How do we know how much alcohol is in beer? The chemistry of alcohol production was worked out by German chemists Karl Balling, Adolf Brix and Fritz Plato in the 19th century.
Yeast converts simple sugars roughly evenly into ethanol and carbon dioxide (More precisely, 2.0665g sugar yields 1g ethanol, 0.9565g CO2 and 0.11g yeast).
But we never measure sugar or alcohol directly, which would be too involved. Instead the sugar or alcohol content is inferred from the beer’s density. But this causes its own problems.
Density is the ratio of the mass of a substance to its volume. It is not a fixed thing and varies with atmospheric conditions like temperature and pressure. Typically the warmer something is, the greater its volume and the lower its density. As a further complication, the densities of different substances change with changes in temperature at different rates. The density of ethanol is more “elastic” than the density of water. Increasing the temperature from 20˚C to 21˚C decreases the density of water by 0.00021g/ml but decreases the density of ethanol by four times as much (0.00085g/ml). *
In practice this means all brewery density measurements have to be taken at a set temperature, nowadays 20˚C, or brewers must cross-reference actual measurements with correction tables to find a corrected value.
How do changes in a beer’s density over the course of its production reflect its alcohol content? If we know how much sugar is consumed, we can use Balling’s work mentioned above to determine how much alcohol is present. Comparing the density of wort before fermentation with the density of beer after fermentation, we can use the drop in density as an indication of how much sugar was converted into alcohol by yeast. But ethanol itself has a significantly lower density than water so the raw density drop doesn’t actually tell the whole story. It overstates how much sugar has actually been converted. Strictly, to find the real residual extract (or ‘final gravity’) of a beer, it is necessary to remove and replace the alcohol from the sample with pure water and measure the density of what’s left.
In Ireland, the official method for the determination of alcohol is given in part four of Alcohol Products Tax Regulations 2004 (S.I. No. 379/2004).** ABV is determined by distilling off the alcohol from a precisely determined volume of beer at 20˚C and then making the distillate up to the original volume at 20˚C and determining the density of the alcohol/water mixture thus obtained (e.g. using an alcohol hydrometer or simply weighing the sample), and then cross-referencing that density with a book of alcohol-water tables.
UK revenue formula makes more concessions to the technical limitations of microbrewers and uses drop in specific gravity as measured by a calibrated hydrometer to determine ABV. ABV is change in specific gravity multiplied by a factor that depends on the size of the drop. It’s an empirical correlation judged to be close enough for practical purposes and easy for microbreweries to implement. One problem with this method is that it’s useless if the beer needs to be diluted after fermentation. Dilution with water lowers the density but by that very fact increases the gravity drop, leading the simple formula to yield an increased ABV!
All in all, it can be seen that determination of ABV is not a simple matter. To close with one further illustration of this. Molecular interactions make alcohol-water mixtures tricky in another way: 50ml of water and 50ml of alcohol do not add together to give 100ml of mixture but actually only 96.4ml! Paradoxically, in other words, the component percentages of alcohol water mixtures don’t sum to 100. In view of all this, perhaps ABV is not the best basis to tax and regulate alcohol concentration in alcoholic beverages but that’s a topic for another occasion.
*Manual of Weighing Applications, part 1. Density. Sartorius.
<http://www.dcu.ie/sites/default/files/mechanical_engineering/pdfs/manuals/DensityDeterminationManual.pdf> page 5 (accessed 12 August 2017).
**Alcohol Products Tax Regulations 2004 < http://www.irishstatutebook.ie/eli/2004/si/379/made/en/print?q=alcohol+products+tax> accessed 19-08-2017.