This article is intended to give the advanced home brewer a better understanding of attenuation, how it is affected by the brewing process and how it can be used to improve the brewing process.
Attenuation refers to the percentage of original extract that has been converted by the fermentation process:
Attenuation = 100 % * (starting extract - current extract) / (starting extract)
This formula works with extract given in weight percentages and degree Plato. Since, at least in the wort and beer gravities that most brewers work with, an almost linear relationship between (specific gravity - 1) and extract percentages exists, this formula can be changed to:
Attenuation = 100 % * (starting gravity - current gravity) / (starting gravity - 1)
for brewers who prefer to work with specific gravity.
Brewing Process and Wort Composition
To understand the different forms of attenuation we need to take a look at the extract composition first. During mashing, the majority of the grist is converted into water soluble compounds. This is shown in Figure 1.
The conditions during the mashing process as well as the grains will determine the exact ratio between the various compounds (sugars, detxtrines, proteins and others). Once the conversion is complete, the mash is lautered. Due to inefficiencies during this process not all of the dissolved extract ends up in the boil kettle. The percentage of the total dissolved extract and the extract that actually ends up in the boil kettle is called brew house efficiency.
During the boil only minor changes happen to the wort composition. The denaturization of the enzymes finally fixes the ratio between fermentable and unfermentable extract. The coagulation of proteins changes the amount and composition of proteins in the wort. Hops will add additional compounds. But these are of little interest for the discussion of attenuation.
During the fermentation of the wort the fermentable sugars are converted into almost equal parts of CO2 and ethanol as well as much smaller amounts of other compounds (esters, higher alcohols). The yeast will also absorb most of the simple proteins. But not all of the fermentable sugars will have been fermented at the time the beer is ready for consumption. The amount of fermentable sugars left in the beer has an affect on the beer character and different styles of beer oftentimes have different amounts of fermentable sugars left.
Apparent vs. Real Extract
Hydometers are calibrated for measuring the sugar (extract) content of a water solution. This is true for wort. But when used to measure the extract of beer, which contains ethanol, the reading will be skewed by the lower specific gravity of the ethanol. As a result the hydrometer shows a lower extract content than the actual beer has. This measured extract value is called apparent extract (as opposed to the real extract that is measured when there is no alcohol in the solution) and is commonly used when refering to the extract (or specific gravity) of beer. Like the real extract it can be expressed as weight percent, degree Plato or specific gravity. To determine the real extract one can boil-off the alcohol and replace it with distilled water before using a hydrometer. Or, if the original extract is known, the following formula [Realbeer] can be used to calculate the real extract from the apparent extract:
real extract = 0.1808 * original extract + 0.8192 * apparent extract
Apparent vs. Real Attenuation
When the apparent extract of the beer is used to calculate its attenuation it is called apparent attenuation. The use of the real extract will give the real attenuation. When brewers speak of just attenuation they are most likely to mean apparent attenuation since it can easily be calculated from the hydrometer readings.
Limit of Attenuation
The limit of attenuation is the sum of all the sugars, that the yeast is able to ferment, expressed as a percentage of the total extract content. Since ale yeasts (saccharomyces cerevisiae) can only ferment maltotriose to a third and lager yeasts (saccharomyces uvarum) can ferment that wort sugar completely, lager yeasts will show a slightly higher limit of attenuation for the same wort.
Other than this the limit of attenuation of a given wort does not depend on the yeast strain. It is solely set by the mashing process. All yeasts are able to ferment all the fermentable sugars (save for the lager - ale difference mentioned above) in a given wort. This fact is taken advantage of in a [forced ferment test]. The forced ferment test uses a sample of wort and pitches it with a large amount of yeast. To ensure complete fermentation this sample is kept warm and the yeast is regularly roused (shaking or stirring). Once the fermentation is complete the apparent extract is measured and the limit of attenuation for that wort can be calculated.
Yeast Strain Differences in Attenuation
If there are no limit-of-attenuation differences between the different yeast strains, why are there more and less attenuative yeast strains available?
Though yeast strains are able to ferment all the sugars in the beer, they usually don't get to. In contrast to a forced ferment test, beer is generally fermented at lower temperatures, with smaller pitching rates and without constant rousing. Because of that the yeast will not get a chance to ferment all fermentable sugars in the wort. Flocculation will cause it to drop to the bottom or collect on the surface where it doesn't have as much contact with the sugars anymore. Because of nutrient depletion and or high alcohol levels cells die before they get a chance to ferment every last bit of sugar in the wort. The result is left over fermentable sugars that play an important role in the character of the finished beer. The closer a beer's attenuation is to its limit the drier and less sweet it will taste. When looking at the attenuation ranges given for commercial yeast you will notice that the less flocculating a yeast is, the more attenuative it will be. This makes sense as the poorly flocculating yeasts will remain in contact with the wort for a longer time. The beech wood aging process used by Anheuser-Bush to brew Budweiser does exactly that without relying on poor flocculation alone; it maximizes the contact area between beer and yeast.
[Narziss, 2005] lists ranges for the differences between finished beer attenuation and limit of attenuation for some German beer types:
- Helles : 2 - 4%
- Export : 0.5 - 2 %
- Pilsner : 0.5 - 4 %
- Bock, Dunkel : up to 6 %
Example: A Helles with a target attenuation difference of 3% should be brewed. The wort has an original extract of 12.0 *P and a forced ferment test extract of 2.0 *P. The resulting limit of attenuation is 100% * (12-2.0)/12 = 83%. The target attenuation of the finished beer is 83% - 3% = 80%. This attenuation is reached when the beer has an apparent extract of 12 - 12 * 80%/100% = 2.4 *P. But hitting this number exactly is difficult. One way would be to regularly measure the extract of the lagering beer and pull it off the yeast into a serving keg when the desired final extract is reached.
Even if a brewer doesn't go to this extend of control over the beer, performing a forced ferment test and comparing the limit of attenuation to the current attenuation can be helpful in understanding the taste character of a beer. It shows if an unexpected high finishing gravity is due to problems during fermentation (low attenuation but high limit of attenuation) or due to problems during the mashing process (the limit of attenuation is also high).
Attenuation Listed for Yeast Strains
As mentioned above, choice of yeast strain also affects the actual attenuation of the beer. As a guideline yeast vendors like Wyeast and White Labs list attenuations ranges with their yeasts. But since the yeast strain is only one factor in attenuation (other important factors are mashing and yeast health) these attenuation values are only useful to compare yeasts with each other and cannot be used to predict the final extract (or final gravity) of the beer. This can only be done with a forced ferment test.
When designing a recipe, or using an existing recipe, the brewer generally has a targeted attenuation (original and final extract/gravity) in mind. Unfortunately there is no formula that can be used to calculate this attenuation upfront, though some recipe design programs attempt this. Such programs generally use the attenuation given for the selected yeast which may not be the attenuation that you will actually be getting.
As shown above, there are 2 parameters regarding attenuation, that the brewer can affect. The first one is the limit of attenuation and the second one is the difference between the final attenuation and the limit of attenuation. The limit of attenuation is set by the wort production. Here is a list of factors that can affect it:
- all grain brewers:
- saccrification rest temperature: This is the first factor that comes to mind for all grain brewers. For a single step saccrification rest, the mash temperature has a great affect on the fermentability of the resulting wort. The lower the temperature (within a given range of course) the longer the beta-amylase will be able to work and produce maltose.
- mash schedule: the choice of mash schedule also affects the fermentability. Some beta amylase activity is already present during a protein rest and the time it takes to heat to the saccrification rest. Another mash schedule factor is the length of the mashing time and the time the wort spends below 175 *F (80 *C). Below that temperature the alpha amylase is still active and can produce fermentable sugars, though not as effective and quickly as the beta amylase.
- water to grist ratio: the enzymatic activity of the amylases is affected by the thickness of the mash. Thinner mashes enhance the maltose production and therefore increase the fermentability.
- grain bill composition (base malt): mashes with high diastatic power (Pilsner, Pale) will produce more fermentable worts since they contain a lager amount of beta-amylase which can produce more maltose than mashes with lower diastatic power (Munich or large amounts of unmalted grains) assuming the same saccrification rest temperature.
- grain bill composition (specialty malts): crystal and roasted malts add unfermentable sugars to the wort which lowers its overall fermentability.
- mash pH: the beta and alpha amylase enzymes have diffent optimal pH ranges (beta amylase : 5.0 - 5.5 pH; alpha amylase : 5.3 - 5.8 [Palmer 2006]) and therefore the mash pH can affect the activity balance between these enzymes. Though the effect is only marginal.
Don't let the number of factors confuse you. The most practical way of controlling the fermentability is adjusting the saccrification rest temperature and time. Because of the other affects, the rest should be kept the same between batches.
- extract brewers:
- type of extract: different extracts have different levels of fermentability. Get to know the extract that you are using
- specialty malts: specialty malts like crystal and roasted malts can add unfermentable sugars thus lowering the fermentability.
- blending extracts: If a lot of control over the fermentability of the extract is desired, the brewer can blend highly fermentable and less fermentable extracts to achieve a desired limit of attenuation. Though this is more predictable than achieving the desired fermentability through mashing most extract brewers rather start all grain brewing before blending extracts
- adding unfermentable sugars: malto dextrin and lactose (milk sugar) are examples of unfermentable sugars that can be added to the wort to make it less fermentable.
- partial mash: this is basically a mix between all grain and extract brewing. The more of the final extract is produced by mashing the more the factors shown for all grain will matter for the fermentability of the wort.
The other important parameter is how close will the final attenuation come to the limit of attenuation. This parameter is influenced by the fermentation process:
- lager vs. ale: lager yeasts are able to completely ferment trisaccarides (maltotriose and raffionse) which ale yeast can only ferment partially
- yeast strain: less flocculant yeasts will remain in suspension longer which gives them the ability to ferment more of the available sugars. Rousing a flocculant yeast can improve the attenuation if that is desired.
- yeast health: healthy yeasts are able to withstand the increasingly toxic environment (ethanol is toxic for yeasts and they rely on healthy cell walls to keep it out of the cells) better than weak ones. This is especially important for high gravity beers where a large amount of healthy yeast is needed to ferment the beer before the high alcohol level starts to kill the yeast.
- pitching rate: The more yeast is pitched the quicker it can ferment more fermentable sugars in wort. But more is not always better. Overpitching a wort, which can happen when harvested yeast is used, can lead to negative impacts on the flavor of the beer as the reduced yeast growth will change the flavor compounds produced by the yeast. A common rule of thumb for ales is 0.75 million cells per milliliter of wort per degree Plato for ales and 1.5 million cells per milliliter of wort per degree plato for lagers [Zainasheff].
- fermentation temperature: Higher temperatures accelerate the yeast's metabolism and the yeast will be able to consume the sugars faster and gerenally more complete. But the production of unwanted flavor compounds limits the fermentation temperature. With good yeast health and sufficient pitching rate the fermentation temperature can be kept fairly low while still ensuring sufficient attenuation.
- agitation: If the yeast is roused regularily, more of the yeast cells will be in contact with unfermented sugars and the yeast will be able to ferment the beer faster and more complete. But since this oftentimes goes against the flavor characteristics desired from a yeast strain, it may only be done in rare occasions. It is suggested to regularily rouse the yeast in a fast ferment test to ensure a complete fermentation even with well flocculating yeasts.
- fermentation time: This is more practical for lager brewing than for ales. During the long and cold lagering phase a very slow fermentation is in progress. By taking the beer off the yeast once the desired attenuation has been reached, a targeted attenuation/limit of attenuation difference can be achieved if the attenuation was already close enough when lagering started. The latter is important since only a few percent of attenuation gain can be expected during lagering.
- [Realbeer] realbeer.com Attenuation and related formulae
- [Narziss, 2005] Prof. Dr. agr. Ludwig Narziss, Prof. Dr.-Ing. habil. Werner Back, Technische Universitaet Muenchen (Fakultaet fuer Brauwesen, Weihenstephan), Abriss der Bierbrauerei. WILEY-VCH Verlags GmbH Weinheim Germany, 2005
- [Palmer, 2006] John J. Palmer, How to Brew, Brewers Publications, Boulder CO, 2006
- [Zainasheff] Jamil Zainasheff, mrmalty.com