There is a common belief that 1st fill casks are superior to refill casks. With words such as ‘fresh’ and ‘active’ often associated with these particular barrels, hogsheads and butts.

The majority of casks used in the maturation of scotch are ex-bourbon. A US law states that bourbon can only be matured in virgin oak casks. And due to this law, these barrels and hogsheads will have been filled only once with bourbon whisky for an average of 2-3 years.

WHERE DOES THE FLAVOUR COME FROM?
The first time a bourbon cask is filled with scotch whisky, the wood is still fresh and active. With the sweet flavours of bourbon helping to season the wood. The more active the wood, the quicker the maturation and the more flavour extracted from the cask. Oak contains natural sugars that give the rich flavours of caramel and toffee, and vanillin gives classic vanilla notes. Lactones result in sweet coconut notes. And spicy-tasting compounds such as eugenol, iseugenol and cinnamaldehyde (from the charring process)  give us those familiar oak-spice, nutmeg and cinnamon flavours.

VIRGIN OAK
Virgin oak casks are perfect for the relatively quick maturation of bourbon whisky. But these would give far too much oak-influence to scotch which often matures for a considerably longer time. The previous liquid takes the ‘edge’ off the oak. Which ensures that the rougher, harsher elements of the cask are tempered (the really ‘woody’ and ‘varnishy’ flavours often associated with bourbon).

1ST FILL BOURBON CASKS
1st fill bourbon casks are perfect for the maturation of Scotch whisky up to a certain age (around 15-18 years). However, a fresh and active cask will mature quicker. Breathe more and subsequently result in a greater angel’s share with greater loss of liquid and a faster reduction in ABV. Furthermore, whisky left for too long in a 1st fill cask runs the risk of becoming too woody. In which case, the softer and more subtle flavours and all of the distillery character is lost.

REFILL BOURBON CASKS
The wood of refill bourbon casks are less active than 1st fill; a portion of the flavour and colour from the oak will already have been extracted. However, a refill cask still contains plenty of the natural chemical compounds present in the oak. And we must not forget that the previous whisky will have also helped to season and flavour the oak. It will take longer for the new spirit to extract the flavours, and the whisky will mature more slowly. The cask will breathe less, but this will also result in a lower angel’s share. And make a refill cask much more suitable for long maturations from 18 to over 40 years old. The oak flavours will be more subtle, and the distillery character will play a greater part in the overall flavour.

REFILL FLAVOURS
Smokey flavours, caused by phenolic compounds from the peated barley, will be more distinctive and take longer to mellow. As a cask breathes, spirit is drawn in and out of the oak, with the layer of charcoal from the charring process acting as a natural filter to fatty acids and phenols. These fatty acids are often the result of impure compounds from distillation through shorter, squatter stills and less copper contact in the condensers. This is what makes some whiskies such as Mortlach and Springbank so distinctively rich, fruity and meaty.

The compound B-damascenone gives floral notes, with diacetyl giving a buttery and creamy taste. A refill cask will naturally showcase these flavours better than a 1st fill cask. Other flavour compounds from the distillation process are esters such as ethyl hexanoate, which gives those fresh apple flavours. Another ester, isoamyl acetate, gives classic banana and pear-drop notes. These flavours are still present in 1st fill casks, but are often so subtle that the big toffee, caramel and vanilla flavours overpower them.

If you were to line up several 15 year old first-fill cask samples, you’d find a lot of similar flavours. The whisky would be rich, golden and sweet, but you might find it difficult to guess which distillery they are from. Were you to do the same with refill cask samples, the distillery character would be much more obvious.

A 1st fill cask might give 80% of its flavour from the wood and 20% from the distillation process. On the other hand, the refill cask will give closer to 60% of its flavour from the oak and 40% from the distillery.

SHERRY BUTTS
The same rings true with sherry butts. It must be noted that the large size of the butt results in a smaller surface area to volume ratio. They naturally mature slower and have a correspondingly lower angel’s share than the smaller barrels and hogsheads.

Spanish sherry butts are traditionally made of European Oak, which is less porous than American oak. There is less sugar and vanillin, but higher concentrations of spicy compounds. The previous fill of rich and nutty amontillado and oloroso sherry soaks into, and flavours the oak which often leaves several litres of liquid in the bottom of a butt (to ‘keep it fresh’) when it’s filled with whisky. 1st fill sherry butts result in an extremely rich, nutty and often dark-coloured whisky with a lot of flavour imparted from the sherry. Due to the colour, 1st fill sherry butts are among the most desirable casks available and fetch the highest prices. They’re also much harder to source, and an empty butt can cost in the region of £1000.

A refill sherry cask will mature more slowly, and the sherry influence and the colour will be less intense. However, this is a serious advantage if the cask is intended for long ageing, and the distillery character and more subtle notes from the sherry and European oak will come to the fore.

IN CONCLUSION
The difference between 1st fill and refill is one between youth and maturity, vigour and complexity, sweet and savoury, oak and copper. There is no right or wrong, and most people enjoy both! The only way to decide your preference is to try as many different examples of single-cask bottlings (blind if possible!) and make up your own mind!