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UNDER THE MICROSCOPE Examining the crucial role yeast play in creating distilled spirits. BY NEVA PARKER
In thinking about influences on flavor and aroma compounds in distilled spirits, most people look at ingredients such as grist or botanicals, barrels and aging, or even flavoring compounds. Very few would consider the art of fermentation as an important part of creating spirits. However, as many professional and hobby brewers know, fermentation plays a critical role in producing unique flavor and aroma compounds in beer and is also true of distilled spirits. Aside from ethanol production, yeast strains have been selected over hundreds of years because of a variety of characteristics they display during fermentation. These secondary metabolites produced by yeast during fermentation are crucial to the sensory profile of any fermented beverage, many of which carry over into the distillate. Since yeast is a living, active component of fermentation, many shy away from learning much about it. Yeast can be complicated and fickle, and can sometimes cause a great deal of head scratching when they don’t perform as expected. Nonetheless, being well-versed in yeast education is one of the most important weapons in a distiller’s arsenal. Yeast are a fungus, and the species used in beer fermentation are the Saccharomyces cerevisiae, one of over 500 species of yeast, which can then be subdivided into countless strains. They are single-celled organisms whose main purpose is to produce ethanol and carbon dioxide from glucose molecules derived from complex carbohydrates in grains and other starches. As secondary byproducts to this metabolic process, the yeast also produce flavor- and aroma-active compounds. The types and quantities of these compounds produced vary by strain. Some strains produce high levels of ester compounds, resulting in such aromas as pear, apple and banana, while other strains produce high
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levels of phenolics which can be attributed to spicy characteristics. In fact, the majority of the aroma characteristics found in fermented beverages are yeast-derived compounds. Today, a diverse collection of strains are available to distillers that vary in fermentation performance and contribution of flavor characteristics. During the formation of ethanol and carbon dioxide in fermentation, the yeast cell also produces the byproducts that make a major contribution to flavor and aroma. The primary aroma-active compounds of interest are npropanol (alcoholic, harsh), iso-amyl alcohol (alcoholic, vinous), ethyl acetate (solvent, nail polish remover), iso-amyl acetate (fruity, banana), acetaldehyde (grassy, green apple), diacetyl (buttery, butterscotch), and sulfur compounds. The level of compound produced during fermentation is strain-dependent, and many are also controlled by external factors. Aside from simple ethanol, yeast will also produce fusel alcohols as a fermentation byproduct. The biosynthesis of amino acids from nitrogen (free amino nitrogen) from barley malt is one of the largest contributors to the formation of these flavor-active compounds. These alcohols contribute to the harsh alcoholic, vinous character and can be typical in many washes with high sugar concentrations. In large quantities, these compounds can be “hot” or solventy, and will be carried over into the distillate. The reaction of these fusel alcohols with other metabolic intermediates (acids) result in the production of iso-amyl acetate, ethyl acetate and other trace ester compounds. Ethyl acetate is the most common, producing a solvent or nail polish remover quality in large amounts. Isoamyl acetate, when present and detectable, is the larger flavor contributor, giving beer and resulting spirit a pronounced fruitiness even in small quantities.
All spirits can benefit from the use of a carefully selected yeast strain. Other common byproducts are the carbonyls—diacetyl and acetaldehyde. Diacetyl is always produced as an intermediate in amino acid synthesis. Diacetyl can contribute a buttery or butterscotch character, which can add complexity to malt-forward spirits such as bourbons or molasses-based spirits such as rum. Acetaldehyde is commonly produced by most strains as well, but typically in lower quantities. It is a very volatile substance and can be captured in the distillate if some of the heads are collected, resulting in an apple or nutty character. Some less detectable flavor compounds are phenolics—vinyl phenols, guaiacol and eugenol. These typically display spicy, peppery characteristics, which can benefit whiskey or bourbon. There is a consideration to be made regarding the use of active dry yeast or live liquid yeast. While both types of cultures perform similarly in terms of fermentation speed, however, there is an unmistakable difference in the congeners produced. The use of live yeast in fermentation commonly produces more notable metabolic byproducts, resulting in more complex characteristics in the final distillate. This is especially favorable in malt or corn-based spirits, such as bourbon and whiskey, as well as more aromatic spirits like rum and brandy. All spirits can benefit from the use of a carefully selected yeast strain. In brewing, yeast is known to be one of the biggest play-
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