Why does whisky mature in oak and not in other wood?

For Scotch, oak is legally required: the Scotch Whisky Regulations 2009 require maturation in oak casks with a capacity of at most 700 litres.

Regulation 3 of those regulations states that the whisky is matured only in oak casks of no more than 700 litres, and for no less than three years. The industry guidance of the Scotch Whisky Association is just as strict: casks for maturing Scotch whisky must be made of oak. Other woods therefore fall outside the regulations.

No. EU Regulation 2019/787 requires whisky to mature for at least three years in wooden casks of at most 700 litres, without prescribing the type of wood.

Annex I, category 2 of that regulation speaks of maturation of the final product for at least three years in wooden casks of no more than 700 litres. The Japanese labelling standard of 2021 works the same way: wooden casks of at most 700 litres and then at least three years of maturation in Japan, without any type of wood being named. Oak is therefore mandatory where the product rules expressly say so, and elsewhere wood is the criterion.

Oak holds liquid because tyloses seal the vessels in the heartwood, which makes the wood dense enough to keep a cask full for years on end.

A review article on polyphenols in oak wood puts it this way: the combination of ellagitannins and tyloses in the heartwood of oak makes these wood species unsurpassed in cooperage. On top of that come the mechanical properties of the wood, which allow staves to be split and formed into a round cask. Wood species without that sealing do not keep a cask of strong spirits tight for years.

Oak releases vanillin, oak lactones, ellagitannins and wood sugars to the maturing whisky.

The overview article in International Journal of Molecular Sciences distinguishes three groups of polyphenols in oak: volatile phenols, phenolic acids and ellagitannins. The monomers vescalagin and castalagin are dominant in this and make up 40 to 60 percent of the ellagitannins. The amount differs per species: American oak contains considerably more vanillin than French oak. That difference between Quercus alba, Quercus robur and Quercus petraea is the reason distilleries pay such close attention to cask origin.

Casks of other wood are ruled out for Scotch: the industry guideline admits only oak, and a cask type must also have an established prior history in the sector.

The guideline of the Scotch Whisky Association uses a clear starting point for this: if a type of cask has been in general use in the industry for many years and has thus contributed to the characteristics on which the reputation of Scotch whisky rests, it may be considered acceptable. Accepted previous contents include still wine, fortified wine, beer or ale, whisky, grape brandy and rum. A new or unusual type of wood does not pass that test.

Mizunara is not an exception to the oak rule: this Japanese type of wood is itself an oak of the genus Quercus.

Maturation or finishing in mizunara therefore stays within the oak tradition, even though the wood differs in structure and release from American or European oak. The Japanese labelling standard from 2021 only requires wooden casks of at most 700 litres and three years of maturation in Japan, and names no wood species. Anyone looking for mizunara on the shelf is therefore looking for a variety of oak, not an alternative to oak.