The present invention relates to methods of isomerisation of X (xanthohumol) into IX (isoxanthohumol), by incubating a dry spent hop material containing polyphenols under inert conditions at a temperature of between 20 and 100 °C, until a product is obtained with a ratio of IX (w/w %) / X (xanthohumol) (w/w %) which is above 0,035. Typically the dry spent hop material which is used in this method is the residual fraction obtained after the extraction in whole or in part of the soft resins and the hop oil from hops and wherein said dry spent hop material conditions is kept under inert conditions in the period after the extraction and before the isomerisation.
