What is Attenuation?

So you are at a party looking to impress the non-brewing muggles, but instead a brewing geek comes up and starts talking about original extracts, apparent extracts, and ABVs.  Here’s how to tell if he really knows what he’s talking about:

Attenuation is nothing more than the percentage of the original extract that has been converted via fermentation to CO2 and alcohol (and a few lesser compounds like esters in small quantities).  Recall that the basic brewing process for all grain starts with the mashing process, which converts your barley grain into sugary wort.  If you are an extract brewer, then you just start with sugary wort syrup.

You boil the sugary wort, cool it, add some yeast, and fermentation starts.  During fermentation a portion of the sugary wort is converted to alcohol (primarily ethanol).  That portion of the sugar, expressed as a percentage, is the attenuation of the beer.   Apparent attenuation is very easy to calculate as follows:

Apparent_Attenuation_in_% = 100 * (OG – FG)/(OG – 1.0)

where OG is your original gravity and FG is your final gravity.  So if you have a beer with an original gravity of 1.050 and it finishes with a gravity of 1.010, the math works out to be  100*(1.050-1.010)/(1.050 – 1.000) which is exactly 80%.  So for this example, 80% of the available extract in the wort fermented to become alcohol and CO2.

What is Apparent Extract and Real Extract?

The gravity of beer is most often measured using hydrometers.  However, hydrometers are calibrated to measure the sugar content of a solution of water.  Finished beer, however, contains alcohol (ethanol) which skews the hydrometer reading because alcohol is less dense than water.  Therefore, a hydrometer reading taken on finished beer will show lower (less extract content) than the beer actually contains.

Apparent extract (often written as AE) is the measured hydrometer reading for the finished beer, usually expressed in degrees plato by professional brewers.  For a homebrewer, this is the same as your final gravity (FG), but convert it from a specific gravity to degrees plato if you want to sound like the pros.  To  do a rough Plato calculation in your head, one degree plato is approximately 4 points of specific gravity, so a finished beer with a specific gravity of 1.012 (1.012 is “12″ points) is approximately 3 degrees plato.  If you want an exact calculation you can use a tool like BeerSmith or an online converter.

Real extract (often written as RE) is the real extract content of the finished beer, accounting for the actual alcohol content and imperfect nature of hydrometers.  Real extract can be calculated from the starting gravity and apparent extract (final gravity) as follows:

Real_extract = 0.188 * Original_extract + 0.8192 * Apparent_extract

where Real_extract, Original_extract (which is just your OG) and Apparent_extract (your FG) are all in degrees plato.

Now you know enough to be dangerous at dinner parties.  Please join us next week for part two of this article, where we explore real attenuation, and also how to make proper use of attenuation when designing your own beer recipes.  Thank you for joining me on the BeerSmith Home Brewing Blog.  Don’t forget to subscribe to our weekly brewing newsletter, and have a great brewing week!

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