Attenuation reflects the amount of reduction in wort concentration resulting from fermentation. It is typically reported as a percentage. It tells a brewer how much the specific gravity of his beer will drop during fermentation. The most common form used by brewers is called apparent attenuation or apparent degree of fermentation (ADF). It is expressed as 100 * (OG-FG / OG). For example, a beer with a wort density of 20°P and a final gravity of 6°P has an ADF = 0.70 or 70% (20-6) / 20 = 0.7 and 0.7 * 100 = 70%.
You might be asking yourself why you should care about 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).
Factors Affecting ADF
These are the things that affect ADF:
- Mash temperature regime - Lower mash temperatures ie. 148-150°F (64-66° C) yield a more fermentable wort since the beta-amylase will have a chance to produce more maltose, long mashes with multiple rests tend to produce a more fermentable wort with higher attenuation.
- Water to grist ratio - thinner mashes enhance the maltose production and thus increase the wort's fermentability and attenuation.
- Mash pH -
beta and alpha amylase enzymes each have their own optimal pH ranges, and mash pH can affect the activity balance of the two enzymes, although the effect is only minimal.
- Recipe's grain bill - more base malt and less speialty grains will yield a more fermentable wort.
- Yeast strain chosen - some yeast strains just naturally ferment longer and can tolerate higher alcohol levels.
- Type of extract - for extract brewers, you will find that different types of extracts have different attenuations. Experience with a particular extract will give you an idea of its ADF.
- Yeast Health - healthy yeast will ferment cleaner and further and will be able to withstand the toxicity of the ethanol being produced. Pay close attention to nutrients, oxygenation, contamination, number of generations, etc. of your yeast for optimal results.
- Fermentation temperature -
higher fermentation temperatures accelerate the yeast's metabolism and the rate of fermenation. There are only a few times when a homebrewer would want to raise the fermentation temperature to increase metabolism. This is mainly done to emulate the fermentations of the Trappist and abbey ales from Belgium which may be fermented at temperatures nearing 90°F (32° C). At these temperatures, many unwanted compounds will be produced that just don't fit many beer styles.
- Pitching Rate -
when more yeast is pitched fermentation will progress quicker. It's always better to use a pitching calculator and know the correct amount of yeast you should be pitching. Always pitch the proper amount of healthy yeast, oxygenate well, use nutrients when needed, and pay close attention to the other factors and you will always make more a better more consistent beer. Overpitching can lead to reduced yeast growth and less ester production (thus less fruity esters), not to mention the yeasty flavors one gets in the finished beer.
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 fermantation, there is more alcohol being produced which is less dense than water. To find the real attenuation would require you to distill the alcohol from the sample and then adjust the sample's volume back to its original volume with 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.
Dealing With Final Gravities
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.
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. 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. Sometimes just moving the fermenting wort to a warmer temperature is enough to get the yeast going again and will increase the attenuation. Rousing a very flocculant yeast may start a stuck fermentation, giving the yeast another chance at the unfermented sugars.
When our final gravity doesn't end up where you think it should you need to figure out what happened. 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 don'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 inside the fridge. 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.
References: http://www.homebrewtalk.com/wiki/index.php/Understanding_Attenuation, http://www.brewingtechniques.com/library/backissues/issue6.6/depiro_sb2.html, The Home Brewer's Answer Book , Solutions to Every Problem, Answers to Every Question, by Ashton Lewis.
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