The “Reinheitsgebot” -- meaning "purity order" -- is a series of regulations limiting the ingredients in beer. It applies to brewing in Germany and to some states of the former Holy Roman Empire. A well-known version of the law was adopted by William IV, Duke of Bavaria in 1516. However, similar regulations predate the Bavarian order, and the detail of modern regulations deviates significantly from the 1516 Bavarian version.

Although the “Reinheitsgebot” is mentioned in a number of texts about the history of beer today, historically it was only applied in the duchy of Bavaria; it applied in Germany as a whole from 1906 but before that had little impact elsewhere.

According to the 1516 Bavarian law, the only ingredients that could be used in the production of beer were water, barley and hops. Interestingly, the legal text does not mention yeast as an ingredient, although it is known that, at that time, yeast was used in the brewing process. However, it may have been regarded as a continuous by-product of the brewing process, because yeast produced in one batch of brewed beer was commonly transferred to a subsequent batch. A full understanding of chemical basis of yeast and the fermentation process did not come until much later.

The “Reinheitsgebot” remains the most famous law regulating the brewing of beer, and continues to exert influence on brewing not only in Germany, but around the world.

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