Originally Posted by Kaiser
To get real extract you need to boil off the alcohol. To do that you take a beer sample of known weight, boil it down by half and then add water to get back to the same weight. Make sure to use this water to rinse all the sugars from the pot back into the sample....
Nothing wrong with the concept of doing it gravimetrically but it is usually done volumetrically as that is easier. Fill a volumetric flask to the mark with cold beer. Use a pipet to suction off the foam that rises into the neck. Put the flask in a water bath and, when at equilibrium at 20 °C, use the pipet to bring down to the mark (the beer will have expanded as it warms to 20). Now transfer quantitiatively to a distillation flask. "Quantitiative" transfer means that you rinse the volumetric flask with two 25 mL portions of DI water and transfer the rinsings to the boiling flask. Now boil until down to 1/3 the original volume. Turn off the heat and allow to cool. Transfer quantitatively back to the volumetric flask, fill up to the neck with DI water and put back in the water bath. When equilibrated make back up to the mark with DI water, stopper and mix thoroughly. Measure the extract of this 'reconstituted' beer and convert to specific gravity. The mass of the extract in the reconstituted beer is V*(°Pr/100)*SGr*.998203 (r designates 'reconstituted'). This all came from the beer. The true extract is the number of grams of extract per 100 grams of beer. The mass of the beer was V*SGb*.998203 (b designates 'beer'). The true extract is, thus 100*V*(°Pr/100)*SGr*.998203/V*SGb*.998203 = °Pr*(SGr/SGb).
Note that this is the ASBC procedure (MOA Beer-5,A) for determining the true extract. It is usually done as part of the alcohol determination procedure in which the the 'steam' from the boiling is condensed and the density of the condensate measured in order to determine how much alcohol was in the beer.
[Edit] Upon reflection I guess whether the volumetric method is easier than the gravimetric (ASBC MOA Beer-5, B) would be a matter of personal preference. It may or may not be easier to bring volume to 100 mL than weight to 100 grams but if you are weighing you do not need to worry about temperature (no water bath required) and you don't have to do any calculations (other than converting SG to Plato if your instrument does not read Plato).