What is an STR cask?
Shaved, Toasted, Re-Charred.
A cask style developed & optimised for spirits maturation by the late Dr. Jim Swan, the STR cask was born out of the need for more wood for single malt whisky production, and an abundance of wine casks across Europe and America. Initially used for red wine maturation, in our case a Spanish Rioja, and coopered from American oak. Following a first use, our STRs are taken in for refurb by J. Dias cooperage (Porto, Portugal) a fourth generation family business now run by Sandra Dias. We’ve taken some time to visit the Dias team and their process is unchanged in years with coopering skills passed down through the generations.
S – step 1
3-5mm is Shaved from the inside oak surface of the cask to remove some of the red wine residue that could otherwise impart unwanted tannic and sulphurous notes (and potentially home to unknown bacteria), but not so far that it removes all of the wine touched wood that can impart desirable fruity notes.
T – step 2
The casks are then Toasted to break down the hemicellulose and lignins in the wood structure into wood sugars and other desirable flavour compounds that are alcohol soluble.
R – step 3
Re-Charring with intense flame that caramelises the wood sugars, allows deeper penetration of the staves by the spirit, increases the surface area of the inside of the cask by producing shallow cracks, and acts as an in-built ‘carbon filter’ system that can clean-up unwanted off-notes in newly distilled spirits.
When selecting our STR casks for our whisky, there’s always a conversation with J Dias about a number of variables. Firstly the oak type, either American oak (Quercus Alba) or French oak, of the European variety (Quercus Robur).
American oak has a tighter wood grain suitable for long term maturation, and imparts notes of vanilla, caramel, citrus, and coconut. French oak, with its coarser grain, allows the spirit faster access to the flavours within, but also more oxidation to its surroundings, and imparts rich spicy, and sometimes savoury notes.
Toasting temperature is also a factor, as this gives access to different ranges of flavours available in the oak: anything from woody, to sweet, vanilla, toasty, and nutty.
Finally, there’s char level, graded from 1 to 4 which relates to the amount of time exposed to intense heat: typically ranging from 15 to 60 seconds.
The STRs we choose
We select two main types of STR cask split between (i) 100% American oak (staves and heads) and (ii) American oak staves with French oak heads. Our STR casks are generally medium toasted, giving access to the broadest spectrum of flavours, with a level 2 char (although we have a range of char levels in our filled cask inventory).
ROS STR casks
There is also the opportunity to specify a further treatment of ‘extra smoke’. Signified on the cask by the letters ROS (Re-charred Over Smoke), through this extra smoke we add subtle smokey characteristics to our single malt spirit. The coopers at Dias achieve this at the moment they suffocate the flames when charring by not allowing the smoke to escape from the cask, forcing it to be absorbed into the wood. We fill a small proportion of these ROS STRs, giving us the option to incorporate an additional flavour dimension into our releases or to showcase it as part of a limited special release.