Percentages… in moderation -----------------------------
Wine bottles often display a percentage—12%, for example, on a good bottle of Bordeaux—and people then say that the wine is “twelve degrees.” What does this expression mean?
In winemaking, the percentage is the volume of ethanol—that is, pure alcohol—as a percentage of the beverage's total volume at 20 °C. Thus, one litre of wine at 10% contains 10 cl of ethanol. Since ethanol has a density of 0.79 (0.78935, to be precise), the bottle contains 79 g of ethanol, or pure alcohol. This is therefore a percentage by volume, denoted % vol.
The notion of percentage by mass is also used. In our example, the volume of liquid is one litre, but its mass is only 979 g, since the 10 cl of ethanol weighs just 79 g. The percentage by mass (denoted % mas) is therefore 79/979, or 8.07% mas.
But why speak of degrees? In 1821, Louis-Joseph Gay-Lussac was commissioned to devise a measure of a liquid's alcohol content so that spirits could be taxed. He proposed using the liquid's percentage of ethanol by volume. For many years, the Gay-Lussac degree was used; it corresponds exactly to 1% vol.