This refers to the yeast’s efficiency in converting the fermentable sugars to alcohol and Co2. This is expressed as a percentage and most yeast strains attenuate between 65% to 80%, but it is important to note that this is the apparent ‘attenuation’ and not the ‘actual’ attenuation. It is very difficult to get the actual attenuation and is only done by large breweries. Attenuation gives you an idea when your brew is finished and can be measured using a hydrometer as it measures the decrease in the density or specific gravity of the brew.
Reasons for poor attenuation and how to solve this
The attenuation will vary depending on your fermentation conditions such as temperature for example. If the temperature is too low this will slow down the yeast and may affect attenuation so achieving the correct temperature for your brew helps with an efficient fermentation. You will sometimes find that a yeast does not perform well which can sometimes be due to a poor yeast but most of the time is due to the environment the yeast is in. For example if there is not enough oxygen in the brew to begin with the yeast will struggle to reproduce therefore there may not be enough yeast cells to perform an efficient ferment. Introducing oxygen by giving the brew a gently stir can often improve this. Oxygen is crucial to yeast health specifically the cell membrane which is semi permeable allowing sugars in and waste products out. It also helps the yeast to reproduce through a process called budding. Poor attentuation can also be the result of your ingredients as some malts have less fermentable sugars or the mash temperature could have been too high which produces more complex sugars that cannot be broken down by the yeast and if this is the case you just to need to put it down to experience and mash at a lower temperature next time.
Attenuation and fermentability
Generally darker malts such as chocolate malt have a higher level of unfermentable sugars and will therefore affect attenuation. So your attenuation depends on what kind of beer you’re wanting to make, if you’re making a chocolate stout for example you’ll be aiming for some sweetness in the end product and therefore the use of darker malts is critical to the beer style. Other malts such as Briess Carapils are designed to have a lower fermentability.
The Mash and Attenuation:
The fermentability can also be affected in the mash as mentioned above. You can mash your grain at different temperatures depending on what style of beer you are making. A low temperature will produce a wort with a higher fermentability and a high mash temperature will produce a wort with a low fermentability. This is due to the the different enzymes at work in the mash which have different optimum activity temperatures. Beta Amylase will produce most of the fermentable maltose sugars in the mash, the optimum mash temperature for Beta Amylase is 66 °C. Alpha Amylase produces much more unfermentable sugars and it’s optimum mash temperature is 68 °C. Most brewers make a compromise between these two enzymes mashing between 66°C and 68 °C when doing a single infusion mash. This produces good wort fermentability.
You can add extra enzymes to your mash which break the sugars down even further allowing for even higher attenuation depending on your yeast. White labs ultra ferm is an example of a product that will do this. This allows ‘total hydrolysis of dextrins to fermentable glucose, from all types of starch.’ https://www.home-brew-online.com/ingredients-c45/white-labs-ultra-ferm-10ml-vial-p3391
Pitching the Yeast:
Making sure you pitch enough yeast is also crucial to good attenuation so if you are making a big beer you will need enough healthy yeast to cope with it. This can be done through a yeast starter which increases your cell count, the video below explains how to do this: