work in progress
--Kaiser 22:51, 25 November 2007 (CST)
Attenuation refers to the percentage of starting 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, there is an almost linear relationship between (specific gravity - 1) and extract percentages. This formula can be changed to:
Attenuation = 100 % * (starting gravity - current gravity) / (starting gravity - 1)
to work for brewers who measure extract as 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 and then to beer. 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 compopunds (sugars, detxtrines, proteins and others). Once the conversion is complete, the mash is lautered. During this process not all of the dissolved extract is extracted. The perentage 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. Coagulation of proteines change 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.
When during the wort fermentation, 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 comsumed. 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 calibrtated for measuring the sugar (extract) content of a water soulution. This is true for wort. But when they are 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. 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 * apparant extract