The STR cask was developed and optimised for the maturation of spirits by the late Dr Jim Swan. The barrel arose from the need for more wood for single malt whisky production and an abundance of wine barrels in Europe and America. Initially used for maturing red wine.
S - step 1
3-5 mm is scraped from the inside of the oak surface of the barrel to remove some red wine residue that might otherwise give unwanted tannic and sulphurous notes (and possibly a home for unknown bacteria), but not to the point of removing all the wood stirred by the wine that might give desirable fruity notes.
T - step 2
The barrels are then toasted to break down the hemicellulose and lignins in the wood structure into wood sugars and other desirable flavour compounds that are soluble in alcohol.
exposed to intense heat:
R - step 3
Re-curing with an intense flame that caramelises the wood sugars, causing the spirit to penetrate deeper into the staves, increasing the surface of the inside of the barrel by creating shallow cracks, and acting as a built-in ‘carbon filtering system’ that can clear up unwanted off-notes in just-distilled spirits.
First, the type of oak, 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 produces notes of vanilla, caramel, citrus and coconut. French oak, with its coarser grain, gives the spirit quicker access to the flavours inside, but also more oxidation to the environment, and provides rich spicy and sometimes savoury notes. Roasting temperature is also a factor, as it gives access to different flavours available in the oak: from woody to sweet, vanilla, toasty and nutty. Finally, there is the char level, graded from 1 to 4, which relates to the time exposed to intense heat: