There are a lot of terms out there in the beer and brewing world that we hear and might even use, but don’t quite understand. Attenuation is one of those words. So what is attenuation? Why does it matter to brewing? Hopefully I can answer those questions and maybe even a few more. Here’s your introduction to attenuation.
Wikipedia defines attenuation as “the gradual loss in intensity of any kind of flux through a medium.” So when talking about brewing this basically measures how much sugar is lost in the wort. Beer is basically made when yeast eat the sugar in the wort and turn it into alcohol and CO2. When the yeast have eaten all they can eat, they kind of “turn off” and sink to the bottom of the fermentation vessel. This is known as flocculation. When the yeast flocculates, it’s a pretty sure sign that fermentation is over and it’s time for the next steps.
So visually inspecting is the easiest way, but what if it’s not that easy? Sometimes flocculation doesn’t happen as you would expect, leaving it up to the brewer to figure it out. So, how does you, the brewer do that? Well, it’s done by measuring the degree of attenuation. This is basically the percentage of sugars that the yeast consume. The conditions you are fermenting in, as well as the gravity of the beer will cause variation in this, but overall you’re looking at 65-85% attenuation.
Break It Down
This can be kind of tricky, so let’s look at it another way. If a freshly fermented beer has an attenuation of 75%, that means that three quarters of the sugars in that beer have been gobbled up by the yeast. Remember, not all sugars are fermentable, and yeast will never work completely, so that left over 25% sugar will stay in solution.
Ok, hopefully you have a rough idea what we are talking about now. The next step is actually doing the calculations. The basic way is done by pretty much every home brewer. You take a hydrometer reading before pitching the yeast. This is measuring the specific gravity of the liquid. Remember, specific gravity is a measurement of density. You are checking for the density of the wort. Pure water sits at exactly 1.000. Wort will have a higher reading because of the sugars in the wort.
As soon as you add yeast, they begin to eat the sugar, causing the density of the liquid to go down. This, paired with the creation of alcohol, which is lighter than water, will give you a lower reading on the hydrometer. As we talked about before, you can do some math with these two numbers and get the ABV of the beer. While this is fairly simple, there is a lot more to it.
Actual vs Apparent
Using OG and FG to determine alcohol content and attenuation is not perfect. This is known as apparent attenuation. While this is perfectly fine for home brewers, the big guys need better measurements than this. Since alcohol is lighter than water, this can throw off readings. Professional breweries can heat the sample, releasing the alcohol and leaving only water and the dissolved sugars. They now can measure gravity. This gives them actual attenuation, or the true amount of sugar that has been converted to alcohol and CO2.
Gravity is taken once per day, and when it does not change for three days in a row, the beer is considered to be done fermenting. To determine attenuation at this point, you use the gravity equation. For reference, here it is:
So, how do you know when your beer is done fermenting and ready to keg or bottle? This is something I’ve worried about with my first few batches of beer. Attenuation takes the worry out of this. If you measure the gravity and it doesn’t change for 3 days, changes are you’re good to go. For most beers it’s really this simple. If you’re making a higher gravity beer there is a little more math involved, and an attenuation table is needed. The higher level of leftover sugars will throw your numbers off.
What Does This Mean For Me?
Here’s the important part. If you are making your own recipe, choosing a yeast that has proper attenuation for they style you are brewing is vital. This will make all the difference. A simple example is a good American Pale Ale. You want this to have a clean, dry finish to accentuate the happiness of this beer. A yeast with a high attenuation would be best here.
People talk about beer when sampling it and sometimes say that a specific beer is “nicely attenuated” or “has a dry finish.” Basically they are talking about how much sugar is left. The less sugar, generally the more dry the beer will finish. This also has to do with astringency and other factors, but attenuation, or the presence of sugars, plays a big role here.
Wrapping It Up
To sum this up, attenuation and gravity tend to go hand in hand. as the beer ferments, sugars are turned into alcohol and CO2. The more this happens, the lower the FG will go, and the higher attenuation the beer will have.
I hope this helped you understand attenuation. You already talk about this, but might not be using the proper wording. Knowing how to describe taste and the brewing process as a whole is an important step in learning all about beer.
Until next time,