Understanding Attenuation in Brewing
Attenuation is a critical concept in fermentation, referring to the percentage of sugars that yeast consumes for fermentation. It is one of the key parameters linked to a beer’s final gravity and alcohol content.
Attenuation Calculation: Apparent vs. Real
Attenuation is typically expressed as a percentage and can be calculated in two ways: apparent attenuation and real attenuation.
Apparent attenuation is the most common method for tracking fermentation progress, based on the difference between OG and FG measured with a hydrometer. It is calculated with the following formula:
Apparent Attenuation (%) = [(OG – FG) / (OG – 1)] × 100. Note that because ethanol has a lower density than water, the final gravity (FG) reading is skewed. This bias leads to an underestimation of residual sugar, making the apparent extract seem lower than it truly is. In reality, the actual sugar content (real extract) is higher, meaning real attenuation is always lower than apparent attenuation. Understanding this difference is important when calculating actual sugar content, estimating calories, or troubleshooting fermentation performance.
Real attenuation corrects for the presence of alcohol by estimating the real extract ie. the actual sugar content remaining after fermentation. It provides a more accurate picture of how much sugar the yeast has truly consumed.
Since real extract is higher than the hydrometer reading (apparent extract), real attenuation is always lower than apparent attenuation.
Example:
OG = 1.050 (12.4°P) and FG = 1.010 (2.5°P) → Apparent Attenuation = 80%
Real Extract = 1.017 (4.3°P) → Real Attenuation = 65%
While apparent attenuation is most commonly used in day-to-day brewing, understanding the difference is valuable when troubleshooting fermentation issues or comparing yeast strain performance across different worts.
Why Do Different Yeast Strains Have Different Attenuation Ranges?
Differences in attenuation between yeast strains are primarily due to yeast genetics. Each strain possesses a unique enzymatic profile, which governs its ability to metabolize specific sugars in wort.
For example, Hornindal Kveik exhibit lower attenuation due to poor maltotriose utilization. Without assistance, these strains may struggle to achieve a low final gravity.
In contrast, diastaticus strains like Farmhouse Vibes possess the STA1 gene, which enables them to break down and ferment nearly all available sugars and dextrins. These strains are typically reported with attenuation values exceeding 90%.
Suppliers usually report average attenuation ranges based on fermentation in a standard wort, using a typical mash profile.
Why Your Measured Attenuation May Differ from Reported Values
Several factors can cause your attenuation values to diverge from the figures provided by yeast suppliers. The main driver is wort fermentability, which is influenced by both ingredients and mash procedure:
Malt Composition: The diastatic power of malt affects the amount and activity of enzymes available during mashing. This can vary between maltsters and crop years. Raw grains and adjuncts contribute little to no enzymatic activity, which can reduce overall fermentability.
Mash Temperature: Lower mash temperatures (e.g., 63–65 °C) favour maltose production and lead to more fermentable wort. Higher mash temperatures (e.g., 68–70 °C) result in more unfermentable dextrins, reducing attenuation.
Brew System Design: In systems without a direct-heated mash tun, enzymes may continue breaking down starches during transfer to the kettle. This can lead to unexpectedly high fermentability and attenuation.
Enzymes: The use of exogenous enzymes (e.g., glucoamylase, beta-glucanase, or amyloglucosidase) can significantly impact fermentability by breaking down complex starches or dextrins into simpler, fermentable sugars. This is especially relevant in high-adjunct beers or when aiming for ultra-low final gravities.
As an example, Saturated is reported with an attenuation range of 73–77%, based on a standardized mash. However, it is entirely possible to brew a beer with 80% attenuation using the same strain by increasing wort fermentability. This can be achieved by extending mash time, mashing closer to 63–64 °C, or skipping the mash-out before lautering to allow enzymes to remain active longer.
Under Attenuation Due to Yeast Performance
Attenuation may also be limited by poor yeast health. Symptoms include sluggish or incomplete fermentation. Contributing factors include:
Inadequate nutrients in the wort
Deficiencies in key micronutrients (e.g., zinc, nitrogen, vitamins) can impair yeast metabolism and reduce attenuation.
Underpitching
Using too little yeast can overwhelm the cells, delaying fermentation onset and resulting in incomplete sugar consumption.
Overpitching or overgrowing yeast
Although often assumed to be safe, excessive pitching or overgrowing yeast (eg. adding oxygen to a dry pitch) can lead to rapid nutrient depletion, which may negatively affect fermentation completion and yeast performance in future generations.
Improper yeast cropping or handling
Poor harvesting, storage, or propagation practices can compromise yeast viability and vitality.
Poor temperature control
Maintaining consistent fermentation temperature is critical. Fluctuations can stress yeast and affect enzyme activity, slowing or halting fermentation.
Fermenting too cold for the strain
Yeast activity is temperature dependent. Fermenting below a strain’s optimal range can reduce metabolic rate, resulting in incomplete attenuation or extended lag phases.
Insufficient oxygenation (for repitch)
Oxygen is critical for sterol and membrane synthesis during the early fermentation phase. Oxygen-starved yeast is less resilient and less efficient at completing fermentation.
High alcohol stress
In high-gravity or high-ABV fermentations, ethanol toxicity can impair yeast function, especially if cells are not in peak condition.
Predicting and Troubleshooting with a Fast Ferment Test
To determine whether attenuation issues originate from the yeast or from the wort itself, brewers can carry out a fast ferment test. This test is designed to reveal the maximum fermentability of a given wort under ideal conditions.
In a small sample of sterile, cooled wort (around 500 mL), pitch a large amount of healthy yeast. Use the same strain used in your full-scale fermentation, at approximately 10 times the standard pitching rate.
Seal with an airlock or sanitized foil and place the vessel in a warm environment (typically 24–27 °C). Agitate continuously (using a stir plate or by regular swirling) to ensure thorough yeast contact and oxygenation. Fermentation is usually complete within 24 to 36 hours.
The final gravity represents the lowest possible gravity achievable in your wort, assuming healthy yeast and optimal fermentation conditions.
Comparing the FFT final gravity to the actual fermentation final gravity helps pinpoint the limiting factor in attenuation:
If the FFT final gravity is close to the actual batch final gravity:
The wort is the limiting factor. This suggests that fermentability is determined by mash composition, enzyme activity, or process conditions, not by yeast performance.
If the FFT final gravity is significantly lower than the actual batch final gravity:
The yeast in the main fermentation may have underperformed due to factors such as poor health, low viability, incorrect pitching rate, oxygen deficiency, nutrient limitations, or poor temperature control.