November Outturn 2024 Feature Article
Words: Chris Middleton
Casks have legal lives and working lives. The minimum legal life for whisky in-cask in Scotland, Ireland and Canada is three years. Straight American and Australian whisky is two years. Casks have much longer working lives and can be productive for over a hundred years.
In the United States, straight bourbon and rye whisky are legally required to be stored in ‘charred new oak containers’ to better harmonise with the assertive sweet and spicy flavour compounds and congeners distilled from their corn and rye mash bills. However, with lighter malt and grain distillates, new oak flavour astringency tends to quickly overpower the whisky’s character. Every cask and class of whisky behaves a little differently — variances in distillates, entry proofs, capacities, maturation environments, and differences in the oak staves. Every cask has its own life and discreet flavour signature.
The life of the cask involves two processes: seasoning and treatment. Seasoning prepares the wood staves before filling the cask with spirit, while treatment prolongs the productive use of the container after emptying the whisky cask.
Seasoning is the first stage of reducing or degrading undesirable flavour compounds by drying newly cut oak timbers, either by traditional air drying or mechanical intervention in kilns. American cooperages pioneered industrial kilns in the 1950s and became the accepted practice for American whisky by the 1970s, economising manufacture by accelerating seasoning.
Open-air drying is preferred but is a longer process and more expensive as the sunlight, wind, and rain dry and leach the timber while allowing microflora to invade the wood. Staves exposed to nature for five years are deemed superior to those only seasoned for three or eighteen months. Even different techniques of stave stacking (close, open, edge or solid piles) retards air circulation for drying, reduced rainfall penetration affects leaching, and slows fungi infestations, affecting the flavour structure in the wood. Microbiologists discovered various fungi species arriving in two population waves. Three colonising species first appear during the first two years, and another six fungal communities invade after two years, each improving the flavonoids within the stave oak, as their roots secrete hydrogen peroxide, hydrolysing the wood sugars and reducing tannins, explaining why the best staves are air-dried naturally for five years.
Another invasive seasoning intervention for casks is exposure to liquid, fire, or both. Since the late 18th century, British cooperages and distilleries soaked new casks in water or brine solutions called pickling. Discovering breweries used pressurised steam to clean stinky and tannic kegs, distilleries began applying steam to season virgin casks from twenty minutes to eighteen hours. A range of diluted chemicals was also used to steam and steep casks in solutions of caustic soda (alkali metal oxide), lye (metal hydroxide), ammonia (binary hydride), quicklime (calcium oxide), and sulphuric acid to potash (chloride). These solutions leached, masked and diminished objectionable odours and flavours. In America, where new and often green oak staves were prevalent, they quickly learned to set the cask ablaze, burning off green sap and caramelising the wood sugars, purifying and ‘sweeten the barrel’. European coopers used scorching methods for centuries; discovering gentle toasting or sometimes light charring the internal wood membrane complemented the subtle character of the distillate and filtered offensive fusel oils in rye-mashed spirits.
The other process to prolong the life of a cask is treatment. Due to whisky’s soluble properties of water and alcohol, time and temperature, and multiple refills, the cask oak flavour compounds will eventually exhaust. A technique borrowed from the wine industry in the 19th century to rejuvenate casks was to shave or scrape back the spent membrane surface to expose fresher stave wood, then re-toast or re-char. These techniques are back in vogue, notably the STR method (Shave, Toasted Recharred), as the cost and supply of casks force distilleries to cost-effectively recondition casks to limit the rising cost of sourcing new wood and by extending a cask’s working life.
Another treatment adapted from the sherry bodegas is wine suffusion, which occurred when the volume of imported sherry casks declined in the second half of the 19th century, and whisky production increased. Suffusion proved a permeable feature of Scotch cask maturation for over one hundred years.
Coda: Distilleries sourcing ex-bourbon barrels now face a loss in flavour due to a new cask treatment. Twenty years ago, the largest two distilleries in Kentucky and Tennessee began barrel rinsing or swishing. A similar practice in Australia was once called ‘bunging.’ A quick and effective way to liberate whisky soaked in the cask staves, especially when the bulk of Scotch was locally bottled throughout much of the 20th century, where workers emptied Scotch casks, added some boiling water and rolled the casks around to draw out the whisky absorbed in the staves. The American patented rinsing process releases 11 litres of proof whisky from each barrel over three weeks of rinsing. The rinsed whisky is generously oaken-flavoured and is used primarily for food service goods (whisky BBQ sauces) or blended with dumped whisky for RTDs, pre-packaged cocktails and whisky expressions known as Devil’s Cut. The Scotch distillers noticed a diminution of flavour and colour in the 1980s from kiln-seasoned ex-bourbon barrels, now rinsed ex-bourbon barrels will add to the industry’s changing maturation palette.
Next in the Mighty Oaken Cask series is ‘paxarette’ sherry casks, the leading suffusion treatment, and its sensory implications on once-called ‘genuine Scotch.’
This is the 6th instalment in the Mighty Oaken series by Chris Middleton. The series can be read online at smws.com.au/news.
This article is featured in November 2024 Outturn — bottles will be available to purchase on Friday the 15th of November at midday AEST exclusively to members of The Scotch Malt Whisky Society. Not a member? Click here to learn more about the world’s most colourful whisky club.
