September 18, 2015 by fhsteinbart
In the beer and wine trade, the traditional way of determining the alcohol content in a given sample is to weigh out 100 grams and apply heat until all the ethanol is driven off and weighing the difference. This can be done either isobarically, or isothermally. In this case, we are going to assume an isobaric situation in which the pressure remains the same, while the temperature varies. Traditionally an Ebulliometer was used, and can also be used by intrepid home brewers; however, instead of reflux condensing the solute back into the water or ethanol substrate to keep the volume consistent, we are going to evaporate the solute (aka beer) into the atmosphere. No chart required, as in the Ebulliometer, so it makes for a quick and dirty estimation to within 3% of the actual amount. Typical low end Ebulliometers are around $900 but I’m going to show you a far less expensive way to do this at home. All you need is an Erlenmeyer flask (150ml), double hole rubber stopper, long stem spirit thermometer, glass tube, rubber hose, ring stand with burette clamp, and an alcohol burner. Weigh the flask and tare the weight until your gram scale reads zero. Now add 100 grams of beer into the flask. Have the burette clamp hold the Erlenmeyer flask just above the ethanol burner, then seal with the two holed stopper. Insert the thermometer into one of the holes in the stopper. Insert the glass tube with rubber hose attached to the other end into the other hole in the stopper so that the rubber hose is on top, and the free end of the hose goes into the sink or small sauce pan. Gently heat the beer until the thermometer reads 174°F., and adjust the burette clamp on the ring stand until a constant temperature is achieved. After all the alcohol is boiled off (the temperature will start to increase), allow to cool, then weigh the flask again, subtracting the weight of the flask. Take the new weight and subtract it from the original weight as this will be your total amount of alcohol as found in your beer. Dividing the amount of alcohol by the original weight of your beer will result in your alcohol by weight (ABW) which is 1.25 times the alcohol by volume, so if you got 4 grams of alcohol, then 4%ABW X 1.25 = 5%ABV. All very simple, all very easy to do with low cost equipment available locally or online. If you have any questions, feel free to post comments, email me at the store, call me at 503-232-8793, or drop on by when I’m working at the store!