RANDY MOSHER, THE BREWER'S COMPANION, ALEPHENALIA PRESS, 1994. ...

17. Randy Mosher, The Brewer's Companion, Alephenalia Press, 1994. E. Yeast and Fermentation by Chuck Hanning and Scott Bickham Introduction Most beer styles are made using one of two unicellular species of microorganisms of the Saccharomyces genus, more commonly called yeast. Generally, either an ale yeast strain (known as S. cerevisiae) or a lager yeast strain (known as S. pastorianus or by older terminology S. carlsbergensis or S. uvarum) is used for the appropriate style. Functionally these yeasts differ in their optimum fermentation temperatures, ability to ferment different sugars, environmental conditions, and ability to settle out upon completion of fermentation, and production and/or metabolism of fermentation by-products. The choice of the strain of ale or lager yeast and how these factors are controlled during the various stages of fermentation will determine how well a beer is made to style. While a list of all the possible strains is beyond the scope of this guide, readers are encouraged to review reference (1) for a more thorough review. Most yeast suppliers provide detailed descriptions of the various commercial strains they carry (2), as do homebrew shops or other retailers of their products. One of the common terms used to describe yeast is apparent attenuation. The attenuation of a particular yeast strain describes its ability to decrease the original gravity of wort upon fermentation. It is commonly listed as a percent, in which the numerator is the difference between final and original gravity and denominator is the original gravity. Because the density of ethanol is less than water, when a hydrometer is used to measure this attenuation, it will be measuring the apparent attenuation not the real attenuation (if the alcohol was replaced by water). Another common term used to describe different yeasts is flocculation, which is the ability of the yeast to settle out of the beer upon completion of fermentation; it can vary significantly with strain. The environmental conditions that differ with each yeast type and strain are alcohol tolerance, oxygen requirements, and sensitivity to wort composition. Alcohol tolerance describes how well a yeast strain will continue to ferment as the alcohol concentration increases during fermentation. Most lager yeast can ferment up to about 8% alcohol by volume, and some ale strains can ferment up to 12% (2, 3). Oxygen requirements can differ with each strain as well; some need much more oxygen to be able to ferment without problems. Lastly, different worts will have different relative amounts of sugars present. The various strains can respond differently to the same wort upon fermentation. The by-products that are produced (and also metabolized) by the yeast are esters, fusel alcohols, diacetyl, and sulfur compounds. Esters are produced by yeast combining an organic alcohol and an acid. While approximately 90 different esters have been identified in beer, ethyl-acetate, isoamyl-acetate and ethyl-hexanoate are most commonly above their flavor thresholds. These impart a fruity, sweet aroma to the beer. Another by-product of fermentation is fusel alcohols, which contain more carbon atoms than the most common alcohol, ethanol. These are produced by the metabolism of amino acids (4), and tend to add harsher, more solvent-like tones the beer. Yet another by-product is diacetyl, which is generally reduced to more benign compounds during the secondary fermentation, but premature removal of the yeast (among other things) can lead to elevated levels. Its presence imparts a buttery note to the beer. It is produced by an oxidation reaction, which can be repressed by the production of the amino acid valine (5). Lastly, there are several sulfur compounds that can be produced by the yeast. One of these is hydrogen sulfide, which smells like rotten eggs. Other sulfur compounds exist, but their production is Ale Yeast, for the purposes of beer fermentation, tend to work best in the 55-75 °F (13-24 °C) temperature range. Apparent attenuation can range from 69 to 80%. These yeasts can fully ferment the common sugars glucose, fructose, maltose, sucrose, maltotriose and the trace sugars xylulose, mannose, and galactose. They can partially ferment raffinose. These yeasts have traditionally been called top fermenting because they form colonies (groups of yeast that cling together) that are supported by the surface tension of the beer. Ale yeasts produce esters since they require higher temperatures to remain active. Styles that use these yeasts have varying degrees of fruity and sweet smelling aromas. It should be noted that the yeast used to produce the German weizen style are special strains that generate high concentrations of the clove-like phenols and “bubblegum” and “banana” esters, which are the signatures of this style. Lager Yeast generally tend to work best between 46-56 °F (8-13 °C), but California Common Lager yeast is an exception, having a range of 58-68 °F (14-20 °C). Apparent attenuation usually ranges from