🥃 The Chemistry of Flavor in Whiskey - Consolidated Guide

Whiskey is not just a great drink, it's a solution... perhaps to not all of your problems, but chemically. In chemistry, a solution is a substance with another substance homogenously (somewhat evenly) distributed throughout it. Within a glass of your favorite brown fluid, there are a wide variety of flavoring molecules, each with its own multifaceted impact on flavor and satisfaction. Below we will explore the major families involved in the whiskey flavor trade both from a chemical and experiential perspective.
Major Flavoring Compounds in Whiskey

Flavors in whiskey can come from the different grains in the mash, type of yeast used, additives, and the wood in which it is aged. While the exact percentage is tough to pinpoint, sources cite that between 50% and 75% of a whiskey's palate flavor comes from the wood barrel and aging process. At the same time, the NIH and others say that aromas primarily come from the grain carbohydrates subjected to fermentation and then concentrated during distillation. This may explain why the nose and palate often differ in terms of nuanced flavors. It is worth mentioning that the perception of a given substance is not a result of concentration alone. Different substances may have different minimum odor perception thresholds depending on how well they bind to sensory organs in the body. The same substance may also be perceived differently as its concentration increases or decreases. To truly understand everything in a given bottle of whiskey, you would have to contract with a laboratory with special equipment and qualified staff, and producers both big and small are starting to do this with increasing frequency. For those of us without a corporate bankroll, we'll continue to rely on our tongues and can make rough attributions back to the flavoring molecules present based on taste characteristics. 

Before we start, some important vocabulary:

• Congener - substances other than the desired type of alcohol/ethanol produced during fermentation, including small amounts of chemicals such as methanol and other alcohols (fusel alcohols), acetone, acetaldehyde, esters, tannins, and aldehydes (e.g. furfural). They are responsible for most of the taste and aroma of distilled spirits. For an exhaustive academic analysis of congeners, check out this paper from MDPI.
• Foreshots - initial volatile compounds such as methanol. These are typically discarded as part of the distillation process. 
• Heart - middle and desired portion of the distillate
• Feints - final portion of the distillate, typically with lower concentrations of flavoring compounds which may result in oilier or more pungent flavors. 

In a given bottle of whiskey, there are likely hundreds if not thousands of different flavoring compounds. The interaction of these together is more than the sum of their individual parts. As you look over the most common chemicals below, know that this is just scratching the surface. With so much going on chemically, it is a marvel that brands are able to blend and batch barrels to achieve a relatively consistent product. 

Common Flavoring Compounds

These are some of the more famous whiskey flavoring compounds though groups such as Phenols and Esters include hundreds of different compounds, each with their own impact on flavor. 

Lactones 

Found in oak barrels and lovingly extracted by changes in temperature, Lactones are known for giving whiskey a coconut flavor. 

• cis-3-Methyl-4-octanolide - known as the "whiskey lactone", it was discovered in 1970 and produces the sensation of coconut, celery, or fresh wood depending on concentration. Fun fact, it also repels flies and mosquitos! It has a stronger and more dominant flavor than the trans isomer.
• trans-3-methyl-4-octanolide - more of a woody coconut as opposed to the cis spicy coconut

Phenolic Compounds

Phenols add smoke or bitterness. In scotch, phenols often come from the use of peat to dry malted barley. Phenolic compounds can also arise from the toasting or charring of barrels. While smoke or medicinal character is common, Eugenol and Guaiacol often layer with other flavoring chemicals to produce certain herbal notes.

• m-Cresol - smoky, woody, musty/dusty, petroleum-like
• o-Cresol - adds Cedar at high concentrations, smoky, acrid, pungent, petroleum-like, 
• p-Cresol - similar to m-Cresol, smoky, woody, musty/dusty, petroleum-like
• Xylenol  - smoke with seaweed notes, common in higher roasted coffee, potentially mushroom
• Eugenol - found in cloves, earthy, alternative flavors: cinnamon,  nutmeg, bay leaf, and basil
• Guaiacol - smokey chemical common in smoked meats and coffee
• Quercetin - Apples, Onion, broccoli, an antioxidant and anti-inflammatory 

Aldehydes

Produced primarily during the fermentation process, Aldehydes can inform a pungent or sharp nose. 

• Syringaldehyde - occurs naturally in Spruce and Mable trees as well as in Oak Barrels, creating spicy or smoldering wood aromas. Technically Syringaldehyde is a phenolic aldehyde. 
• Vanillin - is created through pyrolysis of the lignin in wood and has a distinctive vanilla flavor
• Hexanal - results in grassy or hay type flavors, off notes may resemble green pea
• Acetaldehyde - Green apple-like aroma.
• Benzaldehyde - Almond-like aroma.

Esters

A truly staggering number of different esters are produced during the fermentation process as a result of the combination of alcohols and fatty acids. Lighter esters may have fruity flavors and largely get removed in the foreshots of the distillation process. Some esters are removed during chill filtration which does prevent cloudiness in lower ABV whiskies. Many say that chill filtration produces a more muted flavor profile and chemically I feel like that must be the case, though I am still looking for experimental proof. 

Used with Permission - jameskennedymonash.wordpress.com 

You can also find a fairly comprehensive list of esters and their odorants here.

Some quick Examples:

• Ethyl acetate: Fruity, apple-like aroma.
• Ethyl lactate: Buttery, milky aroma.
• Ethyl butyrate: Pineapple-like aroma.

Other Compounds

Outside of these major families, you may see references to other specific compounds or flavoring components, though their impact will be less typically than the others discussed above.

Furfural

Formed by the charring of  hemicellulose in oak barrels, Furfural imparts a warm, nutty, and complex taste. The level of furfural can vary depending on the whiskey's specific aging conditions and degree of barrel toasting, in the best cases yielding a "Baked goods" tasting note. 

• 5-Methyl Furfural - Toasty, Sweet, Caramel
• 5-Hydroxymethyl Furfural - Creamy Fudge

Cyclotene

A cyclic ketone, Cyclotene has a sweet flavor that may present as that of strong caramel, maple, or floral taste depending on the presence of other chemicals. 

Minerals

Mostly sourced from the water used in the mashing process, minerals typically do not make it into the final distillate unless the still is operating at abnormally high temperatures, but do have an impact on the fermentation process which certainly will impact the flavor outcomes.

• Calcium - contributes to a fuller body as it helps increase the efficacy of the yeast, strengthening their cell walls and increasing both nutrient uptake and enzymatic activity. This allows for the yeast to reproduce faster, crowding out undesirable microbes and also last longer before they expire due to the concentration of alcohol. 
• Magnesium - may yield additional sweetness by increasing the overall activity level of the yeast
• Iron - May inhibit yeast activity leading to off flavors or lower alcohol yields
• Sulfates - add a slightly astringent or dry quality
• Bicarbonates - increase the pH of the mash, increasing the extraction of flavor from grains

Tannins

Tannins, and their active hydrolysable compatriots, ellagitannins, are key non-volatile compounds in the maturation process. These are a class of polyphenolic biomolecules that affect the whiskey by adding bitterness, astringency, and some herbaceous notes while also affecting color Both ellagitannins and lignans help to remove unwanted sulfur notes. They are a group of organic compounds found in many plants, including grapes, tea leaves, and oak wood and are responsible for the astringent or "mouth-drying" sensation common in wine. In whiskey, tannins may contribute to a darker amber color and more substantial mouthfeel while balancing sweetness with bittering notes, helping to create complexity. American white oak is much higher in tannins than European Oak, which is yet another reason for its use in most whiskey. 

Conclusion

Many similar articles are chock full of molecular diagrams, but it is not truly necessary to develop an understanding of chemical structures in order to speak confidently about what may be influencing flavor. For my part, I focus on the tasting notes and relationships of the various compounds most commonly in whiskey. Flavor is a multi-sensory experience, a computer model would be very challenged to predict precisely how a given dram would taste. While I am excited to see more chemical analysis of whiskey over time, the human mouth remains the best determinant of quality for now.