Yeast Attenuation and Flocculation Explained
Yeast Attenuation:
Yeast is responsible for turning sweet wort into beer. Yeast consumes the sugar in wort, and converts that sugar into carbon dioxide, alcohol, and flavors. When yeast completes the fermentation process, they begin to clump together, and fall to the bottom of the fermenter, 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 stay suspended in the beer? How do you really know when fermentation is done? The answer is by testing the degree of attenuation. Apparent attenuation percentage is the percentage of sugars that yeast consume. Attenuation varies between different strains of yeast. The fermentation conditions and gravity of a particular beer will cause the attenuation to vary; therefor, each strain of brewer’s yeast has a characteristic attenuation range. The range for brewer’s 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. The density and therefore specific gravity measurement lowers as these sugars are consumed by yeast during fermentation. 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 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, use this calculation:
[(OG-FG)/(OG-1)] x 100 = Attenuation Percentage
To determine if the yeast has completed fermentation, check the expected attenuation. Many home brewers 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 determining the attenuation. 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 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 neutral yeast with an attenuation of 70-80%. If a brewer wants to make 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 English style mild ale? No, but the beer would not taste true to style.
Yeast Flocculation:
When the yeast in the fermenter comes together, dropping to the bottom of the fermenter, it’s called flocculation. Flocculation is a desirable and important characteristic that is unique to brewer’s yeast. When brewer’s yeast nears the end of fermentation, single cells aggregate into clumps of thousands of cells, and drop to the bottom of the fermenter, leaving clear beer behind. If yeast flocculate too early, the beer will be under-attenuated and sweet. If yeast do not flocculate, the beer will be cloudy and have a yeasty taste. Most strains of yeast, which brewers call “wild” yeast, do not flocculate well, and remain in suspension for extended periods of time. The ability to flocculate is a product of natural selection. Brewers have continually collected yeast either from the bottom or top of a fermenter and in doing so, selected for increasingly flocculent stains. Yeast flocculation can be classified as high, medium, or low. Ale yeast strains are found in each category, while lager yeasts are predominantly medium flocculators. An English/London Ale strain would be a high flocculator, while an California/American Ale strain a medium flocculator. A Hefe-Weizen strain is an example of a low flocculator.