Beer attenuation is one of those seemingly complicated topics thrown around on the forums. It can get confusing for even advanced brewers who don't use the terms often. Before we get into the calculations, I believe a few definitions of the terms are in order.
You might be asking yourself why you should care about beer attenuation or ADF. Well, a beer's degree of fermentability determines many of its final properties after it goes through fermentation.
These include the beer's mouthfeel, body and alcohol content. A beer which has a very high apparent degree of fermentability (ADF) will end up very dry after fermentation. These beers usually end up lighter in body and mouthfeel than a beer with a lower ADF and they will also have more alcohol (assuming both beers have the same OG).
These are the things that affect beer attenuation and the Apparent Degree of Fermentation:
Note: As a homebrewer, you probably won't have to worry too much about the term "apparent" in apparent attenuation. It is used to get around having to deal with the fact that when you get further along into a fermentation, there is more alcohol being produced which is less dense than water.
To find the real beer attenuation would require you to distill the alcohol from the sample and then adjust the sample's volume back to its original volume with distilled water and then remeasure the density.
The word apparent just means that our sample has alcohol in it and that we are not worrying about it in our calculations.
Final Grafity Too Low
So what do you do when you find the final gravity is not what your brewing software says it should be? If the final gravity is too low, and all other factors such as fermentation temperature etc. were correct, then you may have mashed at a temperature that was too low and produced a wort that was more fermentable than you needed. If the fermentation is MUCH lower than you expected, and there are unexpected phenolic flavors in the beer, the problem is probably wild yeast contamination. The common sense solution to this problem is better control of your sanitation.
Final Gravity Too High
If the final gravity is too high, it could indicate that you mashed at too high of a temperature which produced more long chain unfermentable sugars. Beta-amylase which produces most of the simple sugars in a mash gets denatured quickly at temperatures greater than 158° F (70° C). Calibrate your thermometer frequently with ice water and boiling water. If you are high on the strike temperature, don't let it go thinking that it will come into range as it cools. By that time most of the beta amylase will be denatured and you will end up with a very viscous overly sweet beer
Other Solutions to Fermentation Problems
When our final gravity doesn't end up where you think it should you need to figure out what happened to your beer's attenuation. It can come from two places.
One is the fermentability of the wort or its ADF, and another is the fermentation itself, which includes the yeast health. Taking good notes is key to figuring this out.
If you missed your strike temp, make a notation in your brewing log. If you didn't do an iodine starch conversion test, you can't know if the wort was completely mashed and all of the starch converted to sugars.
Try to avoid temperature fluctuations during fermentation and tape the temperature controller's sensor to the side of the fermenter or use a thermowell to control the temperature of the fermenting beer and not the ambient temperature around the beer.
If your final gravity is too high, give it another week and monitor it closely. If you can't blend it with a thinner beer, figure out what went wrong and correct the problem when you brew the recipe again. If you went too far, and you don't have a sweeter beer to blend with, use a higher strike temp next time you brew the recipe.
There is also the possibility that you have a stuck fermentation. A stuck fermentation is usually caused by not pitching enough healthy yeast or not oxygenating or aerating the wort enough prior to pitching, or possibly fermenting at too low of a temperature.
1. Sometimes just moving the fermenting wort to a warmer temperature is enough to get the yeast going again and will increase the attenuation.
2. Rousing a very flocculant yeast may start a stuck fermentation, giving the yeast another chance at the unfermented sugars.
3. But sometimes, you must repitch a fresh batch of yeast. To do this, first transfer your beer to a secondary fermentation vessel to get the beer off of the old yeast cake and trub. Read about doing a secondary fermentation by clicking here. To determine what your final gravity should be, do a forced fermentation test.
4. As a last ditch effort to get your beer attenuation corrected and your beer fermenting again, you can try adding Brettanomyces, a yeast which attenuates further than saccharomyces.
It may require warming up the fermentation, and will definitely require more time than you had originally planned on, but the end result may well be worth the wait. Be aware that you will not end up with the beer you originally envisioned.
Brett will create a tangy and many say "funky" nature to the beer. You may or may not see a pellicle form on the top of the beer. Continue measuring the SG and tasting the beer sample when you do.
Expect the beer to attenuate more than you expected for the original beer. When the beer appears to have stopped fermenting and tastes good to you, rack off the sediment and either age further, where the beer will continue to develop brett-derived flavors, or cold crash, fine with gelatin (or not) and package the beer.
No matter what happens, you will have saved a beer who's fermentation was permanently stuck (or so you thought) and possibly gained a wonderful sour funky beer instead. To see the interview with Michael Tonsmeire, the author of American Sour Beers, click here.
References for Beer Attenuation: http://www.homebrewtalk.com/wiki/index.php/Understanding_Attenuation, http://www.morebeer.com/brewingtechniques/library/backissues/issue6.6/depiro_sb2.html, The Home Brewer's Answer Book , Solutions to Every Problem, Answers to Every Question, by Ashton Lewis. For a nice beer attenuation calculator, click here.
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