Ardent Spirits: The Essential Science

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Featured Speakers: 

  • Ken Kuno, Professor of Chemistry and Biochemistry and Concurrent Professor of Physics, University of Notre Dame
  • Ron C. Runnebaum ’96, Associate Professor, Department of Viticulture & Enology, Robert Mondavi Institute of Wine and Food Science, UC Davis

The first virtual event in the series The Secret Life of Spirits introduces the world of spirits– the engineering technology, scientific factors of production, and how to properly taste spirits. This event was moderated by Ken Kuno, a Professor of chemistry and biochemistry and concurrent professor of physics at the University of Notre Dame. Professor Kuno was joined by Ron C. Runnebaum, an Assistant Professor in the Department of viticulture & enology, Robert Mondavi Institute of Wine and Food Science at UC Davis. This discussion took a science-heavy look at the world of spirits, as the speakers thoroughly explained the chemical components and processes to make spirits safe, delicious, and differentiable. The session concluded with an opportunity for the speakers to answer some questions from online viewers.

We all know the general distinctions between beer, wine, and spirits. Most can distinguish clear alcohols from  dark alcohols. However, little is known about what distinguishes a scotch from a bourbon on a technical level. Kuno and Runnebaum began their discussion explaining common elements found in all alcoholic beverages or production processes. Distilled spirits are made from various agricultural products that are fermented. Then, the ethanol and water within these liquid mixtures are vaporized, condensed, and manipulated to become specific spirits with different flavors, ABVs, and consistencies. 

The three main elements of distillation that differentiates types of alcohol are the raw materials, the distillation process, and the yeast strain used (Saccharomyces strain). The ability of saccharomyces cerevisiae, also known as brewer’s yeast, is also a major differentiating factor in distillation processes. Saccharomyces cerevisiae can live in beverages up to 20% of alcohol; however, enriching beverages beyond that threshold requires distillation to increase the concentration of ethanol and other materials to change the chemistries of the product. Brewers select the yeast strain they want to use based on their personal access to yeast, as some are more available than others. Over time, specific yeast strains have been used with specific raw materials to produce specific, marketable alcoholic products, so the yeast strains used also depend on the raw materials available in a certain region. Another key aspect of the distillation process is the amount of times whiskey is distilled (e.g., Irish whiskey is distilled 3 times). Distillation is a proudly imperfect process, but commercial fermented beverages makers also want some uniformity among batches. This imperfection can result based on skill, climate conditions, or raw materials available.

Whiskey is barley-based, but it does depend on the agricultural products that were available in the relevant regions. Bourbon whiskey and other more specific whiskeys/whiskies are typically made from corn. These different grains depend on what is available in different geographic areas and are also responsible for whether whiskey is marketed as “whiskey” (like in Ireland) or “whisky” (like in Scotland). Tequila, mezcal, and other Mexican clear spirits rely heavily on agave supplies. Grapes and grape-based raw materials are the foundations for wine and cognacs. Additionally, fruit is the primary raw material in many fruit-based brandies. Finally, vodka is more versatile, able to be made from a broad range of raw materials including potatoes, sweet potatoes, and whatever other grains are grown locally.

The different raw materials are particularly important given that Saccharomyces consumes the sugars found in the raw ingredients to convert them into alcohol. These sugars are often simple sugars that we are familiar with such as glucose and fructose. However, others are more complex. Raw materials have different types and ratios of sugars. For example, the sugars within barley or corn, used to make whiskey, require polysaccharides to break them down. Whiskey undergoes a malting process to refine the complex sugars into simple sugars. Conversely, agave stores sugars as fructose polymers, similarly to grapes; saccharomyces must navigate these different environments and starting conditions, which is why the strain of brewers’ yeast, and the strains familiarity with certain raw materials, is so critical to the distillation process.

Kuno and Runnebaum also discussed how distillation can affect the flavors within an alcoholic beverage. For example, they considered the smokiness of scotch. This smoky flavor comes from the treatment process of grain, avoiding enzymes consuming all the sugars and meaning that some polysaccharides are consumed but not all. The heat source for this process has commonly been peat, as that is what is available in Scotland. Peat creates a smoke that penetrates the grain and gives the beverage its smoky flavor. The smoky scent comes from phenolic compounds or is based on the distillation process and environment. This is a smoky effect similar to that derived from fuel in mezcal. 

The chemistry behind the combination of ethanol and water is also essential in creating alcoholic products. Many distillers use pot stills and column stills. In this process, the materials are heated so that the water and ethanol (in vapor form) can be separated. The vapor is then condensed back down into a liquid; as this happens, the alcohol by volume– proof– changes. The mixture can continue to be vaporized and condensed to further manipulate the ABV. Pot stills are fairly inefficient; however, they are good at producing characteristics necessary for whiskeys. There is also the potential of methanol being present in alcoholic beverages. Commercially, there are regulations put in place to avoid this; however, methanol is a serious risk in less regulated alcohols or environments. Some raw materials have higher methanol (e.g. fruit for fruit-based brandies), and these high methanol levels can be very physically damaging, leading to illness and death. 

Before jumping into questions, Kuno and Runnebaum concluded by talking about consumption. Due to the diverse nature of spirits, they can be enjoyed in many ways. These methods of consumption might be different based on the specific spirit and consumer. The higher ethanol content means the product can be enjoyed in even more ways compared to wine or beer. One can consume spirits as an ingredient in cocktails, in a tasting environment, or casually drinking the product straight (neat). Runnebaum encouraged the audience to appreciate the complex flavors and trace compounds present in spirits. Because the aromas are stacked, different aromas are revealed throughout the drinking experience.

Visit the event page for more.

  • Spirits are distinguishable by the base ingredients used (corn, grapes, etc.), the ethanol concentration, and specific fermentation process. (3:22)
  • Distillers must select the best yeast strains that align with the raw materials available and desired product. Over time, these strains become associated with certain specific variations of a spirit. (9:09) 
  • The interaction between yeast and sugar produces a slew of other chemicals that create a complex aroma and flavor to the final alcoholic beverage. (15:47)
  • Commercial-grade spirits are required by law to not surpass a certain methanol concentration; however, these regulations shift internationally or may be neglected by homemade distillers who do not reject the high-concentration methanol product early in the distillation process. (28:44)
  • Different markets require specific information to categorize a beverage as a particular spirit versus a miscellaneous liquid with alcoholic content. These standards of identity include the raw materials used, distillation requirements, aging requirements, bottling guidelines, and alcohol concentration. (43:32)

  • “In the distillation process, when we’re basically bringing over our product, we’re also bringing over impurities from some of the starting ingredients and also the processing here in the case of peat smoke… it’s this imperfect distillation that allows us to get this characteristic Scotch flavor aroma profile.” (Ken Kuno, 16:01)
  • “The distillers are trying to understand the process so that they can capture these trace compounds that really give the products a distinctive character, whether that be a result of the raw materials or the distillation process or even the choice the distiller is making, using almost an identical process to present different and unique products to us as consumers.” (Ron Runnebaum, 17:59)
  • “Even if your intent is to mix a product with a cocktail or so forth, is that you can just smell the product out of the bottle. And because we have different volatilities, the one thing that we can also do is smell underneath the rim– so we’ll have some aroma compounds that if we’re holding the glass at an angle some of the aroma compounds will spill out, others will go and partition a little bit higher above the liquid– so we can slowly move up and down and appreciate different flavors and trace compounds in a particular product. That might be a way of differentiating similar products just by their relative amounts of the flavors that were captured during fermentation.” (Ron Runnebaum, 38:23)
  • “[Change in temperature] adds another dimension to the concentration which again can change the volatility. One of the things that can be helpful in this is to reduce the volatility of ethanol relative to the other components, so that’s where diluting it can be quite helpful, but also going to a lower temperature can help be a tool for enjoyment as well.” (Ron Runnebaum, 41:07)
  • “There might be a reason from a chemistry standpoint that one finds [certain spirits] less desirable, whether it’s from a phenolic standpoint that might not be as appealing depending on what our sensitivity is from a sensory standpoint. We all have different capabilities from a taste and flavor and aroma standpoint that might keep us from enjoying something that one of our best friends thinks is the best product ever.” (Ron Runnebaum, 51:43)

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