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.
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 area 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.