The bitter substances will degrade in beer during storage,and the change of bitter substances in beer during storage was studied by determining cis/trans stereoisomers of iso-α-acid using HPLC. As a result,the trans form of iso-α-acid decreased faster,while the cis form decreased scarcely. The same brand with forced aging and natural aging except of the different brands of beers with forced aging were reviewed. The T/C ratio showed an evident linear relativity with staling time and stale flavor intensity,so it can reflect the extent of beer staling excluding the difference of manufacturing dates even brands,and it would become a new index to estimate beer staling better.

An analytical method of cis/trans stereoisomers of iso-α-acid in beer was established. The sample was obtained by liquid/liquid extraction with iso-octane. Then aim compounds was separated by Purospher STAR RP18 (4.6min×250mm, 5μm) under an isocratic elution system of methanol-ammonium acetic buffer and completed within 50 minutes. The HPLC-diode array detector-electrospray ionization-mass spectroscopy (HPLC-DAD-ESI-MS) was used to differentiate these compounds according to the selectivity of the ESI-MS. The aim peaks were identified by the retention time, UV and mass spectra. The method is sensitive and accurate with a detection limit of iso-α-acids at 0.21mg/L.