The influenceofdryingmethodonthemoisturesorptionisothermsoflemonbalmleaves
(Melissa officinalis L.) wasinvestigatedat25and50 ◦C usingthehygrometricmethod.Equi-
librium relativehumidity(aw) intheheadspaceofthesampleswasmeasuredatseveral
moisture contents(0.04≤Xe ≤0.45 kg/kgdrybasis)developedbyconvective-,vacuum-and
freeze drying.TheessentialoilconstituentsweredeterminedbyGC/FID,whilerosmarinic
acid wasanalysedbyHPLC.Theexperimentalsorptiondatawasmodelledbythemodi-
fied Halseyequation.Theisostericheatofdesorption(Qst) wascalculatedasafunctionof
moisture contentandtemperaturedirectlybythemodelinwhichtheClausius–Clapeyron
equation hasbeenincorporated.Thedifferentsorptioncharacteristicswereascribedto
modifications inthematrixstructurecausedbythemodeofdrying.Sublimationofwater
vapour duringfreezedryingresultedinahighersorptioncapacityfor0.1≤aw < 0.8. Qst
increased asmoisturecontentandtemperaturedecreased,increasingsharplyfor Xe below
0.2 kg/kg. Qst of freezedriedleavesat Xe of 0.10kg/kgwassignificantlyhigherthanthat
of vacuum-andairdriedmaterial.Geranialcontentwashigherafterfreeze-orconvective
drying, whilevacuumdryingledtolowercontentsofneralandgeranial.Freeze-andvac-
uum driedsamplesexhibitedgreaterrosmarinicacidcontentthanthehot-airdriedleaves
of M. officinalis.