The Chemistry of Flowers and Whisky
A Valentine’s Day dramming session got us to some serious thought about the presence of floral aroma and flavour in Whisky. Most specifically Rose, but also true of Lavender, Violet and also many of the more herbal notes you will find, coriander, fennel, cardamom, clove, etc. Where or how do these aromas and flavours arise, and how similar are the chemistries of Flowers and Whisky?
The short answer is Very! In fact the exact same compounds produced by flowers to attract pollinating insects can be generated during the production of whisky. It appears that we are also incredibly sensitive to these aroma compounds and can detect them even at very low concentrations.
Aroma compounds in common flowers
Of the major components of Rose aroma, both β-Damascenone and Phenethyl Alcohol can be found in Whisky. Where they come from is debatable, both may be produced by yeast activity during fermentation but equally could come from both distillation and maturation reactions or in the latter case extraction from wood of the oak cask. β-Damascenone naturally occurs in Apple juice, apricot, black currant, grape, raspberry, strawberry, cognac, rum, whiskey and scotch. β-Damascenone belongs to a family of chemical entities known as rose ketones, with its aroma being detectable at just 0.009 parts per billion (ppb) .β-Damascenone has been proven to play a part in beer flavour also, with aged beers revealing a reduction of β-Damascenone in line with a deterioration in flavour, particularly under conditions of low pH(1).
The Chemistry of Whisky
Furthermore the Terpene, Linalool is also found in whisky and may be responsible for a vast array of aromas and flavours. Linalool is a chiral molecule and exists as two mirror images (termed Enantiomers).
Enantiomers of Linalool
The beauty of this is that each enantiomer is recognised as a separate entity by our sense of smell. (S)-(+)-Linalool is detectable (Odor Threshold) at 7.4 ppb and presents as a sweet and floral and is present in Coriander seed and Sweet Orange whereas (R)-(–)-Linalool is detectable at 0.8 ppb is more woody and Lavender-like and present in Lavender, Bay Laurel and Sweet Basil. Linalool is produced by over 200 species of plants including [Lamiaceae] mint and fragrant herb families, [Lauraceae] laurels, cinnamon and rosewood and [Rutaceae] Citrus fruits it is also present in tea and coffee, whilst it is an important part of the aroma of hops (and beer). It is found in many other fruit, including guava, peach, plum, pineapple and passionfruit. Most natural materials contain (almost) exclusively one enantiomer. Genuine lavender oil is at least 85% (R) –enantiomer; likewise the (S)-isomer of linalool predominates (94-96%) in authentic sweet orange oil.
Another monoterpene compound found to be produced by Yeast is Citranellol, again another chiral molecule. Both enantiomers occur in nature. (+)-Citronellol, which is found in citronella oils, including Cymbopogon nardus (50%), is the more common isomer. (−)-Citronellol is found in the oils of rose (18-55%) and Pelargonium geraniums.
Enantiomers of Citranellol
During whisky production it appears mixtures of both may be produced, but under certain conditions one or other may be favoured? For example during wine production higher concentrations of assimilable nitrogen increased accumulation of linalool and citronellol. Also microaerobic compared with anaerobic conditions favored terpene accumulation in the ferment. The production of these compounds was directly attributable to the fermenting yeast Saccharomyces cerevisiae (2).
The most famous example is the perhaps over-ascribed tasting note FWP (for French Whore Perfume) a heady mix of lavender and to a lesser extent violets – indicating a heavy lean towards (R)-(–)-Linalool which we are almost 10-fold more sensitive to detecting. Several distilleries have had batches of whisky heavily affected by this phenomena, most noticeably Bowmore expressions from the mid-1990’s – more discussion can be found here: http://whiskyscience.blogspot.co.uk/2011/06/lavender-in-whisky-conspiracy-of-french.html.
Though Ardbeg’s recent flirtation with ‘Space Whisky’ may have been more PR than Science, one of the main aims of this endeavour was to study how terpene alcohols were extracted from wood by whisky under differing conditions. The results may well have real world applications in changing how wood flavours the whisky as it matures. Our own investigation however has revealed that many of these terpenes may actually be produced by Yeast during the fermentation process and further studies looking at differing fermentation conditions (over and above the current understanding of long and short fermentation times) may be much more readily applicable in terms of altering whisky flavours. As such further investigation of the fungal terpene biosynthesis pathway and their metabolism of mevalonate are worthy candidates.
How Low pH Can Intensify β-Damascenone and Dimethyl Trisulfide Production through Beer AgingLaurence Gijs,Fabienne Chevance,Vesna Jerkovic, and, and Sonia Collin*Journal of Agricultural and Food Chemistry 2002 50 (20), 5612-5616
De novo synthesis of monoterpenes by Saccharomyces cerevisiae wine yeasts
Francisco M. Carrau,*, Karina Medina, Eduardo Boido, Laura Farina, Carina Gaggero, Eduardo Dellacassa, Giuseppe Versini and
Paul A. Henschke
FEMS Microbiology Letters Volume 243, Issue 1, pages 107–115, February 2005