No mere alcoholic malt drink WITH films, exhibitions and a commemorative stamp, Germany is marking the 500th anniversary on April 23rd of an event considered central to the national identity. It is not Martin Luther’s nailing of his 95 theses to a church door in Wittenberg—that anniversary falls next year. Rather, it is the passing of a law concerning beer by two Bavarian dukes, Wilhelm IV and his brother Ludwig X. Henceforth, they decreed, brewers in Bavaria were to use only three ingredients: water, barley and hops. (They did not yet know about yeast, which occurs naturally in beer but was officially permitted as an additive only later.) Subsequently dubbed the _Reinheitsgebot, _or “German beer purity law”, the regulation spread. Otto I, a Bavarian prince who became king of Greece in the 19th century, subjected his new country’s brewers to its stringent rules (becoming perhaps the first German to impose austerity on the Greeks). Other German regions adopted it after unification in 1871, and from 1906 the whole of Germany did. Today the _Reinheitsgebot_—which governs lagers but...