Rejuvenation

Scotch whiskies are predominantly matured in ex-bourbon casks. The previous filling with bourbon extracts much of the flavours, but there is still a lot left for the refills, too. The water-soluble extracts seem to exhaust faster than the lignin-derivatives or the lipid-solubles, although the filling strenght does influence extraction rates. For example oaklactones and soluble oaktannins, especially ellagitannins are largely depleted during the bourbon fill and the first Scotch fill. By extending the maturation period it is possible to create whiskies with considarable oak flavours, although the proportions of flavour compounds tend to differ from the first fill maturations (see table 1 below).

 

Table1. Extracts from ex-bourbon casks used for Scotch malt whisky. From left: 1st, 2nd and 3rd/more refill (modified from Piggott&Conner 2003)

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Lignin does break down in presence of alcohols, but most of the linkages are resistant to ethanol. Therefore lignin does not degrade completely even in the presence of high filling strenghts during long maturation, but the release of monoligninderivatives tends to slow down considerably leading to an exhausted cask. The relative absence of guaiacol ja syringyl compounds is thought to be a good marker for cask exhaustion. The guaiacol concentrations are probably quite similar to those of ferulic acid concentrations shown in table 1 and the amount of syringic acid is likely to represent the amount of total syringyl compounds (for information on lignin construction see previous blog 15.2.2011).

Exhausted casks can be rejuvenated by scraping the inside of the cask, removing the used wood and bringing new wood in contact with the spirit. Untoasted wood however gives green and raw aromas and scraped casks are usually toasted or charred again. Recharring might impart too much colour and change the aromas towards bourbon-style whiskies and therefore retoasting in apparently preferred in Scottish cooperages. Regenerated recharred casks differ considerably from ex-bourbon casks used for the first fillings with Scotch. In table 2 are presented the sensory profiles of 3 year old grain whiskies matured in refill bourbon casks compared to decharred recharred exhausted ex-bourbon casks. The recharred casks seem to produce more sweet and woody notes as refill casks tend to bring out the drier woody notes. In terms of flavour compounds the recharring promotes fast lignin breakdown and caramellisation of hemicellulose thus extracting considerably more guaiacols, isoeugenol and vanillin commonly associated with new wood barrels. Surpisingly the concentrations of oaklactones are only slightly greater in recharred casks compared to refills. Perhaps oaklactones are already extracted from the deeper layers of the wood exceeding the scraping depth in rejuvenation and the recharring affects primarily wood layers that are already poor in oaklactones. Another possible explanation is the lack of seasoning of rejuvenated casks after the scraping which might lead more green wood notes and less oaklactones. A recharred cask with more guaiacol and vanilla extractives is more likely to promote smoky and sweet flavours of the spirit than a refill cask, although the sweet notes differ from those of the first fills as they are probably caused more by caramellisation products and vanilla than oaklactones. The charred layer is likely to remove some of the off-flavours, especially some sulphury aromas, but as seen in table 2 toasting also considerably reduces suphury aromas, so the filtration by active carbon layer might not be the only explanation for this.

A common practice during the 20th century was to season the rejuvenated casks as well as some of the new casks with pajarete to create a sherry-cask profile. The first intentional wine-treatments were made in 1880s and the last larger scale treatments in the whisky industry were terminated prbably during the 1980s.

A recent develepment has been a treatment of exhausted casks with salt catalyst before recharring or retoasting; this "curing" increases the extraction from rejuvenated casks (tables 2&3 below). The salt solution probably increases the heat influence in deeper layers of wood especially during medium heating (toasting). It is also likely that salt increases the solubility of extractives into spirit, although the precise mechanism is not yet described.

Tables 2 and 3. Sensory profiles of 3 year old Scotch grain whiskies matured in different ex-bourbon casks (brutal modification from Reid KJG et al 2008, sorry...)

Rejuvenated casks are not particulary appreciated by malt whisky aficionados, but they probably are of good use in maturing young grain whiskies as they remove effectively some of the common off-notes associated with new make spirits. Also lighter style malt whiskies might benefit from a subtler cask influence lacking especially the overly sweet oaky notes.

REFERENCES

Boudet AM et al. Biochemistry and molecular biology of lignification. New Phytol 1995; 129; 203-236 

Conner JM et al. Changes in wood extractives from oak cask staves through maturation of scotch malt whisky. J Sci Food Agric 1993;62;169-174

Clyne J et al. The effect of cask charring on scotch whisky maturation. Int J Food Sci Tech 1993;28;69-81

Lea GH, Piggott JR. Fermented beverage production 2nd ed. Kluwer Acad 2003.

Mosedale, JR. Effects of oak wood on the maturation of alcoholic beverages with particular reference to whisky. Forestry 1995; 68; 3; 203-230

Piggott JR, Conner M. Whiskies. In Fermented beverage production. Kluwer Acad 2003.

Piggott JR et al. Effects on scotch whisky composition and flavour of maturation in oak casks with varying histories. Int J Food Sci Tech 1993;28;303-318

Reid KJG et al. Understanding and enhancing cask performance. The Scotch Whisky Research Institute 2008.

Russell I (ed). Whisky, technology, production and marketing. Academic Press 2003.

Viriot C et al. Ellagitannins and lignins in aging spirits in oak barrels. J Agric Food Chem 1993; 41; 1872-1879