yeast , the single-celled fungus that causes fermentation, has been the constant companion of humans since before we even knew that it existed. It’s no wonder that early words describing fermentation are often related to the phrase “godlike.” In the 1718 guide to spirits production “The Practical Distiller,” fermentation was described as follows:
“The exaltation of parts is most conspicuously discerned in those things destined to human use, in as much as by fermentation they obtain a greater vigor and perfection… . Amongst the aliments applicable to human use, bread, beer, wine, cyder etc. deserve the first rank: for whatsoever they have of virtue or power, may be most justly ascribed to fermentation.”
All virtue to fermentation—and hence to yeast.
Yeasts are truly exceptional creatures. They are approximately ten times larger than bacteria, but still many times smaller than a single human hair. There are hundreds of different species of yeast and countless strains within each species, each specialized to thrive in a particular microscopic environment. Yeasts are ubiquitous in the natural world. They are present on your skin, your plants, your pets, and even in the air, resting on dust motes or bits of pollen.
Some spirits incorporate wild yeasts in their production processes, especially in the tropics, where in particular Schizosaccharomyces pombe, or pombe yeast, often plays a part in fermentation for rums and arracks. See baijiu; fermentation; and Kweichow Moutai. Brewers and distillers, however, most commonly rely on one particular species of cultivated yeast, Saccharomyces cerevisiae, which has evolved to thrive in the acidic, oxygen-limited environment of beverage fermentations. In order for this yeast to be readily available to commercial brewers and distillers, the strain must be carefully separated out and precisely reared to maintain a healthy, consistent population free of contamination—no simple task when dealing with a creature characterized by a rapid life cycle, fluid genetic exchanges, and high sensitivity to environmental changes and possessing not one but two possible methods of reproduction.
Why go to all of this trouble? Brewers and distillers need yeast for two very important tasks in beverage production. The first is fermentation, the process by which yeast metabolize simple sugars such as glucose, fructose, sucrose, and maltose and in turn excrete carbon dioxide and the hallmark of social beverages—ethanol. Yeast will take, for instance, one fructose molecule (C6H12O6) and break it down into two ethanol molecules (C2H5OH) and two carbon dioxide molecules (CO2). Other, more complex sugars go through intermediate stages but end up in the same place.
The other task assigned to these industrious creatures by hopeful brewers and distillers is to improve the flavor of their fermented beverage through the production of a coterie of desirable compounds, such as esters (fruity, floral, spicy), and the destruction of undesirable compounds, such as diacetyl (old buttered popcorn) and acetaldehyde (sour, green apple, solvent). See esters.
The role of fermentation in creating the character and flavor of a spirit is often underappreciated. It is more complex than distinctions in ingredients (e.g., different mash bills) and less romantic than flowing water springs and warehouses full of maturing casks. To appreciate the impact of fermentation, consider the difference between ale and lager beer. Both generally begin with the same raw ingredient—malted barley—but are fermented with different yeasts under different conditions to produce vastly different flavor profiles in the finished beer.
The same principle holds true for distilled spirits, and in many ways the effect is amplified through the selection and concentration of flavors that occur during the distillation process. For unaged spirits, the fermentation phase is the producer’s last opportunity to create new flavors. The distillation phase can only select and separate compounds that already exist; it cannot generate new ones. The amount of flavor that can be generated reaches its apex in strong-aroma- and sauce-aroma-style baijius, where repeated fermentations of the same, bland grains produce a uniquely broad palette of strongly asserted aromas and flavors, from the crispest, fruitiest green apple to the most concentrated wild-mushroom broth, all in the same spirit.
The factors that influence the outcomes of fermentation are numerous. The selection of the yeast strain; the quality and availability of nutrients in the mash; environmental conditions such as temperature, pH, oxygen level, and osmotic pressure; the initial health and size of the yeast population; the presence of competitors; alcohol levels—all will impact the yeast’s activities and therefore impact the final character of the fermented product. In order to produce a consistent product, the distiller must work diligently to control each of these factors throughout the fermentation process. Natural changes in the raw materials (quality of growing season, storage conditions, etc.) and in the distillery environment (seasonal changes, cleanliness, etc.) introduce further potential for variability in the fermentation.
A “dona can” aka “dona tub,” in which yeast is propagated before being added to an American whisky mash.
Courtesy of Michter’s.
Examples of Fermentation Products
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In facing the challenge of controlling fermentation, distillers have both advantages and disadvantages over their brewer cousins. For distillers, the beer is not the final product. Distillers have further production phases (distillation and maturation) that will present opportunities to modify the flavor and character of their final product. Many undesirable compounds that a brewer would be stuck with can be effectively removed or diminished during distillation, while many desirable compounds can be selected, concentrated, and emphasized. This is a distinct advantage for removal of undesirable compounds, but it creates also a new set of questions. Which fermentation products should be removed, and which should be kept?
The rich, subtle complexity of fermentation is so chaotic and multifarious that it can feel magical to the most seasoned distillers. Even now the nature of yeast eludes artificial reproduction and the achievement of soulless perfection. Of course, that is also what makes these beverages so seductive. This is both the challenge and art of spirits production, which requires a careful, purposeful, and thoughtful producer to produce a truly exceptional elixir.
Russell, Inge. Whisky Technology, Production and Marketing. Amsterdam: Academic Press, 2003.
Walker, Graeme M. Yeast Physiology and Biotechnology. Chichester, UK: J. Wiley & Sons, 1998.
White, Chris, and Jamil Zainasheff. Yeast: The Practical Guide to Beer Fermentation. Boulder, CO: Brewers Publications, 2010.
Nicole Austin
ethyl acetate, amyl acetate
fruity, solvent, pear
higher alcohols or fusel alcohols (propanol, butanol, amyl alcohol, glycerol, phenylethanol)
musty, oily, floral
sulfur compounds
rotten egg, vegetal
organic acids (acetic, caproic, caprylic, lactic, pyruvic, succinic, butyric)
rancid, sweat, barnyard, cheesy, sour
phenols
spicy, pepper, clove, smoke, medicinal
esters
citrus, floral, fruit, spice
A “dona can” aka “dona tub,” in which yeast is propagated before being added to an American whisky mash. Source: Courtesy of Michter’s.