Know Your Yeast: Attenuation

Brewing yeast is a domesticated beast. Researchers estimate that we have been selectively choosing and breeding yeast to produce specific outcomes since the 1500s.

The same holds true today: broadly speaking, we choose a certain yeast strain in order to get a specific type of beer we want as a result. There are a number of characteristics that yeast can impart to a beer, and today we’ll be focusing on attenuation.

Attenuation is a measure of the amount of sugars that have been consumed by yeast during fermentation. It can be measured with simple math, by figuring out the difference between your starting gravity and final gravity. Calculating attenuation in this way is considered apparent attenuation because it doesn’t take into account ethanol content or any other compounds present that may influence the final density of the beer.

The various attenuation ranges listed for brewing strains are relative, and not absolute. Inherently yeast strains vary in their ability to metabolize certain carbohydrate sources. Brewing strains have been domesticated and selected for their ability to metabolize maltose, but the degree to which they metabolize maltotriose and dextrins varies widely. 

To use an extreme example, British Ale V (OYL-011) is capable of 100% apparent attenuation if the recipe consists of 100% dextrose (e.g., in a seltzer) even though the ​“official” attenuation range when used in beer is listed as 71 – 75%. Fermentation ranges differ from strain to strain and give you an idea of what to typically expect, especially when comparing strains. However, those ranges can be influenced (and even pushed out of range) by a number of factors.

Even though attenuation ranges vary from strain to strain, there are still factors that can contribute to higher or lower attenuation than expected:

Hot side

Mash temperature: one of the primary ways to influence the overall fermentability of wort and your overall wort composition (more on that next). Consider the ​“sweet spots” where alpha-amylase and beta-amylase activity coincide (about 146 – 158°F/63 – 70°C). Very generally speaking, the lower the mash temperature within that range, the more fermentable the wort will be. Mashing closer to the high end of the range may denature those enzymes, leaving you with a less fermentable wort.

Mash pH: just as enzymes have ideal temperature ranges, they also have pH ranges for optimal activity. The beta-amylases and alpha-amylases in malt prefer pH ranges from 5.1 – 5.3 and 5.3 – 5.7, respectively. If you want a dry finishing gravity, you would target the pH range for beta-amylase of 5.1 – 5.3. Conversely if you want a higher finishing gravity, you would target the pH range for alpha-amylase 5.5 – 5.7. If you want a middle ground 5.3 – 5.5 is a good compromise.

Water-to-grist ratio: mashes that are both too thin and too thick can lead to poor conversion, so you’ll need to aim for a sweet spot in the middle. In a thin mash, enzymes and substrate interactions are limited and can lead to lower enzymatic activity. In a thick mash, enzymes and substrate are at a high concentration and very active, but at some point the high accumulation of sugars can reach a point of diminishing returns, also leading to lower enzymatic activity.

Cold side

Pitch rate & yeast health: issues with either of these can lead to what most refer to as a​“stalled” fermentation. While overpitching could lead to attenuation issues, underpitching is a more likely culprit — you need to have enough healthy yeast cells available to take care of business. Plus, unhealthy and/or stressed yeast is less efficient and may underattenuate. Fresh yeast is always best — when in doubt, make a starter.

Oxygenation: certain strains have higher oxygen demands than others, and we tend to see a direct correlation between under-oxygenation and under-attenuation. Since oxygen is primarily used for yeast growth, low oxygen levels will not only reduce growth but also inhibit performance for those cells that are present. Read more about an experiment involving oxygenation.

Nutrient: low nutrient levels can stress out yeast, so you may opt to add a yeast nutrient to ensure a healthy fermentation. Take seltzer fermentation for example: even happy, healthy yeast in the presence of 100% fermentable sugar will massively underperform without the proper nutrients.

Alcohol tolerance: high-alcohol environments are stressful for brewing yeast. Once the yeast approaches the limit of how much alcohol it can tolerate, fermentation will slow down even if there are still unfermented sugars in the wort.

Dry hop creep: adding hops on the cold side can introduce enzymes that liberate more sugars for fermentation. The beer drops below what was originally intended to be the terminal gravity, as the yeast that’s still present is now able to consume those sugars and further attenuate the beer. Read more on hop creep over here.

Temperature: fermentation temperature typically won’t have as much of an effect on attenuation as the rest of these factors. However, fermenting too cool can cause the yeast to flocculate too early, leaving behind some unfermented wort and in turn resulting in a lower rate of attenuation than desired.

The percentage of attenuation a yeast strain is capable of depends on how well they take up sugars, and most strains of brewing yeast consume sugars in a specific, predictable way — beginning with the smallest, simplest sugars (glucose, fructose, maltose) and moving to the larger, more complex ones next (maltotriose, dextrins, etc.). With this knowledge, you can design your recipes to maximize your fermentables according to what strain of yeast you plan to use. Many British-type strains, for example, do not take in maltotriose. Others, like West Coast Ale I, handle maltotriose fermentation pretty well.

What about when the beer is done and you’re finding that it’s off by <0.5°P? Sometimes it happens. Ultimately, it’s usually not a big enough difference to cause concern, but if it is happening regularly, it may be time to adjust your processes.

It’s important to keep all of these factors in mind when designing a recipe, brewing the beer, and managing fermentation. Diligent note taking will help you track your successes, as well as checking that your thermometers and pH probes are calibrated to ensure consistency from brew day to brew day. While yeast attenuation is largely predictable, it all comes down to dialing in what works on your system, and that will sometimes take a few brews. Keep at it and find what works best for you.