The exclusive use of Gruit herbs in Europe started to change around 1100AD when it was learned that hops were a very good antiseptic, meaning that they retarded the growth of bacteria and preserved the ale longer then the herbs did. Gruits received their walking papers in 1516 when Bavarian brewers adopted the tenets of the Reinheidsgebot. Remember also that there was no refrigeration in those days and a batch of brew would last only so long with just alcohol and the herbs or hops to prevent spoilage. Because of this bacteria problem the brews of that era were stronger in alcohol and bitterer then today’s beers.