Understanding Yeast Attentuation
Yeast is responsible for turning sweet wort into beer. Yeast consume the sugar in wort and turn that sugar into carbon dioxide, alcohol, and flavor compounds. When yeast finish the fermentation process, they shut down, clump together, and fall to the bottom of the fermenter. This process is called flocculation. 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, often called simply attenuation, is the percentage of sugars that yeast consume. Attenuation varies among different strains. Fermentation conditions and gravity of a particular beer cause the attenuation to vary. Hence each strain of brewer’s yeast has a characteristic attenuation range. The range for brewer’s yeast is typically between 65 percent and 85 percent.
A Little Math
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 brewer’s notebook. Then during fermentation, the specific gravity can be re-checked. The specific gravity will fall toward 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 three 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, use the following equation:
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 than 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 brewer’s yeast list the attenuation ranges of their yeast strains. This can be very useful in matching strains to beer styles. For an American-style pale ale, for example, select a yeast strain 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 percent to 80 percent.
To make an English-style mild ale, choose a yeast strain that does not attenuate as much. An attenuation range of 65 to 70 percent would be more appropriate. Would a yeast strain that attenuates to 80 percent taste bad in an English style mild ale? No, but the beer would not taste true to style.
A Troubleshooting Aid
Many brewers attempt to elevate their brewing skill to hand-craft the best beer possible. One simple step is to check attenuation. Not only will this give the brewer information about the batch of beer, but it will also aid in troubleshooting problems. Perhaps the fermentation temperature is too low, and the yeast is not active enough to complete fermentation. The pitching rate of yeast may be too low, so there are not enough cells to complete fermentation. Or in all-grain brewing, the mash temperature may be too high or too low, which will affect the availability of fermentable sugar.
During Prohibition, homebrewers did not regularly check attenuation, hence the stories of bottles blowing up. Bottles will do that today, too, if the beer is bottled well before attenuation is complete. Checking attenuation can improve your brewing skill, improve your recipe formulation, and improve the taste of your homebrew!
