The ingredient rarely mentioned is charcoal. By law, white oak barrels must have their interior staves carbonised under intense naked flames from around 15 seconds to a minute. The duration of the burn determines their char grade and depth of the charcoal. The vulcanisation of the oak creates a carbon membrane inside the barrel that plays a crucial role in developing Bourbon’s flavour and colour. So important is the charcoal treatment, the ‘charred white oak container’ was legislated under US law in July 1936.
Before US Prohibition the charcoal rectification process was ubiquitous to North America, both for American and Canadian whisky producers. After Prohibition, only one distillery in Tennessee returned to the tradition of charcoal filtration; others joined later. The world’s most popular American whiskey calls this process ‘leaching’ or ‘mellowing’. The spirit trickles through deep vats filled with sugar maple charcoal before being put into charred oak barrels to become whisky. This process was legally recognised as distinctive to Tennessee whisky by the Treasury Department in March 1941 and given geographic protection under State legislature in 2013.
In 1776, Joseph Lowitz at the St Petersburg Academy conducted a systemic study assessing the relative merits of different carbonised woods, absorbency properties and their suitability for rectification. His discoveries were immediately adopted by Russian vodka distillers and quickly capitalised upon in the western hemisphere for sugar-making, rum and American whiskey. While distillers had used charcoal for centuries, including early whiskies in Virginia and Pennsylvania since the mid-18th century, Lowitz was the first to codify its applications and attributes.
Whether small distillers, massive combines or the network of large rectifiers most whisky manufactured in America during the 19th century was charcoal filtered. It was the most efficient and cost-effective method to purify, polish and refine whisky for consumption. Today, most whisky still gets clarified through activated carbon filters before bottling to remove fine particles and to prevent the risk of flocculation by fatty proteins when exposed to cold temperatures. The porousness and absorbency in charcoal’s matrix of pores capture impurities, especially unpleasant congeners like sulphur compounds and solvent-tasting fusel oils. The intense heat also caramelises the polysaccharide wood sugars, cellulose and hemicellulose representing more than 60 per cent of the wood’s structure. It’s behind the charcoal skin that much of the complex flavours, and all the colour is rendered endowing the spirit with mellowness, flavour and colour. Add oxygenation, and the maturation process is at full effect. ‘Leaching’ vats of different wood charcoal species and the method of preparation extract subtle flavour variances. Tennessee whiskey is renowned for seeping Bourbon distillate through sugar maple charcoal to add a little sweetness and strip-out disagreeable congeners, helping hasten maturation to make the distinctive Tennessee sour mash whisky.
It’s not only American whiskey and vodka that uses charcoal. The invention of Cuban white rum by Facundo Bacardi Masso used charcoal from coconut shells to remove colour and strip-out cane congeners from poorly made light Cuban rum. The French engineer Charles Derosne incorporated Lowitz’s charcoal discoveries into his 1811 sugar purification plant.
In a twist, vodka and white rum, both reliant on charcoal filtration, were the greatest threat to American whiskey sales in the 1960s.
To participate in the growth of the white spirits category, a strange creature called American white whiskey launched in the early 1970s. The law was changed to permit legally matured Bourbon and whisky to be charcoal filtered to strip out the colour and remove much of the whisky flavour. By the late 1970s, a host of bland white whisky brands were rejected by consumers as a chimera.
It’s ironic that drinkers could accept white rum from black charcoal, but not white whisky.