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Yeast

Brewer's Yeast (Saccharomyces cerevisiae) is considered to be a type of fungus. It reproduces asexually by budding- splitting off little daughter cells. Yeast are unusual in that they can live and grow both with or without oxygen. Most micro-organisms can only do one or the other. Yeast can live without oxygen by a process that we refer to as fermentation. The yeast cells take in simple sugars like glucose and maltose and produce carbon dioxide and alcohol as waste products.

Along with converting sugar to ethyl alcohol and carbon dioxide, yeast produce many other compounds, including esters, fusel alcohols, ketones, various phenolics and fatty acids. Esters are the molecular compound responsible for the fruity notes in beer, phenols cause the spicy notes, and in combination with chlorine, medicinal notes. Diacetyl is a ketone compound that can be beneficial in limited amounts. It gives a butter or butterscotch note to the flavor profile of a beer and is desired to a degree in heavier Pale Ales, Scotch Ales and Stouts. Unfortunately, Diacetyl tends to be unstable and can take on stale, raunchy tones due to oxidation as the beer ages. This is particularly true for light lagers, where the presence of diacetyl is considered to be a flaw. Fusel alcohols are heavier molecular weight alcohols and are thought to be a major contributor to hangovers. These alcohols also have low taste thresholds and are often readily apparent as "sharp" notes. Fatty acids, although they take part in the chemical reactions that produce the desired compounds, also tend to oxidize in old beers and produce off-flavors.

The following are some terms that are used to describe yeast behavior:

Attenuation This term is usually given as a percentage to describe the percent of malt sugar that is converted by the yeast strain to ethanol and CO2. Most yeast strains attenuate in the range of 65 - 80%. More specifically, this range is the "Apparent" attenuation. The apparent attenuation is determined by comparing the Original and Final gravities of the beer. A 1.040 OG that ferments to a 1.010 FG would have an apparent attenuation of 75%.

(From FG = OG - (OG x %) => % att. = (OG-FG)/OG)

The "Real" attenuation is less. Pure ethanol has a gravity of about 0.800. If you had a 1.040 OG beer and got 100% real attenuation, the resulting specific gravity would be about 0.991 (corresponding to about 5% alcohol by weight). The apparent attenuation of this beer would be 122%. The apparent attenuation of a yeast strain will vary depending on the types of sugars in the wort that the yeast is fermenting. Thus the number quoted for a particular yeast is an average. For purposes of discussion, apparent attenuation is ranked as low, medium, and high by the following percentages:
65-70% = Low
71-75% = Medium
76-80% = High

Flocculation This term describes how fast or how well a yeast clumps together and settles to the bottom of the fermenter after fermentation is complete. Different yeast strains clump differently and will settle faster or slower. Some yeasts layers practically "paint" themselves to the bottom of the fermenter while others are ready to swirl up if you so much as sneeze. Highly flocculant yeasts can sometimes settle out before the fermentation is finished, leaving higher than normal levels of diacetyl or even leftover fermentable sugars. Pitching an adequate amount of healthy yeast is the best solution to this potential problem.

Lag Time This term refers to the amount of time that passes from when the yeast is pitched to when the airlock really starts bubbling on the fermenter. A long lag time (more than 24 hours) indicates that the wort was poorly aerated, not enough yeast was pitched and/or that the yeast was initially in poor shape.