Apparent and Real Yeast Attenuation in Beer Brewing

Explore the role of attenuation in beer brewing. Learn how yeast converts sugars to alcohol and CO2, impacting flavour, alcohol content, and mouthfeel.


Mat Stuckey

9/2/20237 min read

Demystifying Yeast Attenuation in Beer Brewing

If you've done any reading on homebrewing, you've likely encountered the term "attenuation" used to describe yeast performance. But what exactly does it mean and why does attenuation matter?

This expanded guide breaks down the science in simple terms along with how brewers can leverage attenuation principles to improve their beer. After reading, you'll be an attenuation expert!

Yeast attenuation refers to the extent to which yeast consumes sugars during fermentation. It is one of the key factors that determine the final alcohol content, flavour profile, and body of beer.

By understanding attenuation, brewers gain control over the finished product. Let's dive deeper on this critical brewing concept.

Key Takeaways on Beer Attenuation

  • Attenuation measures how thoroughly yeast ferments wort sugars

  • Apparent and real attenuation quantify the drop in wort density

  • Many factors impact attenuation including yeast strain and health

  • Attenuation level affects alcohol content, body, sweetness, and more

  • Brewers select yeast and control processes to reach target attenuation

  • Watch for too high or low attenuation compared to expectations

What Causes Beer to Attenuate?

Attenuation occurs as yeast ferments wort, converting sugars into ethanol and carbon dioxide. The degree of attenuation depends on variables like:

1) Yeast strain: Different strains of yeast have different characteristics and attenuation capabilities. Some yeast strains have a higher tolerance for alcohol and can ferment more sugars, resulting in higher attenuation.

2) Fermentation temperature: The temperature at which yeast ferments the wort can have an impact on attenuation. In general, higher fermentation temperatures can lead to more rapid fermentation and potentially higher attenuation, but it can also produce off-flavours. Lower fermentation temperatures may result in slower fermentation and lower attenuation.

3) Wort composition: The composition of the wort, including the types and amounts of sugars present, can affect attenuation. Yeast can ferment different sugars at varying rates, with simple sugars being more easily fermented than complex sugars. A wort with a high proportion of fermentable sugars will generally have higher attenuation.

4) Mash conditions: The mash is the process of converting grain starches into sugars. Variables such as mash temperature and duration can affect the amount and type of sugars available for fermentation. Optimal mash conditions can result in higher attenuation.

5) Oxygen availability: Oxygen exposure during fermentation can impact yeast health and performance. Excessive oxygen can lead to oxidative stress, affecting yeast metabolism and attenuating the fermentation process.

6) Yeast pitch rate: The amount of yeast added to the wort (pitch rate) can influence fermentation and attenuation. Overpitching (adding too much yeast) can result in faster fermentation and potentially higher attenuation, while underpitching (adding too little yeast) may lead to slower fermentation and lower attenuation.

7) pH levels: The pH of the wort can affect yeast metabolism and attenuation. Yeast generally thrives in a slightly acidic environment, so maintaining appropriate pH levels can promote optimal attenuation.

8) Fermentation vessel: The type of fermentation vessel used can impact yeast performance and attenuation. Factors such as the vessel's size, shape, and material can influence temperature control, oxygen exposure, and yeast behaviour during fermentation.

These variables, among others, can influence the degree of attenuation achieved during the fermentation process. Brewers often manipulate these factors to control the final flavour, aroma, and alcohol content of the resulting beer.

The level of attenuation is revealed by comparing the specific gravity before and after fermentation using a hydrometer. But first, let's break down how attenuation is measured.

How is Attenuation Measured?

Attenuation reflects the change in wort density as sugars are fermented. Two key metrics are used:

Apparent attenuation (AA)

  • Measures change between original gravity (OG) and final gravity (FG)

  • Reflects ALL density reduction - from alcohol, CO2, and any unfermented sugars

  • Formula: AA = (OG - FG) / (OG - 1) x 100

Real attenuation (RA)

  • Accounts for the gravity effects of alcohol and CO2 produced

  • More accurate measure of the true percentage of sugars fermented

  • Requires an adjustment formula using OG and FG

How Does Attenuation Impact Finished Beer?

The level of attenuation directly affects multiple beer qualities:

  • Alcohol content - High attenuation = more sugar to alcohol conversion.

  • Body and mouthfeel - Lower attenuation leaves more unfermented sugars, increasing body.

  • Sweetness/dryness - High attenuation leaves little residual sugar, increasing dryness.

  • Yeast character - Attenuation limits can stress yeast, producing off-flavours.

  • Head retention - Unfermented dextrins support lasting foam.

Brewers balance attenuation along with malt bill to achieve the desired beer profile.

What are Typical Attenuation Ranges?

Attenuation varies widely by yeast strain. Average ranges:

  • American ale - 70-80% apparent attenuation

  • English ale - 65-75% apparent

  • Lager - 70-80% apparent

  • Belgian - 65-85% apparent

  • Wheat - 70-80% apparent

  • Wine/champagne - 80-90%+ apparent!

Consult manufacturer data for your specific yeast's expected attenuation performance.

Tips to Reach Optimal Attenuation

Follow these best practices to promote thorough yeast attenuation:

  • Make a yeast starter for healthy, active cells

  • Oxygenate wort fully before pitching

  • Control fermentation temperatures for strain

  • Test yeast viability if attenuation is sluggish

  • Ensure balanced water profile and mash pH

  • Limit highly roasted grains reducing fermentability

  • Pitch adequate cell volumes for wort density

  • Provide nutrients like zinc, nitrogen, and vitamins

What if Attenuation is Too High or Low?

Too high attenuation - Stressful for yeast. Potential ester/phenol off-flavours. Thin body. Overly dry beer.

Fixes: Mash hotter for less fermentable wort, reduce pitch rate, choose less attenuative yeast strain

Too low attenuation - Unfermented sugars leave sweeter beer. Potential flavour imbalances. Stuck fermentation risks.

Fixes: Ensure adequate oxygenation. Make a starter. Rouse yeast. Pitch more cells. Raise temperature.

With careful brewing process and yeast selection, brewers can achieve their desired attenuation outcomes.

How Wort Composition Influences Attenuation

The types of sugars present in wort directly affect how thoroughly yeast can attenuate. Wort contains:

  • Glucose - Ferments readily

  • Fructose - Ferments readily

  • Sucrose - Ferments readily

  • Maltose - Ferments readily

  • Maltotriose - Somewhat less fermentable

  • Dextrins - Unfermentable, contribute body

Wort with higher proportions of simpler sugars like glucose will attenuate further. More complex sugars limit attenuation.

Why Mash Temperature Matters

Higher mash temperatures increase wort fermentability and attenuation. At higher saccharification rests:

  • More starches are converted to fermentable maltose

  • Beta amylase produces more maltose and glucose

  • Alpha amylase is denatured, limiting unfermentable dextrins

A more fermentable wort composition promotes higher attenuation.

The Role of Yeast Health and Pitching

Vigorous, healthy fermentation is key for full attenuation. Consider:

  • Make a starter to ensure adequate, active yeast cells

  • Oxygenate wort prior to pitching - yeast need O2 to reproduce

  • Pitch proper cell volumes for wort density - underpitching stresses yeast

  • Control fermentation temperature for optimal yeast activity

Robust yeast means attentive, thorough attenuation.

Calculating Apparent Attenuation

Apparent attenuation compares wort gravity before and after fermentation:

  • Use a hydrometer to measure original gravity before pitching

  • Take a final gravity reading once fermentation is complete

  • Plug OG and FG into the AA formula:

    AA = (OG - FG) / (OG - 1) x 100

This reveals the percentage drop in gravity and apparent attenuation.

Adjusting for Real Attenuation

Real attenuation accounts for gravity effects of alcohol and CO2 produced:

  • Requires an adjustment to apparent attenuation

  • Adjustment formulas factor in OG and FG

  • More complex but provides actual % sugars consumed

For simplicity, apparent attenuation serves homebrew needs.

Troubleshooting Low Attenuation Issues

If attenuation is lower than expected, try these fixes:

  • Improve yeast health with starter and oxygen

  • Ensure fermentation temperature in strain's ideal range

  • Gently rouse yeast to avoid premature flocculation

  • Add yeast nutrient to replenish depleted vitamins and minerals

  • Repitch with a fresh batch of healthy yeast cells

  • Increase fermentation temperature slightly

With some adjustments, sluggish yeast can often finish attenuation.

The Role of Residual Sugars and Dextrins

Besides alcohol and CO2, attenuation also impacts residual beer sweetness:

  • Unfermented sugars contribute sweetness and body

  • Wort dextrins are non-fermentable but add body and mouthfeel

These residuals create beer sweetness without needing to back-sweeten.

Balance Attenuation With Your Goal Profile

Attenuation helps brewers fine-tune the final beer character:

  • Lower attenuation leaves residual sweetness and body

  • Overly high attenuation risks thin, dry beer with potential off-flavours

  • Control mash, yeast selection, pitch rates and more to hit your target

There is no universal ideal attenuation - it all depends on your recipe goals!

Evaluating Attenuation Characteristics

To predict attenuation, understand your yeast strain:

  • Manufacturer specs provide attenuation range guidelines

  • Attenuation can vary batch to batch with yeast handling

  • Keep notes on your observed attenuation with different yeasts

  • Compare against expectations and troubleshoot outliers

Know your yeasts' tendencies to select the right strain for your wort.

Now that you're an attenuation expert, you have the knowledge to produce beautifully balanced beers! With experience, you will get a feel for designing recipes and fermentation to achieve your ideal attenuation outcomes. Want more info on homebrewing? We have recipes, techniques and gift ideas for brewers, along with a wealth of information in the Brewpedia blog. Check it out!

Frequently Asked Questions

How Does Yeast Attenuation Affect Beer Flavour?

Yeast attenuation plays a crucial role in determining the beer's flavour profile. High attenuation yeast strains will consume more sugars, resulting in a drier, less sweet beer. On the other hand, low attenuation yeast strains will leave more residual sugars, giving the beer a sweeter taste.

What Are the Best Practices for Achieving Desired Attenuation in Brewing?

To achieve your desired level of attenuation in brewing, it's essential to consider factors like yeast health, pitch rate, and fermentation temperature. Using a yeast starter, oxygenating the wort, and maintaining optimal temperature conditions can help you reach your attenuation goals.

How Do You Calculate Apparent Attenuation in Homebrewing?

In homebrewing, apparent attenuation is calculated using a formula that involves the original gravity (OG) and final gravity (FG) of the wort. The formula is: AA = (OG - FG) / (OG - 1) x 100. This gives you a percentage that represents the drop in wort density due to fermentation.

Can High Attenuation Yeast Be Used for All Beer Styles?

High attenuation yeast is not suitable for all beer styles. For example, if you're brewing a style that requires a fuller body and sweeter taste, like a Milk Stout, a high attenuation yeast may not be the best choice.

What Factors Can Lead to Low Attenuation in Beer?

Factors like low fermentation temperature, poor yeast health, and inadequate oxygenation can lead to low attenuation in beer. This will result in a sweeter, fuller-bodied beer that may not align with the intended beer style.

Is Attenuation the Same in All Types of Brewing?

Attenuation can vary depending on the type of brewing. For instance, the attenuation levels in wine or champagne brewing are generally higher than in beer brewing due to the different yeast strains and sugar content involved.

How Does Wort Composition Influence Attenuation?

The types of sugars present in the wort can significantly impact attenuation. Simple sugars like glucose and fructose are more easily fermented by yeast, leading to higher attenuation levels. In contrast, complex sugars may result in lower attenuation.

What Role Does Mash Temperature Play in Attenuation?

Mash temperature can influence the types of sugars produced, thereby affecting attenuation. Higher mash temperatures can lead to more unfermentable sugars, resulting in lower attenuation, while lower temperatures may produce more fermentable sugars, leading to higher attenuation.