TABLE 1:
FLAVORS ASSOCIATED WITH THE PRODUCTION OF DISTILLED SPIRITS
THE GOOD, THE BAD AND SOMETIMES THE UGLY FLAVOR NOTE
TYPICAL DESCRIPTORS
Acetaldehyde
Green apples, bruised apples, grassy, latex paint, Florists shop (green stems, cut grass), melon, pumpkin, ethereal.
FLAVOR THRESHOLDS* COMMENTS AND NOTES 10-25 ppm.
CH3CHO [Ethanal]
Acetic acid
[Range in spirits 8-240 ppm. Whiskies (incl. Bourbon): 16-100, Brandies: 52-240, Rum: 8-60 ppm.]
Originates from poor/stressed fermentations and from oxidation aging reactions in the wood. May also arise from bacterial contamination (incl. Acetobacter). Oxidative formation of acetaldehyde from ethanol or from reduction of acetic acid may occur during maturation. However, most aldehydes are formed during fermentation.
100-200 ppm?
Acetic acid is the major component of the total acids in matured spirits. It may be produced from ethanol via acetaldehyde during maturation. It can also be a bacterial contamination issue.
CH3COOH [Ethanoic acid, Acetate in the anion form]
Vinegar-like, pungent, sour, acidic
Acrolein
Piercing, disagreeable odor, peppery, hot/acrid, horseradish. Lachrymator (irritant causes tearing). Known by the term “pepper” by grain alcohol distillers.
Threshold in water – 0.04 ppm. [10 ppm In low proof distillates?]
Butter, butterscotch, movie popcorn. As an oily compound it may be present as an oily “slickness” on the palate.
Odor threshold cited at From fermentation and bacterial spoilage. Yeast strain 0.02-0.04 ppm (or 20-40 dependent. Produced by wild yeast and Lactobacillus and ppb) in imitation whiskey. Taste at 0.2 ppm (200 ppb). Pediococcus bacteria. Controlled by proper care of yeast (nutrient levels) and fermentation conditions. It can be Very low amounts in vodka, controlled at the end of fermentation – this complex topic low ppm. Values in Scotch is not discussed here. The precursors of diacetyl can be whiskies, 70-300 ppb in converted rapidly to diacetyl during the early stages of Brandies, 30-4400 ppb in distillation when heat is applied. With similar volatility to Rum (high in the “Buttery ethanol it is not easily removed once formed. Rums”).
CH2CCHO [Acraldehyde, Acrylic aldehyde]
2,3 Butanedione [Diacetyl] CH3COCOCH3
Butyric acid
CH3CH2CH2COOH [Butanoic acid, Butyrate as the anion form]
Acetic acid also has a key role in fatty acid metabolism and in ester formation (See Esters).
Rancid, sharp cheese, baby vomit/sickly sour, pungent/ putrid, sour spent grains.
Cresols
(ortho and para: o-cresol aka. 2-methylphenol, p-cresol aka 4-methylphenol) CH3C6H4OH
Pungent, creosote, coal tar [o-cresol – medicinal, tar] [p-cresol – medicinal, smoky]
Aroma threshold in beer (possibly also in whisky?) at 3 ppm. High levels (10-14 ppm) may be present in Rum and Brandy.
Thresholds in water: o-cresol 40 ppb, m-cresol 200 ppb. Detected in the ppb range in spirits also. Amounts said to vary from 2 ppm to 54 ppm in Highland and Islay (peated) whiskies.
A low boiling point compound – readily detectable as a pungency in spirits. Often noted in poorer quality GNS (grain neutral spirit). Some lactobacilli convert glycerol excreted by yeast into 3 or β-hydroxypropionaldehyde (peppery note) which is then converted to acrolein by the heat of distillation. Some acrolein is always produced from glycerol in pot still distillation. Acrolein has been reported to disappear over twothree years of aging to yield a less flavorful compound.
From bacterial contamination in many cases except in certain rum fermentations. Produced if appropriate strains of bacteria are present during low temperature mashing. Once formed it is not easy to remove. Notably positive in some rum fermentations (heavy rums). Butyric acid bacteria (various species) can convert carbohydrates to yield acetic acid and butyric acid. While often unpleasant as the acid it can combine with ethanol to form the ester ethyl butyrate with pleasing fruity notes. Other acids form other complex esters with butyrate. Phenolic compounds which contribute significantly to the smoky peaty flavor of certain whiskies. Peating (smoking), killing of malt (thermal degradation reactions) and maturation (aging in oak barrels) are the main sources of phenolic compounds in whiskies. See Figure 2.
* Thresholds mostly known for Scottish Whisky—assumed source unless otherwise noted. ~ ppm. Parts per million—also expressed as mg/L. ppb—parts per
billion (micrograms/L). A few dozen other compounds could have been added to this list of the core or better known flavor contributors to various types of distilled spirits. Metabolic reactions (biology) during fermentation, chemistry in the heat of distillation and surface chemistry in the case of barrel aging provide the majority of flavor active substances in distilled spirits. The Scottish Whisky Flavor wheel and research by the Scottish Whisky Research Institute into whisky flavors would be worth a search to add depth to our brief coverage here. Recently other spirit flavor wheels (Rum, Gin, and Brandy, for example) have been developed and will assist the reader in expanding their flavor terminology knowledge. A series of proceedings volumes from the Barrel Symposium series sponsored by the Independent Stave Company will add depth to the maturation issues and flavor production involved in spirit aging.
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