congeners

are produced (excreted) by yeast along with ethanol during fermentation and contribute flavor to alcoholic beverages. The formation of congeners depends on the genetic makeup of the yeast culture and on fermentation conditions such as temperature, type of raw material, carbohydrate concentration, fermenter design, pH, oxygen, and growth nutrients. There are five main categories of yeast excretion byproducts that influence flavor: higher alcohols, esters, carbonyls, organic and fatty acids, and sulfur-containing molecules. All of these compounds affect the quality and uniqueness of alcoholic beverages.

Higher alcohols are also called fusel oil, an older term derived from the old German word Fusel, “rotgut” or “hooch.” High concentrations of fusel oil can contaminate beverage alcohol with harsh flavors, but smaller amounts can make a beverage distinctive. The main fusel oils include propyl alcohol, isobutyl alcohol, amyl alcohols, and phenyl ethyl alcohol; however, forty different types of alcohols made by yeast have been identified. These contribute subtle winey, fruity, banana, or floral notes to alcoholic beverages when in the parts per billion (ppb) to parts per million (ppm) range. When fermentation conditions are favorable for yeast growth, such as higher temperatures or increased oxygen or nitrogen levels, those yeast tend to excrete higher concentrations of fusel oils. Likewise, conditions that do not favor yeast growth prevent higher alcohol production.

In general esters impart fruity or flowery characters such as strawberry, apple, banana, pineapple, lilac, or pear notes to alcoholic beverages. The main esters are ethyl acetate, isoamyl acetate, isobutyl acetate, ethyl caproate, and phenylethyl acetate, but as many as 90 different esters have been identified. Even small changes in ester concentrations can influence the quality of the alcoholic beverage, as they are very potent in terms of flavor profile—thus fermentation consistency is critical.Carbonyls include various aldehydes, diacetyl, and vicinal diketones. Over two hundred have been identified. Even in small quantities, these compounds can have a negative effect on alcoholic beverages. Acetaldehyde is the most important aldehyde and is an intermediate in the production of ethanol, but at its threshold level it imparts an undesirable green-grass note to alcoholic beverages. Diacetyl, considered a defect in most beverages that gives a buttery impression, is the second notable carbonyl compound. See acetaldehyde.

Over 110 organic and fatty acids have been identified in yeast. Yeasts produce short chain acids such as pyruvic, succinic, citric, or acetic acids when they consume carbohydrates. These can give a spirit structure. They also form longer-chain acids that, as they break down with exposure to oxygen, influence the flavor of spirits maturing in barrels.

Occasionally yeast also produces sulfur-containing molecules, which contribute a significant flavor to alcoholic beverages. Even levels as low as one part per trillion (ppt) can give rise to unpleasant off odors. Some major sulfur defects are sulfur dioxide (burnt match), hydrogen sulfide (rotten egg), dimethyl sulfide (cabbage), or mercaptans (rubber). Sulfur aromas are not necessarily byproducts of yeast metabolism; they may also arise from feedstock or exposure to ultraviolet light.

Hazelwood, L. A., J. M. Daran, A. J. A. van Maris, J. T. Pronk, and J. R. Dickinson. “The Ehrlich Pathway for Fusel Alcohol Production: A Century of Research on Saccharomyces cerevisiae Metabolism.” Applied and Environmental Microbiology 74, no. 8 (2008): 2259–2266.

Mauricio, J. C., J. Moreno, L. Zea, J. Ortega, and M. Medina. “The Effects of Grape Must Fermentation Conditions on Volatile Alcohols and Esters Formed by Saccharomyces cerevisiae.” Journal of Science and Food Agriculture 75 (1997): 155–160.

Saerens, S. M., F. R. Delvaux, K. J. Verstrepen, and J. M. Thevelein. “Production and Biological Function of Volatile Esters in Saccharomyces cerevisiae.” Microbial Biotechnology 2 (2010): 165–177.

Stewart, G. G., and I. Russell. An Introduction to Brewing Science and Technology. Series III: Brewer’s Yeast. London: Institute of Brewing, 1998.

Van Meer, G., D. R. Voelker, and G. W. Feigenson. “Membrane Lipids: Where They Are and How They Behave.” National Review Molecular Cell Biology 9 (2008): 112–124.

By: Don Livermore