Yeast is responsible for turning sweet wort into what we call "beer". Yeast consume the sugar in wort, and turn that sugar into CO2, alcohol, and flavor compounds. When yeast finish the fermentation process, they shut down, clump together, and fall to the bottom of the fermentor, or "flocculate". When yeast flocculate, it is easy to see that fermentation is done. But how can the brewer be sure? What if the flocculation is minimal, and yeast and CO2 stay in solution. How does the brewer really know when fermentation is done? The answer: by testing the degree of attenuation. Apparent attenuation percentage is the percentage of sugars that yeast consume. Attenuation varies between different strains. The fermentation conditions and gravity of a particular beer will cause the attenuation to vary, hence each strain of brewers yeast has a characteristic attenuation range. The range for brewers yeast is typically between 65-85%.
How does a brewer calculate attenuation?
First, the specific gravity must be checked with a hydrometer before the yeast is pitched. Specific gravity is a measurement of density. The specific gravity of water is 1.000, and wort has a higher density relative to water because of the sugars present in wort. As these sugars are consumed by yeast during fermentation, the density and therefore specific gravity measurement lowers. The yeast also produce alcohol, which is lighter than water, so to obtain the "actual" attenuation, alcohol must be removed by heat and replaced by water. Only large breweries go to such lengths to report the "actual" attenuation, while the attenuation most homebrewers measure is "apparent" attenuation. The specific gravity measured before pitching, after correcting for temperature, is called the "OG", or Original Gravity. The OG needs to be logged into a brewers notebook. Then during fermentation, the specific gravity can be re-checked. The specific gravity will fall towards 1.000 during fermentation, and a brewer can learn much about the fermentation by checking the specific gravity of the beer daily. Once the gravity remains the same for 3 days in a row, the yeast is most likely done with fermentation. The specific gravity at the end of fermentation is called "FG", or Final Gravity. To calculate attenuation percentage, the following equation can be used:
[(OG-FG)/(OG-1)] x 100
The only way to know if a yeast has completed fermentation is to check the expected attenuation. Many homebrewers make the mistake of worrying about a beer before they even check the attenuation. A simple check of the specific gravity at the end of fermentation will help in this regard. It is not completely accurate without computing the attenuation. For example, if a high gravity beer is made, the FG will be higher then normal, but the expected attenuation for that yeast strain may have been obtained. To obtain expected attenuation numbers, consult a list of yeast strain attenuation figures.
Most manufactures of brewers yeast list the attenuation ranges of their yeast strains. This can be very useful to a brewer in matching a yeast strain to a beer style. An example would be a brewer wishing to make an American-style Pale Ale. A yeast strain should be selected that will produce a dry finish, and allow for hop flavors to come through. A good choice would be a neutral yeast with an attenuation of 70-80%. If a brewer wants to make an English style mild ale, a yeast strain that does
not attenuate as much would be desired. An attenuation range of 65-70% would be more appropriate. Would a yeast strain that attenuates to 80% taste bad in an English style mild ale? No, but the beer would not taste true to style.
As found here: White Labs