak, with hemicellulose a secondary structural element, while lignin is the glue that holds it all together,” says Swan.
These compounds lack aroma and flavour until they’re toasted.
“Heat caramelises and breaks down hemicellulose, with cellulose slightly caramelised as the heat can only partially break it down. Lignin breaks down into several components including vanillin, which contributes vanilla to the maturing spirit,” adds Swan.
Rachel Barrie, whisky creator and master blender at Glenmorangie, details the schedule during toasting: “At 300 degrees fahrenheit hemicellulose starts to break down, and above 350 the sugars caramelise to give a sweet, fudgy character. At about 400 degrees lignin starts breaking down to create vanillin.”
The level of vanillin a barrel provides also depends on the charring level.
“A number three char is the peak for producing vanillin, while a number four gives more oakeyness and less vanilla, so it all depends,” says Morris.
To extract flavour compounds such as vanillin, the spirit must enter the toasted layer. This happens when the ambient temperature rises, making the spirit expand and initially penetrate the char. The greater the degree of charring the more the surface of the oak is broken up, which means a number three or four char gives the spirit more immediate access to the toasted layer than a number one or two.
When the temperature cools down the spirit contracts and exits the toasted layer, carrying flavour compounds extracted from the oak back into the bulk of the spirit. This journey, known as a cycle, requires a significant change of temperature in order to occur. In Scotland spirit expands during the summer and contracts in winter.
Meanwhile, the layer of char absorbs a certain level of undesirable notes from the maturing malt whisky. These include meaty, sweaty socks and burnt match notes which mask the esters (fruity notes). Consequently, the char helps to ‘unmask’ esters, which then show more clearly.
“The char is most active in terms of absorption in the early stages of maturation, and does the job of unmasking within five years,” says Rachel Barrie.
Bourbon barrels account for the vast majority of casks in Scotland, with sherry casks taking most of the balance. Sherry casks are toasted on a scale specified as light, medium or heavy. A light toast affects the first millimetre of oak, with a heavy toast affecting an additional two mms (staves in a Sherry hogshead are about 32 mms thick, and 35 mms in a butt).
Toasting instigates the same reactions in the oak as when the toasted layer is created by charring a bourbon barrel. Toasting also opens up the surface of the oak (but not to the same extent as charring), giving the spirit greater access to the flavour compounds.
Both types of casks contribute significantly different flavours to malt whisky. Bourbon barrels provide vanilla, butterscotch, honey, fruit and coconut flavours, for example, while sherry casks typically give fruit cake, raisin and fortified wine notes. Sherry casks also add richer sweetness and a darker colour.
These differences are essentially down to Sherry casks being made from Spanish oak (quercus robur), a different species from the American oak (quercus alba) used for bourbon barrels. (Some sherry casks are American oak, but the vast majority used to age malt whisky are Spanish oak). However, it’s also toasting and charring that enables casks to deliver their range of characteristics.