A classic problem with home brewed beer is “under-attenuation,” which means that a beer did not fully ferment.
Under-attenuation may be caused by an issue with your yeast, but it may also be due to rushing the process. Sometimes the recommended fermentation time on a brewing recipe kit, for example, is not long enough given the fermentation conditions in your house.
First, Here’s a simple description of attenuation:
Attenuation is how much of the sugar in the wort (un-fermented beer) is fermented away by the yeast.
The yeast that you add to the wort at the end of the brewing process basically “eat” the sugars that you provide by adding malt. The yeast then give off CO2 and alcohol as by-products of “digesting” the sugars.
Under-attenuation means that the yeast was not able to “eat” enough sugar to bring the beer down to normal sugar levels (and up to the expected alcohol level). The excess sugar left over results in an overly sweet taste to the beer.
Here’s a slightly more detailed description of attenuation and how it can be determined:
1. How “heavy” is the beer to begin with?
It turns out that attenuation refers to the conversion of sugars in the beer into alcohol and carbon dioxide. When we brew beer we use malted barley, which contributes sugars to the beer. The amount of sugar in the beer can be measured by taking a measurement of the specific gravity, which tells you the “weight” of the beer at the start of fermentation (referred to as “original gravity”) compared to the “weight” of pure water.
The specific gravity can be taken using a hydrometer, which requires a larger volume of the liquid for the measurement, or a nifty but affordable little device like this one which can take the measurement with only a few drops of liquid and give an even more accurate reading than a hydrometer.
2. How “heavy” is the beer after fermentation?
When the beer is finished fermenting you can measure the specific gravity of the beer again and find out how much of the sugar was eaten (this number is referred to as the “final gravity”). This is how most home brewers determine the alcohol content of their beer, as there is a direct relationship between how much sugar is eaten by the yeast and how much alcohol is given off.
3. Is the difference in these “weights” enough?
The percentage of the sugars that were converted is the attenuation. Under-attenuation means that not enough of the sugar was converted and the beer will taste very sweet. It may require that more yeast be added to the beer to finish the job. Over-attenuation can occur, but that’s a whole different beast.
What are Typical Attenuation Levels?
The style of beer is largely going to determine the numbers you should expect, so there can be a lot of variability here.
This is painting with a very broad brush, but in general a standard ale will come in with an original gravity between 1.040 and 1.060. You might expect the final gravity to end up between 1.010 and 1.015.
Very rarely will you see a final gravity much below 1.010. This can lead to other issues anyway, so you do not want that.
A stronger beer that starts with an original gravity of 1.060 or higher may only finish between 1.020 and 1.030. This may be ok for some beer styles, but for other styles you may have to make adjustments to your yeast.
Is the Beer Fully Fermented?
The best practice in determining whether your beer is full fermented/attenuated is to take a measurement of the specific gravity each day once fermentation slows down. The specific gravity will decrease from day to day if the beer is still fermenting.
When the reading is steady for three consecutive days you can be reasonably confident that the yeast has finished doing its work.
From there you can determine whether the beer is ready to be moved along to secondary fermentation or bottled, or whether it is under-attenuated and something must be done to promote more fermentation.
Have you brewed or come across under-attenuated beers? What did it taste like to you?