The Bavarian purity law of 1516
On 23 April 1516 the Bavarian dukes Wilhelm IV and Ludwig X issued an ordinance at the assembly in Ingolstadt. It is now called the Reinheitsgebot, the purity law, and is often described as the oldest food regulation still in force. That description is generous. The ordinance of 1516 is one of several earlier city and territorial rules, and much of its text sets maximum prices for beer in summer and winter.
The clause that made it famous says that beer may be made only from barley, hops and water. Yeast is missing because nobody knew what it was. Fermentation was still a mysterious process, and brewers kept a vessel of yeasty residue from one batch to start the next.
Why the clause was written
The rule limited the use of wheat and rye, which were needed for bread, and it kept out additives that were believed to be harmful or were used to disguise poor beer. It also protected the ducal interest, since Bavarian rulers later held a monopoly on wheat beer.
The law spread slowly. Bavaria insisted on keeping it when it joined the German Empire in 1871, and a version applied to the whole of Germany from 1906. Today brewers in many countries cite it in their marketing, and others point out that it excludes many good beers, among them Belgian and British styles that use sugar, spices or other grains.