The organic acids and bacteria that sour or acidify the mash have become synonymous with Tennessee and Kentucky whisky, where distillers use a portion of spent distiller’s wash to lower the pH for fermentation. However, the fog of time makes it difficult to identify the origin of this now well-known practice.
Anecdotal evidence indicates German aqua vitae distillers of the 13th century applied the principle of rye sourdough inoculation, and Dutch distillers adapted this method to manufacture their grain spirits in the 1500s. At their Brazilian sugar colony at Permbucco, the Dutch modified it for rum production, later taking it to Barbados and ‘sugar islands’.
Called dunder in Jamaica, the spent distiller’s wash obstructs bacterial infections, stabilises flavour, and adds nutrients and microflora. West Indies sugarcane plantation landholders owned property in Colonial America, providing a conduit from Caribbean dunder methods to enter the American rum distilling industry. After the Revolutionary War, sour mashing transferred to whisky production from Virginia to the Carolinas before settlers trekked through the Appalachian Mountains, disseminating its use throughout the Ohio Valley.
The sour mash method has a variety of names: yeast-backing, stillage, acidification, spent beer, pot/burnt ale, lees, dunder, bottoms, and spent distiller’s wash. Some terms describe the distilled residuum as slops and, when added to the new mash, it became backset. In Scotland, Highland distillers used this method when unable to procure fresh yeast and to enhance the distillate flavour.
In the 1770s, testimony by a Dr Jeffrey said, ‘Whiskey distilled from sour wash is mild to the taste and highly fragrant.’ Called ‘bottoms’ in the Highlands, ‘distillers are allowed to mix or distil them with the wash.’ At a distillery near Oban, an excise officer called Delamie explained, ‘I then examined his wash, and it was sour, very sour indeed; yet this man-made excellent whiskey.’ Late 18th-century government enquiries reported: ‘Pleasantness of Highland whiskey arises from it being distilled with acid wash,’ and ‘[the] presence of acid contributes to improve the flavour of the spirits.’ And elsewhere: ‘Highland distillers, at least those of the old school... studiously ferment so that will contain much acid.’
The British practice of sour wort became widespread, requiring the government in March 1809 to legislate the Act for Charging with Duty Spirits Wash Pre-distilled in Great Britain, stating: ‘Whereas certain distilleries in Great Britain are in practice of refermenting and redistilling with commonly called spent wash after the same has been through the still’. In July 1820, another act also mentioned the practice: ‘If any distiller shall referment or redistil wash commonly called spent wash, after it has been through the still, or shall distil any fermented liquor whatsoever.’
As sour mashing using spent distiller’s wash fell out of favour in Scotland, Robert More of the Underwood Distillery in Stirlingshire applied for an unenforceable patent in July 1827. He described a ‘process rendering distillery refuse productive’ using one-third spent wash to worts before being distilled just before the yeast is added for fermentation. He claimed Dutch distillers long-practised the method. More was also part-owner of the Schiedam Distillery in London, employing Dutch distillers.
British-trained distillers in Holland, such as Robert Haig back in the 1640s, likely brought this method to Scotland 200 years earlier to make Scottish gin. In 1743, Henricus Van Wyngaerden, a Dutch distiller in Edinburgh, was recruited by the Society of Improvers of Agriculture in Scotland to counsel estate distillers. He recommended they ‘put the barm [yeast] & lees into the still along with the liquor, but if the lees be thick, put likewise in water, or rather feints to make thinner. Wort should be allowed to approach the same Degree of Acidity that vinegar hath.’
Not surprisingly, Scottish emigrant James Crow, in 1840s Kentucky, prescribed the procedures and formulas that advanced bourbon’s sour mash method.