the main factor for the replacement of spices by hops is that hops were cheaper (especially in the gruit area, where the price of beer flavouring spices was artificially kept high) and were better at rendering the beer keepable. this preservative effect is thought to have had a large impact on the early movement to switch over, although other plants commonly used in gruit mixes, for example sage, rosemary or bog myrtle, also have antiseptic properties likely to extend the shelf life of beer.